Tk. Fowler et al., 2-PHASE EVOLUTION OF THE SHADOW VALLEY BASIN, SOUTH-EASTERN CALIFORNIA - A POSSIBLE RECORD OF FOOTWALL UPLIFT DURING EXTENSIONAL DETACHMENTFAULTING, Basin research, 7(2), 1995, pp. 165-179
Miocene strata of the Shadow Valley Basin rest unconformably on the up
per plate of the Kingston Range - Halloran Hills detachment fault syst
em in the eastern Mojave desert, California. Basin development occurre
d in two broad phases that we interpret as a response to changes in fo
otwall geometry. In southern portions of the basin, south of the Kings
ton Range, phase one began with near synchronous initiation of detachm
ent faulting, volcanism and basin sedimentation shortly after 13.4 Ma.
Between c. 13.4 and c. 10 Ma, concordantly bedded phase one strata we
re deposited onto the subsiding hangingwall of the detachment fault as
it was translated 5-9 km south-westward with only limited internal de
formation. Phase two (c. 10 to 8-5 Ma) is marked by extensional dismem
berment of the detachment fault's upper plate along predominantly west
-dipping normal faults. Phase two sediments were deposited synchronous
ly with upper-plate normal faulting and unconformably overlie phase on
e deposits, displaying progressive shallowing in dip and intraformatio
nal onlap. Northern portions of the basin, in the Kingston Range, expe
rienced a similar two-phase development compressed into a shorter inte
rval of time. Here, phase one occurred between c. 13.4 and 12.8-12.5 (
?) Ma, whereas phase two probably lasted for no more than a few 100 00
0 years immediately prior to c. 12.4 Ma. Differences in the duration o
f basin development in and south of the Kingston Range apparently rela
te to position with respect to the detachment fault's breakaway; north
ern basin exposures overlie the upper plate adjacent to the breakaway
(0-15 km) whereas southern basin exposures occur far from the breakawa
y (20-40 km). We interpret the phase one to phase two transition as re
cording breakup of the detachment fault's hangingwall during footwall
uplift. We propose a model for supradetachment basin evolution in whic
h early, concordantly bedded basin strata are deposited on the hanging
wall as it translates intact above a weakly deforming footwall. With c
ontinuing extension, tectonic denudation along the detachment fault le
ads to an increasing flexural isostatic footwall response. We suggest
that isostatic footwall uplift may drive internal breakup of the upper
plate as the detachment fault is rotated to a shallow dip, mechanical
ly unfavourable for simple upper-plate translation. Additionally, we a
rgue that continuing hangingwall thinning during phase two places geom
etrical constraints on the timing, amount and, thus, rate of footwall
uplift. Kinematically determined footwall uplift rates (0.5-4.5 mm/yr)
are comparable with rates determined independently by thermochronolog
ical and geobarometric methods.