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Cross-Strike Basement Faults, Lateral Ramps, and Tectonic Framework of Central and Southern Appalachians

Howard A. Pohn

Field mapping of faults combined with analysis of radar data and examination of proprietary seismic data in the central and southern Appalachians shows that, although the tectonic regime is thin skinned, the effects of cross-strike basement discontinuities are conspicuous at the surface.

Cross-strike basement block or strike-slip faulting in basement rocks produces significant changes in regional scale fold wavelength in Paleozoic cover rocks in the Valley and Ridge province. It is believed these changes in fold wavelength are produced by lateral ramps that connect different decollement levels along strike.

The basement faults appear to have been reactivated through Mesozoic time because east-west border faults and Precambrian highs separating Mesozoic basins are along extensions of lateral ramps. More recent reactivation is strongly indicated because more than 35% of modern earthquakes are coincident with mapped lateral ramps.

Many of the cross-strike basement discontinuities are aligned with transform faults in the oceanic crust of the Atlantic. It is hypothesized that these cross-strike features acted as zones of least resistance along which modern transform faults formed during episodes of sea-floor spreading.

Reactivation of basement faults through time has the effect of shattering the overlying Paleozoic cover rocks and destroying the integrity of hydrocarbon traps. A map of gas fields in central Appalachian Valley and Ridge province shows that these fields terminate abruptly against the lateral ramp zones and that most gas fields occur only between lateral ramps.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.