Interpretation of gravity and magnetic data and development of two-dimensional cross-sectional models for the Border Ranges fault system, south-central Alaska

Abstract

Extensive Quaternary glacial cover and a lack of dense geophysical data within the Cook Inlet basin (CIB) of south-central Alaska make locating and determining the geometry of the Border Ranges fault system (BRFS), a major feature of the AlaskaAleutian forearc region, diffi cult. We use recently collected gravity data, available aeromagnetic data, and other geophysical information as constraints to develop plausible two-dimensional cross-section models that better image the BRFS and related geologic structures of the CIB. Our integrated models show a thick sequence of late Mesozoic sedimentary rocks and the Peninsular terrane basement (6-20 km depth) overlying a serpentinized body at a depth of 16-34 km. The late Mesozoic rocks and serpentinite are interpreted as possible sources of the south Alaska magnetic high over the CIB. The eastern boundaries of the CIB are characterized by gravity and magnetic highs of the emplaced Border Range ultramafi c and mafi c assemblages (BRUMA). Formation of the BRUMA may be related to the serpentinized rocks that composed a Jurassic oceanic arc. Our models constrain the BRFS as a structural boundary between the overthrusted BRUMA and the Chugach terrane to the east. The BRFS dips 50°-70° toward the west-northwest and extends to at least 15 km. The BRFS may penetrate steeply or shallowly to a form a décollement at greater depths. A model that includes underplated sediments at the base of the accretionary complex (12-40 km) is consistent with the observed gravity low over the Chugach Mountains (Chugach terrane). The underplating may be associated with the subducting and shortening of the Yakutat microplate in south-central Alaska.

Focused exhumation in the syntaxis of the western Chugach Mountains and Prince William Sound, Alaska

“…The subduction of these sediments may have caused duplex stacking and regional antiform development beneath the coastal mountains, which drives the long-term rock uplift and rapid exhumation observed there. New gravity and published magnetic data and 2.5-D cross-sectional models across the western Chugach Mountains are consistent with low-density sediments being subducted beneath the accretionary complex since the Neogene (Mankhemthong et al, 2013). We speculate that for the last ~5 m.y., coincident with deposition of the Yakataga Formation and foreland basin development in the Saint Elias orogen (Plafker, 1987;Lagoe et al, 1993), fl atslab subduction may have carried the sediments northwest with the Yakutat microplate, where they were focused into the region beneath the Chugach core and underplated.…”

“…Refl ectors from deep seismic-refl ection profi les from just southwest of the Kenai Peninsula are interpreted to represent Eocene underplating southeast of the Border Ranges fault system (Moore et al, 1991;Ye et al, 1997). New gravity and published magnetic data and 2.5-dimensional cross-sectional models across the Cook Inlet basin and Border Ranges fault system are consistent with low-density sediments being subducted beneath the accretionary complex (Mankhemthong et al, 2013). Bulk uplift is also consistent with the thermal-mechanical models of Koons et al (2010), which show increased vertical velocities in their convergent sub-boundary area above the subducting Yakutat microplate and adjacent to the Chugach core region.…”

Introduction: Neogene tectonics and climate-tectonic interactions in the southern Alaskan orogen themed issue

“…The Benowitz et al (2011) paper on the exhumational history along the Denali fault and the Trop et al (2012) study of the Wrangell Mountains volcanic and sedimentary basins contribute important new regional information on the general consequences of the fl at-slab subduction of the Yakutat microplate beneath southern Alaska. Mankhemthong et al (2013) provide new data on the history and geometry of the Cook Inlet Basin, immediately west of the St. Elias orogen, that give information on the western fringe of the orogenic system. Thus, these studies, together with the STEEP studies in the volume, provide an important new regional data set for synthesis of this important orogenic system.…”

“…Possible reasons for this increased exhumation are discussed, and include a tectonic forcing and a climate driven signal due to the beginning of glacial erosion. Mankhemthong et al (2013) seek to locate and determine the geometry of the Border Ranges fault system, a major feature of the Alaska-Aleutian forearc region, using gravity, aeromagnetic, and other geophysical data in the Cook Inlet Basin region. They present 2.5D cross sections showing 6-10-km-thick sedimentary cover over the Peninsular terrane, the lithospheric mantle of which is serpentinized at depths of 14-34 km.…”

Focused rock uplift above the subduction décollement at Montague and Hinchinbrook Islands, Prince William Sound, Alaska

“…Pavlis et al (2012) suggested that sediments originally shed from the Saint Elias orogen 2-3 Ma in response to cooling climate and glacial erosion caused duplex stacking and underplating under the Yakataga segment farther to the southeast. Mankhemthong et al (2013) used gravity and magnetic data from the Chugach Mountains north of Prince William Sound to suggest that sediments shed from the Saint Elias Mountains were carried along and underplated above the Yakutat microplate. Thus, the increased volume of subducting sediments starting ca.…”