usu extensional processes in the northern rocky mountains susanne u. janecke utah state university...

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Extensional processes in the northern Rocky Mountains

Susanne U. JaneckeUtah State University

David A. FosterUniversity of Florida

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Sonder and Jones, 1999

Extensional processes in northern Rockies

This talk:

Selective overview of evolution of this province

A few questions about the province

forgotten part of the Basin-and-Range north of San Andreas

MTJ

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Long history of extension:

Belt basinPassive marginSevier orogeny?Eocene core complexesMany younger phases of extension-

Too many to name

Extension continues today

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Cenozoic extension is superimposed on:

Belt basin Paleozoic hingeline West edge of

continental crust Fold-and-thrust belt

Lewis and Clark line Idaho batholith Laramide foreland

province Mitra (1997)

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Thrust belt-mostly NW strikes near ESRP

Thrust belt has salients and reentrants-

Culminations

Note bend in thrust south of ESRP

•Laramide province has diverse trends:

•NE NW E N

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Overall curvature of the normal faults parallels the fold-and-thrust belt

•Strikes of normal faults changes south of ESRP

•Largest faults collapsed culminations

Modified from Carney and Janecke, 2005 GSAB

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Parallel strikes are a bit puzzlingWestern part has normal faults that clearly CUT the thrusts

Lost River fault-planar to ~15 km depth

Wasatch fault

Yet strikes of those normal faults grossly parallel the thrust belt

Eastern part of thrust belt shows much simple “backsliding” on thrusts

Model of West (1992, 1993) explains this pattern

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•Older Grasshopper thrust fault guided the extension

•Ramps and flats in Muddy-Grasshopper detachment fault-

•Inherited?

•Shallow depth of faults :1.5 to 2.5 sec

Seismic data reveal reactivated thrust faults in the east

seismic line ML-83-01

MGF

Grasshopper basin

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Predictions of geologic data Depths of normal faults in thrust belt should

increase westward Will we see this in locking depths?

Extension directions would be dip-slip if bouyancy from thrust belt drives normal

faulting Oblique-slip if some other process dominate

Test with GPS campaigns Focal mechanisms Slickenline data

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Figure from Foster et al. in press.Metamorphic core complexes: Oldest widespread extension:•Late Paleocene to Eocene-narrow belt

•Mostly top ENE to top ESE slip (some top west slip too)

•Less extension in the south?

•Several pairs of complexes-even triplets

•Bivergent slip

200 km

USUFigure from Foster et al. in press.

Large magnitude of

Eocene extension

north of 45° N

•30-40 km of displacement across Anaconda MCC

•45-50 km across Bitterroot MCC Foster et al. in press.

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Lewis and Clark line: Transfer fault within the northern Rockies

Left step and left bend of core complexes (Foster et al., in press.)

>300 km step

Clearwater core complexes lies within the stepover zone

Figure from Foster et al. in press.

200 km

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Lewis and Clark line: Many unknowns

Figure from Foster et al. in press.

Does it cut across Precambrian crustal boundaries?

Why did it have such a profound effect?

From Belt time to mid Tertiary?

Depth extent? Geometry?

Still active?

Modern strain profile

This feature has partitioned strain for ~1.5 Ga. Why?

USUModified from Janecke (1994)

A

BA

Eocene to Oligocene (± e. Mio.) basin-forming

event

•Paleogene rift zone-narrow

•Moderate displacement

•Detachment faults

•Sedimentary basins formed-supradetachment basins

•Two major accommodation zones-one at Lewis and Clark line

•Quiescent Renova basin east of the rift zone (debated)

•Core complexes (pink)

•Remnants of sedimentary basins-pale orange

Core complexes were just the first of many periods of

extension: Robust extension continued into early Miocene

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Complex fault systems reflect changing strain field

Janecke et al., 2000 USGS OF

10 km

Beaverhead Range

Tendoy

Mountains

Pre-Eocene to Recent normal faults

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Many changes in extension direction over time Extension direction

can change dramatically (up to 90°)

in short periods of time.

Many changes can occur

up to 6 temporally and geometrically distinct sets of normal faults

Largest normal faults parallel the thrust belt

Geodynamic models should consider such variability VanDenburg et al., 1998

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Idaho batholith little extension

Former fold-and-thrust belt regular spacing and moderate

extension long fault systems N to NW strikes

Laramide foreland Many trends Shorter faults

Anomalous trends near the Yellowstone hot spot and its former positions?


Altogether Basin and Range faults-4 domains?

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Few normal faults cut the Idaho batholith:•Strong crust and lack of preexisting thrusts probably explain this pattern

•Is this strain pattern evidence in GPS data sets?•Do adjacent areas compensate for this slip deficit?

•e.g. Western Snake River Plain? Gentle tilts suggest modest extension there but crustal geophysics is needed

WSRP ESRP

IB

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Normal faults in the former fold-and-thrust belt:

Many long fault zones:

Lost River,Lemhi and Beaverhead faults- 140-150 km long

Grand Valley-140 km

Mission fault- 102 km long

Swan fault- 156 km long

(Haller et al. Quat flts 2004)

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Normal faults in Laramide province


Short faultsGallatin Range-27 km

Emigrant-43 km

Bridger 48 km

Tobacco Root fault-32 km

Madison- 99 km

Ruby Range fault-38 km

Data of Haller et al. 2004

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Anomalous trends of normal faults near Yellowstone Centennial Range

north dipping fault South-dipping rocks

Teton Range- ESE-dipping fault WNW-dipping rocks

Together they define a SW plunging syncline

These faults are cross faults


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Possible effects of Yellowstone hotspot on

extension

Susanne JaneckeRobert Smith

Michael Perkins2000 GSA abstract

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Typical Basin and Range normal faults

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Cross faults:

Numerous normal faults at a high angle to the overall structural grain are localized near ESRP

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Focal mechanisms document anomalous orientation of stress near Yellowstone

Stress map of Waite and Smith, 2004

Waite and Smith (2004) suggest that the N-S extension may be related to viscoelastic relaxation in the upper mantle and lower crust following the 1959 Hebgen Lake earthquake.

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Prior work and new data

Eastern Snake River Plain is a broad SW-plunging syncline (Kirkham, 1931; Myers and Hamilton 1964, Hamilton and Myers, 1966; McQuarrie and Rodgers, 1998)

Active faults adjacent to Yellowstone have anomalous trends and probably formed due to subsidence toward the Plain (Honkala, 1960; Hamilton and Myers, 1966).

Minimum principal stress is perturbed adjacent to the hot spot e.g Waite and Smith, 2004

A mafic sill underlies much of the ESRP (Sparlin et al., 1982)

Differential subsidence is greatest SW of the hot spot (Smith and Braile, 1993)

Ancient cross faults occur throughout the region adjacent to the ESRP.

Strikes of cross faults are E-W north of the plain and NNE to E-W south of the plain.

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Two components of subsidence can produce cross faults near ESRP

SWward subsidence SW of Yellowstone

Subsidence toward the axis of the ESRP

Component 1 + Component 2 = south dips on the NW side of the ESRP and WNW-dips on the SE side of the ESRP.

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Testable predictions of the model1) Teton and Centennial faults initiated in

Pliocene-Quaternary time2) Cross faults initiate earlier in the SW than in the

NE3) Teton area should have ESE extension4) Anomalous strain field is localized near

Yellowstone5) Anomalous strain field should be persistent

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Northward termination of the Basin-and-Range provinceClockwise rotation?

Subdomains of strain?

What happens across and in the ESRP?

GPS and more focal mechanisms could address these questions

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Simple predictions of geologic history

Crust should be thinner near MCC’s beneath intermontane seismic belt probably E and W of Idaho batholith

Crust should be thicker in Great Basin- Fewer double and triple core complexes Extension began later there

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Instead crust seems thick in Rocky Mtns

• 37 km next to ESRP (Peng and Humphreys, 1998)

• 38 km at Hebgen lake Nishimura Thatcher (2003)

• BUT• Normal rifts are 30.5 km

thick (Christiansen and Mooney, 1995

• Some of northern Great Basin is much thinner, <30 km (Gilbert and Sheehan, 2004)

• Thin crust trends E-W there, not N-S

Gilbert and Sheehan, 2004 JGR

Was the crust inflated by magmatism?

Was the crust thicker initially?

Col. Pl.

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Rivers flowed from highly extended core complexes to less extended areas in the Eocene-Oligocene

Arkoses are persistent feature Base to top of Medicine Lodge beds Even in the Everson Creek beds

Source was a 80-70 Ma granite Two micas-both biotite and muscovite Late Cretaceous to Early Tertiary

cooling Chief Joseph pluton in footwall of the

Anaconda MCC? (Thomas , 1995) Persistent north to south fluvial

system along axis of the rift Janecke et al. 2005 Bach Mtn map

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Partial solution:Lower crustal flow toward core complexes? (e.g. Vanderhaeghe et al.) Away from Eastern Snake River Plain? (McQuarrie and Rodgers, 1998

Campbell and John, 2003

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Earthscope facility can help with these problems: Moho mapping across the region Strain patterns How does hotspot influence the stress and strain field? What drives extension now? In the past? Recovery and rescue of industry seismic data

Comparison of GPS and geologic strain rates Rotation and strike slip in the Basin and Range province Deformation within the ESRP Role of crustal inheritance

USU81-4

Nicholia Creek basin

1 sec

2 sec

NESW

Muddy-Grasshopper detachment

Break up fault

Our community should rescue existing industry seismic data

•A wealth of data already exists•Archive in GEON or IRIS?

usu extensional processes in the northern rocky mountains susanne u. janecke utah state university...

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