Interpretation of refraction and reflection stack data over the Brevard fault zone in South Carolina

Abstract

Near surface structures across the Brevard fault zone are studied using the refraction and reflection arrivals recovered from the Appalachian Ultradeep Core Hole (ADCOH) regional seismic Line 1. In using refracted arrivals, a new processing approach is introduced that translates refracted first arrivals from multifold seismic data into a refraction stack of two-way delay time sections. Reprocessing of reflected arrivals has improved shallow reflectors and allowed better imaging of the Brevard fault zone. Following processing of refraction and reflection arrivals independently, both data sets are combined into a <u>composite stack</u> section. The composite stack section displays one bright refractor interpreted as the boundary between the weathered layer and high velocity crystalline rocks. This refractor is continuous in the Inner Piedmont with occasional vertical offsets. The continuity of the refractor diminishes across the Brevard fault zone. In the eastern Blue Ridge, the refractor is discontinuous with high angle truncations. On the composite stack section, the Brevard fault zone can be traced from the surface to 6 km (2 s) where it appears to splay from the Blue Ridge thrust. Different from previous interpretations, the Brevard fault zone is imaged as having both an upper and a lower boundary surface as well as a group of reflectors within the zone. This reflection package initially thickens to 2 km at 3 km depth, then thins as it reaches the Blue Ridge master decollement. The Blue Ridge thrust is as shallow as 1.5 km (0.5 s) at the northwest end of the Line l. A deeper decollement is interpreted below the Blue Ridge thrust. The depth of this deeper thrust is 3 km (1 s) at the northwest end of the line, and also joins to the Blue Ridge thrust at 6 km depth making the structures below the Brevard fault zone more complex than previously published.