Recent investigations of the Talkeetna Mountains in south-central Alaska were undertaken to study the region's framework geophysics and to reinterpret structures and crustal composition. Potential field (gravity and magnetic) and magnetotelluric (MT) data were collected along northwest-trending profiles as part of the U.S. Geological Survey's Talkeetna Mountains transect project. The Talkeetna Mountains transect area comprises eight 1:63,360 quadrangles (∼9500 km 2 ) in the Healy and Talkeetna Mountains 1° × 3° sheets that span four major lithostratigraphic terranes ( Glen et al., this volume ) including the Wrangellia and Peninsular terranes and two Mesozoic overlap assemblages inboard (northwest) of Wrangellia. These data were used here to develop 2½-dimensional models for the three profiles. Modeling results reveal prominent gravity, magnetic, and MT gradients (∼3.25 mGal/km, ∼100nT/km, ∼300 ohm-m/km) corresponding to the Talkeetna Suture Zone—a first-order crustal discontinuity in the deep crust that juxtaposes rocks with strongly contrasting rock properties. This discontinuity corresponds with the suture between relatively dense magnetic crust of Wrangellia (likely of oceanic composition) and relatively less dense transitional crust underlying Jurassic to Cretaceous flysch basins developed between Wrangellia and North America. Some area of the oceanic crust beneath Wrangellia may also have been underplated by mafic material during early to mid-Tertiary volcanism. The prominent crustal break underlies the Fog Lakes basin approximately where the Talkeetna thrust fault was previously mapped as a surface feature. Potential field and MT models, however, indicate that the Talkeetna Suture Zone crustal break along the transect is a deep (2–8 km), steeply west-dipping structure—not a shallow east-dipping Alpine nappe-like thrust. Indeed, most of the crustal breaks in the area appear to be steep in the geophysical data, which is consistent with regional geologic mapping that indicates that most of the faults are steep normal, reverse, strike-slip, or obliqueslip faults. Mapping further indicates that many of these features, which likely formed during Jurassic and Cretaceous time, such as the Talkeetna Suture Zone have reactivated in Tertiary time ( O'Neill et al., 2005 ).

A suite of geophysical data obtained along the Richardson Highway crosses the eastern Alaska Range and Denali fault and reveals the crustal structure of the orogen. Strong seismic reflections from within the orogen north of the Denali fault dip as steeply as 25° north and extend downward to depths between 20 and 25 km. These reflections reveal what is probably a shear zone that transects most of the crust and is part of a crustal-scale duplex structure that probably formed during the Late Cretaceous. These structures, however, appear to be relict because over the past 20 years, they have produced little or no seismicity despite the nearby Mw = 7.9 Denali fault earthquake that struck in 2002. The Denali fault is nonreflective, but we interpret modeled magnetotelluric (MT), gravity, and magnetic data to propose that the fault dips steeply to vertically. Modeling of MT data shows that aftershocks of the 2002 Denali fault earthquake occurred above a rock body that has low electrical resistivity (>10 ohm-m), which might signify the presence of fluids in the middle and lower crust.

Introduction The use of geophysical methods for hydrogeological investigations has, during the last two decades, attracted much attention. The goal of a hydrogeological investigation is to construct hydraulic models of the areas of interest. The models need to be calibrated against traditional hydrogeological data such as hydraulic head, precipitation, infiltration, base flow, hydraulic conductivities inferred from pumping tests, and geological conditions estimated from drillhole information. The models can provide predictions of groundwater movement under specified natural variations and human extraction rates. An obstacle to information on geological conditions is that drillhole information is spatially sparse. This, combined with the high degree of equivalence in inverse hydraulic modeling, makes it necessary to find alternative methods for reducing the number of possible solutions. One way is to make use of improved geological models derived from hydrogeophysical investigations.