NORMAL FAULTS OR LANDSLIDES? NEAR-SURFACE GOUGE FORMATION, MYLONITIZATION AND THE AMBIGUITY OF EXHUMATION FAULTS, DENALI FAULT SYSTEM, YUKON, CANADA

The Denali-Toschunda-Duke River fault system comprises a major juxtaposition of continental terranes within Alaska and Yukon. The 2002 M7.9 earthquake along the central Alaska segments generated rupture of approximately 340 km and displacement of up to 9 metres. Eastward propagation transferred from the Denali to Toschunda fault segment giving about 2 metres displacement. Observations of faulting history along Yukon segments of the fault system and pre-2002 seismic records had earlier suggested that the main active zone branches off from the Toschunda-Duke River faults intersection. Both paleo- and active faults exhibit partitioning into thrust and strike-slip components. Seismic activity along the Yukon segments of the fault system has been limited to <M5.8 events with no historical ruptures; the primary surface expression has been landslides. Notably, the largest events are in regions of major structural segmentation (fault intersections, displacement transfer). Fractures are concentrated in a limited number of orientations that can be correlated that can be correlated with major displacement elements along the regional faults; these orientations are consistent with cyclic displacements, slip transfer and strain partitioning along constraining bend fault segments. Topographic features perpendicular to the main fault trend segment mountain ridges without observable offset and are parallel a major fracture orientation. Typically, established slopes run from valley bottom to peak without break, although surface movement of the slopes is common. When these slops are cut by vertical surfaces, it is possible to observe faults that effectively lie in the same orientation as the topographic slope; in geotechnical parlance, they daylight, whereas they are normally hidden. These faults comprise incohesive to barely cohesive gouge and breccia zones intermixed with mylonitic clasts consistent with progressive uplift. The latter suggest formation at high crustal levels with little opportunity to become indurated; that is, they appear immature (recent). This raises the possibility that certain landslip surfaces can otherwise be examined as normal faults acting as release planes for more regional exhumation.