REFINING SHORTENING STYLES IN THE APPALACHIANS BY THE INCORPORATION OF HIGH RESOLUTION SEISMIC DATA

The Appalachian Mountain belt extends through the eastern United States into southeastern Canada, and is expressed as elongated topographic ridges and valleys of folded and thrust faulted sedimentary and crystalline rock. The Appalachian Plateau (AP) in northwest PA consists of gentle folds and low relief topography, overlying a decollement surface in Silurian evaporites. Rocks sampled at the surface in the AP reveal 10-15% layer parallel shortening (LPS). Due to the deficiency of geophysical and outcrop data, all the strata in the AP above the Silurian detachment has been assumed to be shortening via LPS plus gentle folding.

Increased hydrocarbon exploration led to the acquisition of new high quality 3-D seismic data in the AP. We observe truncated and tilted reflectors in seismic data. Individual truncated reflectors can appear doubled or stacked. We interpret this as macroscale wedge style faulting in strata from the Silurian Salina decollement through the Devonian Marcellus shale. Through kinematically restored and balanced cross sections we show that the magnitude of LPS calculated from the rocks at the surface of the AP is equal to the magnitude of macroscale shortening in the Silurian-lower Devonian units immediately above the salt detachment.

The Valley and Ridge province in southeast PA is a blind fold and thrust belt. The duplexing Cambrian – Ordovician sequence is bound by a deep decollement surface within shales of the Cambrian Waynesboro Formation and an overlying detachment at the base of the Ordovician Reedsville shale. 24 km of shortening in the Cambrian – Ordovician sequence is expressed as LPS in the AP. Previous work hypothesized that 10 km of that shortening was transferred along a thrust ramp under the northwestern most anticline (the Nittany Anticline) of the Valley and Ridge, which provides a pathway to feed slip from Cambrian to Silurian strata.

Seismic data under the Nittany Anticline shows discontinuous and tilted reflectors. Area between seismic reflectors can abruptly thicken. We interpret backthrusts in some Cambrian – Ordovician horses, and wedge style faulting in Ordovician-Silurian units. This allows us present a more complex pathway for shortening to reach the Silurian decollement in the AP, involving triangle zones in both Cambrian-Ordovician duplexes and Ordovician-Silurian units.