Crustal structure and tectonics from the Los Angeles basin to the Mojave Desert, southern California

A seismic refraction and low-fold reflection survey, known as the Los Angel es Region Seismic Experiment (LARSE), was conducted along a transect (line 1) extending from Seal Beach, California, to the Mojave Desert, crossing th e Los Angeles and San Gabriel Valley basins and San Gabriel Mountains. The chief result of this survey is an interpreted cross section that addresses a number of questions regarding the crustal structure and tectonics of sout hern California that have been debated for decades and have important impli cations for earthquake hazard assessment. The results (or constraints) are as follows. (1) The maximum depth of the Los Angeles basin along line 1 is 8-9 km, (2) The deep structure of the Sierra Madre fault zone in the northe rn San Gabriel Valley is as follows. The Duarte branch of the Sierra Madre fault zone forms a buried, 2.5-km-high, moderately north dipping buttress b etween the sedimentary and volcanic rocks of the San Gabriel Valley and the igneous and metamorphic rocks of the San Gabriel Mountains. (For deeper st ructure, see following.) (3) There are active crustal decollements in south ern California. At middle-crustal depths, the Sierra Madre fault zone appea rs to sole into a master decollement that terminates northward at the San A ndreas fault and projects southward beneath the San Gabriel Valley to the P uente Hills blind thrust fault. (4) The dip and depth extent of the San And reas fault along line 1 dips steeply (similar to 83 degrees) northward and extends to at least the Moho, (5) The subsurface lateral extent of the Pelo na Schist in southern California is as follows, Along line 1, the Pelona Sc hist underlies much, if not all of the San Gabriel Mountains south of the S an Andreas fault to middle-crustal depths. North of the San Andreas fault, it is apparently not present along the transect.