• Thomas K. Rockwell, Dept. Geological Sciences, San Diego State University, Earth Consultants International, USA
  • Date: July, 3, 2014 11:00 am
  • Place: Sala d'Actes del Institut de Ciències de la Terra Jaume Almera(ICTJA)
  • Location: C/ Solé i Sabarís s/n, Barcelona

Slip on the Pitas Point thrust produces uplift and folding of the Ventura Avenue anticline (VAA) in the western Transverse Ranges (WTR) of southern California. Rapid convergence has resulted in as much as 2.7 km of uplift on the VAA over the past 200‐300 ka, with as much as 320 m in the past 45‐50 ka, indicating a long‐term uplift rate of 6‐7 mm/yr. Using vintage aerial photography form the 1920’s through 1950’s prior to construction of the 101 freeway, along with differential GPS and LiDAR, we mapped remnants of four emergent Holocene marine terraces between Carpinteria and Ventura, the youngest three of which are reasonably well‐preserved and dated between Punta Gorda and Pitas Point. The most recent emergence event uplifted a Chumash Indian village at Pitas Point by 5‐6 m about 900 years ago; this 1st terrace is nearly continuously preserved to the fold crest reaching an altitude of 7‐8 m, which implies 9‐10 m of slip on the causative fault at depth. Nearly 50 radiocarbon dates on marine shells and culturally derived charcoal indicate terrace emergence at about 0.9 ka, 1.9 ka, and 4.2‐4.7 ka, with the 4th (highest and oldest) terrace inferred to be slightly older than 6.5 ka. The Holocene uplift rate indicated by the emergent terraces is 5‐6 mm/yr, similar to the long‐term rate. Scaling relationships between fault slip, earthquake magnitude, and rupture length suggest that paleo‐earthquakes were close to M8, similar to the 1999 Chi Chi or 2008 Wenchan earthquakes, and that ruptures likely included both onshore and offshore faults. Large uplifts in the offshore may have also resulted in local tsunamis along the Santa Barbara and Ventura coasts. High‐resolution CHIRP seismic reflection data acquired across the VAA offshore Rincon Point in summer 2013 show evidence for discrete folding/uplift events that are captured in the Holocene sediment record and complement the onshore studies.

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