DIGGING INTO THE SOUTH ALASKA DEEP MAGNETIC HIGH – IMPLICATIONS OF BOUNDARY AND INTERNAL COMPLEXITIES FOR CRUSTAL STRUCTURE

Regional magnetic anomaly data for southern Alaska are dominated by the South Alaska Magnetic High (SAMH), a margin parallel domain of mid- to long-wavelength magnetic highs inferred to relate to the Wrangellia Composite Terrane of Plafker and Berg (1994). To a first order, this domain results from an abundance of mid- to deep-crustal mafic material resulting from a combination of (1) arc basement, (2) magmatic intrusion and mafic underplating, and (3) possible deep serpentinization.

As a part of the USGS’ Western Alaska Range Project, we are examining second order variations within the SAMH and details of its boundaries with adjacent domains, such as the Southern Alaska Magnetic Trough (SAMT) to the north. Using matched filtering, we dissect the anomalies in magnetic potential (AKA pseudogravity) based on a new magnetic data compilation. We identify sub-domains based on visual pattern classification of the match-filtered depth slices.

In the eastern part of our study area, magnetic sub-domains of the SAMH agree with sub-domains previously identified and interpreted in the Talkeetna Mountains area by Glen and Schmidt (GSA Special Paper 431, 2007). In that interpretation, a critical boundary was inferred to reflect the transition from Wrangellian to Peninsular basement. By analogy, we extend this region further west (across the Susitna basin, Cook Inlet, and into the Western Alaska Range) and evaluate the implications of this possible terrane association.

The southern boundary of the SAMH (associated with the Border Ranges fault, the northern boundary of a long-lived accretionary complex) is distinct and continuous throughout, and beyond, the study area. The northern boundary of the SAMH, although continuous to the northeast and southwest, is not a simple linear boundary beneath the Tordrillo Mountains. Prominent variations in crustal resistivity structure, as determined from magnetotelluric data, are also observed across this northern boundary, both within and to the northeast of the study area. In addition, the geochemical and isotopic signatures of coeval Paleocene plutons above the SAMH are distinct from those intruding the SAMT. These correlations lead us to infer that that boundary between the SAMH and the SAMT reflects two different basements concealed beneath the flysch of the Kahiltna/Kuskokwim basins.