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

Paleomagnetic and structural data from intermediate to mafic composition la va Rows and related dikes in all major blocks of the late Miocene Hamblin-C leopatra 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 th is part of the Basin and Range province. Sinistral separation along the fau lt system dissected the volcano into three major blocks. The eastern, Cleop atra Lobe of the volcano is structurally the most intact section of the vol cano. Normal and reverse polarity data from paleomagnetic sites collected a long traverses in the Cleopatra Lobe yield an in situ grand mean of Declina tion (D) = 339 degrees, Inclination (I) = +54 degrees, alpha (95) = 3.1 deg rees, k = 27.2, N = 81 sites. The rocks of the central core of the volcano yield an in situ grand mean of D = 3 degrees, I = +59 degrees, alpha (95) = 6.8 degrees, k = 42.5, N = 11 sites (six normal. five reverse polarity). S ites collected within the western Hamblin Lobe of the volcano are exclusive ly of reverse polarity and yield an overall in situ mean of D = 168 degrees , I = -58 degrees, alpha (95) = 6.5 degrees, k = 28.9, N = 18 sites. Interp retation of the paleomagnetic data in the context of the structural history of the volcano and surrounding area, considers the possibility of two diff erent types of structural corrections. A stratigraphic tilt correction invo lves restoring Rows to the horizontal using the present strike. This correc tion assumes no initial, possibly radial, dip of flows of the volcano and i s considered invalid. A structural tilt correction to the data assumes that dikes of the radiating swarm associated with the volcano were originally v ertical and results in block mean directions of D = 9 degrees, I = 53 degre es, alpha (95) = 3.1 degrees, k = 27.2, and D = 58 degrees, I = +78 degrees , alpha (95) = 6.8 degrees, k = 42.5, for the Cleopatra Lobe and the centra l intrusive core, respectively. The data from the Cleopatra Lobe are slight ly discordant, in a clockwise sense, from expected middle- to late-Miocene field directions. The data from the volcano are not consistent with a propo sed structural model of uniform, moderate magnitude, statistically signific ant, counter-clockwise vertical axis rotation of fault-bounded blocks durin g overall sinsitral displacement along the LMFS. We also analyzed dikes of the northernmost part of the Miocene Wilson Ridge hypabyssal igneous comple x, strata of the Triassic Chinle Formation, and basalt Rows of the Miocene West End Wash/Callville Mesa volcanic centers. Dikes in the Wilson Ridge pl uton and the Triassic strata yield magnetizations with directions suggestiv e of statistically significant, clockwise, vertical-axis rotations consiste nt with local, large-magnitude shear of crustal fragments near some of the faults of the LMFS. Late Cenozoic deformation of the Hamblin-Cleopatra volc ano area appears to have been non-uniform in scale and magnitude and no sin gle structural model, involving strictly strike-slip faulting, can account for the observed paleomagnetic data. (C) 2001 Elsevier Science Ltd. All rig hts reserved.