ABSTRACT
Mesozoic thickening and Cenozoic extension resulted in the
juxtaposition of upper and middle crustal rocks in the eastern
Mojave Desert, southeastern California and western Arizona. The
application of 40Ar/39Ar thermochronology and
petrology/thermobarometry to rocks in this region provides
information about the timing and nature of thrusting, plutonism,
metamorphism, denudation, and detachment faulting. 40Ar/39Ar ages
of 175 to 125 Ma from the Clipper, Piute, Turtle, Mohave, Bill
Williams, and Hualapai Mountains are interpreted to be the result
of a middle Mesozoic thermal event(s) caused by crustal
thickening. This is supported by the presence of ca. 150 Ma dikes
that locally intruded Paleozoic meta-sediments at ~3.5 kbars in
the Old Woman Mountains. Orogenesis culminated during the Late
Cretaceous when rocks exposed in the Old Woman-Piute, Chemehuevi,
and Sacramento Mountains attained temperatures >500 °C.
High grade metamorphism in the Old Woman Mountains area was caused
by the intrusion of the Old Woman-Piute batholith at 73 ± 1
Ma; Cretaceous mineral assemblages in Proterozoic pelites increase
in grade from greenschist to upper amphibolite facies, and
40Ar/39Ar hornblende ages from Proterozoic amphibolites decrease
in age from ~1600 Ma to 73 ± 1 Ma, in the direction of 73
Ma plutons. Pluton emplacement and metamorphism occurred at 3 to
3.5 kbars and 400 to >600°C in the Piute Mountains, and 3.5
to 4.5 kbars and 530 to >650°C in the Old Woman Mountains.
Cooling rates following batholith emplacement in the Old Woman
Mountains were ~100°C/Ma between 73 and 70 Ma, and 5 to
10°C/Ma from 70 to ~30 Ma. This rapid cooling between 73 and
70 Ma requires unroofing rates of 1 to 2 km/Ma for this interval.
Following the Cretaceous, the eastern Mojave Desert underwent a
period of cooling at a rate of 2 to 10°C/Ma between 65 and 25
Ma. By 30 Ma rocks exposed in the Old Woman-Piute, Marble, Ship,
Clipper, and Turtle Mountains were below ~100°C. 40Ar/39Ar
ages from the Sacramento Mountains suggest that mylonitization
caused by the onset of regional extension occurred at 23 ±
1 Ma. When extension started in the Chemehuevi Mountains, rocks
exposed in the southwestern and northeastern portions of footwall
to the Chemehuevi detachment fault were at ~180°C and
~350°C, respectively, which suggests that this fault initiated
at a dip of 5 to 30°. Unroofing of the footwalls to detachment
faults in the Sacramento and Chemehuevi Mountains resulted in
cooling rates of 10 to 50°C/Ma between 22 and 15 Ma.
Foster, D.A., 1989. Mesozoic and Cenozoic thermal history of the
eastern Mojave Desert, California and western Arizona, with
emphasis on the Old Woman Mountains area and the Chemehuevi
Metamorphic Core Complex. Unpublished PhD dissertation,
State University of New York at Albany. 310pp., +xiii, appendix
6pp.
University at Albany Science Library call number: SCIENCE
MIC Film QE 40 Z899 1989 F67
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