EVOLUTION OF CRYSTALLINE ROCKS OF THE TRANSVERSE RANGES

Perry Ehlig’s recurring theme in interpreting the geology of the Transverse Ranges was to free geology from the confines of physiography. He emphasized lateral transport in Mesozoic and Cenozoic time, and showed that the evolution of crystalline rocks in the Transverse Ranges spoke to the geologic evolution of a wider region, from the Sierra Nevada into Arizona and northern Sonora. Silver noted that tectonic models of lateral transport require differential exhumation constraints: ‘…we must point out that such high rank igneous and metamorphic assemblages inevitably have been brought up too’ (Silver et al., 1963). Rapid differential exhumation in the Transverse Ranges may be constrained by correlated thermobarometric exhumation estimates derived from structurally high Mesozoic plutonic rocks with 60-65 Ma cooling ages recorded by biotite K-Ar (Miller and Morton, 1980) and apatite (U,Th)/He and fission track ages (Spotila et al., 1998; Blythe et al., 2000). These results, when considered in light of the finding of 70-80 Ma detrital zircons in structurally low Pelona schist (Jacobson et al., 2000), indicate wholesale vertical reworking of crustal material during formation and emplacement of Pelona (and related Rand and Orocopia) schist, although the magnitude of lateral transport is still in some dispute. Quantifying this remarkable Laramide orogenic event then frames understanding of Mesozoic and older elements of the tilted and fragmented crustal column. Preserved Proterozoic and Paleozoic metasediments at the highest structural levels record rifting and initiation of the Cordilleran miogeocline. Beneath them, diverse Early Proterozoic rocks record construction of continental crust through a range of structural levels, and its subsequent modification by Mesoproterozoic and Mesozoic magmatism and deformation.