TERRANE-SCALE CRUSTAL STRUCTURES OF SOUTHCENTRAL ALASKA INFERRED FROM REGIONAL GEOPHYSICAL STUDIES

Recent gravity, aeromagnetic, and magnetotelluric (MT) investigations of the Talkeetna Mountains (TK) of southcentral Alaska (61.5-63.75°N, 145-151°W) were undertaken to study the region?s framework geophysics and to re-interpret structures and crustal composition. Aeromagnetic data for this study were compiled from 13 available regional and local-scale surveys. Over 400 new gravity stations were collected along 12 profiles in the study area (including the TK Transect - from Broad Pass to Kosina Creek) and combined with 3,286 existing regional data. Over 50 MT data were collected along six of the profiles. At the broadest scale, gravity data suggest that a large portion of the TK consists of relatively dense crust, likely of oceanic-composition [Wrangellia (WR) terrane], that may also have been underplated by mafic material during early to mid-Tertiary volcanism. At the NW edge of this block lies a prominent gravity magnetic and MT gradient (~3.25 mGal/km, ~100nT/km) that forms a NE-trending 1st-order crustal discontinuity between dense WR crust and the low-density Jurassic to Cretaceous flysch to the NW. This crustal break lies under the Fog Lakes basin approximately where the Talkeetna thrust was previously mapped. Potential field and MT models, however, indicate the crustal break is a deep (2-8 km) steeply west-dipping structure - not a shallow E-dipping thrust. Most of the crustal breaks in the area appear in the geophysical data to be steep. They are dominated by reverse and strike-slip faults, with some reactivated in Tertiary time. The Mesozoic flysch NW of the crustal break, once considered a single basin is distributed over two distinct geophysical domains separated by a NE-trending gravity gradient aligned with the Broad Pass graben. This suggests the two regions form distinct basins -- the SE portion in the TK being underlain by transitional crust, and the NW portion (Kahiltna basin) being rooted by continental crust along North America?s Mesozoic margin. Sediment studies by Ridgway (2002, GSA Bull. 114) demonstrate the sub-basins received their sediments from two sources: the NW basin from continental NA, and the SE from WR.