No description available.

No description available.

No description available.

No description available.

Jurassic volcanic rocks in southeastern Arizona provide an opportunity to study the paleomagnetism of an autochthonous segment of the Mesozoic Cordilleran magmatic arc. The Corral Canyon sequence in the Patagonia Mountains is a 650 meter thick homoclinal sequence consisting of interbedded volcaniclastic red-beds, welded ash-flow tuff, and lavas. Rb/Sr isotopic analysis of eight whole rock tuff samples yields an isochron age of 171 ± 3 Ma. Welded tuffs in the Corral Canyon sequence possess a stable, primary magnetization carried in both magnetite and hematite that defines a paleomagnetic pole at 61.8°N, 116.0°E, alpha₉₅= 6.2°. This pole is considered to be a reliable Middle Jurassic reference pole for cratonic North America. Paleomagnetic study of the Canelo Hills volcanics welded tuff member also yields a stable, primary magnetization throughout a stratigraphic thickness of 600 meters. However, results from this formation are enigmatic and the mean pole is discordant with respect to Middle Jurassic reference poles. Various aspects of the paleomagnetic data indicate that discordance of the Canelo Hills volcanics pole is probably due to acquisition of remanent magnetization during a period of non-dipole behavior of the geomagnetic field. Dispersion of paleomagnetic directions suggests that the welded tuff member represents at most two cooling units and can be interpreted as a caldera-fill sequence. A revised Jurassic APW path differs significantly from available paths and has important implications for North American plate motion and paleolatitude. The spatio-temporal progression of reliable Jurassic paleopoles, in conjunction with Triassic and Early Cretaceous poles, is well described by paleomagnetic Euler pole analysis. The APW path is divided into three tracks, separated by two cusps. These cusps represent changes in the direction of North American absolute plate motion and can be correlated with global plate motion and intraplate deformation events at approximately 200-210 Ma and 150 Ma. Finally, the APW path presented herein predicts more southerly Late Triassic and Jurassic paleolatitudes for North America than have been suggested by previous authors. Using revised reference poles, there are no inclination anomalies within paleomagnetic data from Late Triassic and Early Jurassic rocks of Stikinia and Quesnellia (B.C., Canada).

The Jurassic J-2 surface is one of the more extensive unconformities of the Western Interior (U.S.A.). Stratigraphic relationships show that this is an angular unconformity, which developed first in the north and progressively moved south. An examination of features associated with the J-2 surface reveals detrital and diagenetic chert grains, ventifacts, polygonal fracture patterns, oscillation ripples, and paleotopographic relief. Some of these features indicate that the J-2 unconformity was a hot, dry, deflationary surface, and that the level of deflation on this surface was controlled, at times, by the height of the water table. The ventifacts are a lag deposit of the overlying Gypsum Spring Formation. Three possible variables involved in the formation of the J-2 surface are fluctuations in water table, disruption of sediment supply, and tectonic activity. For the Early-Middle Jurassic, these variables can be associated with increased rifting rates in the Atlantic Ocean. The increase in rifting rate raised eustatic sea level, which resulted in a larger area for deposition of marine sediments and a rise in continental water table. A higher water table would have trapped sediment that was previously available for sand sea (erg) formation. Furthermore, a higher sea level might have changed sediment supply and transportation routes. As the increase in rifting continued, tectonic movement and tilting of the North American plate occurred. This tilting caused the uplift and erosion of some deposits, and influenced the sites available for sediment preservation. / text

No description available.

Ground magnetic data are consistent with the interpretation that Lower Jurassic volcanic rocks of the Twin Buttes mine area (Ox Frame Volcanics) are confined to a distinct block by the northwest trending Sawmill Canyon Fault Zone which was initially active during the Lower Jurassic. Possible reactivation of the Sawmill Canyon Fault zone in the Middle Jurassic as a left-lateral wrench fault is recorded by the deposition of syntectonic red-beds (Rodolfo Formation). Lower Cretaceous rocks (Whitcomb Quartzite, Glance Conglomerate, and Angelica Akrose) were deposited in alluvial environments resulting from additional reactivation of the Sawmill Canyon Fault Zone. Upper Cretaceous (Laramide) deformation involved the formation of northwest trending folds and northwest and northeast trending reverse, tear, and later block faults during the uplift of Precambrian basement. Ore deposits of the Pima mining district are localized along a northeast trending fault zone with evidence for initial activity in the Middle Jurassic and later reactivation during the Laramide.

Rocks west of the Coast Mountains batholith in central southeastern Alaska include the Alexander terrane, Gravina belt, Taku terrane, and newly defined Ruth assemblage. Geologic, geochronologic and thermobarometric studies of these rocks in the Petersburg region provide new constraints on the accretionary history of the Alexander terrane and structural evolution of the Coast Mountains batholith. Paleozoic and Upper Triassic strata of the Alexander terrane were deformed within the Duncan Canal shear zone. Dextral shear in this zone during Early or Middle Jurassic time is inferred to reflect deformation along the eastern margin of the Alexander terrane and record the juxtaposition of the Alexander terrane with the North American margin. Deposition of the Upper Jurassic-Lower Cretaceous Gravina belt occurred within a transtensional back-arc to intra-arc basin that evolved during the northward translation of the Alexander terrane. The Ruth assemblage and Taku terrane were structurally emplaced over the Gravina belt and Alexander terrane along the Sumdum-Fanshaw fault system during mid-Cretaceous time. West-vergent thrusting of the Ruth assemblage was accompanied by metamorphic P,T conditions of 6.8 kb, < 450°C in the Gravina belt and 6.9 to > 7.4 kb, > 550°C in the Ruth assemblage. The age of deformation is constrained by syntectonic and post tectonic intrusive bodies that yield U-Pb lower intercept apparent ages of 92.3 ± 3 Ma and 91.3 ± 6.3 Ma, respectively. Late Devonian-Mississippian orthogneiss and felsic metavolcanic rocks in the Ruth assemblage suggest correlation of the assemblage with continental margin rocks of the Yukon-Tanana and Nisling terranes east of the Coast Mountains batholith. Thus the mid-Cretaceous Sumdum-Fanshow fault system marks the fundamental boundary between the Alexander terrane and inboard fragments. This deformation records the final structural accretion of the Alexander, Wrangellia, and Peninsular terranes to the western margin of North America. The mid-Cretaceous thrust system is truncated to the east by the LeConte Bay shear zone: a complex zone of Late Cretaceous-early Tertiary fabrics that occur within and west of the Coast Mountains batholith. This zone has apparently accommodated both west-side-up and east-side-up displacement during the collapse of the overthickened crust developed during mid-Cretaceous time.

Not available / text