TOPOGRAPHIC RESPONSE TO MANTLE LITHOSPHERE REMOVAL IN THE SOUTHERN SIERRA NEVADA REGION, CALIFORNIA

Geological studies of mantle xenoliths that were entrained in late Neogene-Quaternary lavas from the southern Sierra Nevada region, and regional geophysical studies suggest that the high-density mantle lithosphere that formed beneath the Sierra Nevada batholith in conjunction with arc magmatism is being convectively removed as a “drip” structure. In its place asthenosphere has and may still be actively ascending to the base of the crust. This drip structure, as imaged seismically, is roughly cylindrical in shape with a diameter of ~100 km, and extends to ~250 km depth. Centered above this structure is a region ~150 km in diameter which is undergoing active subsidence, relative to adjacent regions. Such subsidence is seen in the active fluvial-alluvial sediment flooding of mountainous topography of the southwestern Sierra Foothills in the regions of the Kings, Kaweah and Tule Rivers. This active basement onlap pattern of sedimentaion is distinct from the general offlap pattern in Quaternary sedimentation which typifies the Great Valley-Sierran Foothills transition in regions to the north and south. The zone of subsidence is also expressed by the development of the adjacent Tulare Lake sub-basin of the San Joaquin Valley. The Tulare Lake sub-basin has developed over the past 2 to 3 m.y. which matches the xenolith data indicating that lithosphere removal had progressed to its advanced stages by 3-4 Ma. Dynamic modeling of such upper mantle drip structures predict a phase of overlying surface subsidence during the most vigorous phase of drip formation. The southern Sierra region upper mantle drip and the overlying crust appear to be in this phase of their dynamically coupled evolution. The westward offset of the drip from the axial Sierra source region is hypothesized to have resulted from the preferential nucleation of the drip beneath the most mafic and densest (western) domain of the batholith; a process also predicted by dynamic modeling of such structures.