Louie et al., 1996, Geophysical Data in Tecopa Valley

Geophysical Constraints on Extension East of Death Valley, California

John N. Louie Seismological Laboratory and Department of Geological Sciences, The University of Nevada, Reno Lauren Wright, Charles A. Langston Department of Geosciences, The Pennsylvania State University Roger B. Morrison Morrison and Associates

Spring 1992 Geol 453/653 participants: Arif Cetintas, Vijay Chekuri, William D. Corchuelo, Yutian Lei, Li Li, Govardhan Mekala, Serdar Ozalaybey, John Raskulinecz

Spring 1996 Geol 453/653 participants: David Aglietti, Steve Bowman, Michael Hasting, Andrew Hessel, Gene Ichinose, Zakir Kanbur, Shean-Der Ni, James Ollerton

Accepted for: L. A. Wright and B. W. Troxel, eds., 1998, Cenozoic Basins of the Death Valley Region: Geol. Soc. Amer. Spec. Paper

Nature and timing of So. Great Basin extension

Deep-seated faulting of Amargosa Valley

Dating the extensional pulse from basin reflections

Chicago Valley: Differences in Extensional Style

Nature and Timing of Southern Great Basin Extension

Late Cenozoic extension near Death Valley is recorded in the geometry of basins at shallow depths. Amargosa Valley is the first major structural basin east of Death Valley.

Stewart characterized Great Basin normal faulting as a product of 20% crustal extension. Wright and Troxel proposed for the SGB that shallow-dipping normal faults and complementary pairs of strike-slip faults (pull-aparts) allow up to 50% extension.

Wernicke et al. challenge such models with correlations between widely separated Devonian to Mesozoic folds and thrust sheets. They derive >100% extensions for the Death Valley region, accommodated by a shallow, gently dipping regional detachment extending from Pahrump Valley west to the Sierra Nevada.

(Photo by Mike Hasting)
Under a rolling hinge model the Pahrump, Stewart, California, Chicago, and Tecopa Valleys should have opened in westward order, with tilted range blocks as at left in Chicago Valley. Now the hinge has progressed to central Death Valley.

COCORP collected 250 km of deep-crustal seismic surveys in the Death Valley region, with survey parameters to give high resolution in the deep crust, limiting resolution of the shallow crust.

Serpa et al. demonstrated mid-crustal bright spots and possible 15 km-deep regional detachments in the COCORP images, but did not show upper-crustal faults or shallow-basin geometries.

Tecopa Valley contains a sequence of Recent to Miocene mudstones, tufas, and volcanic ashes deposited in Pleistocene Lake Tecopa. The sediments show little deformation and only subtle faulting. Morrison attributes them to slow deposition in a quiet tectonic environment. In the surrounding ranges 5.3-11 Ma tuffs outcrop, which normal faulting has broken and exposed.

The draining of Lake Tecopa and erosion of Tecopa Valley by the Amargosa River since 0.16 Ma is due to the subsidence of the central Death Valley graben and the lowering of hydrographic base.

Deep-seated faulting of Amargosa Valley

We use small-scale geophysical methods to search for shallow basin geometry. In the Basin and Range, basin fill has physical properties that contrast sharply with older-rock basement.

(Photo by Gene Ichinose)

Four years of one-week geophysical field camps operated out of courses at Penn State and UNR have conducted gravity, magnetic, seismic, and electromagnetic surveys in Tecopa Valley.

Work of PSU and UNR Field Camps in Tecopa Valley

	Spring 1990	Spring 1991	Spring 1992*	Spring 1996*
University	Penn State	Penn State	UNR	UNR
Profiles (Fig. 2)	SHO OST	THS	AR CCV BMR	OSTH*
Gravity stations	77	75	82	31
Magnetic stations	74	150	142	120
Seismic source points	34	29	107	*
EM measurements	54	8	204	*
Results	600 m deep basin, steep range-front fault	Deep basin is continuous	Early Lake Tecopa stratigraphy, geometric differences of basins	Range-front fault and listric fault blocks

* Additional exercises in 1994 and 1996 focused on Pahrump and Stewart Valleys and are reported elsewhere.

The Spring 1990 camp found unexpectedly thick basin fill in Tecopa Valley. Previous, widely spaced gravity data had missed the 13 mGal negative anomaly, in a classic example of spatial aliasing.

On the east side of Tecopa Valley, a 700 nT magnetic anomaly and seismic refraction measurements along the trace of the Resting Spring Range-front fault at profile SHO demanded a 200-500 m thick sliver or intrusion of basalt within the fault zone.

We estimated the total thickness of Plio-Pleistocene basin fill to be ~580 m, using a conservative estimate for the density contrast between the basin fill and the older substrate.

The Spring 1991 exercise confirmed the 600 m deep trough along the west side of Tecopa Valley. An unknown sequence of Miocene alluvium and volcanics (Tm) fills most of the basin. Above is a 170±20 m section of Tecopa lacustrine mudstone (Qtlm).

This large thickness indicates strong extension and sediment deposition before Tecopa Valley was breached and drained.

Dating the Extensional Pulse from Basin Reflections

The Spring 1992 camp imaged Lake Tecopa stratigraphy with a high-resolution seismic reflection survey. Ideal conditions after wet weather allowed recording reflections from 180 m depths using a sledgehammer source.

The stacked section shows that the regular and undeformed Plio-Pleistocene lake-beds continue to a depth of 134±20 m below the 2.01 Ma Huckleberry Ridge tephra, extending the 72 m exposed section by 100 m.

Using Hillhouse's post-Huckleberry Ridge average rate of 27 m/m.y., the seismic section pushes the base of the sequence back to 7 Ma. Given Morrison's evidence for slower early deposition rates, the 7 Ma age is a minimum.

Since the Resting Spring Range-front fault breaks 11 Ma pyroclastic deposits, Tecopa Valley experienced rapid (>0.15 mm/yr) downdropping for 4 m.y., ending at 7 Ma.

Chicago Valley: Differences in Extensional Style

The 1992 and 1996 camps also extended potential-field profiles to Chicago Valley.

Despite use of a ±1 mGal Worden gravimeter, the BMR profile in northern Chicago Valley shows up to 500 m basin depth. Unlike Tecopa Valley, it is divided into two sub-basins by a detached block. The sole magnetic anomaly appears over the fault of the western sub-basin, but unlike the Resting Spring Range-front is <50 nT.

The short CCV profile on the western side of central Chicago Valley shows a ramp to 200 m basin depths; from eastward tilting of a detached Resting Spring Range block.

The OSTH Old Spanish Trail Highway profile traverses southern Chicago Valley between the Tecopa block and the Nopah Range, showing up to 300 m basin depths and up to 3 sub-basins. The only magnetic anomaly is a 700 nT feature off the southern tip of the Resting Spring Range, similar to that found along the range front at the SHO profile. 1992 PSU students also found this anomaly on three other profiles of the range front.

Detached, rotated blocks along the OST and OSTH profiles would be (W-E) Tecopa Peak, Tecopa Hill, the Resting Spring Range, and a block buried along the middle of Chicago Valley.

The linear magnetic anomaly and thick basalt along the Resting Spring Range front suggests intrusion up a deeply penetrating and relatively steeply dipping fault plane during 11-7 Ma Tecopa Valley extension.
Opening of Chicago Valley was by detachment and rotation against more shallowly dipping faults, evidenced by sub-basins and eastward ramps. Similar faults are exposed throughout the Dublin Hills and Resting Spring Range.
Lack of magnetic anomalies suggest Chicago Valley detachments are not basalt-intruded, and have an earlier age and shallower penetration than the Resting Spring Range front.

Conclusions

Surprisingly great age of the undeformed Lake Tecopa sequence, and cessation of extension at 7 Ma.
Rapid, basin-forming Cenozoic extension between 11 and 7 Ma.
Rapid extension took two phases: shallow detachment; and steep normal-faulting with volcanism.
Any Wernicke rolling hinge must compensate isostatically within the 4 m.y. period.
Simple, shallow geophysical surveys of basins put physical constraints on regional extensional models.

ACKNOWLEDGMENTS

Drs. Douglas R. Schmitt of the University of Alberta, Chris Sanders of Arizona State, Laurie Serpa and Terry Pavlis of the University of New Orleans; and Bennie W. Troxel have generously contributed to this work. Undergraduate and graduate students from Penn State and UNR carried out most of the survey planning, fieldwork, and analysis. PSU participants included Dr. Abu Asad, Dr. Michael Gross, John Hammer, Greg Jablunovsky, Raymond Laird, Brian Lassige, Steve Nichols, Jordi Prims, Dr. Sathish Pullammanappallil, Dr. David Verdonck, Nancy Yonkers, and Dr. Jie Zhang.

Supported in part by the National Science Foundation under grant EAR-9405534; Chevron USA, Inc. grants to the Dept. of Geosciences at The Pennsylvania State University; the S. F. Hunt Fund at the Mackay School of Mines, University of Nevada, Reno; and the W. M. Keck Foundation.