Geophysical Data Reveal the Crustal Structure of the Alaska Range Orogen within the Aftershock Zone of the Mw 7.9 Denali Fault Earthquake

Abstract

Geophysical information, including deep-crustal seismic reflection, magnetotelluric (MT), gravity, and magnetic data, cross the aftershock zone of the 3 November 2002 M w 7.9 Denali fault earthquake. These data and aftershock seismicity, jointly interpreted, reveal the crustal structure of the right-lateral-slip Denali fault and the eastern Alaska Range orogen, as well as the relationship between this structure and seismicity. North of the Denali fault, strong seismic reflections from within the Alaska Range orogen show features that dip as steeply as 25Њ north and extend downward to depths between 20 and 25 km. These reflections reveal crustal structures, probably ductile shear zones, that most likely formed during the Late Cretaceous, but these structures appear to be inactive, having produced little seismicity during the past 20 years. Furthermore, seismic reflections mainly dip north, whereas alignments in aftershock hypocenters dip south. The Denali fault is nonreflective, but modeling of MT, gravity, and magnetic data suggests that the Denali fault dips steeply to vertically. However, in an alternative structural model, the Denali fault is defined by one of the reflection bands that dips to the north and flattens into the middle crust of the Alaska Range orogen. Modeling of MT data indicates a rock body, having low electrical resistivity (Ͼ10 X•m), that lies mainly at depths greater than 10 km, directly beneath aftershocks of the Denali fault earthquake. The maximum depth of aftershocks along the Denali fault is 10 km. This shallow depth may arise from a higher-than-normal geothermal gradient. Alternatively, the low electrical resistivity of deep rocks along the Denali fault may be associated with fluids that have weakened the lower crust and helped determine the depth extent of the aftershock zone.

Spatial variations in focused exhumation along a continental-scale strike-slip fault: The Denali fault of the eastern Alaska Range

“…The dip of the Denali fault is near vertical along the Delta River corridor where the topography is low (Fisher et al, 2004). The measured surface dip of the Denali fault is ~80° N in the region between the Balchen and Deborah transects , but it is not well constrained beneath the glaciers or at depth.…”

“…Along the Denali fault, maximum aftershock depths of ~11 km suggest the brittle-ductile transition and approximate a regional geothermal gradient to ~30 °C/km (Fisher et al, 2004), which we use to estimate the depth to closure temperature (Tc) for the various thermochronologic systems. Approximately 30 °C/km is also the same standard regional geothermal gradient constrained by other thermochronology studies in Alaska (O'Sullivan and Currie, 1996;Haeussler et al, 2008;McAleer et al, 2009).…”

Imaging the transition from Aleutian subduction to Yakutat collision in central Alaska, with local earthquakes and active source data

“…As part of this transect, the Trans‐Alaska Crustal Transect (TACT), several controlled source studies, including both deep crustal reflection and refraction profiles, provide a detailed and continuous velocity and structural model for the crust along the transect. Transect results from onshore were presented in numerous individual publications [ Fisher et al , 1983, 1989, 2004; Brocher et al , 1989, 1991, 1994, 2003, 2004; Brocher and Moses , 1990, 1993; Flueh et al , 1989; Goodwin et al , 1989; Fuis and Plafker , 1991; Fuis et al , 1991, 1997; Wolf et al , 1991; Beaudoin et al , 1992; Beaudoin , 1994; Levander et al , 1994]. The crustal structure of the southern continental margin in Prince William Sound by TACT and in the Cook Inlet by EDGE was studied using marine air guns, marine reflection profiling, and ocean bottom and land‐based recorders [ Moore et al , 1991; Brocher et al , 1994; Ye et al , 1997].…”

Alternating asymmetric topography of the Alaska range along the strike‐slip Denali fault: Strain partitioning and lithospheric control across a terrane suture zone

“…In the transition between the eastern and central Alaska Range, the Denali fault bisects the ARSZ but appears confined to the upper and middle crust and does not offset the moho [ Brennan et al ., ]. To the east of the ARSZ the Denali fault appears to flatten into the middle crust [ Fisher et al ., ]. Fluids are likely present in the middle and lower crust of the ARSZ [ Glen et al ., ] with a low‐resistivity body beneath the Denali fault just east of the ARSZ also suggesting fluids [e.g., Fisher et al ., , ].…”

“…To the east of the ARSZ the Denali fault appears to flatten into the middle crust [ Fisher et al ., ]. Fluids are likely present in the middle and lower crust of the ARSZ [ Glen et al ., ] with a low‐resistivity body beneath the Denali fault just east of the ARSZ also suggesting fluids [e.g., Fisher et al ., , ].…”