Stratigraphic effects and tectonic implications of the growth of normal faults and extensional basins

Abstract

Recent research on normal faults has established that (1) cumulative displacement is highest near the fault center and decreases toward the tips and (2) faults lengthen as cumulative displacement increases. Half-graben-type basins are a fundamental manifestation of displacement on large normal fault systems, and thus are expected to be deepest near their centers and to grow in depth, width, and length through time. Basin growth models predict that progressively younger synextensional strata will onlap basement rocks, especially if sedimentation keeps pace with increasing basin capacity. Under certain circumstances, the models predict a transition from conditions in which sediment supply exceeds capacity (predominantly open basin) to one in which the basin is underfilled (predominantly closed basin) if sedimentation cannot keep pace with increasing basin capacity. Some basins evolve through the merger of originally isolated subbasins whose border faults grew toward one another. The oldest strata form restricted sequences in each subbasin. Strata deposited shortly after consolidation thin toward the intrabasin high that forms in the merger zone, a region of short-term displacement deficit. For two merging coplanar faults, the merger zone is located at the center of the combined fault system, and displacement must increase to conform to the typical displacement profile and the scaling law between fault length and displacement. Thus, the youngest strata thicken toward the former location of the intrabasin high. If the growing faults overlap in the extension direction, displacement is distributed on multiple splay faults. The intrabasin high then has long-term expression, even though the summed fault displacement within the high is equal to or slightly greater than that of the deeper, flanking subbasins. Oblique-slip accommodation zones form in the overlap zone of basins whose propagating faults dip in opposite directions. In general, faults growing in length obviate the need for transfer faults at the fault tips. Transfer faults may form if both fault tips cannot propagate. If only one tip is fixed, the other tip propagates away from the fixed tip, and the depocenter migrates in the same direction. The fault and basin growth models described above provide a useful framework for interpreting the stratigraphic record of extensional basins and extracting their tectonic development, as demonstrated by examples from the Basin and Range and Mesozoic rifts of eastern North America. Schlische, R.W., and Anders, M.H., 1996, Stratigraphic effects and tectonic implications of the growth of normal faults and extensional basins, in Beratan, K.K., ed., Reconstructing the History of Basin and Range Extension Using Sedimentology and Stratigraphy: Boulder, Colorado, Geological Society of America Special Publication 303, p. 183-203. 2 R. W. Schlische and M. H. Anders