Nathaniel E. Putzig

Master of Arts Thesis
Rice University
Department of Geology and Geophysics
Houston, Texas
April 15, 1988


    Rice conducted an onshore-offshore seismic survey across the central California transform margin. Refraction velocity modeling of shot and receiver gathers has generated two models with similar near surface features. Model synthetics produce excellent fits to first arrivals and good matches for later arrivals in the data. CMP stacking of nearer offset traces imaged an event interpreted as reflections from the top of a dipping lower crustal layer. Model raytracing indicates layer flattening landward and seaward of the coast. High amplitude late arrivals seen at long offset on receiver gathers are modeled by imbricating the lower crust beneath the coast. This feature is interpreted as a result of overthrusting of the continental and Farallon plates onto the Pacific plate. The models differ in the middle crust, where one includes a laterally discontinuous low velocity zone. No direct indication of this LVZ exists in the data or in offshore reflection profiles.


    I have produced a 120 km wide velocity and structure profile across the transform margin in central California, extending 25 to 30 km below the surface to Moho depths. Together with the reflection processing of the same data, the refraction velocity modeling provides the most complete image of a crustal transition zone ever obtained anywhere. The near surface of the models made here produces excellent matches to the first breaks of the field data and shows a strong correlation with the surface geology. At depth, two different models have been proposed. Both provide an imbricated lower crust which has oceanic crustal velocities and is interpreted as Pacific plate beneath Farallon plate material. One model shows a laterally discontinuous LVZ above the lower crust, demonstrating the feasibility of including such a feature at depth as has been done by other authors. The other model contains no such LVZ but rather a slightly deeper lower crust. I prefer this simpler model because I have not seen direct indication of any extensive low velocity zones in either the continuous offset data or in offshore reflection profiles.