Five Triassic and Jurassic tectonosequences recognized on the Colorado Plateau have been identified within the Caborca terrane in Sonora, Mexico. Piercing lines defined by truncation of tectonosequence boundaries constrain the pre-offset position of the terrane. Restoration of 1040 ± 290 km of Middle to Late Jurassic left slip along the Mojave-Sonora megashear places the Caborca block in a paleogeographic position that satisfies the constraints provided by truncated tectonosequences and yields predictable regional facies distributions for all tectonosequences. The restoration is consistent with previous estimates of displacements based upon offset of correlative (1) terrains of crystalline basement and (2) pre-Oxfordian cover.

Shallow-marine Triassic red sedimentary rocks and diabase intrusives were investigated on the Caborca Block in Sonora, Mexico. The lower 212 m half of the sequence was sampled as a magnetostratigraphic section. Samples exhibit exceedingly linearly decaying remanent magnetization and pass a fold test. Unblocking temperatures suggest that the remanence is carried by magnetite. The beds are inferred to be Early Triassic in age because they overlie Permian strata and are overlain by late Early Triassic (Spathian) Tirolites -bearing strata. The red bed samples exhibit an apparently reversed polarity (northern-hemisphere) remanence. Comparison of this polarity to a global compilation of Early Triassic magnetostratigraphy, combined with the age of the superposed beds and the sequence stratigraphic framework, suggests that the age of these beds and their magnetization may be middle Early Triassic (Dienerian). The remanence suggests a paleolatitude of magnetization of 21° N (±4°), so that in the Early Triassic, the Caborca Block may have lain off of western North America near the present location of Seattle, Washington. The overlying red sedimentary rocks containing Spathian ammonites have been remagnetized in a recent geomagnetic field direction. The entire sedimentary section has been intruded by diabase sills; yet oddly, diabase samples gave only widely scattered directions. The sampling site and Caborca Block are bordered by the left-lateral Mojave-Sonora megashear, but the paleopole is rotated clockwise relative to the North America Early Triassic reference pole, compatible with transport of the terrane in conjunction with right-lateral strike-slip faulting. Many terranes along the western North American margin have been shown to exhibit a history of Jurassic left-lateral transport followed by Cretaceous-Tertiary right-lateral movement (Beck, 1991). The current location of Caborca relative to its inferred Early Triassic paleolocation and the clockwise displacement of the Early Triassic paleopole may stem from a Jurassic left-lateral transport as postulated for the Mojave-Sonora megashear, followed by post-Early Cretaceous right-lateral motion, as observed in numerous other western North American terranes. The important point is that because of the multiplicity of terrane histories, e.g., northward then southward motion relative to cratonic North America, the inference of right-lateral transport for the Caborca Block does not, and cannot, disprove the existence of the left-lateral Mojave-Sonora megashear.

On the Colorado Plateau of southwestern Utah, the Lower Jurassic Glen Canyon Group comprises, in ascending order, the Moenave and Kayenta Formations and the Navajo Sandstone. In southern Nevada and southeastern California, the lithostratigraphic equivalent of the Navajo Sandstone is the Aztec Sandstone. In southern Nevada, the Aztec Sandstone is conformably underlain by four informally recognized stratigraphic units (A-D) of the undifferentiated Moenave and Kayenta Formations. The Glen Canyon Group unconformably overlies the Upper Triassic Chinle Formation above a regional unconformity. In addition to the Petrified Forest and Shinarump Members, the Chinle Formation contains a distinctive limestone-pebble conglomerate at its base. Using the Aztec Sandstone as a distinctive reference unit, the Glen Canyon Group and its relation to underlying and overlying lower Mesozoic depositional sequences are traced southwestward from the Las Vegas extensional domain, along the eastern edge of the relatively unextended Las Vegas Range-Spring Mountains block, into the Jurassic arc terrane of the eastern Mojave Desert. Southwestward, the regional unconformity at the base of the Glen Canyon Group truncates progressively older strata into the arc terrane. Although Middle Jurassic strata have been erosionally or tectonically removed from the Las Vegas extensional basin, volcanic-clast-bearing marginal marine facies of the Middle Jurassic Carmel Formation are tentatively correlated with silicic volcanic, volcaniclastic, and epiclastic rocks of the southern end of the Spring Mountains extensional domain. Stratigraphic and facies boundaries in lower Mesozoic strata potentially serve as important strain markers to test models of Cenozoic extension. Restoration of structural blocks containing lower Mesozoic outcrops to their pre-Tertiary positions is based on restoration of the Las Vegas Range-Spring Mountains block to its preextension position. The reconstruction reveals that (1) the limestone-pebble conglomerate at the base of the Chinle Formation is truncated on the east by the north-south-trending Vermilion Cliffs paleovalley; (2) the undifferentiated Moenave and Kayenta Formations were deposited in a north-south-trending, incipient foreland basin that deepened to the north; (3) alluvial fans were shed northeastward, at right angles to the Triassic paleoslope, into this basin from the arc terrane; and (4) volcanic centers lying east of the present Colorado River served as the source of volcanic clasts in the Carmel Formation.

Abstract Geologic relationships observable at this site are displayed Paleozoic sedimentary and Mesozoic sedimentary and rocks preserved beneath a cap of Jurassic rhyolite on a herein referred to as Rhyolite Ridge. Rhyolite Ridge liesat northeastern corner of the Cowhole Mountains approximately 13 mi (21 km)south of Baker, California, comprising portions sections 12 and 13 of T12N, R9E and 7 and 18 of T12N, R1OE, San Bernardino Principle Point. Rhyolite Ridge (not labeled) be found in the southwest corner of the Seventeenmile Quadrangle, 7 1/2-minute Topographic Series of the U.S. Geologi cal Survey. The ridge may be reached by taking the paved baker Road south from the four-way stop in central Baker or turning south from either of the central Baker off-ramps 1–15. Figure 1 indicates mileage between points mentioned in following text.