International Geology Review, Vol. 38, 1996, p. 979-994. Copyright © 1996 by V. H. Winston & Son, Inc. All rights reserved.

Tectono-Metamorphic Impact of a Subduction-Transform Transition and Implications for Interpretation of Orogenic Belts

JOHN WAKABAYASHI

1329 Sheridan Lane, Hayward, California 94544

Abstract

Subduction-transform tectonic transitions were common in the geologic past, yet their impact on the evolution of orogenic belts is seldom considered. Evaluation of the tectonic transition in the Coast Ranges of California is used as an example to predict some characteristics of exhumed regions that experienced similar histories worldwide.

Elevated thermal gradients accompanied the transition from subduction to transform tec­tonics in coastal California. Along the axis of the Coast Ranges, peak pressure-temperature (P/T) conditions of 700 to 1000° C at a pressure of ~ 7 kbar, corresponding to granulite-facies metamorphism, and cooling to 500° C, or amphibolite facies, within 15 million years, are indicated by thermal gradients estimated from the depth to the base of crustal seismicity. Greenschist-facies conditions may occur at depths of 10 km or less. These P / T estimates are consistent with the petrology of crustal xenoliths and thermal models. Preservation of earlier subduction-related metamorphism is possible at depth in the Coast Ranges. Such rocks may record a greenschist or higher-grade overprint over blueschist assemblages, and late growth of metamorphic minerals may reflect dextral shear along the plate margin, with development of orogen-parallel stretching lineations.

Thermal overprints of early-formed high-P (HP), low-T (LT) assemblages, in association with orogen-parallel stretching lineations, occur in many orogenic belts of the world, and have been attributed to subduction followed by collision. Alternatively, a subduction-transform transition may have caused the overprints and lineations in some of these orogenic belts. Possible examples are the Sanbagawa belt of Japan and the Haast schists of New Zealand. P / T conditions of inferred granulite-grade metamorphism in the Coast Ranges, and predicted cooling of these rocks through lower thermal gradients, resemble the P / T evolution of many granulite belts, suggesting that some granulite belts may have formed as a result of a subduction-transform transition. Arc­like belts of plutons also can form as a consequence of subduction-transform transition.

Introduction

TRIPLE-JUNCTION MIGRATION along trenches involving either migrating transform faults or spreading ridges is common in Earth history (Sisson et al., 1994). Accordingly, it is reason­able to surmise that past plate interactions, similar to the subduction-transform transition occurring in present-day northern California, have left their imprint on many orogenic belts. Nelson and Forsythe (1989) suggested that ridge-trench collision is an important process in crustal growth, and speculated that this pro­cess played an even greater role in Archean crustal growth. However, the impact of ridge-trench interactions or subduction-transform transitions on the development of orogenic belts still is largely unappreciated.

The Coast Ranges of California are a type example of subduction-transform transition.

Coastal California has been the site of subduc­tion or transform tectonics for the last 160 m.y. (Engebretson et al. 1985). During the late Mesozoic and Tertiary, the Franciscan subduc­tion complex formed over a period of ≥140 m.y. of continuous subduction (Wakabayashi, 1992). Subduction terminated with the north­ward passage of the Mendocino triple junction, and a transform plate boundary developed south of the triple junction (Atwater, 1970). General geologic relations are shown in Fig­ure 1.

In addition to the different style of deforma­tion, higher thermal gradients followed the sub­duction-transform transition (Dickinson and Snyder, 1979; Lachenbruch and Sass, 1980; Furlong, 1984). Dickinson and Snyder (1979) proposed that a "slab window" trailing in the

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FIG. 1. Tectonic elements of the California Coast Ranges, showing major strike-slip faults of the San Andreas system, and upper Cenozoic volcanic rocks (v). Also shown are exposed blueschist-facies rocks of the Franciscan Complex (bsch.). Abbreviations: KJf/g = areas underlain by Franciscan Complex or Great Valley Group rocks (includes areas where these rocks are overlain by Tertiary and Quaternary deposits); Ksb = granitic and high-grade metamorphic basement of the Salinian Block and overlying sedimentary deposits. Map derived from Jennings (1977).

wake of the triple junction allowed astheno-spheric upwelling, causing a significant increase in the thermal gradient. Thermal mod­eling based on the slab-window concept predicts a thermal peak just after the passage of the Mendocino triple junction, followed by cooling as new lithosphere forms in the window region (Furlong, 1984). This model is consistent with heat-flow data of Lachenbruch and Sass (1980) and the occurrence of late Cenozoic volcanism in the Coast Ranges. Recently, the slab-window concept has been challenged as a framework for the late Cenozoic tectonics of coastal California (Bohannon and Parsons, 1995; Beaudoin et al.,

1996), and no single model proposed thus far is completely consistent with some of the recent seismic data obtained (Hole, 1996). However, the thermal effects associated with the passage of the triple junction and the transform nature of the plate boundary south of the Mendocino triple junction are not disputed. These points of consensus, rather than any specific tectonic model, will serve as the foundation of this paper.

This paper relates field, thermal, structural, and geophysical data to inferred metamorphic mineral assemblages and structures at depth in the Coast Ranges of California, as a case

SUBDUCTION-TRANSFORM TRANSITION 981

FIG. 2. Longitudinal section of the California Coast Ranges along the line depicted in Figure 1, showing inferred depth to the ~300° C isotherm based on the base of crustal seismicity from Hill et al. (1990). The screened dashed line shows the line in the crust that experienced a temperature of 300° C during earlier passage of the thermal peak.

study; the attempt is to assess what the deep levels of the transform plate margin in Califor­nia may look like if exhumed and to speculate on the imprint of similar, past plate inter­actions on exhumed metamorphic belts of the world. Among the topics addressed in this paper are: (1) an alternative tectonic mecha­nism to thrusting-thermal relaxation (subduc-tion followed by collision) models for thermal overprinting of some high-pressure, low-temperature (HP/LT) metamorphic rocks; (2) an alternative tectonic mechanism for develop­ment of some granulite belts, or other high-temperature metamorphic belts; (3) problem­atic arc-like belts of plutons; (4) possible examples of exhumed equivalents of subduc-tion-transform orogens; and (5) distribution of blueschists and granulites in time.

An Example of a Subduction-Transform Transition: The California

Coast Ranges at Depth

Elevated thermal gradients following subduction-transform conversion

Modeling of heat-flow data by Lachenbruch and Sass (1980) and Dumitru (1989) suggest peak thermal gradients of 35° C/km, following subduction-transform transition in coastal Cal­ifornia. The modeling of Furlong (1984) pre­dicts peak temperatures of about 700° C at about 25 km depth, 1 m.y. after passage of the triple junction, subsequently cooling to about 450° C at the same depth 20 m.y. after triple-junction passage. In addition to heat-flow data, high thermal gradients in the California Coast Ranges are indicated by the 10- to 18-km depth

of the base of crustal seismicity in the Coast Ranges, which is inferred to coincide with the brittle-ductile transition (e.g., see Hill et al., 1990). The depth of the base of crustal seis­micity varies from north to south along the strike of the Coast Ranges in the wake of the triple junction, first shallowing to an average of about 10 km (locally as shallow as 7 to 8 km in the Clear Lake area), then deepening pro­gressively to the south and leveling out at an average of about 15 km south of San Francisco Bay (Fig. 2; cross-sectional view shown in Fig. 3). The brittle-ductile transition in quartz-rich rocks, inferred to constitute most of the Coast Ranges at depth on the basis of seismic veloc­ities (Holbrook et al., 1996), is estimated to take place at a temperature of ~300° C (Sibson, 1982). The base of crustal seismicity indicates thermal gradients ranging from 30° C/km at the thermal peak region (35-40° C/km in the Clear Lake region), cooling to 20° C/km in the southern Coast Ranges. The migration rate of the Mendocino triple junction indicates that the cooling to 20° C/km thermal gradients, in the wake of the passage of the thermal peak, took 10 to 15 m.y. All of the above studies suggest high (≥30° C/km) peak thermal gra­dients in the California Coast Ranges after the subduction-transform transition, followed by cooling to lower gradients. Superimposed on the regional thermal effect noted above are local effects, possibly related to space problems in the area of the migrating triple junction (Dumi­tru, 1991; Underwood, 1989; Underwood et al., 1995). These local processes have resulted in very high local uplift rates and elevated thermal gradients of ≥50° C/km (Dumitru, 1991). In addition, pull-apart basins along the major

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FIG. 3. Cross-section of the California Coast Ranges along the line depicted in Figure 1, showing major strike-slip faults and thermal structure. Adapted from Fuis and Mooney (1990) with some modifications from Holbrook et al. (1996) and Wakabayashi and Unruh (1995). "300" is the 300° C isotherm, and the screened "300" represents the location of the corresponding isotherm at the time of the passage of the thermal peak. The 300° C isotherm is estimated from the base of crustal seismicity (Hill et al., 1990). Lower-case single letters correspond to the locations of hypothetical P / T paths on Figures 4, 7, and 8. Abbreviations: fr = mostly Franciscan Complex; gv - Great Valley Group (sandstone and shale); fr/oph = Franciscan Complex (mostly sandstone and shale, subordinate volcanic rocks and pelagic rocks), ophiolite, and underlying mantle rocks, and possibly Sierran basement (which may include volcanic-arc and ophiolitic rocks); sal = Salinian Block (granitic basement and high-grade metamorphic rocks).

strike-slip faults may facilitate local upwelling of hot material and/or intrusion of plutons (Jové and Coleman, 1992) and may have an impact on the local thermal structure.

Late Cenozoic volcanism in the Coast Ranges locally has brought up xenoliths of deep crustal material (Brice, 1953; Hearn et al., 1981; Stimac et al., 1992; Jove and Coleman, 1992; Nakata et al., 1993), including high-grade, silicic metamorphic rocks. For example, numerous xenoliths of high-grade, schistose metamorphic rocks have been found in ande-sites from the 10-ka to 2-Ma Clear Lake volcanic field (Brice, 1953; Hearn et al., 1981). The mineral assemblages in these xenoliths include orthopyroxene-cl inopyroxene-plagioclase-quartz ± biotite ± garnet and orthopyroxene-plagioclase-biotite ± sillimanite ± cordierite ± spinel (Stimac et al., 1992). Thermobarometry on these rocks has yielded estimates for their crystallization conditions of 800 to 900° C at 5 to 8 kbar. These P / T estimates are consistent with temperatures estimated for the deeper lev­els of the California Coast Ranges discussed above (Fig. 4). On the basis of chemical and isotopic data, Stimac et al. (1992) concluded that these rocks most likely are metamorphosed Franciscan greywackes, formed by regional metamorphism in the wake of triple-junction migration. The well-developed schistosity or foliation of these rocks indicates regional (rather than contact) metamorphism, and the

P / T conditions of metamorphism are grossly incompatible with original subduction-related metamorphism of Franciscan rocks, the coun­try rocks of the Clear Lake area (McLaughlin and Ohlin, 1984), or Sierra Nevada regional metamorphic rocks, which have been suggested to tectonically underlie the eastern Coast Ranges (Jachens et al., 1995).

Jove and Coleman (1992) analyzed gabbroic xenoliths from Pliocene volcanic rocks near Coyote Lake and estimated crystallization con­ditions of 915 to 1000° C at 9 to 10.5 kbar, based on thermobarometry. The P / T conditions calculated for these gabbro xenoliths also are consistent with the hypothetical thermal gra­dients discussed above (Fig. 4). Nakata et al. (1993) reported xenoliths that include biotite and sillimanite-corundum schists (in addition to gabbroic xenoliths) from the andesite of Dowdy Ranch in the Diablo Range, north of the Quien Sabe volcanic field and east of the Coyote Lake volcanics. Nakata et al. (1993) obtained K/Ar ages of 8.5 to 12.3 Ma from igneous xenoliths from the andesite of Dowdy Ranch, although the ages may reflect resetting by the heat from the andesite in which they were entrained, which yielded a date of 8.2 Ma. The mineral assemblages in schist xenoliths described by Nakata et al. (1993) are indicative of the same type of metamorphism under high thermal gradients recorded in xenoliths studied by Stimac et al. (1992) from the Clear Lake

SUBDUCTION-TRANSFORM TRANSITION 9 8 3

FIG. 4. Hypothetical P / T paths for metamorphic rocks at depth in the California Coast Ranges following conversion to transform tectonics. The dotted lines with arrows repre­sent the prograde metamorphic path after the conversion. Solid paths with arrows show cooling from the thermal peak at locations in the crust (lower-case letters) shown on Figure 3. The 20° C/km line corresponds to estimates of the thermal gradient in the central and southern Coast Ranges, 15 m.y. or more after the passage of the thermal maximum, based on the depth to the base of crustal seis-micity (Hill et al., 1990). The "late-subduction thermal gradient" is the pre-transform thermal gradient north of the Mendocino triple junction estimated from the base of crustal seismicity (Hill et al., 1990). The peak thermal gradient in the figure is 35 ° C/km, an average figure within the range of estimates derived from several different methods (see text). The 30° C/km line represents the high-temperature maximum of longer duration (see text) and may be more representative of peak prograde assemblages. Metamorphic conditions for Clear Lake xenoliths (Stimac et al., 1992) and Coyote Lake xenoliths (Jove and Coleman, 1992) are shown.

volcanic field. The xenoliths from the Dowdy Ranch volcanics probably formed in a setting similar to that of the xenoliths found in the Clear Lake volcanics, at a time when the triple junction, and trailing thermal peak, was farther south in the Coast Ranges.

The thermal pulse that followed the triple-junction migration also produced volcanism (Johnson and O'Neil, 1984; Fox et al., 1985). The volcanic rocks mostly are silicic to inter­mediate rocks of calc-alkalic affinity (Hearn et al., 1981, Johnson and O'Neil, 1984), and apparently involved melting of the crustal rocks above underplated or intruded basaltic magma (Liu and Furlong, 1992). There is evidence of a

magma chamber at depth below the most recent of these volcanic rocks, the Clear Lake volcan­ics (Isherwood, 1981). This magma chamber is estimated to have a diameter of 14 km and to extend from 7 km to 21 km in depth (Isher­wood, 1981), or alternatively to a depth of 30 km (Iyer et al., 1981), in the crust. Analogous solidified magma chambers probably exist at depth below older volcanic fields, such as the Quien Sabe and Sonoma volcanics. Most of the magma generated by partial melting of the crust apparently does not reach the surface and should form plutons at depth (Johnson and O'Neil, 1984; Liu and Furlong, 1992). The amount of melt generated may be linked to the velocity of the triple-junction migration (Liu and Furlong, 1992). Significant volumes of plu-tonic material are expected at depth in the Coast Ranges, although they probably do not form a contiguous belt similar to the Sierra Nevada batholith.

Present-day plate boundary kinematics

The present-day California Coast Ranges are part of the transform boundary between the Pacific and North American plates. Dextral shear in the Coast Ranges totals about 35 to 40 mm/yr, about 80% of the motion being parallel to the plate boundary (DeMets et al., 1990) and nearly all of the small contractional component of ≤3 mm/yr being perpendicular to the plate boundary (Argus and Gordon, 1991; DeMets et al., 1990; Gordon and Argus, 1993). The domi­nant structural features of the Coast Ranges are the major strike-slip faults of the San Andreas system. These strike-slip faults account for essentially all of the margin-parallel plate motion within the Coast Ranges (e.g., Kelson et al., 1992).

The contractional component of plate motion contributes to the development of fold-and-thrust belts within the Coast Ranges and the uplift of the range (Mount and Suppe, 1987; Zoback et al., 1987); much of the shortening is concentrated along the eastern border of the Coast Ranges (Wakabayashi and Smith, 1994). The present contractional component of plate motion in the Coast Ranges has persisted since a change in plate motions at 3.4- to 3.9-Ma (Harbert, 1991). Local shortening and some extension also occurs within the Coast Ranges as a consequence of constrictional and releas­ing bends in the major strike-slip faults (e.g.,

984 JOHN WAKABAYASHI

Aydin and Page, 1984). Prior to the 3.4- to 3.9-Ma change in plate motions, the transform plate boundary may have had a slight divergent com­ponent across it, resulting in the formation of some of the Tertiary basins of the Coast Ranges (Graham et al., 1984; Engebretson et al., 1985). The pre-3.4- to 3.9-Ma history of the transform margin constitutes, for most of the present transform margin, the larger part of the total elapsed time as a transform plate boundary. Uplift rates in the Coast Ranges locally are high, ranging up to several mm/yr (Merritts and Bull, 1989; Dumitru, 1991), but are generally between 0.1 and 0.5 mm/yr (Lettis, 1982; Bürg-mann et al., 1994; Lettis et al., 1994; K. Lajoie, pers. commun., 1994; M. Angell, pers. com-mun., 1995). Apatite fission-track ages for most of the northern and central Coast Ranges are greater than 30 Ma, indicating that the average Cenozoic uplift rate for rocks now exposed at the surface has been relatively low (Dumitru, 1989). Because young (post-20 Ma) fission-track ages are rare, except in local areas of high uplift rates, the average exhumation rate of rocks exposed on the surface must have been less than ~0.15 mm/yr since the thermal peak that followed conversion to the transform plate margin; otherwise, much more of the Coast Ranges would yield younger apatite fission-track ages. These uplift rates place constraints on the hypothetical retrograde P / T paths of rocks at depth in the Coast Ranges.

Inferences regarding deep structure The character of structures present at depth

in the Coast Ranges probably depends, in part, on their depth relative to the brittle-ductile transition. Above this transition, structures are dominated by the strike-slip faults of the San Andreas system. Second-order features are folds and thrust faults that are a consequence of both plate-normal contraction and local restraining bends along major strike-slip faults. A major reflector, interpreted as a possible crustal detachment, has been imaged at a depth of ~15 km at the latitude of San Francisco Bay; this reflector is near the depth of the inferred brittle-ductile transition in the area (Brocher et al., 1994). A similar reflector is offset by major strike-slip faults in the northern Coast Ranges (Beaudoin et al., 1996), indicating that such a reflector probably does not represent a detach­ment in the northern Coast Ranges. As an

alternative to a detachment, the reflector may be a consequence of a major metamorphic-facies change, or a fluid-rich zone in the crust (that also may be a consequence of metamor-phic reactions). At present there are no direct data to determine whether a detachment occurs at the brittle-ductile transition, or if such a structure is present locally, but not universally, in the Coast Ranges. Numerous examples can be found in exhumed orogenic belts of either detachments at the brittle-ductile transition (e.g., Coney, 1980) or discrete shear zones that extend below the brittle-ductile transition (e.g., Hurlow, 1993). It follows that only indirect inferences can be made regarding structures below the brittle-ductile transition in the Coast Ranges.

Although there are many permissible inter­pretations of deep structure, there is general agreement that dextral strike-slip motion domi­nates the kinematics of the Coast Ranges and should strongly influence structures at all lev­els of the crust. This dextral shear may be either localized in discrete shear zones or accommo­dated in a broad zone of distributed ductile deformation; present data do not favor or elimi­nate either end member. Shear-zone rocks should reflect significant stretching parallel or subparallel to the plate margin. As a result, new mineral growth and stretching lineations devel­oped below the brittle-ductile transition may be subparallel or parallel to the plate margin (e.g., Ellis and Watkinson, 1987; Ave Lallemant and Guth, 1990), although local complexities may be expected in some shear zones because of the small component of convergence (e.g., Robin and Cruden, 1994; Tikoff and Teyssier, 1994).

Fuis and Mooney (1990) developed an inter­pretive cross-section of the Coast Ranges, based on seismic refraction and reflection studies. The cross-section in Figure 3 is adapted from Fuis and Mooney's Figure 8.4, with some rein-terpretation of subsurface structure per Holbrook et al. (1996) and Wakabayashi and Unruh (1995). The structure of the deeper parts of the plate boundary are an unresolved issue, and several competing models have been proposed (Hole, 1996). The deep plate bound­ary may be expected to have some vertical component of motion to accommodate the contractional component of plate motion. Although the magnitude of the shortening is

SUBDUCTION-TRANSFORM TRANSITION 9 8 5

FIG. 5. Cross-section of the Coast Ranges along same transect as Figure 3, showing the distribution of the grade of metamorphism from transform-related thermal effects and the pre-transform distribution of blueschist-facies rocks. It should be noted that the facies boundaries are based on the temperatures attained at the thermal peak of 30 to 40 ° C/km that may be of limited duration and spatial extent (see text). The broader and longer-lived thermal high defines a gradient of 30° C/km. If this longer-lived thermal maximum is used, then the facies boundaries will shift downward by a few km. The distribution of blueschist-facies rocks at depth represents the minimum distribution of rocks that at one point in their history were at least 20 km deep, according to the tectonic model of Wakabayashi and Unruh (1995).

small relative to strike-slip displacement (Argus and Gordon, 1991), the component of accumu­lated vertical crustal motion may become signif­icant if the present-day kinematics persist for a long time (≥30 million years or so). Such long-term vertical movement would be important in the future exhumation of the deeply buried parts of the present Coast Ranges.

Inferred metamorphism at depth in the Coast Ranges and relationship with deep structure

Higher thermal gradients associated with the transform margin compared to those associated with the previous subduction zone will produce a quite different metamorphic suite at depth than the earlier subduction (Franciscan) H P / LT metamorphism for which the California Coast Ranges are well known (e.g., Ernst, 1970). Instead of the facies series prehnite-pumpellyite, blueschist, eclogite that charac­terizes the subduction-zone metamorphism of the Franciscan Complex, the predicted facies series at depth in the present-day Coast Ranges should be greenschist, amphibolite, granulite. Cloos and Dumitru (1987), recognizing the thermal significance of the ongoing subduc-tion-transform transition, concluded that the lack of greenschist overprints in exposed Fran­ciscan rocks indicated that no subduction-transform transitions occurred during the span of Franciscan accretionary history (approx­imately 160 Ma to 20 Ma). The 300° C isotherm

in the present-day Coast Ranges corresponds to greenschist-facies metamorphism (Figs. 2, 3, and 4). The elevated temperatures at depth should result in significant recrystallization and growth of new metamorphic minerals. The peak conditions of metamorphism at the base of the Coast Ranges crust are predicted to be ~700 to 1000° C at a pressure of about 7 kbar, or granulite grade (Fig. 4), with subsequent cool­ing to lower thermal gradients. The distribution of metamorphic facies boundaries in cross-sec­tion view is shown in Figure 5. It should be noted that if the Clear Lake region (300° C isotherm at 7- to 8-km depth) is considered to be a local effect, or too short-lived to cause signifi­cant recrystallization, then the "sustained" peak thermal gradient that is likely to result in significant recrystallization is 30° C/km (300° C isotherm at 10-km depth) (see Fig. 2). If the 30° C/km gradient is used as the thermal peak that causes major recrystallization, then the facies boundaries would shift a few km deeper in the crust than shown in Figure 5, a result that is in accord with recent estimates of seismic velocities (John Hole, pers. commun., 1996).

On the basis of structural studies of Cenozoic rocks along the eastern margin of the Coast Ranges, the only significant period of regional shortening and uplift affecting the Coast Ranges since the passage of the triple junction is the period of time since the plate-motion change at 3.4 to 3.9 Ma (Namson and Davis,

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FIG. 6. Approximate distribution of transform-related metamorphism, transform-related shallow plutons, and blueschist-facies relics at 10-km depth in the Coast Ranges. This diagram is based on the point in the future when this level of the crust is exhumed (tens of millions of years from now), so that the Mendocino triple junction has migrated well north of the northern California border. Exposure of deeper levels of the crust will yield a metamorphic belt with higher-grade metamorphism and a greater volume of plutons. Such a level of exposure may appear more "arc-like."

1988). Calculation of P / T paths for rocks at depth, as shown in Figure 4, is based on the depth to the 300° C isotherm from the data of Hill et al. (1990), with an assumed average uplift rate of 0.3 mm/yr, applied only since the plate-motion change at 3.4 to 3.9 Ma. Because the peak metamorphism is inferred to corre­spond to a thermal transient, cooling will occur under conditions of lower thermal gradient with time. Extended uplift of these rocks depends on their location relative to major fault systems in the present and future Coast Ranges. The deepening of the base of crustal seismicity southward in the Coast Ranges (Hill et al., 1990), shown in Figure 2, and the inferred rate

of triple-junction migration (from Engebretson et al., 1985) indicate that rocks that experience granulite-grade metamorphism cool to 500° C, or amphibolite grade, within 15 million years after the passage of the thermal maximum (Fig. 4).

Preservation of the earlier Franciscan high-P / T metamorphic assemblages at depth may occur in the areas affected by recent green-schist (and possibly higher-grade) metamor­phism, because the duration of heating by the thermal peak is probably not sufficient to com­pletely erase pre-existing high-P/T relics. Because the notable recent metamorphic recrystallization (greenschist grade and above)

SUBDUCTION-TRANSFORM TRANSITION 987

is associated with ductilely deforming rocks, new metamorphic mineral growth may in part define stretching lineations parallel or sub-parallel to the plate boundary, reflecting the dominant sense of motion at this transform plate boundary, with minerals such as amphi-bole elongated parallel to this lineation (Fig. 6).

Overprinted high-P/T rocks may occur as two general types in the Coast Ranges and represent types of rock with contrasting histo­ries (Fig. 5). At significant (20- to 30-km) depths in a relatively narrow belt along the eastern margin of the Coast Ranges, metamor­phic conditions were in the blueschist facies prior to conversion to the transform margin, and the rise in thermal gradient was relatively small following subduction-transform transi­tion (Fig. 5, points f and g). Following the transition to a transform margin, these rocks were heated to greenschist-facies conditions (Fig. 5). In this case, rocks resided in high-P/T conditions until the time of tectonic transition and heating (path f, g in Fig. 7).

The other type of greenschist-overprinted high-P/T rock at depth in the Coast Ranges probably occupies a much greater volume of the crust than those discussed above (Fig. 6). These high-P/T rocks in the core of the Coast Ranges were uplifted from their original depth of meta-morphism (≥20 km) and should be overprinted by greenschist or higher-grade assemblages fol­lowing subduction-transform transition (Fig. 5; path a', b ' in Fig. 7). These rocks may preserve their peak high-P/T assemblage, with negligible mineral growth during synsubduction uplift, typical of exposed Franciscan blueschists that were exhumed under low thermal gradients (e.g., Ernst, 1988). Thus the "apparent" P / T path for overprinted rocks such as these in the core of the Coast Ranges would be the super­position of the peak overprint assemblage (and subsequent retrograde assemblages) over the peak high-P/T assemblage, yielding apparent P /T paths similar to those shown in Figure 7 (paths a', b ' , f'). These paths should vary as a function of uplift of the rocks prior to subduc­tion-transform transition and the peak condi­tions of high-P/T metamorphism experienced by these rocks.

The likelihood of preservation of high-P/T relics would decrease with depth, because the grade of overprinting metamorphism would

FIG. 7. Hypothetical P / T path of rocks at depth in the Coast Ranges, compared with P / T paths for the Sanbagawa Belt (Otsuki and Banno, 1990) and the Haast schists of New Zealand (Yardley, 1982). This diagram shows how "clock­wise" P / T paths similar to those preserved in the San­bagawa Belt and in New Zealand could form in the California Coast Ranges as a result of the conversion from a subducting to a transform plate boundary. Paths a, b, f, g, and h correspond to P / T paths at the points on Figures 3 and 5. The arrows along the dashed "subduction thermal gradient" line show the direction of P / T evolution for Coast Range rocks experiencing uplift prior to conversion to the strike-slip thermal regime. The "prograde" apparent P / T path preserved in such rocks would be an overprint of the peak subduction assemblage by the peak post-subduc-tion assemblage; examples of such P / T paths are the screened and dashed paths labeled a', b ' , and P. Depending on the amount of uplift of blueschists prior to subduction-transform transition, different P / T trajectories are possi­ble. The horizontal paths at a, b, f, g, and h show apparent prograde paths of rocks that do not record an earlier uplift history prior to transform-related metamorphism.

increase with resulting faster reaction kinetics. In the case of older high-P/T rocks that have been uplifted prior to overprinting, the high-P /T metamorphism and overprinting may be separated by 20 to 150 m.y., based on the duration of Franciscan subduction and the tim­ing of the subduction-transform transition. For the deeper overprinted rocks, the temporal sep­aration between high-P/T metamorphism and overprinting may be much shorter.

988 JOHN WAKABAYASHI

The pattern of transform-related metamor-phism prior to disruption by post-metamorphic faulting may be approximately symmetrical, with the highest-grade zone flanked by parallel lower-grade zones, in contrast to the pro­nounced asymmetrical pattern of subduction-related metamorphism. Post-metamorphic faulting, however, should greatly complicate the distribution of transform-related metamor­phism. For example, if the deep crustal plate boundary is a relatively narrow zone of deforma­tion, one side of the boundary should rise relative to the other in response to the contrac-tional component across the plate boundary, although the dominant sense of motion would be strike-parallel. An extended period of move­ment along such a boundary zone, during which time the Coast Ranges cooled following their thermal peak in the wake of triple-junc­tion migration, would lead to higher-grade rocks on the upthrown side and lower-grade rocks on the downthrown side; with sufficient time (tens of m.y.) the cumulative vertical dis­placement across the boundary would become significant. A major deep crustal fault zone striking obliquely across the metamorphic belt also may juxtapose terranes formed under dif­ferent thermal gradients, such as the axial Coast Ranges (high thermal gradients) against the eastern margin of the Coast Ranges (low ther­mal gradients). If the deep crustal plate bound­ary is a broader zone of distributed shear, the exhumed plate boundary zone may have an apparent inverted metamorphic gradient across it.

Probability of future exposure The probability of future exposure of

rocks from a given depth in the Coast Range within a period of 200 m.y. or less should decrease with increasing depth, because with greater depth, the amount of exhumation and time required to accomplish the exhumation would increase. Accordingly, within that time frame, exposure of the greenschist-facies level of the Coast Ranges would be more likely than exposure of the granulite level of metamor­phism. Conversely, preservation of the shal­lower levels of the plate margin becomes less likely with a large amount of elapsed time—say 500 million years or more—and exposure of the deepest portions of the plate margins is more likely.

Re-examination of Metamorphic Belts Considering the Role of Subduction-

Transform Transitions Past ridge-trench interactions have been

inferred for the Shimanto Belt of Japan (e.g., Hibbard et al., 1993), the Chile margin triple-junction area (both presently occurring and exhumed rocks that experienced high thermal gradients) (Forsythe and Nelson, 1985), and the southern Alaska forearc (Sisson and Pavlis, 1993). The consequences of a ridge-trench interaction may be similar to the transform-trench interaction described here, and the early history of subduction-transform transition in southern coastal California probably involved ridge-trench interaction as well (e.g., Atwater, 1970, 1989; Bohannon and Parsons, 1995). The following discussion focuses on the types of features of metamorphic belts that can result from a triple-junction migration of the Califor­nia type, but may apply broadly to ridge-trench-type interactions as well.

Thermal overprinting of some high-P/T rocks associated with orogen-parallel stretching lineations

As noted earlier, h igh-P /T assemblages should be overprinted by assemblages of higher thermal gradient over large areas of the Coast Ranges at depth. This type of overprint repre­sents a "clockwise" P / T path that is typical of most high-P/T terranes of the world (Ernst, 1988). Such P / T paths generally are suggested to have been formed as a consequence of sub-duction, followed by collision of an island arc or continental margin and subsequent cessation of subduction. Clearly, the late Cenozoic thermal event in the California Coast Ranges differs because it involves no collision.

Major strike-slip faults typically are associ­ated with belts of high-P/T rocks (Ernst, 1971), and orogen-parallel stretching lineations also are common features of many orogenic belts (e.g.,. Faure, 1986; Ellis and Watkinson, 1987; Brown and Talbot, 1989; Ratschbacher et al., 1989; Ave Lallemant and Guth, 1990; Wallis, 1990). These stretching lineations typically postdate high-P/T metamorphism and are tex-turally related to the thermal overprinting of the earlier high-P/T metamorphic assemblages (Ratschbacher et al., 1989; Hara et al., 1990; Wallis, 1990). A subduction-transform transi-

SUBDUCTION-TRANSFORM TRANSITION 989

tion may be an alternative explanation to colli-sional orogenesis for development of orogen-parallel stretching lineations and overprints of high-P/T rocks in some of these orogenic belts.

The influence of a subduction-transform transition on orogenesis versus the influence of collision is difficult to evaluate for several rea­sons. The most important may be: (1) geo-chronologic studies are not sufficiently detailed in many orogenic belts; and (2) collision appar­ently did occur at least at some time during the development of many orogenic belts.

The Sanbagawa Belt of Japan and the Haast schists of New Zealand are two possible exam­ples of overprinted high-P/T belts with orogen-parallel stretching lineations that may have been affected by a subduction-transform transi­tion. Figure 7 shows the similarity of the P /T paths of metamorphism from the Sanbagawa Belt (Otsuki and Banno, 1990) and the Haast schists (Yardley, 1982) to the apparent P /T paths that may form beneath the California Coast Ranges as a consequence of the subduc­tion-transform transition.

In the Sanbagawa, the clockwise P /T evolu­tion has been attributed to subduction followed by collision (e.g., Ernst, 1988), possibly of the Kurosegawa tectonic zone in the Late Jurassic (Maruyama et al., 1984). However, if the Kurosegawa composite terrane had indeed buoyantly clogged the subduction zone and halted subduction, then it should form the "lower plate" of the orogen and be more likely to be overprinted with high-grade metamor­phism, analogous to major collisional orogens such as the Alps (e.g., Ernst, 1988). The Kurosegawa tectonic zone apparently lacks San­bagawa or younger metamorphism, and yields metamorphic muscovite K/Ar ages signifi­cantly older than the Sanbagawa metamor­phism (Maruyama et al., 1984), indicating that heating of this terrane during and after the time of Sanbagawa metamorphism did not exceed K/Ar closure temperatures for muscovite. Taira et al. (1983) suggested emplacement of the older rocks of the Kurosegawa zone along a major strike-slip fault as an alternative to colli­sion. The timing of the Cretaceous metamor­phism of the Sanbagawa rocks recorded by Ar-Ar dates (Takasu and Dallmeyer, 1990) reflects cooling from the thermal peak that, in turn, may be the thermal overprint that followed the high-P/T metamorphism (Hara et al., 1990).

The timing of this metamorphism is consistent with the conversion of this margin from a subduction zone to a transform margin as pro­posed by Osozawa (1994) in his plate recon­s t ruc t ions for th is area. A subduct ion-transform transition is a permissible alternate to collision as the cause of the thermal over­printing in the Sanbagawa. The Median Tec­tonic line of Japan separates the Sanbagawa Belt from the HT/LP Ryoke Belt. Brown and Naka-jima (1994) concluded that the metamorphism of the Ryoke Belt was a product of spreading ridge-trench interaction. The Median Tectonic line may be the exhumed analog of the plate boundary at depth in coastal California, jux­taposing two terranes with different thermal histories affected by subduction-transform transition or ridge-trench interaction.

The Haast schists of New Zealand record a similar thermal history of early blueschist-type assemblages overprinted by later greenschist assemblages (Yardley, 1982). Similar to the San­bagawa Belt, the Haast schists are bordered by a major strike-slip fault (the Alpine fault) that separates them from high-grade rocks. The schists also have stretching lineations that are subparallel to the border of the belt (Mortimer, 1992). The origin of the structures and over­print of the Haast schists, like the Sanbagawa Belt, generally is attributed to a collision (Mor­timer, 1992). A subduction-transform transi­tion may be a reasonable alternative model for part of the tectonothermal evolution of these schists.

Alternative explanation for some rock associa­tions interpreted as ancient volcanic arcs

A belt of plutons and associated metamor­phic rocks traditionally is interpreted to be the exhumed root of an ancient volcanic arc. Such a belt of plutons should be present, however, beneath the California Coast Ranges (Liu and Furlong, 1992). An analog of such a plutonic belt may be exposed in the Gulf of Alaska, although the origin of these plutons is still in dispute (Barker et al., 1992).

A possibly similar association of granitic rocks and strike-slip faults, in Hercynian shear zones in Iberia and shear zones in the British Caledonides, was noted by Hutton and Reavy (1992). Although these authors suggest crustal thickening during transpressional deformation

990 JOHN WAKABAYASHI

FIG. 8. Hypothetical retrograde P / T paths for rocks at depth in the California Coast Ranges compared with P / T paths of granulites compiled by Bohlen (1987, 1991). The Coast Ranges paths show cooling from 35° C/km. The thermal peak of longer duration (see text and figure cap­tions for Figs. 4 and 5) is 30° C/km and may be more representative of peak prograde assemblages, in which case Coast Range retrograde P / T paths originate from the 30° C/km line, and the similarity of these P / T paths to the granulite paths would be greater.

as the cause of crustal melting, an alternative for the formation of some of the granitoids they discussed may be a subduction-transform transition.

Granulite belts and other HT metamorphic belts

In many granulite belts, granulite assem­blages have undergone retrograde metamor-phism under conditions of decreasing thermal gradient, or a counterclockwise P / T path (e.g., Bohlen, 1991). Such metamorphism has been attributed to metamorphism at the base of a volcanic arc or subcontinental underplating (Bohlen and Metzger, 1989). It should be noted that the P / T conditions of metamorphism at depth in the Coast Ranges and the predicted cooling path (Fig. 8) are very similar to many of the examples cited by Bohlen (1991). The inferred cooling of deep rocks in the California Coast Ranges may have been close to isobaric, because of the minimal uplift since the passage of the plate triple junction (see discussions in earlier sections; Figs. 4 and 8). The cooling paths of rocks in a general subduction-trans­form transition orogen may vary, however,

depending on the component of shortening (or lack thereof) that accompanies the strike-slip motion and drives uplift.

In addition to some granulite belts, other belts of HT metamorphism may be a conse­quence of a subduction-transform transition, representing somewhat shallower levels of crust. An example of such a metamorphic belt may be the Salmon River suture of Idaho, a terrane that features regional metamorphism of greenschist to upper amphibolite facies, with structures suggestive of major strike-slip dis­placement along the suture (Lund and Snee, 1988). The main stage of regional HT metamor­phism associated with this belt may be a conse­quence of a subduction-transform transition.

Probability of exposure and the age of granulite and high-P/T belts

As indicated previously, the probability of future exposure of different depths of the present-day California Coast Ranges varies with the amount of elapsed time in the future; that is, exclusively shallow levels of exposure are more likely with less elapsed time, and deep levels are more likely with long elapsed time. As one of several types of plate-boundary changes that can influence the evolution of mountain belts, the probability of preservation subduc­tion-transform transition effects provides insight into general problems of preservation of various rock types on Earth. For example, it is likely that a long-lived active plate margin will experience a collision, subduction-transform transition, or ridge-trench interaction at some point in its history. The longer the elapsed time since the formation of a blueschist belt, the less likely blueschists along any plate margin will be preserved, because: (1) there is increased prob­ability of a plate-boundary transition that stops subduction or causes an increase in thermal gradients; and (2) the longer elapsed time allows for greater exhumation of rocks, expos­ing deeper levels of the crust where older high-P / T rocks are more likely to have been com­pletely overprinted by the thermal effects of the plate boundary interactions. It therefore is not surprising that the vast majority of high-P/T metamorphic belts are Phanerozoic in age (Ernst, 1972; Liou et al., 1989). The scarcity of older blueschist has been attributed to a decrease in thermal gradients as the Earth cooled (Burke et al., 1977). Although the

SUBDUCTION-TRANSFORM TRANSITION 991

decrease in thermal gradients with time may have affected the age distribution of exposed blueschists, exposure time to later thermal pro­cesses, as discussed here, also may play a major role.

Conversely, any granulites that form in an environment analogous to the present-day Cali­fornia Coast Ranges are not likely to be exposed for a long time, because of the magnitude of cumulative exhumation necessary to expose them. The fact that the majority of granulite belts are Precambrian (Bohlen and Metzger, 1989) is consistent with this observation.

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