Boulder, CO, USA — The August LITHOSPHERE studies large igneous provinces through examination of the South Mountain region, Pennsylvania; structural style across the Kaiparowits Basin, Utah; timing and rate of extension in the Anaconda metamorphic core complex, Montana; slip-rates at four new locations along the Kunlun fault, Tibet; unusual behavior in Earth’s crust that can bring rocks to the surface from more than 50 km deep; and paleo-earthquakes along the Calico fault, Eastern California shear zone.
Keywords: rhyolite magmas, Laramide deformation, Anaconda metamorphic core complex, Kunlun fault, eclogites, Calico fault.
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A model for the origin of rhyolites from South Mountain, Pennsylvania: Implications for rhyolites associated with large igneous provinces
Tyrone Rooney, Michigan State University, East Lansing, MI 48824, USA. Pages 211-220.
Large igneous provinces are regions on Earth where vast volumes of magma are erupted forming regions covered in lava. Most research of these provinces has focused on the more common basaltic type of lavas; however, the rarer rhyolites may preserve more information as to how large igneous province magmas interact with and evolve in the continental crust. Tyrone Rooney of Michigan State University and colleagues show that rhyolite magmas in the South Mountain region of Pennsylvania have two distinctive trends that result from the removal of differing minerals from the magma. These results help scientists understand how these rhyolites evolved and reveal clues as to the volatile content (water and carbon dioxide) of the magmas.
Growth faults in the Kaiparowits Basin, Utah, pinpoint initial Laramide deformation in the western Colorado Plateau
Sarah Tindall, Kutztown University, Physical Sciences, P.O. Box 730, Kutztown, PA 19530, USA. Pages 221-231.
The Kaibab uplift in southern Utah lies near the spatial transition from the Sevier, thin-skinned, thrust belt structural style to the Laramide, basement-cored uplifts of the Colorado Plateau. The temporal progression of deformation across this boundary and into the Colorado Plateau is difficult to trace because of limited preservation and exposure of key Cretaceous-Tertiary syntectonic strata. Sarah Tindall of Kutztown University and colleagues explore relationships between Upper Cretaceous sedimentary rocks and two enigmatic faults crossing the steep eastern limb of the Kaibab uplift. Although fault orientations and slip vectors are consistent with an oblique-reverse interpretation, recognition of syntectonic strata deposited during faulting clarifies that the faults formed as normal faults at the onset of Laramide deformation. The syntectonic strata define the timing of onset of the Laramide orogeny in the western Colorado Plateau as between 80 and 76 Ma. The change in structural style across this region makes it a key area for understanding kinematic and tectonic development of mountain systems, particularly the transition from continental margin to continental interior deformation.
Extension of the Anaconda metamorphic core complex: 40Ar/39Ar thermochronology and implications for Eocene tectonics of the northern Rocky Mountains and the Boulder batholiths
David Foster, University of Florida, Geological Sciences, Gainesville, FL 32611-2120, USA. Pages 232-346.
David Foster of the University of Florida and colleagues used temperature-sensitive isotopic dating methods and field data to define the timing and rate of extension in the recently discovered Anaconda metamorphic core complex in Montana. This hyper-extended terrain is the easternmost Cenozoic core complex in the northern Rocky Mountains and contains the Boulder batholith and world-class Butte ore body in the hanging wall. The results of the study show that the metamorphic and plutonic rocks in the footwall of the Anaconda detachment were transported west about 28 km between about 50 and 25 million years ago, from beneath the western Boulder batholith region. This suggests that the deep-seated hydrothermal systems that deposited the metals in the Butte deposit were driven by intrusions that are now beneath the Deer Lodge Valley. This study was supported in part by a grant from the U.S. National Science Foundation.
Millennial slip rates along the eastern Kunlun fault: Implications for the dynamics of intracontinental deformation in Asia
Nathan Ward Harkins, ExxonMobil Upstream Research Co., 3120 Buffalo Speedway, Houston TX 77098, USA. Pages 247-266.
Harkins et al. use fault-offset Quaternary landforms to document geologic slip rates at four new locations along the easternmost portion of the Kunlun fault, a major Tibetan strike-slip structure in northeastern Tibet. The authors combine these new slip-rate determinations with extant estimates of fault displacement rates to provide a detailed description of an eastward decreasing gradient in slip-rates along this portion of the fault. Slip-rate constraints are considered on other, nearby active fault zones in an analysis of regional kinematics. In the context of these observations, the authors discuss the implications that the slip-rate gradient along the Kunlun fault holds for the geodynamics of the Tibetan Plateau. Notably, the apparent termination of the Kunlun fault internal to the plateau topographic margin implies that this fault zone does not accommodate the eastward extrusion of Tibetan crust, as has often been interpreted by previous workers. Instead, the authors take the slip-rate gradient to reflect either the ongoing eastward propagation of the fault zone or strain accommodation via distributed crustal shortening and/or shear.
Age, structural setting, and exhumation of the Liverpool Land eclogite terrane, East Greenland Caledonides
Lars Eivind Augland, University of Oslo, Geosciences, Sem Slands vei 1, Oslo, Oslo 0371, Norway. Pages 267-286.
During the continent-continent collision between the Baltican plate (Scandinavia, Finland, and western Russia) and the Laurentian plate (North America and Greenland) 400 million years ago, a large mountain belt (about the size of today’s Himalaya) was formed and the crust was considerably thickened. During this thickening event, some rocks were subjected to very high pressures by burial to more than 50-km depth (some rocks to even more than a 100 km). Because they resided at such immense depths, they were transformed into very dense rocks called eclogites. Some of these rocks can now be found at Earth’s surface. It is important to understand how and why these deeply buried rocks made it up to the surface because that can tell us what processes are active at depth in mountain belts as well as what processes may be active today. This paper provides an explanation for how some eclogites in Liverpool Land, East Greenland (Laurentia), were transported from a depth of more than 50 km to the surface (exhumation). To understand how the eclogites in East Greenland made their journey, Lars Eivind Augland of the University of Oslo and colleagues looked at structures in the rocks that retain the history of different types of displacements the rocks have experienced along the way. By dating rocks that have experienced different displacements at different depths and times they have been able to reconstruct the path these eclogites took on their way toward the surface of Earth. One of the surprising discoveries from the Liverpool Land eclogites was that these rocks were brought closer to the surface at the same time as they experienced subhorizontal shortening. Usually during mountain building contraction leads to thicker crust and burial of rocks, but in this case contraction at a deep level in the crust was accompanied by extension and thinning at a higher level in the crust (closer to the surface). This apparently unusual behavior in the crust provides an explanation for how deeply buried rocks can get back to the surface when there is still crustal thickening occurring. These results from the ancient Caledonian mountain chain may give important information on what is happening at large depths in mountain belts that are active today (e. g. Himalaya).
Paleoseismologic evidence for multiple Holocene earthquakes on the Calico fault: Implications for earthquake clustering in the Eastern California shear zone
Plamen N. Ganev, Univ. of Southern California, Dept. of Earth Sciences, 3651 Trousdale Parkway, Los Angeles, CA 90089, USA. Pages 287-298.
Paleoseismologic data from trenches excavated across the Calico fault in the Eastern California shear zone (ECSZ) reveal evidence for four surface ruptures during the past ~9,000 years. Twelve optically stimulated luminescence dates constrain the timing of these surface ruptures to 0.6-2.0 ka, 5.0-5.6 ka, 5.6-6.1 (or possibly 7.3) ka, and 6.1 (or 7.3)-8.4 ka. Geomorphologic mapping of the 8 km section of the fault extending southward from the trenches reveals two sets of displacements which record the slip in the past two or three surface ruptures. The slip caused by the most recent event was ~2.0 m, while the cumulative slip during the penultimate (and possibly the ante-penultimate) event was ~4.5 m. The ages of the paleo-earthquakes coincide with periods of clustered moment release identified previously on other faults in the Eastern California shear zone at 0-1.5 ka, 5-6 ka, and ~8-9.5 ka, with two Calico fault surface ruptures occurring during the 5-6 ka ECSZ cluster. These data strongly reinforce earlier suggestions that earthquake recurrence in ECSZ is highly clustered in time and space. Such seismic clustering suggests that at least some regional fault networks undergo distinct periods of system-wide accelerated seismic moment release that may be driven by feedbacks between fault-loading rate and earthquake activity.
