Boulder, Colo., USA - The February 2014 Lithosphere is now online. Papers cover strain rates measured in travertine in the Rio Grande rift, central New Mexico; age dating of the granitic Neoarchean Rum Jungle complex, Australia; the first lithosphere-scale illustration of the structural evolution of the Black Sea Basin; geodynamic modeling of craton formation; evaluation of the central Garlock fault in Pilot Knob Valley, California; and analysis of Anza network-USArray station seismic records of San Jacinto fault (California) activity.
Abstracts are online at http://lithosphere.
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Quaternary extension in the Rio Grande rift at elevated strain rates recorded in travertine deposits, central New Mexico
Jason W. Ricketts et al., University of New Mexico, Earth and Planetary Sciences, Albuquerque, New Mexico 87131, USA. February 2014 issue, http://dx.
This paper documents persistent E-W regional extension through the Quaternary in the Rio Grande rift that bridges geologic, paleoseismic, and GPS rates. Anomalously high strain rates in the Quaternary were facilitated by ascent of travertine-depositing CO2-rich waters along rift-bounding normal faults, leading to locally very high stain accumulations. These sites also provide examples of natural leakage of deeply sourced CO2 interacting with regional tectonism, and they emphasize that rift maturation is a highly dynamic process, both spatially and temporally.
Low δ18O zircon grains in the Neoarchean Rum Jungle Complex, northern Australia: An indicator of emergent continental crust
J.A. Hollis et al., Northern Territory Geological Survey, P.O. Box 3000, Darwin, NT 0800, Australia. February 2014 issue, http://dx.
This paper discusses the timing of widespread continental emergence and its generally considered dramatic effect on the hydrological cycle, atmospheric conditions, and climate. It presents new secondary ion mass spectrometry (SIMS) oxygen and laser-ablation-multicollector-inductively coupled plasma-mass spectrometry (LA-MC-ICP-MS) Lu-Hf isotopic results from dated zircon grains in the granitic Neoarchean Rum Jungle complex provide a minimum time constraint on the emergence of continental crust above sea level for the North Australian craton.
Lithospheric structural control on inversion of the southern margin of the Black Sea Basin, Central Pontides, Turkey
Nicolas Espurt et al., Aix-Marseille Université, CNRS, IRD, CEREGE UM34, 13545, Aix-en-Provence, France. February 2014 issue, http://dx.
To illustrate the structural evolution of the Black Sea Basin in the context of Neotethyan subduction and subsequent continental collisions, this paper presents the first lithosphere-scale, ~250-km-long, balanced and restored cross section across its southern continental margin, the Central Pontides. Cross-section construction and restoration are based on field, seismic-reflection, geophysical, and apatite fission-track data. Data propose that the location of Cenozoic contractional deformation is related to the absence of lithospheric mantle below the southern Pontides (forearc) zone as a consequence of the Cretaceous high-pressure wedge exhumation.
Craton formation: Internal structure inherited from closing of the early oceans
C.M. Cooper, School of the Environment, Washington State University, PO Box 642812, Pullman, Washington 99164-2812, USA; and M.S. Miller, Department of Earth Sciences, University of Southern California, 3651 Trousdale Parkway, Los Angeles, California 90089, USA. February 2014 issue, http://dx.
This paper presents geodynamic models demonstrating the internal structures produced during the formation of cratonic lithosphere as well as new seismological observations of mid-lithospheric discontinuities in the West African craton, together with reassessment of mid-lithospheric discontinuities observed in the North American, South African, Fennoscandia, and Australian cratons. It suggests that the mid-lithospheric discontinuities observed in these cratons could be remnants of deformation structures produced during the formation of the cratons after ancient oceans closed.
Temporal variations in Holocene slip rate along the central Garlock fault, Pilot Knob Valley, California
William M. Rittase et al., University of Kansas, Department of Geology, Lawrence, Kansas 66045, USA. February 2014 issue, http://dx.
Average geologic slip rates along the central Garlock fault, in eastern California, are thought to have been relatively steady at 5 to 7 mm/yr since at least the Late Pleistocene, yet present-day rates inferred from geodetic velocity fields are indistinguishable from zero. This paper evaluates the possibility of non-steady slip over millennial timescales using displaced Late Holocene alluvium along the central Garlock fault in Pilot Knob Valley. Truncation of a Late Holocene alluvial fan deposit against a shutter ridge requires a minimum of 30 to 37 m of displacement since deposition of the fan; maximum allowable displacement is 43 m. The extent of soil development atop the fan surface and optically stimulated luminescence ages bracket fan deposition between 3.5 and 4.5 thousand years ago (ka). Together, these data require that slip rates during the Late Holocene were ~7 to 14 mm/yr, with a preferred rate of ~11 to 13 mm/yr. These results, in conjunction with previous estimates of displacement over the past ~15 ka, require significant temporal variations in strain release along the Garlock fault and confirm previous suggestions that interactions among fault systems in eastern California give rise to alternating periods of fault activity and quiescence.
Moho structure across the San Jacinto fault zone: Insights into strain localization at depth
Meghan S. Miller et al., Department of Earth Sciences, University of Southern California, 3651 Trousdale Parkway, MC740, Los Angeles, California 90089, USA. February 2014 issue, http://dx.
Ten years of teleseismic earthquakes recorded by broadband seismic instruments from the Anza network-USArray stations around the San Jacinto fault were used to create P receiver function images of the lithospheric structure beneath this major strike-slip fault. Analyses of back-azimuthal variation and location of the conversion points near the fault suggest an ~8 km vertical offset of the Moho directly beneath the San Jacinto fault. This implies that the fault extends through the entire crust and into the mantle lithosphere, supporting the idea that the strain in the lower crust is localized within a narrow zone. The Moho offset and surface trace of the San Jacinto fault zone are coincident with a compositional boundary in the Peninsular Ranges batholith previously identified in potential field geophysical data and Sr isotope analyses. The position of the offset with respect to this relict geologic feature, which predates the pluton emplacement that formed the batholith, may be a controlling factor in strain location and plate-boundary fault initiation.