AN ISOSTASTIC AND MASS BALANCE MODEL OF THE MESOZOIC SIERRA NEVADA ARC DURING TECTONIC THICKENING AND MAGMATIC FLARE-UPS: IMPLICATIONS FOR SURFACE UPLIFT, EXHUMATION AND MATERIAL TRANSFER

The crustal column (surface to seismic Moho) of the Mesozoic Sierra Nevada arc and its isostastic state link tectonopetrologic processes at lithospheric levels to the surface processes, playing roles in long-term, large-scale processes such as tempos of magmatism and deformation, landscape evolution and global climate change (Lee et al., 2013). The thickness of the arc crustal column through time can be described by dH/dt = M+T-E. While magmatic addition rate (M) and tectonic thickening rate (T) directly affect the thickness, the erosion rate (E) is mainly controlled by surface elevation h, which is in turn affected by the change of the thickness of the crustal column (H) and the thickness of the eclogitic root (R).

Data from Sierra Nevada arc (SNA) such as magmatic addition rates and strain measurements were used to establish a first-order isostastic model of crustal thickness and surface elevation during and after the Late Cretaceous magmatic flare-up event. The Cretaceous SNA crust experienced about 40% thickening by tectonic deformation and 70% by pluton intrusion. The combined effect of the two leads to about 8 km surface elevation gain if the crust was originally 40 km thick. The formation of the eclogitic root reduces the surface elevation by about 3 km, resulting 5 km surface elevation gain before the root gets detached. The thickened crust thus has to transport excessive materials downwards by extending the Moho downwards for about 50 km. The foundering of an eclogitic root leads to isostasitc rebound and additional surface uplift. Erosion functions to exhume upper crustal rocks and reduce the thickness of the crust through time. Due to thickening, crustal materials originally at 10 km depth before deformation and flare-up have to be exhumed by 14-20 km to reach the surface.

This model explains a number of the geological observations: (1) The surface of the arc has experienced elevation gain since Early Cretaceous from sea level to 3 km at the end of Cretaceous. (2) The central Sierras have been exhumed 12-14 km since Early Cretaceous. (3) Rapid (0.3 km/myr) exhumation (~9 km) occurred between ~100 and ~70 Ma during and shortly after the magmatic flare-up event. A more detailed model is being established to investigate the temporal evolution of crustal thickness and surface elevation throughout the Mesozoic.