A PURELY EXTENSIONAL ORIGIN TO THE PELONA-OROCOPIA-RAND SCHIST EXPOSURES?

The exposures of the Pelona, Orocopia, and Rand Schists (POR schists) have been called the “world’s type shallow subduction complex.” (A.D. Chapman et al., Geosphere, 2016). As such, the creation of these exposures is examined for clues to the interaction of continents with shallowing slabs. The inference of an oceanic plateau expands the significance of these rocks to another cryptic event, the Laramide orogeny. But there are issues. How does wholesale removal of large parts of the crust yield a landscape that accumulated no sedimentary rocks for tens of millions of years? How does it manage to experience profound extensional tectonism even as the crust has already been greatly thinned? If emplaced as the subduction megathrust cut down, how do schists appear inward of the edge of preserved mantle lithosphere? Here I explore an alternative that relies on normal faulting driven by an exceptionally narrow and thick orogen.

A north-south section from the high Sierra to the Rand Schist looks as though the lower crust was gradually removed from north to south and replaced with schist (Saleeby, GSAB, 2003). But accounting for the inferred levels of exposure, it was upper and middle crust that was removed. It would seem the schist flowed in much as lower crust flows in to compensate for extension in core complexes. Might this be a more primary means of emplacing schists?

As noted by J.B. Chapman (Geology, 2021), some exposures of Orocopia Schist are well east of where the edge of preserved Precambrian mantle lithosphere has been placed. Is it possible that this too is lateral flow responding to recently recognized extensional tectonism in the Rawhide-Buckskin Mountains in the Late Cretaceous (Wong et al., Tectonics, 2023)?

Schist emplacements and normal sense shear zones appear to overlap in time and are mostly complementary in space. They could collectively represent deformation that progressed incrementally from northwest to southeast, or they could represent two or three discrete extensional events; what is striking is that they collectively suggest an extensional orogen across much of the Mojave. Given that the modern outcrops of faults separating the schists from the upper plate are interpreted as extensional shears further emphasizes the ubiquity of extension in this region.

Given all this, are the POR exposures from unusually shallow thrusting or an unusually narrow orogen?