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Seismic Evaluation of Differential Tectonic Subsidence, Compaction, and Loading in an Interior Basin

¹ Department of Geology and Geography, West Virginia University, Morgantown, West Virginia 26506-6300; wilson{at}geo.wvu.edu

Quantitative analysis of depth-converted reflection times defines long-term differential motion across individual structures in a central Appalachian interior basin known as the Rome trough. Differential motion decreases exponentially with time. Rotation about a hinge defining the trough's west margin reached approximately 37% of total displacement in about 63-78 million years (m.y.). Displacement across the trough's faulted east margin occurred more rapidly and reached 37% of the total in 13-51 m.y. A major fault in the interior of the trough developed rapidly with 37% of total displacement reached in from 16 to 23 m.y. Longer term rotation across the west margin may be due to its participation in the overall subsidence of the craton during the Paleozoic. The time spanned by the formation of the East-Margin and Interior faults was restricted to the Cambrian in the northern part of the area, but to the south, movement along the East-Margin fault continued through the Middle Ordovician.

The general effects of differential compaction and loading for a single lithology model are computed from the standard compaction and Airy isostasy equations. Depth-dependent compaction requires that thicker strata over a hanging wall or subsiding fault block undergo greater compaction than occurs in thinner strata over the footwall or structurally high areas. Seismic interpretation of trough structures does not reveal the presence of compaction faults or of long-term differential compaction. The observations suggest that in this interior basin differential compaction of major stratigraphic intervals was nearly complete prior to deposition of later sequences. Local isostatic compensation of differential loads across faults or fault blocks requires movement along near-vertical crust-penetrating faults and abrupt thinning by differential amounts across the base of the crust. This possibility seems unlikely within the context of current models of intracratonic extension. Analysis indicates that differential thickening of strata across trough structures portrayed in reactivation history diagrams defines long-term tectonic movement rather than a mixture of tectonic, compaction, and load-related motion. The analysis also suggests that estimates of the time at which a given horizon entered the oil or gas window and estimates of the total depth reached by a horizon during subsidence may also be in error if simple depth-dependent compaction corrections are used.