Aeromagnetic measurements in Dixie Valley, Nevada; Implications on Basin-Range Structure

Abstract

Interpretation of an aeromagnetic survey flown during 1964 suggests that the pre-Tertiary magnetic basement under Dixie Valley, Nevada, forms an asymmetric composite graben whose inner block is approximately 5 km wide and lies under the western half of the valley at an average depth of 1.9 km. Steplike ‘shelf’ blocks bordering the narrow inner graben are also downthrown with respect to adjacent ranges, but to a lesser degree; the western shelf is approximately 300 meters below the surface, whereas the eastern conjugate block lies about 500 meters below the surface. The average depth of valley fill across the composite graben is approximately 765 meters. Depth estimates imply, in addition, that the eastern shelf block is broken by several NW-trending transverse faults of 300- to 600-meters displacement. The magnetic expression of contacts between a Jurassic gabbroic complex and other basement rocks can be traced across both northern and southern Dixie Valley. An absence of appreciable horizontal offset of this contact across most of the major Basin-Range faults indicates that post-Jurassic displacements have been primarily dip-slip. An apparent right lateral offset of 2–3 km may exist along the eastern side of the deepest graben block, however. Models computed from anomalies over the southern gabbro contact tend to verify earlier geologic inferences that this intrabasement complex is of lopolithic form. The apparent northward displacement of the gabbro outcrops and contact in the Clan Alpine Range from the subsurface position of gabbroic basement in eastern Dixie Valley may reflect an uplift of the range, relative to the valley block, with subsequent erosional stripping of the tapered lopolith. Satisfactory alternative solutions of an equidimensional anomaly in southeastern Dixie Valley are either a volcanic cone or an equidimensional volcanic remnant. Both computational models overlie the gabbroic complex and require a high total magnetization.