A PUBLICATION OFTHEDEPARTMENT OF CONSERVATION

DIVISION OF MINES AND GEOLOGY

Slale or Cal,lo<"", GEORGE OEUKMEJIANGovernor

GORDON K VAN VLECKSacretary for Resoorces

In This Issue'

SHORT MINING COURSES , 242GEOLOGICAL SOCIETY OF AMERICA CONFERENCE 242LOMA PRIETA EARTHQUAKE , , 243PLATE TECTONICS AND THE GULF OF CALIFORNIA REGION 252DMG PUBLICATION-SPECIAL PUBLICATION 98 257BOOK REVIEWS , , 258EARTH ON THE MOVE . 261NOMINATIONS FOR ALFRED E. ALQUIST AWARD 263MAIL ORDER FORM , 263SEISMIC INTENSITY DISTRIBUTION MAPS 264

CALIFORNIA GEOLOGY starr

Depa,tment ot Conae,vallon

Dtv'SIOn 01 M,nes & Geology

Technical Editor:Assislant Ed'tor:Graphics and Oes,gn'Publications Supervisor·

RANDALL M WARDDI,actor

JAMES F DAVISSrate GeologISt

Don DuprasLena Tob,llo

Louise HuckabyJell Tambert

Cover: Typical terrain in the Santa Cruz Mountains. View IS to the northeastbetween Milldego Hill and Alpine Creek Aoad. Note the lobe-shapedearthllows. The epicenter 01 the devastaling October 17. 1989 Lorna Prietaearthquake occurred In these mountains southeast of where this photo wastaken. Standard practice for naming a signilicant earthquake is to name itafter the nearest prominent geographic feature to the event epicenter. In thiScase Lorna Prieta peak. the highest peak in the Santa Cruz Mountains. Anarticle about how the regional geology and tectonism of this region is relatedto the Lorna Prieta quake is on page 243. DMG photo tite.

The Division of Continuing Educationat the Mackay School of Mines. Univer­sity of Nevada. is offering the followingmining-related courses.

P"nle<1 [)epi"tmenl of O_al Se,vocesOtl~ 01 Slate Ptlnllng

OWOSIOfl HelKlquarll1f,' 1416 Ninth SI,ee1. Room 1341Saaam""to. CA 9S814(Telephone 916-445-1825)

PU~'Ca!IOfISand InfOfmaHon Oll,ce660 Be,CU1 Otlve. Sec'amento, C" 9S81H1131Pu~oc Inlormahon 916-445-5716

Los Angeles ()lIke 107 South Bloadway, Room 1065Lin AnQaIes, CA 90012·4402tTelephOne 213-620-3560)

Pleasanr Htli ()ltoco 380 CIVIC D'l'o'e. Su"e 100.Pleasanl H,II, CA 9-4523·1921(Telephone 415·646-5920)

CAUFOfl.NIA GEOLOGY (ISSN 0026 455511$ pubI<S/IIdM'lCH'lh1y by tl>e Depa,1rT\ent ot Con_vallon. DWI!.lDII 01M,nel and Geology Tt!e Records OllocelS It lnl·2OthStrHt. Sacramento. CA 95814 second cia,. post~ ISps,d al Sac,amento, CA POSlmasle, Send add,lSscllanges 10 CALIFORNIA GEOLOGY (USPS 350 6401.Bo. 2980. 56c,amento. CA 95812-2980

RePOl"ls COfl(;e,n"\g Olvlloon ot M,r.es and GooIOgyp<otK!land a,llCles and ......s ,Ierns ,,,lallKl to tl\eea'lhlI<;,enceSIfl Cahfo,"la e,e lfIducllKllflllle mllo\lez,r.e C0n­\! ,butlKl a, Ioclel. pIIOtog'8Qhs. r.e"s II eml. """geoI09oca1mee!,ng announclf"fill! a'" welcome

THE CONCLUSIONS ANOQPINIQNS EXPRESSED INARTICLES ARE SOLELY THOSE OF THE AUTHORSAND "RE NOT NECESSARILY ENDORSEO BY THEOEPARTMENT OF CONSERVATION

Correspondence should be add,esled 10 EdIlO'CALIFORNIA GEOLOGY, 66Oa.<CU1 Of_, SacfamenlOCA 9~14-0131

SUbscf'lltlOflS 510.00 PI< yea' S,ngl" copies $1 25Ndl Send ..,bsa""l011 Of"",", Ind cNlnge III addflSl",lo<rnahOf'l1O CAliFORNI'" GEOLOGY PO 80.2960Sacfam""lO. C... 95812-2980

November 1990NoIume 43/Number 11

CGEOA 43 (11) 241-264 (1990)

ShortMining Courses

January 7-11, 1991:Hydrothennal Alterations. $495.

January 8-12. 1991:Elementary Geostatistic5. $626.

January 14·16. 1991:Applied Geostatistics. $395.

January 14-15.1991 orJanuary 17·18: Fire Assaying. $275.

February 11·13. 1991:Blast Fragmentation. $395.

For further information contact:

Leanne Stone.Program CoordinatorDivision of Continuing EducationMS 048. University of NevadaReno. NV 89557-0024Telephone: (702) 784-4046 ~

Geological Societyof America Conference

The 103rd Annual Meeting andExposition of the Geological Society ofAmerica (GSA) will be held Irom Octo­ber 21-24.1991 in San Diego. It willbe held at the San Diego ConventionCenter. III W. Harbor Drive. Theopening session. titled MThe GlobalChallenge. M will include a Earth-wideperspective concerning shrinking re­sources. geologic hazards. and climaticchanges. Technical sessions will includefield trips and a broad spectrum 01geologic disciplines.

For further information contacl:

Geological Society of America3300 Penrose PlaceBoulder. CO 80301-9140Telephone: (303) 447-2020FAX, (303) 447-1133 j<

COrTection, There is an error in thecrossword puzzle on page 213 of theSeplember 1990 issue. Line 11 Acrossasked for an equivalent word for ~spe'

cilic gravity." The answer. ~density"

however, is wrong because these lwoterms have different meanings.Specific Gravity is the ratio of thedensity of a substance to that of water,Density is the mass per unit volume.

,<2 CALIFORNIA GEOLOGY NOVEMBER 1990

Geologic and Tectonic Setting of theEpicentral Area of the

LOMA PRIETA EARTHQUAKESanta Cruz County, California

By

DAVID l. WAGNER. GeologistDivision of Mines and Geology

FlQure 1 Map of the San Fraoosco Bay area shoW1ng lauhs thai make up theSan Andreas fault system.

.'""

~-N-

I

'\

o

u .........-- ". ,uJ' •_........,~ ~"'"

cut through the relatively weak marinesedimentary rocks that underlie much ofthe range and formed deep steep-walledcanyons. Slope failures are commonand. as a result. thick unstable landslidedeposits cover much of the range.

CALIFORNIA

the San Francisco peninsula southwardto the Pajaro River east of Watsonville(Figure I). Most of the uplift of theSanta Cruz Mountains occurred in thelast few million years. In response tothis rapid uplift. streams have vigorously

T he Lorna Prieta earthquake ofOctober 17. 1989 was the latest

displacement along the San Andreasfaull; displacement that has occurredduring the laSI 30 million years.Throughout this span of geologic timedistinctive rock types and geologic fea­tures have been offset 186 to 205 milesalong the San Andreas fault in centralCalifornia (Nilsen and Clarke. 1975;Graham and others, 1989). The SanAndreas fault bisects the Santa CruzMountains in a northwesterly directionand is a boundary bet\Aleen two largecrustal plates. the Pacific plate to thewest and the North American plate tothe east. The rocks. structure. and geo­morphoklgy of the Santa Cruz Moun­tains "'ere formed by movement andassociated defonnation along the SanAndreas fault system.

santa Cruz Mountains

The Santa Cruz Mountains are asparsely populated. moderately rugged.heavily lorested range that extends from

Although shaking from the magnitude7.1 October 17, 1989 Lorna Prieta quakelasted only about 15 seconds. the lemblorresulted in 67 deaths. more than 3,500people injured, and property damage esti­mated at $7.5 billion. It was lelt over anarea of about 400.000 square miles andcaused the btggest dollar loss 01 any natu­ral disaster in United Stales history. Thelollowlng article was onglnaJly published inthe DMslOn of Mines and Geology 5peclalPubbcabOn 104 and describes the fegJOOaIgeology 01 this catastrophIC quake. Suchdetailed invesllgatlonS of earthquake proc­esses aid In our understanding 01 how.wl\ere-and nopelully-when they willhappen In the luture...edltor.

CALIFORNIA GEOLOGY NOVEMBER 1990 '"

REGIONAL TECTONIC SETIING

D

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f

,,,o 100,Mil..

The San Andreas fault system is aboundary between two major parts ofthe Earth's crust. the North Americanplate and the Pacific plate. The Pacificplate moved northwesterly relative tothe North American plate at an averagerate of about 40 millimeters/year (mm/yr) during late Cenozoic time (Stock andMolnar. 1988). Current movementalong this boundary causes displace­ment and associated earthquakes alongthe San Andreas fault system. Overgeologic time. changes in the directionand rate of relative movement of thePacific and North American platescaused periods of folding. faulting. anduplift in the Santa Cruz Mountains aswell as in the rest of central California(Page and Engebretson. 1984).

The Coast Ranges of central Califor­nia are a series of mountain ranges andvalleys that trend northwest parallel tothe San Andreas fault. This region is acollage of fault-bounded 'tectonicblocks' or masses of rock that weretransported by geologic forces. All ofthe boundary faults between the blocksare part of the San Andreas fault sys­tem (Figure 1).

Figure 2. Map of California showing the distribution of basement rock types in partsot central and northern California. Movement along the San Andreas fault hasjuxtaposed the Salinian block composed of Cretaceous age granitic rock withbasement composed of rocks of the Franciscan Complex. Modified from Page (1978).

GEOLOGY OF THESANTA CRUZ MOUNTAINS

Movement along the San Andreasfault juxtaposes tectonic blocks of dis­tinctly different "basement" rock typesthat underlie the central CaliforniaCoast Ranges (Figure 2). Northeast ofthe San Andreas fault. Cenozoic sedi­mentary and volcanic rocks overlie abasement rock that is composed ofheterogenous, highly deformed oceanicrocks of the Mesozoic Franciscan Com­plex. Southeast of the fault the base­ment is. for the most part, continentalcrust of granitic and metamorphic rocksknown as the SaHnian block (Figure 2).

Although the San Andreas fault sys­tem is the boundary between FranciscanComplex rocks to the east and the Sal­inian block to the west. in places Fran­ciscan Complex rocks do occur west ofthe fault. The Pilarcitos block (Figure 3}is a sliver of basement rocks of theFranciscan Complex to the west of theSan Andreas fault. These rocks are

thought to be evidence that FranciscanComplex rocks underlie part of the cen­tral Santa Cruz Mountains (Stanley,1985).

Rock Units Southwest of theSan Andreas Fault

The part of the Salinian block thatunderlies the Santa Cruz Mountains wassuixlivided into sub-blocks by Stanley(1985) as shown on Figure 3. Two ofthese sub-blocks. the La Honda blockand the Ben Lomond block underlie theepicentral area of the Loma Prietaearthquake. The La Honda block iscomposed of the bulk of Tertiary fonna­tions in the central Santa Cruz Moun­tains (Figure 3. Table 1). These forma·lions were deposited in the La Hondabasin (Cummings and others. 1962).

To the southwest. the La Honda basinwas bounded by the Ben Lomondblock. a granitic highland that providedsediment to the La Honda basin (Nilsenand Clarke. 1975; Graham and others.1989).

Stanley (1985) modified the Tertiaryformalions described by previous au­thors (Cummings and others, 1962;Brabb. 1970; Clark. 1981) into live~deposilional sequences

M

(Figure 3.Table 1) and from oldest to youngestare: (1) a Paleocene sequence. (2) anEocene to lower Miocene sequence. (3)a lower to middle Miocene sequence.(4) a middle to upper Miocene se­quence. and (5) an upper Miocene toPliocene sequence. Boundaries be­tween each of the live sequences areunconformities.

'" CALIfORNIA GEOLOGY NOVEMBER 1990

PIGEONPOINT --­BLOCK

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BUTANOSANDSTONE

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3 •Tp.YOUNGE Tp.YOUhGER

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n. '"MINOEGBASALT Tyl.T,.T"*'

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COLUMNS " .. WMIS'"ffi0 "ILL,"DIu' tlo""dOJ • '"MATlON

--{uncon'o,"'it, "IIID ~iM"' "~ ..t;ffi ffi~' ISCAN P,GEON PT FMO· ,~

...." SALIN '"00 ""MENT

Figure 3. Map of fault-bounded blocks and representative stratigraphic columns in the Santa Cru2; Mountains. Tp.- PurismaFormation; Tsc .. Santa Cru2; Mudstone; Tsm .. Santa Margarita Formation; Tm .. Monterey Formation; Tlo .. Lompico Sandstone,Tla .. Lambert Shale: Tmb., Mindego Basalt: Tvc and Tvl .. Vaqueros Sandstone; TST ... Rices Mudstone. Modified from Stanley (1985).

The Paleocene sequence of rocksconsists of the Locatelli Formation.Patches of sandstone. mudstone. andconglomerate of the Locatelli Forma­tion rest unconformably on graniticbasement rock of the Ben Lomondblock. The Locatelli Formation hasnot been positively identified on theLa Honda block.

The Eocene to lower Miocene se­quence of rocks consists of the ButanoSandstone. the San Lorenzo Forma­tion. the Vaqueros Sandstone. the lay­ante Sandstone. the Mindego Basalt.and the Lambert Shale. These unitsaccount for the bulk of sedimentary fillin the La Honda basin.

The Eocene Butano Sandstone isthe oldest formation of the Eocene to

lower Miocene sequence (Photo 1). [t

overlies the granitic Ben Lomond blockbut its base has not been obselVed onthe La Honda block. The Butano Sand­stone is correlated with the Point ofRocks Sandstone of the Temblor Range(Clarke and Nilsen. 1973). 186 to 205miles to the southeast across the SanAndreas fault. This correlation indi­cates that the La Honda basin was con­tinuous with the San Joaquin basinprior to the existence of the modernSan Andreas fault (Stanley. 1985: Gra­ham and others. 1989).

The San Lorenzo Formation. theVaqueros Sandstone. and the layanteSandstone overlie the Butano Sand­stone and are widespread on the LaHonda block. These units are stratigra-

phically complex and are described indetail by Stanley (I985).

The Mindego Basalt was erupted intothe La Honda basin between 20 millionyears ago (Ma) and 25 Ma (Stanley.1985). Rows. tuffs. breccia. pillow la­vas. and intrusive rocks were eruptedfrom submarine vents. probably nearthe center of the La Honda basin.Overlying the Mindego Basalt is theLambert Shale. a unit that is wide­spread on the La Honda block but notknown on the Ben Lomond block.

The middle Miocene sequence of theSanta Cruz Mountains is represented bythe Lompico Sandstone and the Mon­terey Formation. The Monterey Forma­tion consists of siliceous and calcareous

CALIFORNIA GEOLOGV NOVEMBER 1990 '"

Table 1. STRATIGRAPHIC COLUMNS OF THE SANTA CRUZ MOUNTAINS. Stanley (1985) revised the stratigraphy of Brabb (1970),and Clark and Brabb (1978), and separated an upper Miocene deposll1onal sequence wesf of the San Andreas. Beaulieu (1970)concluded that the Butano Formation, shown on older maps on both sides of the San Andreas fault (Dibblee. 1966). are differentformations. The wavy lines are unconlormlties of regional extent

CENTRAL SANTA CRUZ MOUNTAINS EASTERN SANTA CRUZ MOUNTAINS

1lRA11, I,TO, A,NOSTANLET, I9U~L.IJlK A,NO 111""1, ItTI

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MIOCEtl'fro SA,NTA CRUZ MUOSTON£PLIOC£N£ "'IOOL£SEOUEtl~E

SANTA "'AIIGARITA SANTA ~ItUZ "'UOSTONE '"$AN CST ON ESANTA "'AIIGAIIITA

UPPEIIMIOC£NE

SANDSTONE SEOUENCE

LOWEllMIOOLE 1Il0NTEREY SHALE (011 MONT'fIl'fY fOIlMATION '"Mloa:N£ "'ONTEIl[T fORMUIONl 1Il100LESEOU'fI'fCE MIOCENE

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MIOCEN[ YM:ll [flOS SANOSlO/'lt: 1Il1QC[N[,=~ ZAYANT( SANDSTONE --LAUIIEL UHIT ,,~

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SAN LOIIENZO fOllM,uIOf'l SAN LOIIEIlZO fOll"'AliON--RICES "'UDSTONE M~II - -IIICES IIlUOSTONE MOlBEIl.. TwOllAR SHAU MEMBEII ____ ILOOMS CREEK

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shale. mudstone. and sandstone.Patches of Monterey Formation arepresent throughout the Santa CruzMountains suggesting it was once muchmore extensive than it is today (Stanley.19851.

An angular uneonfonnity separatesthe lower Miocene sequence from theupper Miocene sequence. The upperMiocene sequence consists of the SantaMargarita Sandstone and Ihe SantaCruz Mudstone. The thickest part ofthe lower Santa Margarita Sandstoneapparently was deposited in a five to sixmile wide seaway that connected Iheancestral San Joaquin basin to the Pa·cific Ocean (Phillips. 1983). Largecross beds in the sandstone werefonned by strong tidal currents in the

northeast trending seaway and werelater truncated by displacement alongthe San Andreas fault. The overlyingSanta Cruz Mudstone was deposifed in adeepening sea basin that was caused bya marine transgression (Phillips. 1983).

The upper Miocene to upper Plio·cene Purisima Formation is the young­est and most widespread Tertiary for­mation in the Santa Cruz Mountains(Photos 2 and 3). The Purisima Forma·tion is present throughout the SantaCruz Mountains and is widespread off·shore (Greene, 1977). Quaternary de­posits. including the Aromas Sand. ma­rine and river (fluvial) terrace deposits.and alluvium, overlie the PurisimaFormation.

Rock Units Northeasl of theSan Andreas Fault.

Northeast of the San Andreas fault isthe San Francisco Bay block (Rgure 3).Heterogenous sedimentary, igneous.and metamorphic rocks of the Francis'can Complex form the basement of theSan Francisco Bay block. Tertiary andQuaternary sedimentary rocks overliethe Franciscan Complex basement.

Rocks of the Franciscan Complex in­clude graywacke. shale, altered basalticrock (greenstone). chert. limestone. con­glomerate. and metamorphic rocks;all of which are Late Jurassic throughLate Cretaceous in age. Associatedwith the Franciscan Complex is green toblack serpentine which has been incor-

2" CALIFORNIA GEOLOGY NOVEMBER 1990

porated into the Franciscan Complex bycomplicated tectonic processes. Francis­can Complex rocks have diverse geo­logic and geographic origins: they cametogether when the ancestral Faral10nplate was subducted (underthrust) be­neath the North American plate.

The Tertiary formations that overliethe Franciscan Complex are superficiallysimilar to those of the La Honda basinwest of the San Andreas fault, so thesame formation names have been com­monly used for both areas (Dibblee.1966). However. Beaulieu (1970) com­pared the stratigraphic units on bothsides of the San Andreas fault and foundSignificant differences in age and envi­ronment of deposition and renamedsome of the fonnalions (Figure 3. Table1). Beaulieu concluded that the fonna­tions were deposited at sites far apartand were subsequently juxtaposed bymovement along the San Andreas fault.

Landslides

Photo 1. Bedded sandstone of the Eocene age marine Butano Sandstone in the SantaCruz Mountains along Butano Ridge. the type area where this unit was lirst defined inthe geologic literature. DMG photo file.

Landslides are a common geologicfeature of the Santa Cruz Mountains andoccur on both sides of the San Andreasfault. The mountain range has been up­lifted rapidly over the last two to threemillion years and. as a result, the slopesare steep and prone to landsliding.Some coalescing landslides cover entireslopes and are several square miles inextent (Spittler and others. 1990).

FAULTING AND FOLDING IN THESANTA CRUZ MOUNTAINS

Faults and folds in the Santa CruzMountains (Figure 4) are the result ofwrench tectonics caused by the interac­tion of the North American plate withthe Pacific plate and the now subductedFarallon plate (Figure 5). Consequently.most of the faults and folds trend north­west and are parallel to the Pacific-NorthAmerican plate boundary.

San Andreas and Related Faults

Large-scale lateral displacement alongthe San Andreas fault was first recog­nized by Hill and Dibblee (l953). whodocumented 310 to 373 miles of right­lateral displacement since the Late Cre­taceous. They also presented data thatsuggested displacement decreased with

decreasing age of the offset geologic fea­tures. Thus. Hill and Dibblee treated theSan Andreas fault as a continuous faultzone along which movement occurred ata consistent rate. Subsequent studies.however. have shown that the San An­dreas is not a single fault. but a complexsystem of related fault zones (Crowell.1962. Nilsen and Clarke. 1975: Dickin­son and Snyder. 1979). Also. there isabundant evidence that fault activity oc­curs in episodes and is not continuousthrough geologic time (Sims. 1989).

The first application of the plate tec­tonic theory to San Andreas fault move'ment was by Atwater (1970). Sheshowed that the San Andreas fault sys­tem is a boundary between plates of theEarth·s crust and that movement alongthe fault system is a result of interactionbetween plates. The plate tectonicmodel was later refined (Carlson. 1982)to show three stages for the develop­ment of the San Andreas fault systemsince Late Cretaceous time (Figure 5).

Stage I occurred prior to 42 Ma andwas characterized by the subduction ofthe Farallon plate beneath the NorthAmerican plate. Oblique convergencebetween these two plates in this stageresulted in right-lateral displacementalong an ancestral San Andreas fault.During this stage. the parts of the Salin­ian block that would eventually be thebasement of the Ben Lomond and LaHonda blocks were juxtaposed in whatis now the Santa Cruz Mountains.

During Stage 2. 42 to 30 Ma. con­vergence between the Farallon plate andthe North American plate was perpen­dicular to the plate boundary: no right­lateral displacement occurred betweenthe two plates. By 30 Ma. Stage 3 be·gan when the Farallon plate had beenentirely subducted beneath the NorthAmerican plate. marking the birth of theSan Andreas transform fault system(Atwater. 1970). and resumption ofright-lateral displacement along the SanAndreas fault in central California.

CALIFORNIA GEOLOGY NOVEMBER t990

Evidence that the rate of movement along the San An­dreas fault varied over geologic time was shown by Sims(1989). During the past five million years the rate of slipalong the San Andreas fault has averaged about 33 mm/yr.greater than at any time since its inception 30 Ma (Sims,1989).

Presently the Pacific plate continues to move rapidly pastthe North American plate. at a rate of 48 mm/yr (DeMetsand others. 1987). However. in the Santa Cruz Mountainsthe San Andreas fault accounts for only about 13 mm/yr ofthis movement (Minster and Jordan. 1987). Additionalmovement is accommodated by other faults in the San An­dreas fault system, notably the San Gregorio-Hosgri fault aswell as faults associated with tectonic deformation to the eastof the Coast Ranges. Geologic features were shown to havebeen offset 71 miles along the San Gregorio-Hosgri faultsince the Miocene by Graham and Dickinson (1978).

A significant component of compression. about 9 mm/yr.was shown to occur in a northeast-southwest direction acrossthe San Andreas fault by Minster and Jordan (1987). Thiscompression is consistent with subsurface movement that oc­curred during the Lorna Prieta earthquake. Movementcaused by the Lorna Prieta quake occurred in a reverseoblique sense. along a fault plane that dips 70° to the south­west (Plafker and Ganoway. 1989). Right lateral displace­ment from the quake was calculated to be 6.2 feet. reverseslip was 4.3 feet. and the Santa Cruz Mountains were up­lilted about 14 inches {Plafker and Galloway. 1989. p. 6}.

•Photo 2. Sandstone of the PurisimaFormation exposed in sea dills south ofCapitola at the southwestern edge of theSanta Cruz Mountains. Verticle jointsprofoundly intluence the pallern of erosionalong the cliHs. The straight cliff faceoccurs along one joint and a second joint isoriented at right angles to the cliff lace.The slab (center) bounded by these planesis being undercut by waves and could failat any time. The rubble at the base of thecliff is material that has failed previously.Nole house al top of cliff for scale. Photobyaufhor.

Photo 3. Coastal cliHs in the northern ....Santa Cruz Mountains near the mouth ofSan Gregorio Creek. The middle .....hiteunit is a rhyolitic tuff interbed of theMiocene and Pliocene Purisima Formation.This tormation contains massive marinesandstone and is widespread throughoutthe Santa Cruz Mountains. Asand andgravel deposit 01 Quaternary marineterrace uncomlorably overlies the PurisimaFormation. DMG photo file.

'" CAllFORNIA GEOLOGY NOVEMBER 1990

GENERALIZED MAP OFFOLDS AND FAULTSLA HONDA BASIN AREA

~ lh,." r""it______ ••1,., ..1••, 1.ld

...-....-- hl••t "'.J.' ••11011••

1 ~ ....1. ot ".Jo' 'Y.olI.,

Figure 4. Generalized map of faults and folds southwest of the San Andreas 'ault (modified from Sranfey, 1985). Berrocal andSargent fault zones (from McLaughlin, 1974). Major folds are labeled as tollows: SA .. Butano anticline, BBS .. Big Basin syncline,BlMA .. Ben Lomond Mounfaln anticline. GS .. Glenwood syncline. HHA .. Haskin Hill anticline, JA .. Johansen anticline. lHA .. LaHonda anticline, SGS .. San Gregorio syncline, SlS .. San Lorenzo syncline. SMA .. San Morena anticline, SVS .. Scotts Valleysyncline. WCS .. Weeks Creek syncline.

Other Faults in theSanta Cruz Mountains

Between the San Andreas and theSan Gregorio-Hosgri fault zones. thereare several predominantly dip-slipfaults (Figure 4). These are the Zay­ante-Vergeles. Ben Lomond. Butano.La Honda, and Pilarcitos faults. Eastof the San Andreas fault is theSargent-Berrocal fault zone.

The Zayante-Vergeles fault mayhave developed about the same timeas the San Andreas fault and pro­foundly affected the sedimentary his­tory of the Santa Cruz Mountains(Clark and Rietman. 1973). This faultis considered to be a normal fault byClark and Rietman (l973). but it couldbe a strike-slip fault (Stanley. 1985).Fault-related features and the occur­rence of small earthquakes suggest thatthe Zayante-Vergeles fault is an activebranch of the modern San Andreasfault system (Coppersmith. 1979).The epicenter of the Loma Prieta

earthquake coincides with the mappedsurface trace of the Zayante-Vergelesfault. but the mainshock and the after'shock sequence do not (Plafker and Gal·loway. 1989. p. 7).

The Ben Lomond. Butano. LaHonda. and Pilarcitos faults are poorlyexposed and little is known about them.The Ben Lomond fault is cUlved in thesubsurface and may have been activeinto Pleistocene time (Stanley and Mc­Caffery 1983). Uttle is known about theButano and La Honda faults. The Pilar­citos fault could be an inactive and aban­doned trace of the San Andreas fault.

East of the San Andreas. faulting isdominated by reverse faults of theSargent·Berrocal fault zone (Mclaughlin.1974). The Sargent fault is a reversefault that dips steeply to the west and isseismically active (Mclaughlin. 1974;Mclaughlin and others. 1989). Al­though there is evidence of surfacemovement on the Sargent fault duringHolocene time (Bryant and others.

1981). its relationship to the San An­dreas fault is unknown.

The Berrocal fault zone occurs to thenorth of the San Andreas fault zone.This fault is a complex thrust fault thatdips 5"-75" to the southwest beneath theSanta Cruz Mountains and is vertical insome places (Mclaughlin. 1974). Fran­ciscan Complex rocks are thrust as muchas 1.5 miles eastward over Cenozoicrocks along the Berroca! fault zone(Mclaughlin. 1974).

Folds in the Santa Cruz Mountains

Northwest trending folding is a con­spicuous feature of the Santa CruzMountains. particularly on the La Hondablock (Figure 4). Folds on the La Hondablock are isoclinal-or folds with parallellimbs-and overturned. These folds con­trast sharply to the more open folds onthe Ben Lomond block. Stanley (l985)allributes this contrast to the proximityof the La Honda block to the San An­dreas and the rigid granitic rock of theSalinian basement.

CALIFORNIA GEOLOGY NOVEMBER 1990 '"

Figure 5. Schematic illustration of the three slage history 0' plate interactions in thedevelopment of the San Andreas fault (modified from Carlson, 1982. by Stanley. 1985).

Displacement along the San An­dreas fault in central California hasbeen variable over the past 30 millionyears. Changes in direetion and rate ofrelative movement between the NorthAmerican and Pacific plates are re­sponsible for sedimentation pallernsand tectonic events in the Santa CruzMountains. At present. movementalong the San Andreas fault system isrelatively rapid and is oblique to thePacific-North American plate boundary.This type of movement caused the fold­ing. reverse faulting. and uplift duringthe 1989 Loma Prieta earthquake.

ACKNOWLEDGEMENTS

Mountains. West of the San Andreasfault. continental granitic basement ofthe Salinian block is overlain by marinesedimentary and volcanic formations.East of the fault. the basement is aheterogenous assemblage of oceanicrocks of the Franciscan Complex. Ter­tiary sedimentary and volcanic forma­tions that overlie Franciscan Complexbasement rock superficially resemblethose immediately across the fault.however, detailed studies show thatthey originated far apart.

Early versions of this article werereviewed by John Sims, Charles Jen­nings. George Saucedo, and RobertSydnor. Their comments are greatlyappreciated. The figures were pre­pared by Debbie Maldonado. HeidiKruger typed the manuscript.

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REFERENCES

During Cenozoic time, episodes offolding and uplift alternated with epi­sodes of subsidence. basin formation.and marine transgression where theSanta Cruz Mountains are now. Theseepisodes are related to changes in therates and direction of plate movement(Page and Engebretson, 1984). Dur­ing periods of rapid plate movementthat is oblique to the plate boundary,folding. reverse faulting, and uplift oc­cur. During periods of slow platemovement, rifting causes sedimentarybasins to form and marine transgres­sion to occur.

For about the past three million yearsthe plate movement has been relativelyrapid and there is a significant compo­nent of compression in a northeast­southwest direction. perpendicular tothe San Andreas fault. As a result.reverse faulting and uplift is occurring,typified by the 1989 Loma Prieta earth­quake.

SUMMARY

Movement of the Pacific plate rela­tive to the North American plate hasjuxtaposed geologically different base­ment terranes as well as sedimentarycover in the southern Santa Cruz

Atwater, T., 1970, Implication of plate tec·tonics for the evolution of western NonhAmerica: Geological Society at AmencaBulletin, v. 61, p. 3513-3536.

Beaulieu. J.D., 1970. Cenozoic stratigra­phy of the Santa Cruz Mountains: Un­published Ph.D. thesis, Stanford Univer­sity, 202 p.

Brabb. E.E .. 1970. Preliminary geologicmap of the central Santa Cruz Moun­tains, California: U.S. Geological SurveyOpen-File map, scate1:62,500.

Bryant. W.A., Smith, D.P.. and Hart. EW..1981, The Sargent. San Andreas. andCalaveras fault zones: Evidence for reocency in Watsonville east. Chittenden,and San Felipe quadrangles, Monterey,San Benito, Santa Clara. and SantaCruz counties. California: Division ofMines and Geoiogy Opefl-Fne Report81-7.

250 CALIFORNIA GEOLOGY NOVEMBER t990

Carlson, R.l., 1982, Cenozoic convergencealong the California coast A qualitativetest of the hot-spot approxlmallon: Ge­ology. v. 10, p. 191-196.

Clarlt, J.C.. 1981, Stratigraphy. paleontol­ogy, and geology of the central SantaCruz Mountains. California CoastRanges; U.S. Geological Survey Profes­Sional Paper 1168. 51 p.

Clark, J.C.. and Reitman. J.D.. 1973. Oligo·cene stratigraphy. tectonIcs. and pale­ogeography southwest 01 the San An­dreas fault, Santa Cruz Moontains andGabitan Range, California CoastRanges: U.S, Geological Survey Profes­sional Paper 783, 18 p.

Clarlte, S.H" Jr.. and Nilsen, T.H.. 1973,Displacement 01 Eocene strata and 1m·plicatlons lor the history of ollset alongthe San Andreas fault. central, andnorthern California. tn. Koyac. R.L., andNur, A.. editors, Proceedings of the con­ference on the tectOniC problems 01 theSan Andreas fault system: Stanford Uni·yerslty Publications in Geological SCi­ences. Y. 3, p. 367,368,

Coppersmith. K.J.. 1979, ActiYlty assess­ment of the Zayante-Vergeles fault, cen·tral San Andreas fault system. Califor­ma: Unpublished Ph.D. theSIS, UniyerSltyof California. Santa Cruz. 333 p,

Crowell, J.C.. 1962, Displacement alongthe San Andreas fault, California: Geo­logical Society of America Speciat Paper71. 61 p.

Cummings, J.C.. Touring, R.M.. Brabb,E.E.. 1962. Geology of the norlhernSanta Cruz Mountains. California. In

Bowen, O.E.. Jr.. editors. Geologic guideto the 011 and gas fields of norlhern Cali·fomia: California DiYlslon of Mines andGeology Bulletin 161. p. 179-220.

DeMets. C.. Gordon, R.G., Stein. S., andArgus. D.F.. 1987, A revised estimate ofPaCific-North American motion and Im­plications tor weslern Ncrlh Americaplate boundary zone tectonics: Geo·phySical Research Leiters, Y. 14. p. 911­921.

Dibblee, TW., Jr.. 1966, Geologic map andsections of the Palo Alto 15'minutequadrangle. Santa Clara and San Mateocounties, California: California DIYISlOnof Mines and Geology Map Sheet 6,scale 1:62.500.

Dickinson. W.R .. and Snyder, W.S., 1979.Geometry oltople junctions related to theSan Andreas Iransfarm fault: Journal ofGeophysiCal Research. Y. 84, p. 561·571.

Graham. SA and Dickinson. W.A., t978,Apparent oflsets of on·land geologic fea­tures across the San Gregorlo-Hosgrifault trend' California DIVision of Minesand Geology Special Report 137, p. 13­23.

Graham. SA. Stanley. A.G.. Bent, J.V., andCarter. J.B.. 1989, Oligocene and Mio·cene paleogeography of central Californiaand displacement along the San Andreasfault: Geological Sooety of America, Y.101, p. 711-730.

Greene. H,G.. 1977, Geology of the Mon­terey Bay region: U.S. Geological SurveyOpen·File Report 77·718, 314 p.

HIli, M.l., and D.bblee. T.W.. Jr.. 1953, SanAndreas, Garlock, and Big Pine faults,Calitornia-A study of the character, his­tory. and tectonIC SIgnificance of theirdisplacements: Geological Society 01America Bullelln, Y. 64. p, 443·458.

McLaughlin, R,J.. t974, The Sargent-Berro­cal fault zone and ItS relallon to the SanAndreas fault system in the soothern SanFranciSCO Bay region and Santa ClaraValley, California: U.S. Geological Sur­yey Journal of Research Y. 2, p. 593-598.

McLaughlin, R.J., Clark, J.C., and Brabb,E.E., 1989, Geologic map and structuresections of the Loma Prieta 7.5·mlnutequadrangle. Santa Clara and Santa Cruzcounties, California: U.S. GeologicalSurvey Open-File Report 88-752, scale1:24.000.

Minster. J.B., and Jordan. T.H., 1967, Vectorconstraints on western U.S. deformationfrom space geodesy, neotectonics. andplate motions: Journal of GeophysicalResearch, Y. 92, p. 4298·4804.

Nilsen, T.H.. and Clarke. S.H.. Jr, 1975,Sedimentation and tectonICS In the earlyTertiary continental borderland of centralCalilornia: U.S. Geoioglcat Survey Pro­feSSIOnal Paper 925, 64 p.

Page, 8.M., 1976, The southern CoastRanges, In Ernst, W G., editor. The ge·otectonic development 01 California,Rubey Volume I: Prentice-Hall. p. 329­417.

Page, B.M., and Engebretson, D.C., 1984.Correlation between the geologIC recordand computed plate motions for centralCalifornia: Tectonics, Y. 3, p. 133-156.

Phillips, R.L., 1983, Late Miocene tidalshelf sedimentation. Santa Cruz Moun·tains. Call1ornia. in, Larue. O.K.. andSteele, R.J" editors, Cenozoic marinesedimentation Pacific margin, U.S.A.:Society 01 EconomiC Paleontologists andMineralogists. Pacific Section. p. 45-61.

Plafker. G. and Galloway, J.P" editors,1969. Lessons learned from the LomaPfleta, California. earthquake of October17.1989: U.S. Geologica! Survey Clrcu·lar 1045. 48 p.

Sims. J.D.. 1990. Chronology of displace·ment on the San Andreas fault in centralCalifornia: Evidence from reversed posi·tlons 01 exotic rock bodies nearParkfield, California: U.S. GeologicalSurvey Open-File Report 89'571,43 p.

Spinier. T.E.. Harp. E.L.. Keefer. O.K.,Wilson, A.C.. and Sydnor, A.H" 1990.Landslide fealures and other coseismiCfissures triggered by the Lorna Prietaearthquake, central santa Cruz Moun,tains. California: DIVISIon 01 Mmes andGeology Special PublicalJon 104, p.59-66.

Stantey. A.G., and McCaflrey. R.. 1983,Extent and ollset history 01 the Ben Lo·mond fault. Sanla Cruz County. Caillor·nia, tn, Anderson. D.W.. and Rymer.M.J., editors, Tectonics and sedimenta·tlon along faults of the San Andreas sys­tem: Society of Economic Paleontolo­gists and Mineralogists. PaCific Section,p. 79-90,

Stanley. R.G.. 1985, Middle TertIary sedi­mentalion and tectonics of the La Hondabasin. central California: U.S. GeologicalSurvey Open·File Report 85·596, 271 p.

Stock, J.M.. and Molnar, P" 1968, Uncer­tainties and Implications of the Late Cre­taceous and Ternary position of NorthAmerica relatIve to the FaraJlon. Kula,and Pacific plates: Tectonics, v. 7, p.1339-1384. ~

CALIFORNIA GEOLOGY NOVEMBER 1990 '"

Plate Tectonics and theGulf of California Region

By

NANCY SCHMIDT, GeologistArizona Geological Survey

The geology and tectonism 01 California have been Intluencedgreally by the collision and interaction between the Pacinc plateand the North American plate. The lorces generated by thiS inler­aCllon caused substantial honzontal movement along the san An­dreas 'ault system and created the Gut! of California nIt zone. Thefollowing article originally appeared In the summer 1990 issue ofArizona Geology magazine (II. 20. no. 2). published by the AnzonaGeological Survey. 845 N. Park Avenue. 11100. Tucson. AZ 85719.It is repnnled here With permlssion...9cl'!ror.

INTRODUCTION

T he Gulf of California. familiar to many as a recreationmecca and important part of Mexico's fishing and tourist

industries, is unusual in terms of its depth. tidal range. salinity.temperature. and marine life. Many of the singular features ofthe Gulf of California (also known as the Sea of Cortez) maybe explained by its shape and bottom profile. both of whichare reOections of the gulf's geologic history. This article sum­marizes the unique features of the gulf. describes the theoryof plate tectonics. explains how tectonism may have affectedthe geologic evolution and physiography of the gulf. andillustrates the process by which the Colorado River becamelinked to the gulf

NATURAL FEATURES OF THE GULF

TIle Gulf of California is approximately 683 miles in lengthand ranges from 39 to 150 miles in width (Rgures 1 and 2).Its long and narrow shape inOuences its tidal range. which isthe third highest in the VJOrld and can reach almost 33 feet inthe northern section (Ressa and Eckdale. 1987). The shape01 the gulf is analogous 10 a bathtub. and the tides-which aregreatest at the northern and southern ends and almost nonex­istent in the middle-resemble water in a bathtub that sloshesback and forth when disturbed.

The high salinity and temperature of the shallow water inthe gulf are partially due to its shape. which restricts the inter­change of water between the gulf and Pacilic Ocean. Salinityvaries from 36 to 39 parts per thousand. (The salinity of nor­mal seawater is approximately 35 parts per thousand,) In thenorthern gulf. surface water temperature ranges from approxi­mately 88° Fahrenheit in the summer to 52" Fahrenheit in thewinter (Brusca. 1980). The depth of the gulf also affects tem­perature and salinity by increasing heating and evaporationrates. The gulf is shallow (generally less than 650 feet) at itsnorthern end but deepens to the south. Parts of the southernfwo·thirds of the gulf range from 7.900 to 11.800 feet indepth. with troughs plunging as deep as 13.300 feet (Brusca.

Figure 1. Satellite photo 01 the nonhern Gull of California region.C • Colorado River delta: K • Kino Bay (Bahia Kino in Spanish):P. Pinacate volcanic field (Sierra Pmacate): R • Rocky POint(Pueno Penasco): S _ salton Sea: Y _ Yuma.

,'" CALIFORNIA GEOLOGY NOVEMBER 1990

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Plates may move apart. leaving a gap(or rift) in between that fills with newmolten crustal material. The newlyformed crust is hot relative to the crustsurrounding it and rises to form ridges(also called rises: Figure 4A). Spreadingoccurs as the new crust is added on ei­ther side of the rift. being injected be­tween older crust as the plates moveapart. This process occurs in both oce­anic and continental crust. Long con­tinuous systems of troughs and ridgescharacterize spreading areas on oceanic

and Curray, 1982: Stock and Hodges,1989). The geologic events that forcedthe gulf open and split off Baja Califor­nia were driven by global-scale forces.The theory of plate tectonics describesthese geologic processes.The Earth'scrust and uppermost mantle. collectivelyknown as the lithosphere. are com­posed of 8 to 10 large discrete blocks orplates and many smaller plates (Figure3). The oceanic parts of the plates are50 to 62 miles thick; ihe continentalportions are typically 68 miles thick ormore (Sawkins and others. 1978). Eachplate moves with respect to the otherplates. The boundaries between platesaccommodate this motion and are thesites of several types of geologically im'portant processes (Figure 4). Earth­quakes. for example. are often associ·ated with tectonic boundaries.

PLATE TECTONIC THEORY

The Gulf of California began to fonn4 to 6 million years (m.y.) ago during thelate Miocene as Baja California sepa­rated from the Mexican mainland (Moore

1980). Several shallower troughs arealso presenl in the northern sectionclose to the Baja California coastline.The edges of these troughs are the sitesof upwelling. which bring cold nutrient­rich water to the surface. The organismsthat feed upon the nutrients attract, di­rectly or indirectly. a wide variety ofother animals. including shrimp. tuna,whales. dolphins. sea lions. sharks, seabirds. and humans.

WOll:LD PLATE BOONDARIES

Figure 3. Major plates of the world. Thetypes 01 plate boundaries. marked byheavy lines, are as follows: (A) mld­oceanic ridges at which the plales moveapart are represented by double lines: (B)transform fault boundaries are shown bysingle lines; and (C) trenches and othersubduction zones are marked by lineswith teeth on one side: the teeth pointdown the descending slab. Dashed linesare used where the exact location ornature of the boundary is uncertain. FromSawkins and others. 1978. p. 163.

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... Figure 2. Location of the Gulf 01 California,Colorado River. Salton Sea, and SanAndreas faull system. which includesseveral associated faults. Modified fromHamilton. 1961. p. 1309.

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CALIFORNIA GEOLOGY NOVEMBER 1990 253

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Figure 5. Transform faults and spreadingcenters In the Gulf of Cahlornia. Note:Faulls associated With the San Andreasfault system are shown in greater detailthan in Figure 2. Modified from Moore.1973. p. 1886.

KEY

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Paleomagnetism is the remnant magnet­ism of magnetic minerals in rocks andrepresents the orientation of the Earth'smagnetic field at the time the rockformed. The direction of paleomagnet­ism preserved at one location is com­pared with the paleomagnetic orienta­tions preselVed at other locationsthroughout geologic time to determinethe directions and rates of movement for

Evidence lor the existence of Pangaeaprimarily comes from (I) the distributionof ~matching- rocks and fossils of thesame age on different continents. (2) cor­relation of continental coastlines. and (3)paleomagnetic data. For example. theextinct Glossopteris flora is an assem­blage of Permian (286-to 245-m.y.-oldlplant fossils that is present in Africa.Australia. India. Madagascar. SouthAmerica. and Antarctica. Other evi­dence for the existence of Pangaea is thealignment of the African and SouthAmerican coastlines; they fit togetherlike pieces of a puzzle. They formed asingle continent that was rifted apart andseparated by sea-floor spreading. whichcreated the Atlantic Ocean.

Another type of evidence for the exis­tence of Pangaea is paleomagnetism.

Plate t€Ctonic theory not only de­scribes the relatiw! motion between crus­tal plates today. but also provides a use­ful framework for reconstructing the po­sitions of plates in the past. For ex­ample. during the late Paleozoic andearly Mesozoic eras. approximately 225m.y. ago. all of the continents were as­sembled into one large continent. knownas Pangaea.

Pangaea

boundaries of most oceanic and conti­nental plates in the Pacific Ocean. ThePacific plate. for example. is being sub­ducted underneath Asia near Japan; theassociated melting of this oceanic crustformed the Japanese islands and ac­counts for the active volcanism in thisarea. Collision of the Eurasian and In­dian plates. both continental plates. cre­ated the Himalaya Mountains as theplates collided and one overrode theother. The resulting mountains are un­derlain by crust that is approximatelytwice the average thickness of continen­tal crust.

Plates may also move past one an­other by shearing along faults that paral­lel the plates motion (Agure 40. Thistype of interaction along a transformboundary creates large stresses in theEarth's crust. The San Andreas fault rep­resents the boundary between the Pacificand North American plates. which aremoving past each other.c

plates. Spreading in continental settingsbreaks continents apart. The Great Af·rican Rilt Valley in eastern Africa is anactive example of continental rifting.

Figure 4 Three types of plale boundanesand relative plate motions. (A) Divergent(plates move away from each other): theprocess 01 sea-floor spreading createsnew oceanIC lithosphere. (8) convergent(plates approach each other): the process01 subduction destroys lithosphere. (e)transform (plates slide past each otherwithout approachmg or dlVerging);transform faulting is the correspondingprocess. Modified from Sawkms andothers. 'S78, p. 165.

,~,

Driven by spreading forces. platesmay move toward one another and col­lide. One plate may be pushed underanother or the plates may slide pasteach other. Oceanic lithosphere isdenser and heavier than continentallithosphere and. consequently. the oce­anic plate is generally thrust (subductedlbeneath the continent (Rgure 4B). Platedestruction and plate formation balanceeach other. preselVing the Earth's totalsurface area.

Plate tectonic theory suggests thatsubduction is actively occurring at the

CALIFORNIA GEOLOGY NOVEMBER 1m

the plates. Paleomagnetic data. there­fore. may serve as compass needles, al­lowing geologists to reconstruct thegeographical distributions of continentsduring the past.

The changing positions of the crustalplates have affected many of the Earth'ssystems. The positions of continentsand oceans. for example. influence at­mospheric and marine current patlerns,These. in turn. significantly affect globalclimate. Plate tectonics also plays arole in polar glaciation. Along with pe­riodic. favorable climatic conditions.resulting from changes in the Earth'sorbit. the presence of a continent nearor over the pole is necessary for polarglaciation. As polar ice accumulate'S.sea level drops worldwide and ice sheetsspread over the continents.

Tectonic History at the Gull

Both the physical characteristics andthe origin of the Gulf of California areintimately tied to tectonic processes.The gulf is bounded on the northeast bythe North American plate. which in­cludes the North American continent.and on the southwe'St by the Pacificplate. which is currently moving north­west with re'Spect to the North Ameri­can plate. The San Andreas fault sys­tem is part of the boundary betweenthese two plates. The East Pacific Rise.a spreading system on the Pacific platenear its boundary with the North Ameri­can plate (Ftgure 3). has been partiallysubducted underneath the North Ameri­can plate during the past 30 m.y.(Atwater. 1970). In the vicinity of thegulf. subduction occurred on the westside of Baja California. which was thenpart of the Mexican mainland. Subduc­tion ceased. however. off the Baja coast10 m.y. ago. and the juncture betweenthe two plates became a transformboundary (Atwater. 1970; Figure 4q.

By 6 to 4 m.y. ago the transfonnmargin strengthened. probably as itcooled and became more rigid, and aweaker inland zone broke to accommo­date the shearing motion between thetwo plates (Atwater. 1970). The shiftof this plate margin had two importantresults: Baja California was ripped offof the North American plate and be-

came part of the Pacific plate. and theGulf of California was created (Atwater.1970). Both Baja California and alarge portion of southern California arenow part of the Pacific plate, whichcontinues to move northwest with re­spect to the North American plate.Baja California. in fact. has moved ap­proximately 186 miles to the northwestfrom its original position on the NorthAmerican plate (Moore and Curray.1982). Reconstruction of the positionsof Baja California and the Mexicanmainland reveal thai a large embaymentexisted in the continent during the lateMiocene (12 to 5 m.y. ago). prior tothe opening of the Gulf of California.Evidence for this embayment. known asthe "proto-gulf. M includes the distribu­tion of marine and nonmarine sedimen­tary rocks and changes in volcanic pat­terns that predate rifting (Stock andHodges. 1989). Several mechanismshave been invoked to explain the exis­tence of the proto-gulf. including Basinand Range extension. which is relatedto the widespread mountain-formingepisode that occurred in Arizona duringthis time, and extension associated withdevelopment of the Pacific-NorthAmerican plate boundary (Stock andHodges. 1989) This extension mayhave weakened the crust underlying theproto-gulf and facilitated the onset ofrilting (Stock and Hodges. 1989).

TECTONISM AND SEDIMENTATIONPATIERNS IN THE GULF

lOe spreading cenler in the Gulf ofCalifornia does not form a straight lineof ridges and troughs be<:ause bolhspreading and transform motion areoccurring. This combination of motioncreated a zigzag pattern of troughs andfaults (Figure 5). The floor of the gulfcontains a series of parallel faultsaligned with the motion of the Pacificplate and separated by small deeptroughs. which are the sites of spread­Ing and are approximately perpendicu­lar to the faults_

Estimated sedimentation rates withinthe gulf are high. especially in thenonhern part. This area received largevolumes of sediment from the ColoradoRiver before the 20th century. Thethick cover of sediments that blanketsthe floor of the gulf obscures many of

the features associated with both riftingand transform faulting: however. datafrom geophysical surveys and sea-floordrilling throughout the gulf have con­tributed to unraveling its history. Inthe Guaymas basin near the middle ofthe gulf. for example. accumulationrates for the late Pleistocene and Holo­cene (2 m.y. ago to the present) mayexceed 6.5 feet per 1.000 years insome areas (Curray and others. 1982).Bathymetry (the measurement of oceandepth and topography). geophysicalmagnetic. and seismic data reveal thepresence of three active transformfaults separated by two narrow troughsin the Guaymas basin (Bischoff andHenyey. 1974). The troughs are sitesof active spreading. where an esti­mated 2 to 2.3 inches of new crust areadded each year (Larson and others.1972; Moore and Curray. 1982).

THE SALTON TROUGH ANDCOLORADO RIVER

At its northern end. the Gulf 01 Cali­fornia spreading system is linked withthe Salton Trough and the San An­dreas fault system (Figure 5). The Sal­ton Trough is a structural continuationof the Gulf 01 California that has beencut off Irom the gulf by sediments de­posited by the Colorado River. It in'dudes the Colorado River delta and theMexicali. Imperial. and Coachella val­leys. In some areas. the floor of thetrough lies more than 1.318 feet belowsea level (Brusca. 1980). lOe northerngulf and proto-gulf once extended intothe trough. forming a large embaymentat various times during the late Mio­cene and early Pliocene, approximately11 to 3 m.y_ ago (Metzger. 1968).

Although geologists dispute whenand where the Colorado River beganflowing into the gulf. evidence suggeststhat the local drainage system that de­veloped at the northern end of the gulfeventually joined the Colorado Riverduring the late Miocene to early Plio­cene (5.5 m.y. ago: Lucchitta, 1972:1989). The absence of older depositsassociated with a south-draining riversystem in southwestern Arizona andthe presence of basin sediments depos­ited by interior drainages suppon thisscenario (Eberly and Stanley. 1978).

CALIFORNIA GEOLOGY NOVEMBER 1990 '55

REFERENCES

Figure 6. Distribution of the BouseFormation. From Smith. /970. p. 1418.

Bischoff, J.L., and Henyey, T.L. 1974. Tec·tonic elements ot the central part of theGull of California: Geological Sociely otAmerica Bullefin. v. 85, p. 1893-1904.

Brusca, RC.. 1980, Common intertidal in·vertebrates 01 the Gull of California:Tucson. University of Arizona Press,513 p.

Curray, J.R.. Moore. D.G.. Aguaryo, J.E..Aubry, M.P.. Einsele,G., Fornari, 0.,Gieskes. J.. Guerrero·Garcia, J.. Kast­ner. M" Kelts, K., Lyle. M.. Matoba. Y..Molina-Cruz. A., Niemitz. J., Saunders.A.. Schrader. H.. Simonett, B.A.T., andVacquier. V.. 1982. Guaymas baSin:Sites 477. 478. and 481, in CUITay, J.R,and Moore, D.G" editors, Initial reportsof the Deep Sea Drilling Project: Wash­ington. D.C" U,S. Government PrintingOffice. v. 64. pt. 1, p. 211-415.

Eberly, L.D.. and Stanley, T.B.. Jr., 1978,Cenozoic stratigraphy and geologic his­tory of southwestern Arizona: GeologicalSociety of America Bulletin, v. 89, p.921·g40.

Flessa. K.W.. and Eckdale, A.A., 1987.Paleoecology and taphonomy of Recentto Pleistocene intertidal deposits, Gulf ofCalifornia, in Flessa, KW., editor,Paleoecology and taphonomy of Recentto Pleistocene intertidal deposits. Gulf ofCalifornia: Paleontological Society Spe­cial Publication no. 2. p. 2-33,

Hamilton. Warren. 1961, Origin of the Gulfof California: Geological Society ofAmerica Bulletin, v, 72, p. 1307-1318.

Larson. P.A.. Mudie. J.D.. and larson. RL..1972. Magnetic anomalies and fracfure­zone trends in the Gulf 01 California:Geological Society of America BulleM,v, 83. p, 3361-3368.

lucchil1a, Ivo, 1972. Early history of theColorado River in the Basin and RangeProvince: Geological Society of AmericaBulletin, v. 83, p. 1933-1948.

lucchil1a, Ivo, 1989, History of the GrandCanyon and the Colorado River in Ari­zona, in Jenney. J.P., and Reynolds,S.J.. editors, Geologic evolution of Ari­zona: Arizona Geological Society Digesl17. p. 701-715.

Metzger, D.G., 1968, The Bouse Formation(Pliocene) of the Parker-Blythe-Cibolaarea. Arizona and California, in geologi­cal survey research 1968: U,S, Geologl'cal Survey Professional Paper 600-0. p.126-136.

Moore. DG.. 1973. Plate-edge deformallOnand crustal growth, Gulf of Calitorniastructural province: Geological Societyof America Bulletin. v, 84, p. 1883-1906.

Moore, D.G., and Curray, RJ.. 1982. Geo­logic and tectonic history of the Gulf ofCalifornia, in Curray. J.R., and Moore.D.G .. editors, Initial reports of the DeepSea Drilling Project: Washington, D.C.,U.S. Government Printing Office. v. 64,pI. 1, p. 1279-1294.

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.­o"."'~:',,

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Atwater, Tanya. 1970, Implications of platetectonics for the Cenozoic tectonic evolu­tion of western NOrlh America: Geologi·cal Society of America BUlletin. v. 81, p.3513-3536.

almost zero (Brusca, 1980). The aver­age annual flow from 1902 to 1934was more than 15 million acre·feet. asmeasured near Yuma. From 1935 to1964 the annual discharge of river wa­ter into the gulf de<:reased to slightlymore than four million acre-feet. and bythe late 19605 only subsurface percola­tion and diverted flows reached the gull.Colorado River water that does flow intothe gulf is typically very saline and pol­luted by pesticides and fertilizers (Brusca.1980). Both sedimentation and input offresh water, therefore. have been de­creased dramatically by human activities.In the future, the Gulf of California willbe afle<:ted not only by natural proces­ses. but also by human intervention.

,, ,

O,-=='~O",,~2~O~~30 ,-/- I ! I _ Oo;l<:rops 0/ !loUse

Miles Forrnalion

The Bouse Formation consists of Mio­cene-Pliocene estuarine deposits ex­posed along the Colorado River fromYuma to north of Parker (Figure 6),Conditions dUring its deposition becameprogressively less salty to the north(Metzger. 1968: Smith. 1970). Thisevidence indicates that an influx of freshwater occurred during deposition of theBouse Formation. which suggests thatthe ancestral Colorado River had begunto drain into the Bouse embaymentand. ultimately. the Gulf of California(Lucchitta. 1972: 1989).

Human activities have also affectedthe Colorado River delta and the Gulfof California. Before the 20th century.sediments carried by the river madetheir way as far south as La Paz, ap­proximately 620 miles from the river'smouth. With the construction of irriga­tion proje<:ts in the Imperial Valley anddams along the Colorado River, riverflow into the gulf has been reduced to

The Colorado River progressivelyfilled the estuary with sediments until itsdelta reached the Salton Trough. TheImperial Formation (Miocene-Plioceneagel of the Salton Trough area containsa well-defined horizon. above which arefossils in rocks that were derived fromthe Colorado Plateau. These fossilsrecord the integration of the northernColorado River with the southern drain­ages in the Salton Trough area (Luc­chitta. 1972: 1989).

The delta deposits of the ColoradoRiver eventually extended the entirewidth of the northern gulf, isolating theSalton Trough from the gulf. (Withoutthe Colorado River delta. the gulf tooaywould extend northward to approxi­mately Palm Springs.) The river alter­nated its flow between the isolatedtrough. which became a lake. and thenorthern gulf, but currently flows onlyinto the gulf. The lake dried up andremained dry until 1905. when floodsdestroyed the headworks of an irriga­tion canal and diverted the ColoradoRiver into the fertile Imperial Valley.For 2 years. the entire river floweddown the canal and emptied into thetrough. The canal was repaired in1907. leaving behind the no-outletSalton Sea (Reisner. 1986).

256 CALIFORNIA GEOLOGY NOVEMBER 1990

Special Publication 98

(~_D_M_G_P_U_B_LI_C_A_T_IO_N_~)

A typical in-stream mining operation on the Russian River in Sonoma County. Note thepattern of the low /low channel as it meanders through the recently mined riverbed (top ofphoto) towards an unmined gravel bar (bol1om 01 photo). Gravel bar in center of photo isbemg mined. Several leet of alluvium have built up on the gravel bar since it was lastmined. Elevations of the gravel bars are monitored by the Sonoma Coonty Depanment ofPlanmng. the lead agency lor the mlmng operalJon. whICh IS responSible under Callforl1la'sSurface Mlillng and ReclamalJon Act for overseeing reclamatIOn 01 the site. In lutureyears. ilthe monltonng program shows more matenal has depoSited at the site. aggregatemay agam be removed from the bar. Photo by M,ke sandeckl,

Special Publication 98 is availablefor reference at offices of the Divisionof Mines and Geology in Sacramento.Pleasant Hill. and Los Angeles. A copyof the report may be purchased by pre­paid mail order or over-the-counterfrom the Geologic Information andPublications Qlfke in Sacramento for

$8.00. '"

Ruvial geomorphology. the study ofthe physical form and evolution ofstream channels. is a fundamental toolfor geologists. engineers. ecologists.and biologists to investigate~ effectof California's grO\lling population onrivers. This report explains the ration­ale and methods used 10 initiate stud­ies that can determine rates of channelaggradation (building up of the bed bydeposition of materials). and for esti­mating sediment yield (the total passageof sand and gravel through a river sys­tem) for planning and regulating miningactivities. The report discusses thefactors which influence the supply ofsediment to rivers. how rivers transportand deposit sediment. and how sedi­ment transport and deposition interactwith the morphology of~ channel.Evaluation of these factors by qualifiedconsultants can be assessed and appro­priate recommendations made to land·use planners.

Mining alluvium from gravel bars anddry stream beds has long been prac·ticed in the western states. Increasingawareness of ecological resources in­herent in natural rivers and the desirefor the preservation of these valueshave created a need to carefully evalu­ate the actions of man on ri\'ef environ­ments. Special Publication 98 dis­cusses several means of managing theeffects of aggregate mining from barsand the low-flow river bed of gravelchannels. and presents case studieswhere planning and resource agencieshave adopted a feedback-based. sys­tematic approach of management.

FLUVIAL GEOMORPHOLOGY ANDRIVER GRAVEL MINING. A Guide forPlanners. Case Studies Included. ByBrian Collins and Thomas Dunne. 1990.31 p .. 25 figures.

Rersner. Marc, 1986, Cadillac desen: theAmerlcan west and Its disappeanng wa­ter: Viking Penguin Incorporated, 582 p.

Sawklns. F.J., Chase. C.G.. Darby. D.G.and Rapp. George. Jr.,1978. The evolv­109 Earth: A telCt in physical geology:Macmillan Publishing Company. 558 p.

Smith, P.B., 1970. New evidence for Plio­cene manne embayment along the lowerColorado Aiver area, California and Ari­zona: Geological Society of AmericaBulletin. v. 81. p. 1411-1420.

Stock, J.M.. and Hodges, K V., 1989, Pre·Pliocene elCtenSlon aroond the Gulf ofCalifornia and the transfer of Baja Cab·forma to the Pacdic Plate: TectonICS. v8. p. 99-115. "'"

CALIFORNIA GEOlOGY NOVEMBER 1990 '"

(~B---=-O-----,-----,,--O.,..-----K_R----,-E.,..-----V----,--lE..,...--:::w-:::--s_JBooks reviewed in this section are not available for purchase from DMG.

California Indians GUidebook

TIME'S FLOTSAM, Overseas Collectionsof California Indian Material Culture. ByThomas C. Blackburn and Tr<lVis Hodson.1990 Available Irom Ballena Press. Pub­lishers $ervk:es. P.O. Box 2510. Novato,CA 94948. 224 p. $2495. soft cover;$34 95. doth C:O<Vl:r,

Tlme's FJOfsom is a guide to the manyexceptionally fine objects of nali...e Calilor­nia Indian manu/acture presently housed in21 museums worldwide It also serves tofurther enhaoce ethnographic. 3rchaeo!·ogle. and elhnohistoric research. Studentsnew 10 the field and experienced scholarsalike \\IiU be 5lIrprised by the number andqU/lolity althese collectiOns. which oftencontain a richer and more varied assortmentof early ethnographic objects than can befound in all of North America.

This guide is based on extensive colTe­spondence. as ~n as actual visits to over­seas museums by the authors, The bookgives ilem·by·llem lim of overseas holdingsand their prownience. There is even a liSl~

ing of aU the ships that touched the Califor­nia coast in the late 18th and early 19thcenturies These ships are thought to be theprimary source for the Ili'llive Californiaitems later incorjX)fated into the museums,

Photos of collection pieces are includedThis is volume number 35 of the Ballelli'lPress Anthropological Papers.

Geology 01 Canada and Greenland

QUATERNARY GEOLOGY OF CAN­ADA AND GREENlJ\ND. The Geology ofNanh America Volume K-1 Edited by R. JFulton 1989. Geological Society of Can­ada Available from Canadian GovernmentPubUshing Centre. Supply and SeTVices Can­ada. Ollawa. Canada KIA 059 (check Of

money order ~Id be made payable to,Receiver General for Canada). 839 p ..$8400. hard cover.

This detailed synthesis of Quaternary ge­ology in Canada and Greenland provides aninformational source about these areas toslUdents and professional earth scientists.The volume and accompanying maps areorganized into three parts: Part 1 covers theregional aspects of Quaternary geology inCanada such as rock types. stratigraphy.and hIstorical geology: Part 2 covers topicalaspects-wrillen in varying levels of com­plexJly---of Quaternary geology throughoutCanada: Part 3 cowrs the Quatern<lry as'pects of Greenland

The dominant I<Ind sculpting or geomor­phic factor during the Quaternary In Canadaand Greenland was glaciation: nearly all ofCanada and Greenland were covered by iceat least once during this lIrne. The Intervalsand effects of these huge ice masses thatprofoundly affected Quaternary geology inCanada and Greenland are discussed in thisvolume. FIve separate color map sheets atvarious scales are included 10 help illustrateconcepts wIthin the text

GeophysICal Research

MANTLE CONVECTION. PLAlt ltC­TONICS AND GLOBAL DYNAMICS. Vol­ume 4. The AuK! Mechanics of Astrophysicsand Geophysics Series, Edited by W. RPeltier 1989 Gordon and Breach SciencePublishers. Available from Harwood Aca­demic Publishers. PO, Box 786. CooperStation. New York, NY 10276 881 p.$198.00. hard copy,

This volume is a collection of reviews andarticles that the editors combined Into uni­fied. state·of-the-art accounts by adding sup­plementary material Geophysicists have de·veloped new insights into processes at workin lhe Eanh as a whole: the motions of CIUS'

tal pIales. lhelr relation to mantle convection.the topography of the core-mantle boundary.and the lhermal and dynamical interaction 01

core and mantle,Sections of the book locus on an introduc'

tion 10 basic concepts of thermal convection.analyses of the physical Slate 01 the Earth'smanlle from four different perspectives. anda detailed reappraisal oltne extent to whichgeochemical data may be brought to bearupon the issues of mantle mixing and clUstalgrowth, Other se<:tions present material deal·Ing with hydrodynamics modeling 01 themanlle convection process. Including the wayin which new results from global seismic im­aging are influencing mantle convectionmodels.

1be final secllon 01 the book addressesthe issue of what impact the mantle convec­tion had on Earth·s evolution (and other ter­restrial planets) and the crucial role it playedin the process ofllow Earth's magnetic fieldis generated through magnelohydrodynamicsprocesses in the core,

GUIDEBOOK DEATH VAUEY RE·GION. California and Nevada Prepared lorthe 70th Annual Meeting of the CordilleranSection. Geological Society of America F"leldTrip Number 1. 1974 The Death ValleyPublishing Company. Shoshone. CA92384 102 p. $14.00. paper cOYer. in­cludes postage and handling

This guidebook was designed to inlormprofessional eanh scientists aboul the com­plex geology of the Death Valley region andwas prepared for use on a three-day loopthrough this area. The trip stans and endsal Las Vegas. This guklebook has two parts:Part 1 is a general geologic guide of thestudy region. Part 2 includes severaltochnical articles about specific aspects of this reoglon, Geologic features that are most obvi­ous from the roads are emphasized in thisguide. Although no delailed road maps areincluded. specific reference points that aremarked by signs on the road. or are oncommon road maps orient the reader.

Areas covered in this guidebook includetraverses from Las Vegas 10 Nopah Range,Nopah Range to Shoshone. Shoshone toSalsberry Pass. Salsberry Pass to Ashfordmill site. Ashford mill sile to Mormon POint.Monnon Point to Badwater. Badwater toFurnace Creek wash. Furnace Creek Inn 10

Zabriskie Point. Zabriskie POint toShoshone. Shoshone to Furnace CreekRanch. Furnace Creek Ranch to Heirs Gate.Hell's Gate to Daylight Pass. Daylight Passto Beatty. Beatty to Lathrop Wells. "ndLalhrop Wells to Las Vegas Maps, cross'sections. and pholos are included.

The geology of the Death Valley regionhas been dillicult to interpret for many rea­sons. The extensive rock record in thiS reogion indicates long periods of marine depo·sition. shorter periods of toctonic mountainbuilding and granitic intrusion. and bothancient and f"irly recent volcanic activityRegionally significant high·angle faultsbound thiS region to the west (tne. Ste"oNevada frontal fault) and on lhe south {theGarlock fault}. Other major hlgh·angle faultslie along one or both sides of Pallamint Val­ley. Saline Valley. Owens Valley. and DeathValley

CALIFORNIA GEOLOGY NOVEMBER 1990

.- ...(

MetamorphiC Rocks

INTRODUCTION TO METAMORPHICTEXTURES AND MICROSTRUCTURES1990. By A.J. Barker Available from: Rout­ledge. Chapman & Hall. 29 West 35thStreet. New York. NY 10001 162 p,$69.95. cloth cover: $29.95. soft cover

This book provides a comprehensive in­troduction to metamorphic rocks. their lex­tures and microstructures.The book is di·vided Into three sections. Pan A introducessome of the brooder aspects of melilfTlOl"­phism and metamorphic rocks. Part B pro­vides the basis for Ihe description and Inler­pretation of the textual/microstructural fea­lUres of melamorphic rocks in thin section.from the most basic features such as layeringand banding. to more complex features suo:;has symplectic intergrowths. Part C deals withmore advanced interpretations in polyde­formed and polymetamorphosec! rocks. Par­ticular attention is paid to the interrelation­ships between deformation. metamorphism.and fluids. including such topics as porphy­roblast-follation relationships and shear'senseinditators,

Appendices include petrogenetic grids forpelites and metabasites. and a glossary pro­viding defmition of tenns used In the textThe book Is written for advanced under­grllduate and postgrllduate students of goology. and provides a clear and highly illus­trated exposition of these complex rocks.Researchers deaJ1ng with metamorphic rockswill lind it an Invaluable source of reference.

/

Photo Essay

THE WESTERN PIIOTOGRAPt-lS OFJOHN K. HILLERS -Myself m the Water:By Don D. Fowler 1989 Smithsonian Insti­tutiOn Press. Department 900. Blue RidgeSummit. Pfl. 17294. 166 p. $24.95. hardcover (plus $245 postage arxl handling)

Few readers of western American hIstoryand natural science are unfamiliar with Ma­jor John Wesley p()\l,'(':lrs journey by boatdown Ike uncharted courses of the Greenand Colorado rivers in 1869 The trip madePowell a hero and. with the forthcomingcongressional appropriations, opened thedoor to fUrl her scienhfic exploration 01 the~,

In 1871 Powell organized a second expe­dItion to concentrate on studying the geol­ogy and topography and. for the first time.photographing the Colorado River and SlJr­rounding Canyon Country of the AmericanSouthwest, Powell's 1871 trip became thefourth of four "Geological and Geographi·cal~ expeditions which set out between1867 and 1878. led by C1<Irence King. F.V.Ilayden. and George Wheeler These SlJr­veys were designed to scientifically studyeverything In the west from botany to zoo!ogy and to prepare for the lnevuable use ofnatural resources and assessment of landsfor settlement,

While the four SlJrvey5 were not the firstfederally sponsored expeditions to use ph0­tography. they were the first to successfullyrecord landscape impressions with thisemerging technology. As early as 1819 ex'pedltlonary artists accompanied topographical engIneers ....Iestward and recorded themajesty of the West for eager Americansalong the eastern seaboard I towever. the

A well-exposed roadcut 01 the late TertraryPeriod RestIng Spring Pass ash !low tuflunit. This ash flow tuft unIt is of rhyohllccomposition and vanes Irom denselywelded zone lust above the dense blackobsidian unit thaI cuts dlagonally acrossthe roadcut to a non-welded lapilli·tuH andtufHapiliite near the base of the roadcutalong the road. Photo by CUm Dupras.

daguerreotype process and its limitations inlandscape photography precluded muchsuccess.

The Irwention of the colloidal wet-plateprocess in 1851 provided a medium. thoughshll cumbersome, which could be adapted tolandscape photography in the wilderness, Asimple description of the steps employed bythese elCpedition photographers illustratestheir remarkable achievements. At each lo­cale the photographer set up the tripod.aimed and focused the heavy large·format 5 K8 inch or 8 x to inch camera. set up the darktent and chemicals. coated the glass-platenegative with the colloidal emulsion (nitratedCOllon dissolved in alcohol and ether). madethe exposure. developed and processed thenegative in a solution of potassium cyanide.and finally took down the equipment. re­packed It into a wagon. on a pack animal. ar­on his own back and moved on to the neKtspot.

Don D. Fowler records the career of JohnK. HIllers. lield photographer and SlJpervisorof photographic laboratories for the U.SGeological Survey and the Smithsonian Insti­tution's Bureau of American Ethnology. oneKpedilions to southern Utah. Ihe GrandCanyon, Yosemite. the Indian pueblos 01 theSouthwest. and the Indian Territory of Okla­homa. Photography played an important rolein illustrating the broad geological expansesof this region By 1883 over 6.000 printswere developed annually by Hillers for theGeological Survey to use in its repons.

For 25 years John K. Hillers worked as agovernment photographer associated withJohn Wesley Powell. His photographic workshave appeared in hundreds of governmentpublications, stereograph slide sets. 19th­century International fairs and expositions.and continue to be used in university andcommercial press publications

Over 100 19th century photographs arereproduced in this volume to accompany thebiographical material and to iII\.1Strate Hillers'ssense of compositiOn and balance as well ashis skill in using atmospheric light andshadow. The photographs describe and inter­pret the beauty of the American West beforethe changes of the twentieth century.Relliewed by Sylula Bender-Lamb

CALIFORNIA GEOlOGY NOVEMBER 1990 259

Sierras

IN ll-lE HEART OF ll-lE S[ERRA.YOSEMITE VAu"£Y AND ll-lE BIG -mEEGROVES, By James M. Hutchings. Editedby Peler Browning. 1990. Available fromGreat West Books. P.O. Box 1028. Lafay­ette. CA 94549. 592 p. $44,95. hardtOIler; $29 95. soft tOIler. Add $ 1.50 ship­ping for one book. $.50 for each odditiona[book and 725% sales tax for California ad·dresses.

James Mason Hutthings journeyed 10

California along with thousands of othergold seekers in 1849. Alter better than aver'age suttess at gold mining. Hutthings foundhis true tailing as a literary torresponclentextolling the scenit glories of California. [n1855 he led the first tourist party into theYosemite Va[ley. thus begmning his [ifelongfascination wllh the area

In 1886. Hutthings compiled all of hisYosemite knowledge into In the Heart ofthe Sierras, a mix of autobiography. his­tory. geography. geology. botany. anthro­pology and traveL lbe last edition of thiswork appeared in 1888. 1ne first drawingsand photographs ever made of Yosemite areamong the lt8 engravings and 70 photo­graphs induded in the new edition as is theWheeler Survey's 1883 Topographka[ Mapof Yosemite Valley. Thirty of the photo­graphs did not appear in the original edi­tion. A full indelt has been added fortonvenience. Reuiewed by Syluio Bender·Lomb.

S[ERRA SOVTH. 100 BACK COUN­-my TRIPS. Fifth edition, By Thomas Win~

nell. Jason Winnett. and Kathy Morey.1990. Wilderness Press. 2440 BantrohWay. Berkeley. CA 94704 2% p.$1395. soft cover.

Wilderness enthusiasts who desire toleave the crO'>\.d$ behind will appreciate thisgUidebook to Sierra back country trails fromMono Creek to the southern end of SequoiaNationa[ Park 1ne 100 trips selected repre­sent the autooB' personal choices based oncriteria of scenic allraction. wildemess expe~

rience. and recreational potential Each tripdescription includes an elevation profile andhiking difficulty grade. total mileage. day-by­day descriptions of what can be seen anddone jespecially fishing}. suggeslions ofcamping sites. best season for making thetrip. and names of relevant topographicmaps.

Many geologic features are included inthe hike descriptions. Referentes to n3lTa"tives of earlier explorers in the same areas.such as Clarence King and John Muir. pro­vide added depth for the interested readerAdditional material on wilderness pennilSand preservation of the High Sierra wilder­ness is provV::led. A composite map for thearea IndiCating all [railheads. trails. androads is also included. Numerous black andwhile photos illustrate the guidebook.Reviewed bll Syluio Bender-Lomb.

Stratigraphy

INTERPRETING ll-lE STRATIGRAPHICRECORD. By Donald R Prothero. 1990.W.H. Freeman and Company. Publishers.41 Madison Avenue. New York. NY 10010.410 p. $49.95.

Stratigraphy. the science of rock se­quences. describes the original sucCC$.Sionand age relalJons of rock strata. their fonn.distribution. lithologic composition. fOSSilcontent. geophysical and geochemical prop­erties. environment of deposition. and ge0­

logic history. All consolidated and uncon­solidated sedimentary rocks fall within thescope of stratigraphy. lbe stratigraphkrecord. or the "geologic record." is derivedIrom the meticulous study 01 stratigraphicsequences (rocks arranged chronologically)

Stratigraphy and the stratigraphic recordare the foundation of geology. Most of theworld's important mineral resources occur instratified sequences. For example. oil. gas.coal. uranium. limestone. gypsum. aggre­gate. rock salt. and borates are found inS1rati!ied sequences of rocks. Groundwaterstratigraphy is becoming more important tolocate aquifers. Siting hazardous wastedumps requires an understanding of strati­graphy. Stratigraphic data are often the keyto tracing plate motions across the Earth'ssurface. Isotope stratigraphy provides thedata lor understanding past climatic condi·tions. ancestral land configurallons. andoceanographic circulations through geologictime. This book is designed lor earth sci­ence students, although it can be understoodWIth no previous geology training. Part Iexplains the data base from whith the strati­graphic interpretation is derived. Part II describes the interpretation of rock strata andthe environments of deposition. Part IIIeltplains the process of correlating and inter·preting rock strata on a regional scaleOithostratigraphyl. showing how fossils canbe used to determine various stratigraphicparameters. and elt!lmines tectonic proc­esses that control the accumulation of rocksequences. ~

Eastern face 01 Sierra Nevada Irom ONens Valley Photo by Don Dupras.

CAUFQANIA GEOlOGY NOVEMBER 1990

A Page for Teachers

(Repnnted With perrrussion from Ranger Rick's NiiIllureScope "GeoIogy The Actrw Eanh:published by the NaIIOlliilll WlldlJfe FederiilltlOll. Copynghl 1988)

THE EARTH, INSIDE AND om

the mantle. (Convection is the processby which hot material rises to the sur­face. spreads and cools. and then sinksagain. like soup being heated in asaucepan.) These convection currents.which geologists think are fueled byheat given off by the core and fromradioactive decay in the mantle. con·stantly transfer heat from the deepmantle 10 the crust at a very slow rate,

HeallY Metal; Deep within theEarth is the core-a mass of hot. heavymetals (mostly iron and nickel~that

sank. due to gravity. after the Earthformed. 1be core is almost twice asdense as the mantle Geologists knowthat the core is made up of IWO verydifferent layers The outer core is mol­ten and is responsible for the Earth·smagnehc held. And the inner core is

solid

Since the early 19OOs. geologists haveknown that the Earth is divided into threemain layers: a thin outer crust. a thickermantle. and a core. But exactly howthese layers interact and what they aremade of are still open to debate.

The Ct'USI is the orUy layer that ge0lo­gists can really study first-hand So geoJo.gIStS have had to study other data. suchas the path earthquake shock waves takeas they travel through the Earth. to findout more about the mantle and core,Starting \l,ith the crust. here's a closerk:ao&o: at each layer

A Thin Skin. The outermost layer ofthe Eanh--the layer we walk on-is alhin. rocky skin that covers the planet. Inrelation to the Earth. this crust is aboutas thin as a postage stamp stuck on abilliard ball At its thickest. which is undermountain ranges. the crust is only about22 miles thick about 11200th of theEarth's diameter.

By comparing rock samples dredgedfrom the ocean floor with those on thecontinents. scientists found there weretwo distinct types of crust: continentalcrust and oceanic crust. Continental crustmakes up the continents and containslight-colored rocks (such as granite) com­posed mainly of the elements aluminum.silicon. and oxygen. This layer of crust Ismuch thicker than the oceanic crust.which forms the ocean floor. Althoughthe oceanic crust is thinner. it is made ofdenser rocks (such as basalt) containingthe elements iron. magnesium. silicon.and oxygen. Because of the difference indensities. the lighter continental crust~fIoats~ higher on the underlying mantlethan does the oceanic crust.

The MOCJing Mantle: Underneaththe crust is the much denser mantle. Al­though no one has ever drilled into themantle. geologists think it is made up ofmany of the same elements. such as ironand calcium. found in the crust (Themantle is hotter and denser than thecrust because the temperature and pres­sure inside the Earth increase as thedepth increases,)

Subject:Science

C~.

322"*'1""'.

Core2100..-n....

Although most of the mantle Ismade up of solid rock. geologists thinkit is composed of several zones, Theuppermost zone. the area lying directlyunderneath the crust. Is cooler andthus more rigid than the lower parts ofthe mantle. This thin. uppermost layerof the mantle. combined with the thin.rocky crust. forms a rigid layer of rockcalled the lithosphere.

Bek>w the lithosphere geologiststhink there is a hot. weak zone in themantle that is also solid. but can ~fb.v~

at a very. very siow rate. This weakerzone is called the asthenosphere. Ge·oIogists think the ~thosphere ~floats~

on this more mobtle asthenosphere.and slides around on it very slcMiy.

Many geologists are convinced thatstrong convection currents exist "",thin

1

I..

Inner Core

CAlIFORNiA GEOlOGY NOVEMBER 1990 ,.,

leave I·inch border

Outer Core

day supports

Note: The lay(!T:S of the Earth are not be·lieved to be as distinct as is represented inthese models. Between all of the Earth'slayers there are transition zones and eachlayer varies in thickness and density. Manygeologists disagree on exaclly how the layersinteract with each other.

Lithosphere: Trim the box top soit fits inside the walls of the box. Thispiece of cardboard will separate the up­per mantle from the crust. To make theupper mantle part of the lilhosphere,cover the bottom of the cardboard boxtop with a thin. flat layer of dough.Next make the crust by covering the topof the cardboard with a thin layer ofdough and adding extra dough for themountains and hills. Let dry and thenpaint both parts.

Asthenospllere: Add red food col­oring to a handful of dough and mixwell. Put the dyed dough into a smallsandwich bag to keep it soft. Get all theair out. and seal the bag. Use maskingtape to reinforce the seal. Mash thedough in the bag so that it will fit intothe box.

How to Make the Dough

Two batches of this recipe willmake enough dough for five models.

4 cups baking soda

2 cups cornstarch2 1/2 cups cold water

Mix all of the ingredients in a me­dium-sized saucepan and cOOK O\l€rmedium heat. stirring constantly.Cook about 10 minutes or so. untilthe mixture is the consistency ofmashed potatoes, Remove fromheat, turn out onto a plate. and thencover with a damp cloth. After thedough cools, knead it gently into asmooth ball. Then store it in a tightlysealed plastic bag and refrigerate un­til you're ready to use it. ~

Deep Mantle: Make a block ofdeep mantle out of dough (see end ofactivity for recipe). It should lit insidethe box and be about three inchesthick. Let it dty and then paint it withwatercolors or poster paints.

I

cover box with construction paperlithosphere

Asthenosphe";;--\- _\_-"

Deep Manti"

hc..rc,,-,

OJ'" --

Wlli'lt.;:'il ;;Inner Core

Outer Core: Fill a small plasticsandwich bag with approximately oneinch of water. Get all the air out andthen seal the bag. Use masking tape totape over the seal and the top of thebag so the bag stays closed. To keepthe upper layers of the model fromputting too much pressure on the bag.make two clay supports as shown. (Intime. water will leak from the bag. Ifyou want to use the models for a long­term display. ha\l€ the children replacethe water with crumpled plastic wrap.)

Outer Con'

Inner Core: CUi out severalpieces of cardboard so that they fitinside the bottom of the box. Glue thepieces in the stack and let dty. Coverthe stack with construction paper.

In this activity the children in yourgroup can learn about the structure ofthe Earth by building a model Earth. Af~

terward they can make their ownsmaner Earth models. Have them workin groups to make cross sections of theEarth's layers. First divide the childreninto groups of six and give each groupa fairly tall, thin cardboard box (two­pound baking soda boxes. large raisinboxes. and confectioner's sugar boxeswork well) and other supplies. Haveeach group make a model by followingthe directions below. (Encourage thechildren in each group to divide up thework. For example, one person couldbe responsible for making one layer andanother could decorate the box. Whenevetyone's layer is finished, they canassemble their models from inner coreto lithosphere.)

• scissors• small cardboard boxes• small plastic sandwich bags

• clay• masking tape• watercolors or poster paints

• paintbrushes• materials for making dough

(see recipe at end of activity)

• red food coloring

• water• cardboard

• glue• construction paper• plastic wrap (optional)

Materials:

The Box: Cut off the box top andsave it for the lithosphere. Then cut outa panel from one of the sides of thebox. Do not cut of( an entire side-lea\l€about a one-inch border around theedges (see diagram). Cover the box withconstruction paper.

Grades 5· 7

A MODEL EARTH

CALIFORNIA GEOLOGY NOVEMBER 1990

Nominations ForALFRED E. ALQUIST AWARD

Nominations are now being soughtby the California Earthquake Safety

Foundation for the 1991 Alfred E.Alquist Award for Achievements inEarthquake Safety. This annual awardrecognizes individuals who have madeoutstanding contributions to or havehad a major impact-past or presenl­on seismic safety in California. Awardsare given in such areas as applied re­search. public policy advancement.and/or program implementation.

Past awards have been made to politi­cal personages, engineers, governmentaladvisors, disaster spedalists. and archi­tects. One to three awards are giveneach year. Names of nominees can onlybe considered in the years in which theyare nominated. If you nominated some­one unsuccessfully last year. and wouldlike to see that person considered againin 1991, resubmit his or her nominationbefore the deadline noted below. Post­humous awards are not made.

The awards are presented in April,Earthquake Preparedness Month, ofeach year. A candidate may be nomi­nated by another individual. a finn. oran agency. Letters describing the nomi­nee's background and accomplish­ments should be sent to: CaliforniaEarthquake Safety Foundation. P.O.Box 22912, Sacramento. CA 95822.These letters of nomination must bepostmarked no later than February 28.1991 to be considered. X'

The California Earthquake SafetyFoundation is a nonprofit organizationpromoting earthquake awareness andpreparedness in such areas as hazardmitigation. education. and research.

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Seismic Intensity Distribution Maps

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(1) Special Publication 60. Earth­quake planning scenario lor a magnitude8.3 earthquake on the San Andreas faultin southern California;

(2) Special Publication 61. Earth­quake planning scenario for a magnitude8.3 earthquake on the San Andreas faultin the San Francisco Bay area;

(3) Special Publication 78. Earth­quake planning scenario for a magni­tude 7.5 earthquake on the Haywardfault in the San Francisco Bay area: and

(4) Special Publication 99. Earth­quake planning scenario for a majorearthquake on the Newport-Inglewoodfault zone.

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". CALIFORNIA GEOlOGY NOVEMBER 1990