A MAGNETOTELLURIC TRANSECT ACROSS THE TALKEETNA MOUNTAINS, ALASKA

During the 2000 and 2001 field seasons 25 magnetotelluric (MT) soundings were acquired along a 100 km profile across the Talkeetna Mountains, Alaska to delineate faults and large-scale intrusions. The profile, oriented at N49W, is approximately perpendicular to the Talkeetna Fault (TF) and the Broad Pass graben. From NW to SE the profile crosses the Chulitna terrane, the Kahiltna (KH) overlap assemblage, the TF, the Wrangellia (WR) and the Peninsular (PE) terranes. The KH is a flysch basin intruded by Tertiary granitoid plutons. The WR terrane consists of metasedimentary, metagranitic, and metabasaltic rocks, along with mafic and local ultramafic intrusions. The PE terrane is composed of marine sedimentary, volcanic and Jurassic granitoid rocks.

Measurements of the earth’s natural electric and magnetic field are used to infer the subsurface electrical resistivity that can be related to rock units. The horizontal components of the electric field were measured with 30 m dipoles in a ‘L’ array. Station spacing varied from 2 to 10 km. Frequencies were sampled from 70 to 0.004 Hz for a depth of exploration of roughly 0.5 to 50 km. Data are analyzed and interpreted with the MT quantities of apparent resistivity, phase, rotation angle, polar diagrams, and skew in addition to two-dimensional (2-D) inverse and three-dimensional (3-D) forward modeling.

Structures are predominately 2-D as indicated by tensor quantities. Electrical strike directions are between EW and E25N for most stations at most frequencies with some notable exceptions particularly in the KH terrane. The exceptions are quite 3-D in nature consistent with the response of large intrusive bodies. Preliminary 2-D inverse models depict both the WR and PE terranes having resistivity blocks of values ranging from 100 to 1000 ohm-m making the terrane boundary difficult to resolve. Geoelectric units within the KH terrane vary from over 1000 to less than 10 ohm-m representing plutons and flysch sediments, respectively.