GEOMORPHIC EVOLUTION OF A TILTING EXTENSIONAL FAULT BLOCK, AND TEMPLATES FOR TRANSITION TO INTEGRATED DRAINAGE ACROSS IT BY THE COLORADO RIVER, EASTERN LAKE MEAD AREA

During progressive eastward tilting 16 to10 Ma, the rising crest of the Gold Butte tilt block (and narrower southern extensions) fed boulder fans eastward down the dip slope, and rock avalanches and boulders westward down the footwall of the block’s then-active bounding normal fault (SVWH fault). Increased erosive power on the steepening dip slope of the block during tilting likely countered the tilt-produced structural narrowing of the dip-slope side and widening of the footwall side, and thus helped to center the geomorphic crest of the block. The SVWH master fault system locked at ~10 Ma after initial fault dips > 55° had rotated to <30°, and the locus of faulting then jumped 10–25 km east to the steeper Wheeler fault system on the other side of the block. Because rollover hanging-wall folding toward the Wheeler fault added eastward dip (including 5° since the Miocene) to the uppermost, eastern part of the Gold Butte block, the tilt is not uniform across the block. Preserved examples of late Miocene drainages that developed on the block are spaced about the same as modern drainages (median 1.1 km apart). Accumulation rates of coarse sediment shed from the tilt block into adjacent interior basins declined after the death of the block-bounding fault. Limestone deposition expanded as the basin fills rose to overtop low passes (eroded mostly along sinistral faults) traversing the block, thereby forming sediment bridges that connected basins on either side of the bedrock high. In the early Pliocene, the basin on the eastern side of the block served as an undissected limestone floor to deltaic deposition of thin sand and silt derived from the Colorado Plateau, inferred to signal the initial arrival of the Colorado River. The delta presumably expanded westward across one or more of the older sedimentary bridges into the next basin west. River incision across the block began before 4.7 Ma, driven by lower pre-existing local base levels downstream. The previously formed Miocene sedimentary bridges alternately guided 3 different river paths (two of them since abandoned) across the block, through superpositions following fluctuations in river degradation and aggradation. After >150 m of river incision, a Pliocene fluvial depositional pulse nearly refilled the exhumed Miocene basins before renewed incision exhumed them again.