HOLOCENE TECTONIC COASTAL UPLIFT AND TILTING CONSTRAINED THROUGH SURFACE DEFORMATION ON SANTA MARÍA ISLAND, SOUTH-CENTRAL CHILE ACTIVE MARGIN

The Chilean coast constitutes one of the most active convergent margins of the Pacific rim, where major earthquakes (M>8) have repeatedly ruptured the surface, involving vertical offsets of several meters. Earthquakes occurred in distinct seismotectonic segments along this margin and is responsible for broad coastal warping. In order to decipher long-term deformation patterns related to earthquakes, we focus on Santa María Island (SMI) located 20 km off the Chilean coast at about 37ºS and only ~70km east of the trench. It is situated in the transition between the Valdivia and Concepción seismotectonic segments. There, we identified tilting axes, quantified uplift, as well as tilting rates by surveying, analyzing, and dating Holocene strandlines and stream profiles. SMI has been tectonically deformed throughout the Quaternary and comprises two topographic domains that were tilted eastward during recent earthquakes. We use two independent geomorphic markers in each domain to reconstruct tilting axes and uplift rates: First, westward-flowing paleo-channels preserved in the higher-elevation regions of the island show distinctive river knickpoints that we associate with large earthquakes. Second, successively emerged Holocene strandlines in the low-lying and rather flat eastern part of the island are related to coseismic uplift. We measured detailed surface morphology and longitudinal river profiles with a laser-total station to demonstrate northeastward tilting. Strandlines and paleo-surfaces as geomorphic markers were dated by radiocarbon and optical stimulation methods to constrain deformation rates. Uplifted and preserved strandlines indicate an average uplift rate of ~1.5 m/ka throughout the Holocene. The 2.5 km long strandlines have been successively tilted to the NNE with ~0.02º/ka, indicating that tilting was asymmetrically but nearly constant throughout the Holocene-late Pleistocene. Similarly, inversion of channels and formation of river knickpoints suggest the same deformation direction and may be related to the progressive migration of the tectonic tilting axis in the course of active folding. Thus, topography of the SMI as a dynamic response to uplift and erosion suggests that tilting and uplift rates have been nearly constant in the southern sector of the Concepcíon segment throughout the Holocene.