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GMEG - Geophysics Unit of Menlo Park, CA- (GUMP)


Geophysics Unit of Menlo Park, CA (GUMP)

U.S. Geological Survey - Western Region - Geology and Geophysics

A New Aeromagnetic Survey Over Central Death Valley: Implications for Tectonic Evolution and Hydrologic Framework

A Brief Talk Presented at the Devils Hole Workshop, Furnace Creek, California, April 26, 2001

Richard J. Blakely

1999 and 2000 aeromagnetic surveys Location map.  This map shows boundaries of 1999 and 2000 aeromagnetic surveys.  The surveys were flown by different Canadian contractors but with essentially identical survey specifications.
Simplified geology map Simplified geology map. Dark dashed lines indicate limits of aeromagnetic surveys.  Volcanic rocks comprise the most important magnetic lithology, exposed mostly in the Yucca Mountain area and within the Death Valley volcanic complex.
Merged aeromagnetic data Merged aeromagnetic data. Note high-amplitude, short-wavelength anomalies over volcanic exposures, which contrast with subdued anomalies over Mesquite Flat, Furnace Creek Wash, Ash Meadows, and Pahrump Valley.  The large anomaly over the Black Mountains is probably caused by the Willow Springs diorite and indicates its subsurface extent.  It may extend beneath the floor of Death Valley to connect with similar rocks in the Panamint Range.
Residual magnetic anomalies Residual magnetic anomalies.  Here the aeromagnetic data have been processed in order to emphasize shallow magnetic sources.  The margins of volcanic terranes are clearly demarcated.  Also note various strands of the Furnace Creek fault.
High-definition magnetic anomalies High-definition magnetic anomalies.  Residual anomalies of the previous slide have been filtered to emphasize near-surface sources that strike northwest-southeast.  Multiple strands of the Furnace Creek fault are evident, one extending entirely across Mesquite Flat to the Cottonwood Mountains.  A north-south pattern of anomalies connects the volcanic terrane of Yucca Mountain with the Death Valley volcanic complex through the Amargosa trough.
Intepreted magnetic lineations Intepreted magnetic lineations.  Magnetic lineations, interpreted from the previous maps (and others), reflect faults, ancient fluvial channels, and erosional or depositional margins of volcanic deposits.  The State Line and Furnace Creek fault zones are evident.
Isostatic residual gravity Isostatic residual gravity.
Magnetic lineations and Cenozoic basins Magnetic lineations and Cenozoic basins.  This map reflects the last 15 m.y. of tectonic evolution.  Rainbow colors represent Cenozoic basins determined from an inversion of gravity data, whereas lines are interpreted magnetic lineations.  Magnetic lineations manifest near-surface deformation of Quaternary deposits, whereas the basins highlight very deep deformation of pre-Tertiary basement.  The fact that they correlate spatially suggests a kinematic link .
Regional picture Regional picture.  Dashed lines represent fundamental crustal structures, namely major strike-slip fault zones (Furnace Creek, State Line, and Highway 95 faults) and pull-apart zones (e.g., Amargosa trough, northern Amargosa Valley, Ash Meadows, and Pahrump Valley) that transfer strain between the faults.
Zoom-in on Furnace Creek Zoom-in on Furnace Creek.  A major strand of the Furnace Creek fault passes between Nevares and Salt Springs.  Note the narrow lineation that connects Salt, Texas, and Travertine springs.  It presumably is caused by a near-surface fault that focuses the discharge of water at the springs.  This fault may extend to Lemonade Spring, a total distance of 25 km.
Zoom-in on Devils Hole Zoom-in on Devils Hole.  Springs at Ash Meadows lie along near-surface faults expressed in the aeromagnetic data.  These faults in turn lie along the eastern margin of the Amargosa trough, well defined in gravity data.
Zoom-in on Pahrump Valley Zoom-in on Pahrump Valley.  The three southern springs lie along the northeastern margin of a horst that divides two deep sub-basins beneath Pahrump Valley.  The horst marks the State Line fault and apparently acts as a barrier to the flow of water from the Spring Mountains.  The northern spring lies along a magnetic anomaly that traces the deepest parts of the Pahrump Valley sub-basin.  Perhaps the magnetic anomaly reflects near surface fluvial channels, part of the sub-basin's longpterm depocenter.
link between magnetic anomalies and water discharge What is the link between magnetic anomalies and water discharge?  Perhaps faults that influence the flow of water have offset underlying magnetic units.  Or perhaps ground water concentrates magnetic minerals along the fault plane, through precipitation of iron oxides/sulfides or deposition of detrital magnetite.

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