THE MOJAVE – SONORAN MEGASHEAR AND REASSEMBLY OF CORDILLERAN TRIASSIC-JURASSIC TECTONOSTRATIGRAPHHIC ASSEMBLAGES

Piercing points defined by cross-cutting Triassic and Jurassic tectonosequence boundaries document ~ 1000 km of left-lateral displacement of the Caborca terrane along the Mojave-Sonora megashear (MSm). Displacement from the western end of the Luning terrane occurred from late Bajocian to early Oxfordian time. Displacement requires transport from behind the middle Jurassic arc to the present location outboard of the arc. The most parsimonious explanation: collapse of the Late Triassic island arc onto the craton with associated right-lateral transtension of the margin, middle Early Jurassic (~ 195-185 Ma)]; obduction of the cratonal margin onto the craton, late Early to early Middle Jurassic (185-165 Ma); left-lateral translation of the Caborca terrane, late Middle to early Late Jurassic (165-150 Ma). Collapse of the Triassic island arc and obduction of the cratonal margin onto the craton in the Sierra Nevada was contemporaneous with collapse of the Nicola arc and obduction of the Slide Mountain terrane onto the Kootenay terrane in the southern Canadian Cordillera. This explanation is consistent with the Jurassic North American APW path.

The Pan Thalassan – North American boundary is a proxy for the MSm within the Sierra Nevada batholith. The MSm is displaced eastward along the Garlock fault to the northeastern Mojave Desert, then back to the west in the central Mojave. Rocks of the obducted upper plate include Lower Triassic basin strata and Upper Triassic and Jurassic volcanic rocks in roof pendants of the Sierran crest, Lower Triassic outer shelf strata and Jurassic volcanic rocks and of the Inyo and White Mountains, Panamint Range, and volcanic rocks and eolian sandstones in the northeastern and central Mojave Desert. Paleozoic lower-plate rocks are exposed in windows in the upper plate in the Owens Valley. Lower Jurassic strata on Late Precambrian strata in the Snow Lake block are lower plate rocks exposed between the upper plate and the MSm. All pre Late Jurassic stratigraphic assemblages west of the MSm are offset ~ 1000 km left laterally relative to rocks to the east. Late Triassic and Early Jurassic basinal rocks from the Mineral King pendant to the Mojave Desert, and Peninsular Range of southern and Baja California have been displaced from the reentrant in the cratonal margin in northwestern Nevada.