Boulder, Colo., USA – New Geosphere contributions include two additions to the "Origin and Evolution of the Sierra Nevada and Walker Lane" special series. Four articles focus on western North America, while a fifth discusses faulting in the northern Mergui Basin of the Andaman Sea. Authors hale from the University of Arizona, the China Earthquake Administration, the U.S. Geological Survey, Dennison University, and PTTEP ENCO of Bangkok, Thailand.
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Detrital zircon U-Pb geochronology and Hf isotope geochemistry of Paleozoic and Triassic passive margin strata of western North America
George Gehrels, Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA, ggehrels@gmail.com; and Mark Pecha. Geosphere published 19 December 2013, http://dx.doi.org/10.1130/GES00889.1
U-Pb geochronologic and Hf isotopic analyses have been conducted on detrital zircons extracted from 36 samples of Neoproterozoic through Triassic passive margin strata from western North America. The data serve as an improved reference for comparison with inboard strata that accumulated on the North American craton and outboard strata belonging to potentially displaced Cordilleran terranes. As expected, this reference documents significant variations in ages and Hf isotope compositions both north-south and also through time. The data also provide insights into the provenance of Cordilleran passive margin strata. During Neoproterozoic, Cambrian, and Early to Middle Devonian time, most grains were shed from relatively local basement rocks and from Mesoproterozoic clastic strata containing 1.2 to 1.0 billion year old grains that originated in the Grenville orogen. This pattern was interrupted during Ordovician time, when much of the Cordilleran margin was blanketed by detritus shed from the northern Canadian Shield. Beginning in Late Devonian time, and continuing through late Paleozoic and Triassic time, most regions were dominated by locally derived detritus (largely recycled from underlying strata), but also received 0.7 to 0.4 billion year old grains that were shed from the Franklinian, Caledonian, Appalachian, and Ouachita-Marathon orogens. This pattern is complicated in southern transects as a result of mid-Paleozoic emplacement of off-shelf assemblages onto the continental margin (e.g., Antler orogeny) and construction of Permo-Triassic magmatic arcs along the margin. Our data also provide a robust record of the crustal evolution of western North America, with significant production of juvenile crust during late Archean and Paleoproterozoic time and phases of mainly crustal reworking. This history is somewhat different from that of other continents.
Strike-slip faulting along the Wassuk Range of the northern Walker Lane, Nevada
Shaopeng Dong et al., Key Laboratory of Active Tectonics and Volcanoes, Institute of Geology, China Earthquake Administration, Beijing 100029, China, dshaopeng@gmail.com. Geosphere published 19 December 2013, http://dx.doi.org/10.1130/GES00912.1.
A strike-slip fault is present outboard and subparallel to the Wassuk Range front within the central Walker Lane (Nevada, USA). The strike-slip fault trace projects southeastward toward the eastern margin of Walker Lake, which is ~15 km to the southeast. The trace is obscured in this region by recessional shorelines features that record the historical desiccation of the lake caused by upstream water diversion and consumption. The pattern and trend of folding and faulting beneath the lake are not simply explained; they may record development of Riedel shears in a zone of northwest-directed strike slip. These observations begin to reconcile what was a mismatch between geodetically predicted deformation rates and geological fault slip rate studies along the Wassuk Range front, and provide another example of strain partitioning between predominantly normal and strike-slip faults that occurs in regions of oblique extension such as the Walker Lane.
Petrologic, tectonic, and metallogenic evolution of the southern segment of the ancestral Cascades magmatic arc, California and Nevada
Edward A. du Bray et al., U.S. Geological Survey, MS973, Box 25046, Denver Federal Center, Lakewood, Colorado 80225, USA, edubray@usgs.gov. Geosphere published 19 December 2013, http://dx.doi.org/10.1130/GES00944.1.
Understanding the time-space-compositional evolution of magmatic arc rocks in western North America is required in order to clarify the geologic evolution of this region and to understand the character and distribution of mineral deposits associated with these rocks. Ongoing volcanic arc magmatism in western North America was preceded by ancestral arc magmatism that began about 45 million years ago and evolved into modern arc volcanism. The southern segment of this ancestral arc, active from about 30 to 3 million years ago, adjoins the northern segment in northern California across a crustal discontinuity. Products of the southern arc were erupted from stratovolcanoes and lava dome complexes arrayed along the crest of the ancestral arc. Southern arc segment rocks include potassic, intermediate- to silicic composition lava flows, lava dome complexes, and associated sedimentary deposits. Compositional distinctions between the northern and southern arc segment rocks reflect the nature and thickness of the crust beneath which the associated magma systems developed. Northern segment rock compositions are consistent with magma generation beneath thin, primitive crust, whereas southern segment rocks represent magmas generated and evolved beneath thicker, more evolved crust. Although igneous rocks in the two arc segments have similar metal abundances, the types of associated mineral deposits are distinct. Small copper deposits are characteristic of the northern segment, whereas significant precious metal deposits are most commonly associated with southern arc segment eruptive centers. These deposit-type differences are also fundamentally linked to the crustal regimes within which these two arc segments evolved.
Magmatic lulls in the Sierra Nevada captured in zircon from rhyolite of the Mineral King pendant, California
Erik W. Klemetti et al., Department of Geosciences, Denison University, Granville, Ohio 43023, USA Geosphere published 19 December 2013, http://dx.doi.org/10.1130/GES00920.1,
The Mineral King pendant in the Sierra Nevada batholith (California, USA) contains at least four rhyolite units that record high-silica volcanism during magmatic lulls in the Sierran magmatic arc. U-Th-Pb, trace element (single crystal spot analyses via sensitive high-resolution ion microprobe–reverse geometry, SHRIMP-RG), and bulk oxygen isotope analyses of zircon from these units provide a record of the age and compositional properties of the magmas that is not available from whole-rock analysis because of intense hydrothermal alteration of the pendant. Findings in this study likely reflect the transition of the North America margin from one of docking island arcs in the Early Jurassic to one of a more mature continental arc in the Early Cretaceous. This also shows the utility in examining zircon spot ages combined with trace element and bulk isotopic composition to unlock the petrogenetic history of altered volcanic rocks.
Pull-apart development at overlapping fault tips: Oblique rifting of a Cenozoic continental margin, northern Mergui Basin, Andaman Sea
K. Srisuriyon, PTTEP, Enco, Vibhavadi-Rangsit Road, Soi 11, Bangkok, Thailand, 10900, chrissmorley@gmail.com; and C.K. Morley. Geosphere published 19 December 2013, http://dx.doi.org/10.1130/GES00926.1
The northern Mergui Basin (Andaman Sea) contains ENE-WSW to NE-SW striking normal fault-bound basins, and NNW-SSE trending strike-slip faults. The two largest strike-slip faults (Manora and Mergui) pass into extensional or transtensional basins at their tips, consistent with dextral offset. The faults provide examples of early stage pull-apart basin development at fault tips instead of the more common model for development at releasing bends. Offset of isochron markers for the Ranong Formation indicate that ~8 km of dextral offset has occurred along the Mergui fault and 4.5 km of dextral offset has occurred on the Manora fault. The strike-slip faults and associated extensional faults formed relatively late for the history of the entire Mergui Basin during the Early Miocene. The northern part of the Mergui Basin developed after a phase of WNW-ESE extension during the Oligocene in the Mergui Basin to the south, indicating a rotation in the extension direction toward the north-northwest–south-southeast with time. The basin is part of a major transtensional system involving the Sumatra, West Andaman, and Sagaing faults that accommodated the northern motion of western Myanmar as India moved north relative to Southeast Asia. Fault activity in the northern Mergui Basin decreased significantly when the broad zone of Early Miocene transtension became focused on the Alcock and Sewell Rises during the Middle Miocene, and the West Andaman and Sagaing faults began to develop and interacted in a large pull-apart geometry with the Shan Scarp Fault, and later (Late Miocene or Pliocene) with the Sagaing Fault.
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Geosphere