The Growth of Fault-Bounded Tilt Blocks

A series of uniformly tilted fault-bounded blocks is a common feature in actively extending regions such as the Basin and Range province. If the tilted blocks were produced by rigid ''domino-style'' rotation, one would predict large voids at either end of a series of these blocks. Using tilt data and a simple flexural calculation, we suggest that much of the apparent rigid behavior could also be produced by internal block deformation. In our model of normal fault growth, isostatic/elastic uplift of the footwall is coupled with hanging wall downdrop within the region between faults, resulting in the appearance of a tilted rigid block. We present tilt data sampled at varying distances from several block-defining faults within the northeast Basin and Range province. Tilt measurements between a series of 30-km spaced block-defining faults are found to be uniform, while tilts between more widely spaced faults exhibit a pattern of tilt that diminishes to zero in less than 30 km. Using a simple flexural calculation for internal block deformation, we show that for this region the patterns of tilt are consistent with a flexural length scale of approximately 8-12 km and deflections of 2-4 km. These estimates are compatible with both the lower limit to seismicity and basin depth determined from earthquake and seismic reflection studies.