Impact of a complex overburden on analysis of bright reflections: A case study from the Mendocino Triple Junction

Determination of the physical properties and the geometry of features withi n the subsurface is a common application of active source seismic data. We use the example of unusually bright reflections identified in the lower cru st of the Coast Ranges of northern California during the Mendocino Triple J unction Seismic Experiment to investigate the robustness of such determinat ions. The sources of these reflections are significant because they lie in a region of transition between subduction and strike-slip tectonics, but th ey also have an overburden of heterogeneous Franciscan terrane rocks. We us e finite difference synthetic seismograms including a stochastic overburden representative of Franciscan rocks and a realistic target geometry to exam ine how typical seismic diagonostics are affected under these conditions. T he addition of a realistic overburden to the models replicates variations i n the amplitude of the observed P-g arrival and leads to variations in meas ures of the amplitude of the reflected arrivals that are also similar to th ose in the data. Some of these variations are due to the measures of amplit ude used, some represent real variations at the target, and others are due to distortion of the illumination field by the overburden. Overall reflecti on amplitudes are reduced due to transmission losses within the overburden, and the P wave seismic velocity within the reflectors may be as low as 2.5 km s(-1) rather than the 3.5 km s(-1) estimated when the overburden was no t taken into account. Migrations of simulations, including combinations of overburden, target dip, and uneven spatial sampling, recover somewhat consi stent but inaccurate approximations to the true target geometry: Discontinu ous migrated images may result from a discontinuous wave field even where r eflectors were continous. The simulations also imply that neither reversed reflection polarity nor converted shear waves are a reliable aspect of the reflected wave field if the overburden or target is complex. Comparison of migrated simulations to the data suggests that the reflections originate fr om two relatively continuous layers at the top and base of the lower crust, 300 m and 100-150 m in thickness, respectively. We believe that the reflec tions are from lenses of basaltic melt recently emplaced in the crust and g enerated by decompression melting in a "slab-free" window.