Evidence for Pliocene to present thrust faulting on the south side of the Alaska Range in the vicinity of the Peters Hills piggyback basin
Abstract
The topography of the central Alaska Range lies on one or both sides of the Denali fault system and may be related to young or active thrust faulting. Although numerous young or active thrust faults have been identified north of the central part of the Denali fault, few have been identified on the south side of the range. Here we present evidence for young and active thrust faulting south of the Denali fault, in the region of the Peters Hills between Broad Pass and Mt. McKinley. Gravity and outcrop data indicate the presence of a small Neogene sedimentary basin, which we refer to as the Peters Hills basin along the southern flank of the Alaska Range. The basin is elongate in a northeasterly direction, it lies upon Mesozoic rocks, it has several lobes, and its sedimentary fill is up to 300-m thick. Some of the margins of the basin and the lobes are northeast-striking normal faults. One of these, the Pass Creek fault, is active. A detailed gravity survey indicates a more coherent basin at depth, with a gravity low about 80 km long and up to 30 km wide. We dated pollen from the uppermost sediments, which yielded late Miocene or Pliocene ages (<10 Ma). The Peters Hills basin is no longer a depocenter as the major glaciers incised the sediments in Quaternary time, and it is no longer topographically low. The basin appears to have formed as a piggy-back basin on a southeast-directed thrust sheet in Pliocene and Quaternary time. Several lines of evidence support this origin. First, the presence of Mesozoic and Neogene sediments 300 m above the Neogene sediments of the Susitna basin suggests contractional faults were involved in uplift of the Peters Hills basin and the topography of this part of the Alaska Range. Second, regional seismicity shows scattered crustal (<30 km) earthquakes beneath this entire area. The earthquake focal mechanisms show thrust motion and right-lateral strike-slip if the northeast-striking nodal plane is chosen. A stress inversion indicates east-west compression. Third, uplift and exhumation of Mt. McKinley occurred synchronously with deposition of sediments in the Peters Hills, Susitna, and Cook Inlet basins. Southeast-directed thrust faults mapped around the margins of the Denali and Mt. Foraker massifs appear to be involved in the exhumation of these mountains. Thus, there is additional evidence of shortening in this region at the same time as normal faulting of the margins of the Peters Hills basin. If we accept the piggy-back basin origin, it leads us to speculate on the location of the master thrust fault. A NE-trending topographic lineament in Broad Pass suggests a fault, along which there are SE-vergent thrusts in Mesozoic units. Mesozoic rocks form a rough scarp along the northwest edge of the lineament and are topographically above Neogene sediments of the Susitna Basin. At the north edge of the Susitna basin, rivers that cross this bedrock scarp have distinct changes in morphology (from braided to incised back to braided) and an increase in gradient, which are consistent with an active south-directed thrust fault. If so, there could be a master thrust that reaches the surface, or a master thrust that does not daylight, or a series of thrusts or fault-cored folds. Such active thrust faults that branch southwestward off the Denali system on the south side of the central Alaska Range would help to explain an apparent westward- decrease in the Denali fault slip rate, as indicated by cosmogenic studies of offset moraines.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2008
- Bibcode:
- 2008AGUFM.T44A..08H
- Keywords:
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- 7230 Seismicity and tectonics (1207;
- 1217;
- 1240;
- 1242);
- 8102 Continental contractional orogenic belts and inversion tectonics;
- 8107 Continental neotectonics (8002);
- 8175 Tectonics and landscape evolution