Abstract

Rock units representative of most pre-Cenozoic formations in the southern Cordilleran geosyncline, as well as major structures developed during multiple periods of Mesozoic and Neogene deformation, are exposed at Old Dad Mountain, Mojave Desert, California. Precambrian metamorphic basement is overlain by upper Precambrian through Permain miogeosynclinal strata; intruding and (or) resting upon these strata are predominantly igneous rocks, which are interpreted as parts of the eastern margin of a Mesozoic Andean-type volcanic-plutonic arc.

Structures formed during three phases of deformation of probable Mesozoic age are exposed at Old Dad Mountain. An important unconformity gives evidence of Late Triassic and (or) Early Jurassic unrest, and a thrust fault of this age may underlie Old Dad Mountain. A younger northwest-trending shear zone appears to have accommodated a significant component of left slip during Mesozoic time; it may be an analog of longitudinal faults within and behind modern volcanic-plutonic arcs. Remnants of the Playground thrust plate that moved eastward or southeastward at least 2.5 km during late(?) Mesozoic time overlie this shear zone.

Superimposed on these older features are steeply dipping faults, gentle folds, and landslide masses related to movements on the Old Dad normal fault, a Basin and Range fault of Neogene age.

Recognition of early Mesozoic and probable late Mesozoic structures of compressional origin at Old Dad Mountain lends support to the hypothesis that the eastern Mojave Desert is the site of widespread overlap of early Mesozoic and late Mesozoic orogenic belts.

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