NEW FAULTS FOR OLD DAD: STRUCTURAL INHERITANCE, MIOCENE KINEMATICS, AND QUATERNARY DEFORMATION, NE MOJAVE, CA

On strike and 50km SE of the South Death Valley fault zone, Quaternary downcutting has exposed Proterozoic-Mesozoic plutonic, metamorphic, and sedimentary rocks (Xpm) and unconformably overlying, Miocene coarse clastic rocks and minor volcanic rocks (Mc). Mc is 0.5-1.5km thick, moderately-steeply dipping, and unconformably overlain by gently dipping Plio-Quaternary basalt flows and surficial deposits (QPc). Mc is divided into 7 unconformity bounded units, Mc1-7. Mc3-7 contain clasts of welded tuff correlated with the Peach Springs tuff (Tps) (18.71 ± 0.13 Ma). Units Mc3-5 are cut by faults with .01-1000m stratigraphic throw and are overlain by less deformed Mc6-7 and QPc.

Previous workers have mapped the Old Dad fault (ODf) on the east side of Old Dad Mtn (OD), a 20km long, NW trending ridge of Xpm, but location, orientation, slip sense, and age of faulting are unclear. Kinematic data for meso- to map-scale faults that cut Mc3-7 in this area show ODf is a composite structure with a polyphase mid-late Miocene history:

1)In SE OD, a N15-35W striking high angle fault cuts Tps and lower Mc3, is covered by Mc4 in the north and has at least 7km dextral separation. We interpret upper Mc3 sedimentation to be syn-kinematic with WNW trending folds and reverse faults that formed at a left step.

2) In NE OD, Mc4 and Mc5 onlap basement to the north and are cut by numerous N15-55E striking growth faults with 1-100m vertical separation and oblique sinistral-normal kinematics.

3) Faults in 2) are cut by N40W striking, subparallel high angle faults overlain by undeformed Mc6 or QPc that show strike-slip kinematics and at least 1.8 km dextral separation of steeply-dipping resistant strata in Mc4 and Mc5.

4) Along the length of OD, Mc3-7 are faulted against Xpm in a zone of discontinous NW striking dextral strike-slip faults and WNW striking oblique dextral reverse faults that place Xpm over Mc3-5 and are overlain by undeformed QPc. Late Miocene? transpressive deformation reactivated older structures.

Similar relationships are discontinuously exposed over 90sqkm of the NE Mojave. We present a model of mid-late Miocene transtension and transpression in an area of complex pre-existing structure that is predictive of Neogene structure, sedimentation, and the location of mineralization, geophysical anomalies, and Quaternary deformation.