LATE CENOZOIC STRUCTURE AND TECTONICS OF THE NORTHERN MOJAVE DESERT

In the Fort Irwin region of the northern Mojave desert, late Cenozoic east striking sinistral faults predominate over northwest striking dex tral faults of the same age. Kinematic indicators and offset marker un its indicate dominantly sinistral strike slip on the east striking por tions of the faults and sinistral-thrust slip on northwest striking, m oderately dipping segments at the east ends of the blocks. Crustal blo cks similar to 7-10 km wide by similar to 50 km long are bounded by co mplex fault zones up to 2 km wide at the edges and ends of each block. Faulting initiated after similar to 11 Ma, and Quaternary deposits ar e faulted and folded. We document a minimum of 13 bm cumulative sinist ral offset in a north-south transect from south of the Bicycle Lake fa ult to north of the Drinkwater Lake fault. Paleomagnetic results from 50 sites reveal two direction groups in early and middle Miocene rocks . The north-to-northwest declinations of the first group are close to the middle Miocene reference pole. However, rock magnetic studies sugg est that both primary and remagnetized directions are present in this group. The northeast declinations of the second group are interpreted as primary and 63.5 degrees +/- 7.6 degrees clockwise from the referen ce pole and suggest net post middle Miocene clockwise rotation of seve ral of the east trending blocks in the northeast Mojave domain. The Ju rassic Independence Dike Swarm in Fort Irwin may be rotated 25-80 degr ees clockwise relative to the swarm north of the Garlock fault, thus s upporting the inference of clockwise rotation. Using a simple-shear mo del that combines sinistral slip and clockwise rotation of elongate cr ustal blocks, we predict similar to 23 degrees clockwise rotation usin g the observed fault slip, or one-third that inferred from the paleoma gnetic results. The discrepancy between slip and rotation may reflect clockwise bending at the ends of fault blocks, where most of our paleo magnetic sites are located. However, at least 25 degrees-40 degrees of clockwise tectonic rotation is consistent with the observed slip on f aults within the domain plus possible ''rigid-body'' rotation of the r egion evidenced by clockwise bending of northwest striking domain-boun ding faults. Our estimates of sinistral shear and clockwise rotation s uggest that approximately half of the 65 km of dextral shear in the Ea stern California Shear Zone over the last 10 m.y. occurred within the northeast Mojave Domain. The remainder must be accommodated in adjacen t structural domains, e.g., east of the Avawatz Mountains and west of the Goldstone Lake fault.