Part I. Tectonic evolution of the northern Gulf of California, Mexico, deduced from conjugate rifted margins of the Upper Delfin Basin. Part II. Active folding and seismic hazard in central Los Angeles, California

Abstract

Part I of this thesis addresses the tectonic evolution of the Pacific--North America plate boundary through northwest Mexico and its implications for rifting processes. Offset ignimbrites support 255+/-10 km of opening across the Upper Delfin basin of the northern Gulf of California. Additional deformation from the continental margins supports 296+/-17 km total plate boundary displacement between coastal Sonora and the Main Gulf Escarpment in Baja California, of which at least 276+/-13 km occurred since ~6 Ma. This strain history requires that the plate boundary localized into the the northern Gulf of California during latest Miocene time. Only a narrow width of upper continental crust foundered into the Upper Delfin basin, such that most of the crust between Isla Tiburon and Baja California must be new transitional oceanic crust and possibly lower continental crust contributed by inflow from the rift flanks. Extension of the margins of the Upper and Lower Delfin basins is <40% in most places, though the whole crustal column may have been thinned by a factor of two, further supporting that lower crustal flow has operated here. Opening of the Upper Delfin basin was accompanied by a steady or increased strain rate on its continental margins, contrary to the expected rheology of a narrow continental rift. Reevaluation of a critical deposit of marine rocks on Isla Tiburon indicates that initial marine incursion in the northern Gulf of California also occurred during latest Miocene time. Together, these records suggest that opening of the Upper Delfin basin was an abrupt event, accompanied by a localized zone of intense extension, marine incursion, and a rapid increase in strain rate. Continental rupture in the Upper Delfin basin does not appear to have been a response to crustal weakening by intracontinental extension, but rather may have resulted from a significant increase in strain rate, brought on by a change in boundary forces. Part II of this thesis develops methods to estimate seismic hazard from blind reverse faults by analysis of fault-related folding of Late Quaternary strata, with application to the Elysian Park anticline of Los Angeles, California