THE TAYLOR MOUNTAIN BATHOLITH: AEROMAGNETIC EXPRESSION, GEOMETRY, AND IMPLICATIONS FOR TECTONIC MODELS, EAST-CENTRAL ALASKA

The Yukon-Tanana upland of east-central Alaska contains the boundary between the allochthonous Yukon-Tanana terrane (YTT) and parautochthonous North American basement (NAb) which is interpreted to be a low-angle, north-dipping ductile detachment fault. Exposures of the boundary are limited, and lithologic similarities among juxtaposed rocks make it a generally poor geophysical target. An exception is the Late Triassic Taylor Mountain batholith, a strongly magnetized granodiorite thought to have intruded rocks of the YTT prior to their being thrust over NAb during Jurassic collision. The batholith is located near and along the presumed southern margin of the north-dipping YTT-NAb boundary, and published tectonic models envision the batholith as occupying the thin upper plate of the low-angle detachment. The batholith produces a >300 nT aeromagnetic high. Paleomagnetic studies by other workers yielded high-quality magnetic property data relevant to constraining the batholith’s geometry. 244 samples from 18 sites spread across the batholith indicate a mean total magnetization intensity of 0.84 A/m, and a Koenigsberger ratio (Q value) of 0.13, indicating that magnetization is dominated by induction and therefore is essentially parallel to today’s ambient magnetic field. Forward modeling of the aeromagnetic data using these rock magnetic properties indicates a batholith thickness of 4-8 km, which is not consistent with structural models that imply a batholith thickness of only a few km or less in the upper plate of the low-angle detachment. Thus, our findings require reconsideration of the geometry and/or location of the YTT-NAb boundary at least locally. The southern portion of the batholith is characterized by weakly magnetized rocks that are separated from more extensive, strongly-magnetized portions to the north by east-trending aeromagnetic lineaments. These lineaments indicate high-angle structures that may reflect brittle faulting that postdates the detachment. Alternatively, the ductile detachment might not have the same dip along strike and could possibly wrap beneath the large Late Triassic batholith. Weak magnetizations in the southern portion of the batholith may be a result of magnetite-destructive alteration due to fluids concentrated along these structures.