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Upper Triassic strata, termed the Chinle Formation or Group, reveal a complex sedimentologic history, which has led to numerous modifications to the lithostratigraphic nomenclature for this unit. Recent revisions to Chinle nomenclature in New Mexico are based on local stratigraphic section inspection. The utility of some stratigraphic subdivisions, especially at formation rank, are problematic when mapping units away from type areas. New magnetostratigraphic records for the Chinle Group, as well as 1:24,000-scale mapping, stratigraphic section logs, and sedimentologic analyses support the following revisions to current nomenclature of the Chinle: (1) formally raising Chinle strata to group status in New Mexico; (2) abandoning the term Zuni Mountains Formation for lower Chinle strata in the Chama Basin; (3) including the lower member of the Salitral Formation in the Shinarump Formation; and (4) reassigning strata termed Rock Point Formation in the Chama Basin to the Moenave Formation. Sedimentologic and basin-analysis interpretations utilized in earlier studies of the Chinle should be resurrected. The revised stratigraphic nomenclature for the Chinle Group and sedimentologic analyses, both old and new, greatly expands our understanding of depositional changes in the Chinle Basin of northern New Mexico. The trunk stream for Chinle deposition in New Mexico coursed westward through the site of the modern Sierra Nacimiento of north-central New Mexico during Shinarump time (?late Carnian or early Norian), then moved northward to flow to the west in the area of the modern Chama Basin. In addition, north-central New Mexico was probably a topographic high during the time of Owl Rock and Rock Point deposition in the late Norian, and thus no uppermost Triassic (late Norian to early Rhaetian) strata were deposited in the area of the modern Chama Basin. [ABSTRACT FROM AUTHOR]

Determines the paleomagnetic and geochronologic data on the structural evolution of Silver Peak extensional complex in Nevada. Horizontal tilting of mafic dikes; Association of the vertical rotation of crustal-scale blocks with displacement transfer in the Central Walker Lane; Presence of Paleozoic sedimentary rocks in late Neogene.

Kinematic data associated with both Laramide-age and -style and Rio Grande rift-related structures show that the latest Cretaceous to Neogene interaction between the Colorado Plateau and the North American craton was domi-nantly coupled with a component of dextral shear. Consistent with earlier studies, minor-fault data in this study yielded results of varied kinematics. Inverted to a common northeast-oriented hemisphere, the mean trend of kine-matic shortening associated with Laramide-age structures is . Inverted to a common west-oriented hemi- 056 6 sphere, the mean trend of kinematic extension associated with Neogene rifting is . The observed dispersion 300 34 in these directions suggests multiphase deformation, particularly during rifting, along the margin of the plateau since the latest Cretaceous. These data were evaluated using a simple two-dimensional transcurrent kinematic model; assuming a minimal importance of strain partitioning, a mean trend of convergence between the Colorado Plateau and the North American craton was estimated to be . Subsequent Rio Grande rifting, which separated the 055 5 plateau from the craton, was associated with a mean divergence trend of . Analysis of paleomagnetic data 307 5.8 from Pennsylvanian to Triassic red beds along the eastern margin of the plateau and from rocks within the rift indicate clockwise rotations of uplifted blocks. Given the lack of regional strike-slip and dip-slip faults of common trends, the consistent clockwise rotations support an absence of strain partitioning. Correspondingly, for the north-south-trending eastern margin of the plateau, the apparently clockwise-rotated paleomagnetic data are consistent with dextral transpressive shear between the plateau and the craton. Previous data indicating counterclockwise rotations of crust within parts of the Española rift basin are, if reliable, consistent with dextral transtensive shear. Overall, the transition from latest... [ABSTRACT FROM AUTHOR]

Evaluates the paleomagnetism and magnetostratigraphy of uppermost Permian strata in New Mexico. Correlation of the Permian-Triassic boundary in non-marine environments; Components of magnetization with overlapping laboratory unblocking temperature spectra; Comparison of the polarity sequence of Dewey Lake strata to the stratigraphy of marine in Asia.

Volcanic breccias form large parts of composite volcanoes and are commonly viewed as containing pyroclastic fragments emplaced by pyroclastic processes or redistributed as laharic deposits. Field study of cone-forming breccias of the andesitic middle Pleistocene Te Herenga Formation on Ruapehu volcano, New Zealand, was complemented by paleomagnetic laboratory investigation permitting estimation of emplacement temperatures of constituent breccia clasts. The observations and data collected suggest that most breccias are autoclastic deposits. Five breccia types and subordinate, coherent lava-flow cores constitute nine, unconformity-bounded constructional units. Two types of breccia are gradational with lava-flow cores. Red breccias gradational with irregularly shaped lava-flow cores were emplaced at temperatures in excess of 580 °C and are interpreted as aa flow breccias. Clasts in gray breccia gradational with tabular lava-flow cores, and in some places forming down-slope-dipping avalanche bedding beneath flows, were emplaced at varying temperatures between 200 and 550 °C and are interpreted as forming part of block lava flows. Three textural types of breccia are found in less intimate association with lava-flow cores. Matrix-poor, well-sorted breccia can be traced upslope to lava-flow cores encased in autoclastic breccia. Unsorted boulder breccia comprises constructional units lacking significant exposed lava-flow cores. Clasts in both of these breccia types have paleomagnetic properties generally similar to those of the gray breccias gradational with lava-flow cores; they indicate reorientation after acquisition of some, or all, magnetization and ultimate emplacement over a range of temperatures between 100 and 550 °C. These breccias are interpreted as autoclastic breccias associated with block lava flows. Matrix-poor, well-sorted breccia formed by disintegration of lava flows on steep slopes and unsorted boulder breccia is interpreted to represent channel-floor and levee breccias for block lava flows that continued down slope. Less common, matrix-rich, stratified tuff breccias consisting of angular blocks, minor scoria, and a conspicuously well-sorted ash matrix were generally emplaced at ambient temperature, although some deposits contain clasts possibly emplaced at temperatures as high as 525 °C. These breccias are interpreted as debris-flow and sheetwash deposits with a dominant pyroclastic matrix and containing clasts likely of mixed autoclastic and pyroclastic origin. Pyroclastic deposits have limited preservation potential on the steep, proximal slopes of composite volcanoes. Likewise, these steep slopes are more likely sites of erosion and transport by channeled or unconfined runoff rather than depositional sites for reworked volcaniclastic debris. Autoclastic breccias need not be intimately associated with coherent lava flows in single outcrops, and fine matrix can be of autoclastic rather than pyroclastic origin. In these cases, and likely many other cases, the alternation of coherent lava flows and fragmental deposits defining composite volcanoes is better described as interlayered lava-flow cores and cogenetic autoclastic breccias, rather than as interlayered lava flows and pyroclastic beds. Reworked deposits are probably insignificant components of most proximal cone-forming sequences. [ABSTRACT FROM AUTHOR]

Studies paleomagnetic data for the Antimonio terrane in northwest Mexico. Interpretation of the magnetization in Triassic-Jurassic strata of the Antimonio Formation; Stability of the Antimonio terrane; Implications for timing and accretion of the Antimonio terrane.