Jeffrey M. Rahl

Transition from Contraction to Extension in the Northeastern Basin and Range: New Evidence from the Copper Mountains, Nevada

New mapping, structural analysis, and 40Ar/39Ar dating reveal an unusually well‐constrained history of Late Eocene extension in the Copper Mountains of the northern Basin and Range province. In this area, the northeast‐trending Copper Creek normal fault juxtaposes a distinctive sequence of metacarbonate and granitoid rocks against a footwall of Upper Precambrian to Lower Cambrian quartzite and phyllite. Correlation of the hanging wall with footwall rocks to the northwest provides an approximate piercing point that requires 8–12 km displacement in an ESE direction. This displaced fault slice is itself bounded above by another normal fault (the Meadow Fork Fault), which brings down a hanging wall of dacitic to rhyolitic tuff that grades conformably upward into conglomerate. These relationships record the formation of a fault‐bounded basin between 41.3 and 37.4 Ma. The results are consistent with a regional pattern in which volcanism and extension swept southward from British Columbia to southern Nevada from Early Eocene to Late Oligocene time. Because the southward sweep of volcanism is thought to track the steepening and foundering of the downgoing oceanic plate, these results suggest that the crucial mechanisms for the onset of regional extension were probably changes in plate boundary conditions coupled with convective removal of mantle lithosphere and associated regional magmatism and lithospheric weakening. Paleobotanical data indicate that surface elevations in northeastern Nevada were not significantly different than at present, suggesting that gravitational instability of overthickened continental crust was not the dominant force driving the onset of crustal thinning in mid‐Tertiary time.

Tectonic significance of ductile deformation in low-grade sandstones in the mesozoic Otago subduction wedge, New Zealand

This study demonstrates how ductile strain measurements can determine the tectonic evolution of a large and long-lived subduction wedge. We provide a synthesis of the geology of the South Island of New Zealand, with an emphasis on a wedge tectonics perspective that contrasts with a traditional view that interprets New Zealand geology in the context of terrane collision and accretion. We argue that the Otago subduction wedge evolved in a steady fashion throughout its 290 to 105 Ma history in response to accretion of trench-fill and abyssal-plain sediments, and slow erosion of a subaerially-exposed forearc high. Maximum temperatures for rocks in the flanks of the forearc high were no greater than 150 to 300 °C, with solution mass-transfer active as the dominant ductile mechanism. The 54 studied samples provide information about the absolute ductile strains acquired all along their flow-path, from the site of accretion to exhumation in the forearc high. We use tensor-averages to estimate strain at a regional scale. These show plane-strain uniaxial flattening, given that the tensor-averages for Sy and Sx are close to one. On average, Sz is approximately 0.77, and this is balanced by a mass loss of about 23 percent. The average Z direction is sub-horizontal in the prowedge and moderately plunging in the retrowedge, a difference attributed to spatial variations in the mode of accretion. We infer that rocks presently in the pro-side of the Otago high were sourced by frontal accretion, and those in the retro-side were underplated. This result highlights the important role of accretion in determining the style of within-wedge deformation, and also demonstrates the benefit of using a tensor-averaging approach to examine regional strain.

THREE-DIMENSIONALLY PRESERVED SPECIMENS OF AMPLEXOGRAPTUS (ORDOVICIAN, GRAPTOLITHINA) FROM THE NORTH AMERICAN MID- CONTINENT: TAXONOMIC AND BIOSTRATIGRAPHIC SIGNIFICANCE

Graptolites are relatively uncommon fossils in the Middle and Upper Ordovician carbonates of the North American mid-continent. With the notable exception of the Viola Springs Formation in Oklahoma, most units contain a single species, which occur within narrow stratigraphic intervals (often single beds). While uninteresting in terms of their diversity, these monospecific collections commonly yield excellent three-dimensionally preserved specimens that can be isolated from their matrix and studied. In particular, the large number of well-preserved specimens allows for accurate evaluations of intra-specific variation and reliable descriptions of taxa that are generally known from flattened or otherwise distorted material. Additionally, these graptolite collections can provide important biostratigraphic ties between disparate facies—shallow mid-continental limestones and the deeper-water black shales characteristic of continental margins. In this study, we recovered three-dimensionally preserved graptolite specimens from four collections of Middle Ordovician limestones made in the eastern and central United States. All four collections yielded mono-specific assemblages of specimens that belong to the genus Amplexograptus . Specimens isolated from the Lebanon Limestone of Tennessee are particularly noteworthy as they are referable to Amplexograptus perexcavatus (Lapworth)—the poorly known type species of Amplexograptus . The presence of three-dimensional graptolites in the Lebanon Limestone was previously mentioned in the literature (Hofstetter, 1965; Conkin and Conkin, 1992), but the specimens were not described. Two other collections, from the Upper Bromide Formation in Oklahoma and the Mifflin Member of the Platteville Formation in Illinois, yielded specimens of A. maxwelli (Decker). Our last sample, from the Kope Formation in Kentucky, yielded specimens of A. praetypicalis Riva. The purpose of this paper is to re-describe A. perexcavatus based on well preserved three-dimensional material, to quantitatively compare the morphology of A. perexcavatus to the better known species of Amplexograptus found in our other samples, and to discuss the biostratigraphic significance of these samples. …

Rhomb-dominated crystallographic preferred orientations in incipiently deformed quartz sandstones: A potential paleostress indicator for quartz-rich rocks

We describe quartz crystallographic preferred orientations (CPOs) from incipiently deformed quartz sandstones characterized by low-intensity but unambiguous alignment of the poles to positive {r} and/or negative {z} rhombs. These distinctive CPOs appear at minimal strains and in grains with scarcely modified original detrital boundaries. We consider the hypothesis that these patterns reflect Dauphiné twinning (a 180° misorientation about the c-axis) that preferentially affects grains oriented with the elastically stiffer z-rhombs at high angle to the maximum principal stress direction. Twinning facilitates elastic deformation by aligning the more compliant r-rhombs at high angle to the greatest principal stress. Crystallographic maps show that about two-thirds of all grains (by area) are twinned, and untwinned grains are oriented with an r-rhomb perpendicular to the inferred shortening direction. We document this pattern from low-grade quartzite from three locations: the Eureka Quartzite of northeastern Nevada (USA); the Mesón Group of northwestern Argentina; and the Antietam Formation of the Blue Ridge of central Virginia (USA). The widespread presence of these CPOs in minimally deformed quartz rocks suggests that they may be useful in defining paleostress trajectories.