Paleomagnetic data bearing on style of Miocene deformation in the Lake Mead area, southern Nevada

Abstract

Paleomagnetic and structural data from intermediate to mafic composition lava flows and related dikes in all major blocks of the late Miocene Hamblin–Cleopatra Volcano, which was structurally dismembered during the development of the Lake Mead Fault System (LMFS), provide limits on the magnitude and sense of tilting and vertical axis rotation of crust during extension of this part of the Basin and Range province. Sinistral separation along the fault system dissected the volcano into three major blocks. The eastern, Cleopatra Lobe of the volcano is structurally the most intact section of the volcano. Normal and reverse polarity data from paleomagnetic sites collected along traverses in the Cleopatra Lobe yield an in situ grand mean of Declination (D)=339°, Inclination (I)=+54°, α95=3.1°, k=27.2, N=81 sites. The rocks of the central core of the volcano yield an in situ grand mean of D=3°, I=+59°, α95=6.8°, k=42.5, N=11 sites (six normal, five reverse polarity). Sites collected within the western Hamblin Lobe of the volcano are exclusively of reverse polarity and yield an overall in situ mean of D=168°, I=−58°, α95=6.5°, k=28.9, N=18 sites. Interpretation of the paleomagnetic data in the context of the structural history of the volcano and surrounding area, considers the possibility of two different types of structural corrections. A stratigraphic tilt correction involves restoring flows to the horizontal using the present strike. This correction assumes no initial, possibly radial, dip of flows of the volcano and is considered invalid. A structural tilt correction to the data assumes that dikes of the radiating swarm associated with the volcano were originally vertical and results in block mean directions of D=9°, I=+53°, α95=3.1°, k=27.2, and D=58°, I=+78°, α95=6.8°, k=42.5, for the Cleopatra Lobe and the central intrusive core, respectively. The data from the Cleopatra Lobe are slightly discordant, in a clockwise sense, from expected middle- to late-Miocene field directions. The data from the volcano are not consistent with a proposed structural model of uniform, moderate magnitude, statistically significant, counter-clockwise vertical axis rotation of fault-bounded blocks during overall sinsitral displacement along the LMFS. We also analyzed dikes of the northernmost part of the Miocene Wilson Ridge hypabyssal igneous complex, strata of the Triassic Chinle Formation, and basalt flows of the Miocene West End Wash/Callville Mesa volcanic centers. Dikes in the Wilson Ridge pluton and the Triassic strata yield magnetizations with directions suggestive of statistically significant, clockwise, vertical-axis rotations consistent with local, large-magnitude shear of crustal fragments near some of the faults of the LMFS. Late Cenozoic deformation of the Hamblin–Cleopatra volcano area appears to have been non-uniform in scale and magnitude and no single structural model, involving strictly strike-slip faulting, can account for the observed paleomagnetic data.