Mesozoic deformation

Some Mesozoic deformational features of regional extent crop out in the western and north-central parts of the EMNSA. These deformational features are reflections of shortening shown by brittle-style thrust plates developed in the foreland of the Cordilleran thrust belt and ductile-style nappes in southeastern California and Arizona (Burchfiel and Davis, 1971, 1977, 1981; Howard and others, 1980; Snoke and Miller, 1988; Miller and Barton, 1990). Generally east-directed thrust faults, present in the Cowhole Mountains and the Clark Mountain Range areas, may be Middle Triassic(?) through Early Jurassic (Burchfiel and Davis, 1981). In the Cowhole Mountains, metamorphosed Paleozoic rocks were interpreted as having been thrust eastward and then overlapped unconformably by the Lower Jurassic Aztec Sandstone. However, Busby-Spera (1988) and Busby-Spera and others (1989) presented evidence that the sandstone is Middle Jurassic and may overlie a normal, not thrust, fault. In the Clark Mountain Range, some east-directed thrust faults were cut by small dioritic plutons originally dated at 190 and 200 Ma by K-Ar (Burchfiel and Davis, 1981) but now known as Late Jurassic. Latest Early Cretaceous thrusting there placed Paleozoic strata over the Early Cretaceous Delfonte volcanic rocks. This thrust was then intruded by plutons of the mid- Cretaceous Teutonia batholith (Burchfiel and Davis, 1971, 1981; Fleck and others, in press). A similar sequence of faulting, although not as well constrained by dated rock bodies, holds in the New York Mountains.

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