Deformation driven by subduction and microplate collision: Geodynamics of Cook Inlet basin, Alaska

Abstract

Late Neogene and younger deformation in Cook Inlet basin is caused by dextral transpression in the plate margin of southcentral Alaska. Collision and subduction ofthe Yakutat microplate at the northeastern end of the Aleutian subduction zone is driving the accretionary complex of the Chugach and Kenai Mountains toward the Alaska Range on the opposite side of the basin. This deformation creates belts of fault-cored anticlines that are prolifi c traps of hydrocarbons and are also potential sources for damaging earthquakes. The faults dip steeply, extend into the Mesozoic basement beneath the Tertiary basin fi ll, and form conjugate fl ower structures at some localities. Comparing the geometry of the natural faults and folds with analog models created in a sandbox deformation apparatus suggests that some of the faults accommodate signifi cant dextral as well as reverse-slip motion. We develop a tectonic model in which dextral shearing and horizontal shortening of the basin is driven by microplate collision with an additional component of thrust-type strain caused by plate subduction. This model predicts temporally fl uctuating stress fi elds that are coupled to the recurrence intervals of large-magnitude subduction zone earthquakes. The maximum principal compressive stress is oriented east-southeast to east-northeast with nearly vertical least compressive stress when the basin's lithosphere is mostly decoupled from the underlying subduction megathrust. This stress tensor is compatible with principal stresses inferred from focal mechanisms of earthquakes that occur within the crust beneath Cook Inlet basin. Locking of the megathrust between great magnitude earthquakes may cause the maximum prin-cipal compressive stress to rotate toward the northwest. Moderate dipping faults that strike north to northeast may be optimally oriented for rupture in the ambient stress fi eld, but steeply dipping faults within the cores of some anticlines are unfavorably oriented with respect to both modeled and observed stress fi elds, suggesting that elevated fl uid pressure may be required to trigger fault rupture.

Preliminary evaluation of basin margin exhumation and provenance of Cenozoic strata, Chuitna and Beluga rivers area, Cook Inlet forearc basin, Alaska

“…Fold axes are rotated approximately 20° to 30° counterclockwise to the trace of the Capps Glacier fault and approximately 45° to the traces of intrabasinal faults (fi g. 4) based on preliminary fi eld mapping and aerial photograph interpretation. Bruhn and Haeussler (2006) note similar oblique orientations of fault-cored anticlines in Cook Inlet basin with respect to the transpressional Bruin Bay and Castle Mountain faults, possibly implying that the Capps Glacier fault and associated folds evolved within a similar regional stress regime.…”

“…Signifi cant regional motion along the Bruin Bay fault may have ceased by Late Oligocene time in the Katmai area based on the cross-cutting relationship of an apparently underformed igneous body (Detterman and Reed, 1980;Detterman and others, 1996). However, in upper Cook Inlet, deformed strata in the downthrown (southeastern) side of the Bruin Bay fault may indicate Quaternary motion (Bruhn and Haeussler, 2006). In the study area, the Bruin Bay fault is represented by a broad zone of deformation that places lower Miocene Tyonek Formation over upper Miocene Beluga Formation along the Chuitna and Beluga rivers (fi g. 2; Magoon and others, 1976).…”

Location and extent of Tertiary structures in Cook Inlet Basin, Alaska, and mantle dynamics that focus deformation and subsidence

“…Only previously published lines are labeled with their number. The data source of the profile is listed in front of the line number as follows: FM, Fisher and Magoon (1978); HBP, ARCO data published in Haeussler, Bruhn, and Pratt (2000); BH, ARCO data published in Bruhn and Haeussler (2006); NP, not published ARCO data; WG, Western Geophysical data in others (2008, 2010). M o u n t a i n s A l a s k a R a n g e A l e u t i a n R a n g e margin, and rocks of much of the Alaska Range are either arc-derived volcanic or plutonic rocks (Magoon and others, 1976;Plafker and others, 1994).…”

“…The discovery well was drilled in 1957 in the Swanson River field. All known traps of oil and gas in the basin are contraction-related features (for example, Kirschner and Lyon, 1973;Boss and others, 1976;Magoon and Claypool, 1981) and many of these are fault-cored folds (Haeussler and others, 2000;Bruhn and Haeussler, 2006). Therefore, information about the location and extent of the anticlines in Cook Inlet is critical for assessing the oil and gas resources in the region.…”

“…Therefore, information about the location and extent of the anticlines in Cook Inlet is critical for assessing the oil and gas resources in the region. Moreover, stratigraphic evidence indicates at least some folds are growing (Haeussler and others, 2000;Bruhn and Haeussler, 2006). Thus, knowledge of the location and extent of the structures also aids in assessing the earthquake hazard in the region.…”