Proceedings of the 12th International INQUA meeting on paleoseismology, active tectonic and archaeoseismology

198 1 2 T H I N T E R N AT I O N A L I N Q U A M E E T I N G O N PA L E O S E I S M O L O G Y , A C T I V E T E C T O N I C S A N D A R C H A E O S E I S M O L O G Y ( PATA ) , O C T O B E R 6 T H - 1 1 T H , 2 0 2 4 , L O S A N D E S , C H I L E PATA Days 2024 K E Y W O R D S drainage basin development, terraces, uplift, neotectonics (1) Earth Consultants International, Santa Ana, California 92701 USA *Email: gath@earthconsultants.com Eldon Gath (1) UPLIFT AND NEOTECTONIC DEVELOPMENT OF THE PUENTE HILLS IN LOS ANGELES , RIVERSIDE AND ORANGE COUNTIES , SOUTHERN CALIFORNIA A B S T R A C T The Puente Hills are an actively uplifting antiformal structure trending about N65°W through the eastern Los Angeles Basin in southern California. The 2.5-3.0 mm/yr, right-lateral Whittier fault trends parallel across the southernmargin, laterally deforming all fault-crossing streams. The age and uplift rate of the Puente Hills were determined by two different independent approaches: 1) soil development and optically-stimulated luminescence (OSL) age dating of emergent fluvial terraces correlated to eustatic sea level highstands, and 2) drainage basin development rates calibrated by retro deforming the streams across the Whittier fault. Four fluvial fill terraces and three erosional strath surfaces were mapped in the eastern Puente Hills. The ages of the fill terraces were constrained using soil stratigraphy on the upper terraces and OSL dating of the lower ones. A series of parallel, southwesterly trending, consequent streams have formed in response to the uplift. Reconstructing the stream offsets yields an age of formation for each of the drainage basins. The basin areas for 15 Puente Hills drainages were calculated; these range in size from 0.5 to 24 km2. Regressing basin area against basin age (based on offset distance) yields an age of about 600 ka for the largest drainages. The uplift rate has increased exponentially from an initial 0.3 mm/yr to 1.25 mm/yr at the end of the Pleistocene. The rate increase is proposed to be the consequence of the Santa Ana Mountains indenter closing the Santa Ana River gap at ~6 mm/yr and colliding with the Puente Hills about 200-250 ka.