摘要:

The main Cenozoic extensional structure in the central Mojave Desert is the Waterman Hills detachment fault, which places brittlely deformed synorogenic Miocene rocks on ductilely and cataclastically deformed footwall rocks. New data are presented regarding the timing, distribution, magnitude, and significance of early Miocene extension in the area. The mylonitic fabric in the lower plate was formed at 23 Ma, based on a zircon U/Pb age from a synmylonitic intrusion. Upper plate strata consist of rhyolite flows overlain by sedimentary rocks that were apparently deposited during extensional faulting. These strata were tilted, folded, and intruded by synkinematic rhyolite plugs that are cut off at the detachment fault. Potassium metasomatism of the rhyolitic rocks is pervasive. Upper plate detrital sediment was derived from the rhyolitic rocks and from metamorphic and plutonic basement rocks not present in the area. The probable source of the exotic basement clasts is the Alvord Mountain area, presently located 35 km east‐northeast of the Waterman Hills area. This source was probably much nearer to the Waterman Hills during deposition of the synorogenic deposits and has been subsequently shifted by extensional deformation. Distinctive Mesozoic plutonic rocks provide a possible tie between upper and lower plate rocks. Similar poikilitic gabbro bodies in the Goldstone area and the Iron Mountains suggest slip on the Waterman Hills detachment fault to be about 40–50 km. This is also consistent with other offset markers, such as the western edge of a Mesozoic dike swarm. When 15–20 km(?) of Tertiary extension is restored, Paleozoic eugeoclinal rocks are placed structurally above their miogeoclinal counterparts. Combined with the distribution of Triassic and Jurassic rocks, this implies post‐Early Triassic and pre‐Late Jurassic stacking of these li

ISSN:0148-0227    

DOI:10.1029/JB095iB01p00557

年代:1990

数据来源: WILEY

摘要:

Sanidine separates from pumice of the early Miocene Peach Springs Tuff are concordantly dated at 18.5±0.2 Ma by two isotopic techniques. The Peach Springs Tuff is the only known unit that can be correlated between isolated outcrops of Miocene strata from the central Mojave Desert of southeastern California to the western Colorado Plateau in Arizona, across five structural provinces, a distance of 350 km. Thus the age of the Peach Springs Tuff is important to structural and paleogeographic reconstructions of a large region. Biotite and sanidine separates from bulk samples of the Peach Springs Tuff from zones of welding and vapor‐phase alteration have not produced consistent ages by the K‐Ar method. Published ages of mineral separates from 17 localities ranged from 16.2 to 20.5 Ma. Discordant40Ar/39Ar incremental release spectra were obtained for one biotite and two of the sanidine separates. Ages that correspond to the last gas increments are as old as 27 Ma. The40Ar/39Ar incremental release determinations on sanidine separated from blocks of Peach Springs Tuff pumice yield ages of 18.3±0.3 and 18.6±0.4 Ma. Laser fusion measurements yield a mean age of 18.51±0.10. The results suggest that sanidine and biotite K‐Ar ages older than about 18.5 Ma are due to inherited Ar from pre‐Tertiary contaminants, which likely were incorporated into the tuff during deposition. Sanidine K‐Ar ages younger than 18 Ma probably indicate incomplete extraction of radiogenic40Ar, whereas laser fusion dates of biotite and hornblende younger than 18 Ma likely are due to postdepositional alteration. Laser fusion ages as high as 19.01 Ma on biotite grains from pumice suggest that minerals from pre‐Tertiary country rocks also were incorporated in th

ISSN:0148-0227    

DOI:10.1029/JB095iB01p00571

年代:1990

数据来源: WILEY

摘要:

Investigations in the Old Woman Mountains area provide insights into mid‐Tertiary extensional tectonism in southeastern California. The Old Woman Mountains area is a moderately extended region that lies between two highly extended terranes, the Central Mojave Extensional Complex and the Colorado River Extensional Corridor. Two normal faults and a high‐angle fault that we interpret to be a transfer structure divide the area into four distinct structural blocks. These blocks correspond to the four mountain ranges of the area: the Old Woman, Piute, Little Piute, and Ship mountains. Other major faults involved in tilting are inferred to be buried beneath alluvium in surrounding valleys. Deformation occurred in part before, but primarily after, deposition of the Peach Springs Tuff, an 18.5 Ma regional stratigraphic marker. Transport directions of hanging walls are westward on the west side of the Old Woman Mountains area and east‐southeastward on the east side. We suggest that the Old Woman Mountains area is a less extended portion of a continuous extensional terrane that stretches from the central Mojave Desert across the Colorado River to the transition zone of the Colorado Plateau. Lower strain in the Old Woman Mountains area than in most of the Colorado River Extensional Corridor and Central Mojave Extensional Complex and age relations across the terrane can be explained by a model in which the principal locus of upper crustal breakaway to an east dipping low‐angle simple shear zone shifted at about 18–20 Ma from a position far to the west to just east of the Old Woman Mount

ISSN:0148-0227    

DOI:10.1029/JB095iB01p00581

年代:1990

数据来源: WILEY

摘要:

A stratigraphic and structural record of synchronous extension and detachment faulting is preserved in predominantly Miocene deposits of the Whipple detachment system, Colorado River extensional corridor. In the Mohave Mountains and Aubrey Hills of Arizona and the eastern Whipple Mountains of California near Parker Dam, these deposits comprise four unconformity‐bounded sequences composed of locally derived epiclastic and volcanic rocks and the Peach Springs Tuff. The three older sequences represent syntectonic units that were deposited coeval with detachment faulting, and the fourth is interpreted to be postextensional. The sequences are correlated between four fault‐bounded regions, which are the remnants of four different depositional basins. Similar sequences can be correlated over broad areas of the extensional corridor despite the general lack of widespread units. The basins developed in about the same positions, relative to each other and to volcanic sources, as they occupy at present. This is shown by gradational changes of pre‐Tertiary rock types between regions, systematic variations in the abundance of magmatic units, and correlative volcanic units that occur in two adjacent regions. The basins formed in the early Miocene from segmentation of the upper crust into blocks bounded by high‐angle faults that trended both parallel and perpendicular to the direction of extension and which were terminated at middle crustal depths by a low‐angle detachment fault. Extreme rotation of one large crustal block, which constitutes the central Mohave Mountains, is recorded by a major unconformity in the lower Miocene section of one basin. Because coeval sections that formed in adjoining basins do not record this rotation, the underlying crustal blocks must have been separated by transfer faults that allowed them to rotate independently. These proposed transfer faults are represented at present by major faults with trends that parallel the direction of extension on the Whipple detachme

ISSN:0148-0227    

DOI:10.1029/JB095iB01p00599

年代:1990

数据来源: WILEY

摘要:

This study utilizes enhanced Landsat Thematic Mapper (TM) images in concert with field work to solve problems in regional correlation of Miocene rocks in the Colorado River extensional corridor of California and Arizona. These syntectonic volcanic and sedimentary deposits preserve a dateable record of crustal extension, although regional correlation and analysis of the deposits are difficult because of the small sizes of the sedimentary basins in which they accumulated and disruption by postdepositional faulting. Based on field investigations, four sequences of sedimentary and volcanic strata, distinguished by stratigraphic and structural relations, can be recognized in the Mohave Mountains, Arizona, and the eastern Whipple Mountains, California. Although the sequences are found in each of four structural regions, the relative volumes and organization of lithologies within the sequences differ considerably between regions. These sequences have distinctive appearances on the TM image. The volcanic component is particularly important in distinguishing between regions and sequences because differences in color and textural patterns in the image are usually related to variations in the proportion and composition of the volcanic material and in the geometry of the volcanic bodies. The recognition criteria derived from field mapping and image interpretation in the Mohave and Whipple Mountains were applied to an adjacent area in which stratigraphic affinities were less well known. Stratigraphic and structural relations suggested by the TM image were confirmed by subsequent field work.

ISSN:0148-0227    

DOI:10.1029/JB095iB01p00615

年代:1990

数据来源: WILEY

摘要:

Paleomagnetic directions have been obtained from 190 early to middle Miocene (12–20 Ma) mafic volcanic flows in 16 mountain ranges in the Mojave‐Sonora desert region of western Arizona and southeastern California. These flows generally postdate early Miocene tectonic deformation accommodated by low‐angle normal faults but predate high‐angle normal faulting in the region. After detailed demagnetization experiments, 179 flows yielded characteristic directions interpreted as original thermal remanent magnetizations (TRM). Because of the episodic nature of basaltic volcanism in this region, the 179 flows yielded only 65 time‐distinct virtual geomagnetic poles (VGPs). The angular dispersion of the 65 VGPs is consistent with the angular dispersion expected for a data set that has adequately averaged geomagnetic secular variation. The paleomagnetic pole calculated from the 65 cooling unit VGPs is located at 85.5°N, 108.9° within a 4.4° circle of 95% confidence. This pole is statistically indistinguishable (at 95% confidence) from reference poles calculated from rocks of similar age in stable North America and from a paleomagnetic pole calculated from rocks of similar age in Baja California. The coincidence of paleomagnetic poles from the Mojave‐Sonora desert region with reference poles from the stable continental interior indicates that (1) significant vertical axis net tectonic rotations have not accompanied post‐middle Miocene high‐angle normal faulting in this region; (2) there has been no detectable post‐middle Miocene latitudinal transport of the region; and (3) long‐term nondipole components of the middle Miocene geomagnetic field probably were no larger than those of the recent (0–5 Ma) geomagnetic field. In contrast, paleomagnetic data indicate vertical axis rotations of similar age rocks in the Transverse Ranges, the Eastern Transverse Ranges, and the Mojave Block. We speculate that a major structural discontinuity in the vicinity of the southeastward projection of the Death Valley fault zone separates western areas affected by vertical axis rotations from eastern areas that have not ex

ISSN:0148-0227    

DOI:10.1029/JB095iB01p00625

年代:1990

数据来源: WILEY

摘要:

Mantle and crustal xenoliths from volcanic rocks in the southwestern Basin and Range province and Colorado Plateau Transition Zone reveal histories of episodic magmatism and deformation that have profoundly influenced the crustal structure of this region. Seismic transects in this area show a strongly reflective Moho of generally low relief, which, in the area of modern transects, consists of a thin zone (<2 km thick) of short reflectors. The upper mantle is transparent and has aPnof 7.8–8.0 km/s similar to much of the western United States. A lower crustal zone, 2–13 km thick, has variable internal reflectivity and a relatively low velocity of 6.6–6.8 km/s. Upper mantle peridotite xenoliths show both ductile and brittle deformational features and have structures and compositions affected by magmatic intrusion; intrusions form complex dike systems and extensive zones of grain boundary infiltration in peridotite xenoliths. Whereas melt infiltration preceded and followed ductile deformation, brittle deformation, represented by closely spaced joint systems and faults, followed ductile deformation and is related to the youngest magmatic episodes. These structural characteristics and high uppermost mantle temperature (∼1000°C) may combine to explain the relatively lowPn. Alternating layers of ductily deformed and undeformed peridotites, with or without igneous intrusions, may contribute to the reflectivity of the Moho. Lower crustal xenoliths are dominantly igneous‐textured pyroxenites and mafic to intermediate gabbros identical to the dikes in peridotite xenoliths. The crustal xenoliths also commonly are jointed, and in addition many show partial melting and have abundant cavities that probably were filled with CO2‐rich fluids. These rocks are interpreted as products of underplated magmas that were fed through the mantle dike systems and may represent the lowest crustal unit identified in the seismic records. The mafic compositions and high densities of the crustal xenoliths indicate that the low velocity of the lower crust may be caused in part by fracture systems, partial melts, and high temperatures. Garnet granulite xenoliths from a locality with no mantle peridotite xenoliths probably represent crust of the region before late Miocene extension. Felsic granulite xenoliths from two localities have velocities like those of the two lower crustal units identified seismically and could be present in the modern crust as unequilibrated remnants of old crust. The preferred model for the evolution of the lower lithosphere is one in which extension affects the upper mantle as well as the crust and is overlapped in time by multiple magmatic episodes. The earliest magmatic events preceded extension, and later events accompanied and follow

ISSN:0148-0227    

DOI:10.1029/JB095iB01p00649

年代:1990

数据来源: WILEY