TEHACHAPI COMPLEX (TC)-SOUTHERNMOST SIERRA NEVADA BATHOLITH (SNB): TECTONIC STRATIGRAPHY AND THERMOBAROMETRY IN TIME

The TC constitutes the southern end of the SNB as it abuts the Garlock fault.  Its tectonic stratigraphy is comprised of three stacked plates.  The middle plate is "autochthonous" deep-level (8-10 kb) western zone SNB which is in structural continuity with the greater SNB to the north. Such continuity renders a southward deepening oblique crustal section transecting volcanic-hypabyssal to lower crustal levels over a distance of  ~100 km. The deep-level TC rocks are cut below by the £30° N-dipping Rand "thrust" which places the Franciscan-affinity Rand schist in an anomalous position directly beneath SNB lower crust.  Tectonically above these two plates is a complex of shallower-level (£3 kb) granitic plates which sit primarily on the deep batholithic rocks, but also locally on the underplated schist. Batholith and metamorphic framework features of the uppermost plates match the eastern zone of the SNB which along with barometric and structural data indicate SW-directed tectonic descent onto the deep-level rocks.  Broad range thermochronometric and coupled barometric data indicate that following the ca. 100 Ma culmination of lower crust batholith construction the deep TC rocks underwent rapid cooling and decompression to ~500°C and ~4 kb conditions by ca. 94 Ma, and continued rapid cooling to ~200°C by ca. 85 Ma.  Protolith and metamorphic age constraints (Grove et al., 2003), and thermobarometric data on the underlying schist constrain much of it to have entered the subduction zone at roughly 93 Ma reaching its apparent peak metamorphic conditions (~650°C /~9 kb) at ca. 90 Ma, and to have cooled to ~300°C by ca. 85 Ma. Transport of the uppermost granitic plates is constrained to post-88 Ma, and together all three plates were at low elevation surface conditions by Paleocene-early Eocene time.  These P-T-t relations in conjunction with structural relations suggest that following rapid low-angle subduction the Rand schist ascended by channelized flow along the base of the crust back up the subduction zone to its current underplated position while the overlying TC-SNB was partitioned into a SW flowing coherent deep-level plate and higher-level detachment plates.  This transport pattern deflected the southern end of the SNB westward, and together with initial thrusting destroyed the adjacent segment of the forearc basin.