NEW MAPPING IN THE AMARGOSA CHAOS, DEATH VALLEY NATIONAL PARK: MULTIPHASE DEFORMATION, IRON OXIDE-COPPER-GOLD ± U ± LREE DEPOSIT, AND THE AMARGOSA FAULT

The Amargosa Chaos and Fault of southern Death Valley are complex,

poorly understood, and controversial features that play important roles in

various tectonic models. The Chaos was originally defined as a 3-phase,

fault-block mosaic occupying the hangingwall of the Amargosa “Thrust”

(Noble, 1941). The thrust interpretation was later identified as “an illusion”

by Wright and Troxel (1969) who suggested the fault was instead Basin and

Range normal fault. Some recent models claim the fault is a regional

detachment with NW-directed transport of up to 80 km that produced the

Chaos in its hangingwall. In this study, we find the detachment fault

hypothesis also illusive and offer an alternative interpretation: the Virgin

Spring (VS) Phase, which consists of fault blocks of Proterozoic sedimentary

rocks, is a product of multiphase deformation that likely spanned the

Mesozoic and Cenozoic.

This study addresses cross-cutting relationships among fault populations

within the Ashford-Jubilee section of the VS Chaos located south of the Desert

Hound Anticline. We found six major sets of structures: (D1) 25% north-northwest

directed Mesozoic (?) shortening across an imbricate thrust and tight fold system;

(D2) E-SE (?) extension on five presently low-angle normal faults with

hematite-magnetite-chalcopyrite ±U ± LREE mineralized upper plate fractures,

previously unidentified and indicates an Iron Oxide-Copper-Gold (IOCG) deposit that

are elsewhere Oligocene. We've estimated the slip accommodated by the D2

system as 4-8 km; (D3) extension-related folding expressed as the Desert Hound

Anticline and Chaos Syncline, oriented 30°, S80°E and 40°, S35°E, respectively, and

folded the D2 fault system; (D4) dextral, down-to-SW slip on the moderately dipping

Amargosa Fault occurring between ~7-4 Ma and estimated to accommodate <14

km of slip; (D5) E-W extension on high-angle, domino faults after 4 Ma; (D6) ~3km

of down-to-the-west slip on the Black Mountains Frontal Fault.

Relationships indicate this section of the VS Phase is therefore the

product of multiphase deformation. The D2 faults, not the Amargosa, created

the enigmatic attenuation observed in the Chaos. The Amargosa Fault

bounds the northern edge of the VS Chaos against the basement, but is

responsible for relatively minor transport, not the creation, of the Chaos.