Seismic Reflections From the Basal Contacts of Batholiths

Abstract

Although seismic reflection data in batholithic terranes are scarce, datasets available to us from three different regions show surprising similarities. All three profiles, one across Precambrian rocks in Texas, and two across Mesozoic batholiths in California and Nevada, show strong subhorizontal reflections at depths of 6-10 km. These reflections are interpreted to originate from the base of the batholiths, and indicate that these batholiths are tabular in shape. The large amplitudes of the reflections require a marked contrast in acoustic impedance (product of velocity and density) between the batholith and the underlying rocks. Two of the profiles show reflective zones having appreciable thicknesses (0.5-2.5 km), while the third profile shows a single continuous reflection. The four most favored candidates for sub-batholithic reflections are (1) underplated gabbro, (2) cumulate layers, (3) migmatites, and (4) thrust faults. Cumulate layering is the preferred interpretation where reflectors constitute a thick package and are laminated in appearance.

Terrane Boundary Reactivation, Barriers to Lateral Fault Propagation and Reactivated Fabrics: Rifting Across the Median Batholith Zone, Great South Basin, New Zealand

“…Seismic reflections have previously been identified originating from the top and base of granitic bodies (Howell et al, ; Lynn et al, ; McLean et al, ), as well as from internal fracture zones (Mair & Green, ) and layered granitic laccoliths (Evans et al, ; Evans et al, ). When observed in seismic data, granitic intrusions typically display a laccolith‐style geometry, consisting of stacked, lenticular bodies similar to those observed here (Figures , , and a) (Evans et al, ; Lynn et al, ; McCaffrey & Petford, ; Petford et al, ). Across the TBF Footwall Block, we identify some areas displaying relatively acoustically transparent seismic facies, which may correspond to the interpreted granitic laccoliths (Figures and ).…”