GEOLOGIC AND GEOPHYSICAL VIEWS OF THE MULCHATNA LINEAMENT OF SOUTHWEST ALASKA

The Mulchatna lineament runs NE-SW 150+ km in southwest Alaska parallel to and between two major dextral strike-slip faults: the Denali and Lake Clark. The default interpretation is that it has a similar origin to these tectonically significant structures. However, this interpretation has never been critically evaluated. We re-examined available geologic and mostly new aeromagnetic and gravity data in an effort to better understand this feature.

A topographic linear is the most characteristic evidence of this feature; different rock units are not juxtaposed along most of its length. Instead it is bounded on both sides by the Cretaceous Kuskokwim Group (Kk). The Triassic Chilikadrotna Greenstone (Trc) and Jurassic-Cretaceous Koksetna River (KJkr) sequence of Wallace and others (1989) are only known on the southeast side. Structural disruptions have been observed in Kuskokwim Group at Marys Mountain, the only bedrock exposure close to the trace of the lineament. This disruption is not known to be duplicated elsewhere and might have other causes.

New aeromagnetic and gravity data covering the Iliamna, Dillingham, and Taylor Mountains quadrangles and existing data for Lake Clark reveal a strong discontinuity that coincides with the lineament. This discontinuity is best shown in the Lake Clark quad. and weakens to the southwest where rocks tentatively mapped as the Chilikadrotna Greenstone occur northwest of the extension of the apparent magnetic discontinuity. An important feature of the aeromagnetic data is a strong magnetic high northwest of the Mulchatna linear. However, there are no rocks exposed that could yield the observed magnetic signature. Exposed rocks to the west of the trace consist of the distinctly nonmagnetic flysch of the Kuskokwim Group and Paleozoic and Proterozoic carbonates of the Nixon Fork subterrane.

We constructed a series of cross-sections traversing the trace of the linear, plotting mapped geology and constructing magnetic and gravity models to account for the observed geophysical data. The aeromagnetic models were inconclusive as to the nature of the lineament. Gravity data showed a localized high near the linear we correlated with KJkr. Our model suggests the boundary between Kk and KJkr dips 20° west in the vicinity of the lineament, suggesting a thrust fault or depositional contact.