Boulder, Colo., USA - Tubular microtunnels believed to be the trace fossils formed by microbes inhabiting volcanic rock interiors have only been reported in oceanic and subglacial settings. This is the first observation of such features in basaltic volcanic glass erupted in a continental lake environment, the Fort Rock volcanic field.
As a result, the record of subsurface microbial activity in the form of endolithic microborings is prospectively expanded. Our understanding of the range of environments and conditions that microtunnels can form in is enhanced along with our knowledge of potentially habitable environments on Earth and beyond.
The Fort Rock volcanic field has analogous characteristics to locations found on Mars such as Gale and Gusev crater. The presence of these features in this new geologic setting may suggest that subsurface microbes or evidence thereof, if present on Mars, could exist nearer to the surface than previously thought. This knowledge can thus aid future Mars missions (e.g., Mars 2020 Project) with goals that include searching out biosignatures and finding suitable rocks for sample return.
Candidate microbial ichnofossils in continental basaltic tuffs of central Oregon, USA: Expanding the record of endolithic microborings
M.P.C. Nikitczuk et al., Department of Earth Sciences, Brock University, 1812 Sir Isaac Brock Way, St. Catharines, Ontario L2S 3A1, Canada. This article is Open Access online at http://gsabulletin.
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Coseismic horizontal slip revealed by sheared clastic dikes in the Dead Sea basin
R. Weinberger et al., Geological survey of Israel, Jerusalem, Israel. This article is online at http://gsabulletin.
Destructive earthquakes are commonly associated with near-surface horizontal shear. However, field evidence for horizontal slip is exceptionally rare, mainly due to inadequate exposures and markers. Within the seismically active Dead Sea basin, such horizontal hear is evidenced by vertical clayey-sandy dikes that were sheared and laterally displaced along horizontal slip surfaces. We show that this slip simultaneously occurred along eleven horizontal slip surfaces, traced for tens of meters. Field observations suggest that this slip formed by simple shear, akin to shuffling a horizontal deck of cards. The exceptional quality of exposures and markers helped us to document, for the first time, the details of this near-surface horizontal shear. Our data indicates that in seismic areas displacement long bedding planes is a viable deformation mechanism to absorb seismic deformation near surface strata. The implications of this work should extend to other seismically active zones around the globe.
Miocene global change recorded in Columbia River basalt-hosted paleosols
K.M. Hobbs, Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131, USA; and J.T. Parrish, Department of Geological Sciences, University of Idaho, Moscow, Idaho 83844, USA; doi.org/10.1130/B31437.1. This article is online at http://gsabulletin.
During the Miocene Epoch, a number of paleosols (ancient soils preserved in the rock record) were preserved between flows of the Columbia River Basalts in Washington, Oregon, and Idaho. We present chemical, mineral, and physical evidence from some of these paleosols to show that the end of the Middle Miocene Climate Optimum (MMCO), a ~2-million-year-long period of elevated global temperatures, had profound effects on the climate conditions in the study area. Soils in the study area that formed during the relative warmth of the MMCO were similar to modern subtropical soils. Study area soils that formed after the MMCO were similar to modern temperate soils. In addition, we combine our data with other literature to show that a strong precipitation gradient existed during the Miocene across the Columbia River Basalt Province.
Mafic rocks of the Ordovician Famatinian magmatic arc (NW Argentina): New insights into the mantle contribution
P.H. Alasino et al., CRILAR-CONICET, Entre Ríos y Mendoza, 5301, Anillaco, La Rioja, Argentina; doi.org/10.1130/B31417.1. This article is online at http://gsabulletin.
The Famatinian arc is a significant part of the Terra Australis accretionary orogen that fringed SW Gondwana in Paleozoic times. Within this, the Ordovician magmatism of the western Sierras Pampeanas of Argentina provides a "world-class" example of a subduction-related magmatic arc, because a thick vertical section of the arc is exposed, enabling study of the competing roles of mantle and crustal sources beneath the arc. Our investigation suggests that initial pyroxenite-derived melts were generated in the sub-continental mantle due to foundering and removal of the lower lithosphere coeval with the major heat-flux event in the arc (represented by voluminous intermediate and silicic magmatism); more primitive melts resulted from subsequent upwelling and decompression melting of the mantle wedge peridotite. We thus assign an important role in the evolution of the arc to tectonically controlled mafic magmatism.
Cap Carbonate Platform Facies Model, Noonday Formation, SE California
J.R. Creveling et al., Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, USA; doi.org/10.1130/B31442.1. This article is online at http://gsabulletin.
635 million year old sedimentary rocks near Death Valley, California, USA, record profound climate change, including evidence for a global glaciation known as a "Snowball Earth" event. The marine carbonate rocks that directly cap these glacial successions typically record deposition during a single interval of sea-level rise driven by "Snowball" melt. We investigated whether the post-glacial sedimentary succession near Death Valley, known as the Noonday Formation, records the same relative sea-level rise. We reconstructed the depositional environments, relative sea level history, and platform morphology of the Noonday Formation. From this analysis, we found that a major sea-level fall interrupted a broader trend of rising post-glacial sea level. In this regard, the Noonday records a more complicated sea-level history than simple deglacial melt. We speculate that Noonday sea-level fall reflects post-glacial rebound -- the slow rise of a continent in response to the melting of a large ice sheet -- that briefly overwhelmed post-glacial melting.
Shortening rate and Holocene surface rupture on the Riasi fault system in the Kashmir Himalaya: Active thrusting within the Northwest Himalayan orogenic wedge
Y. Gavillot et al., College of Earth, Ocean, and Atmospheric Sciences, Oregon State University, Corvallis, USA; doi.org/10.1130/B31281.1. This article is online at http://gsabulletin.
New mapping and geochronologic data in the Kashmir Himalaya of northwest India demonstrates evidence for active emergent thrust faulting within the fold-thrust belt north of the deformation thrust front. Age and structural data from offset fluvial terraces and Quaternary conglomerates reveal a consistent 6-7 mm/yr-shortening rate for the Riasi fault system for the last 100,000 years. Geodetic data indicate 11-12 mm/yr of arc-normal interseismic plate convergence between India-Tibet. Roughly 50% of the convergence is absorbed by slip on the Riasi fault system, while the remaining convergence is accommodated at the thrust front. Given that high strain rates and the last surface-rupturing earthquake occurred at 4.5 ka on the Riasi fault system, our study demonstrates the presence of a large seismic gap for the Kashmir Himalaya. Slip deficits, scaling relationships, and rupture scenario and slip on the plate boundary décollement parsed onto either the Riasi fault system or the thrust front, or both, suggests that great earthquakes (Mw >8) pose an even greater seismic hazard than the Mw 7.6 2005 earthquake in Pakistan Azad Kashmir.
Distortion of carbon isotope excursion in bulk soil 1 organic matter during the Paleocene-Eocene Thermal Maximum
Allison A. Baczynski et al., Department of Geosciences, Pennsylvania State University, Deike Building, University Park, Pennsylvania, 16802, USA; doi.org/10.1130/B31389.1. This article is online at http://gsabulletin.
Global warming during the Paleocene Eocene Thermal Maximum (PETM), ca. 56 Ma, is an important analogue to current climate change because it, too, was driven by a massive release of carbon to the ocean-atmosphere system. Carbon isotope ratios provide critical information on carbon cycling and feedbacks in the Earth system. Comparing stable carbon isotopes of soil organic carbon and fossil leaf waxes across the PETM, we conclude that the isotopic composition of soil organic carbon was modified by both microbial degradation, which doubled, and inputs of fossil carbon, which increased by about half. These processes must be considered when interpreting the isotopic composition of soil organic carbon. The doubling of soil respiration during the PETM was probably an important positive feedback on warming, and the increase in reworked organic matter suggests increased erosion and redeposition in response to climate change. Both of these have important implications for future climate change scenarios.
40Ar/39Ar chronostratigraphy of Late Miocene-Early Pliocene continental aquatic basins in SE Galilee, Israel
A.G. Rozenbaum et al., Geological Survey of Israel, 30 Malkhe Israel, Jerusalem 95501, Israel; doi.org/10.1130/B31239.1. This article is online at http://gsabulletin.
During the late Neogene the circum-Mediterranean area was characterized by the development of marginal water-bodies that were sometimes invaded by the Mediterranean Sea. The current paper focuses on the marginal water bodies that were developed east of the Mediterranean in the south Galilee area, Israel. The history of the lakes is entwined with several marine ingressions that intruded the early Jordan rift valley and deposited thick sequences of halite. Detailed chronology of the lacustrine formations was established by Ar-Ar dating of basaltic flows below, within and above the sedimentary sequence. The chronology reveals that the lower lacustrine formation (Bira) was deposited during the Tortonian period whereas the upper lacustrine formation (Gesher) was deposited during the Messinian period. While most of the Tortonian and the early Messinian periods were characterized by supply of freshwater to the lakes, during the Messinian salinity crisis lake Gesher dried out, marking arid conditions in the region that accompanied the retreat of the Mediterranean Sea.
Tracking exhumation and drainage divide migration of the western Alps: A test of the apatite U-Pb thermochronometer as a detrital provenance tool
C. Mark et al., Department of Geology, Trinity College Dublin, Museum Building, College Green, Dublin 2, Ireland; doi.org/10.1130/B31351.1. This paper is online at http://gsabulletin.
The sedimentary basins surrounding mountain ranges are archives recording the past development of the sediment sources: the mountains themselves. Like any ancient archive, however, the challenge is to decipher the stored information. Because temperature in the Earth's crust varies largely as a function of depth, and because the processes of mountain-building and the resultant erosion act to move rocks vertically within the crust, geologists can use detrital thermochronometers -- temperature-sensitive radioisotope mineral clocks deposited in sediments -- to unravel past geological processes. However, the temperature sensitivity of the chosen thermochronometer is key. Many classically utilized detrital thermochronometers are either over- or under-sensitive to temperatures that typically occur in collisional mountain ranges. In this study, we utilize the apatite and rutile U-Pb mid-temperature thermochronometers to resolve long-standing uncertainties in the sources of sediment deposited in the foreland of the western European Alps, and therefore to unravel the past development of this major mountain range.
Extension facilitated pulsed S-I-A-type "flare-up" magmatism at 370 Ma along the southeast Gondwana margin in New Zealand: insights from U-Pb geochronology and geochemistry
R. Turnbull et al., GNS Science, Dunedin Research Centre, Private Bag 1930, New Zealand; doi.org/10.1130/B31426.1. This article is online at http://gsabulletin.
This article presents new emplacement and inheritance ages for the c. 370 Ma Karamea Suite in New Zealand that provide constraints on the episodicity and dynamics of a continental arc magma reservoir formed under "flare-up" conditions that punctuated the steady-state arc tempo of the active Gondwana margin. Importantly, magma flux-rates of the Karamea Suite are shown to be comparable to those estimated for volcanic flare-up events, indicating that timescales of magma generation and emplacement in continental arc settings are analogous between the volcanic and plutonic realms. An intimate association between S- and A-type granites indicates that the Karamea flare-up event was probably in response to a brief episode of extension and asthenospheric upwelling that facilitated rapid crustal melting and subsequent voluminous granite emplacement.
Rapid exhumation of the eastern Himalayan syntaxis since the Late Miocene
K.A. Lang et al., Department of Earth and Space Sciences and Quaternary Research Center, Johnson Hall Rm. 070, Box 351310, University of Washington, 4000 15th Ave NE, Seattle, Washington, 98195-1310, USA; doi.org/10.1130/B31419.1. This article is online at http://gsabulletin.
The origin of remote eastern Himalayan mountain ranges in the vicinity of Namche Barwa continues to mystify and inspire Himalayan geologists. Our research indicates that the steep, rapidly eroding topography in this area developed over 5 million years ago and likely dates back to the Late Miocene (less than seven million years ago). Observations from sedimentary rocks adjacent to the mountains seemingly support the controversial "Tectonic Aneurysm" hypothesis -- that rapid erosion in the Himalaya may locally enhance rock uplift, sustaining high topography for many millions of years -- with one important exception: erosion of the mountains did not initiate in response to the capture and reorganization or continental-scale river systems. Instead, mountain erosion and development of the spectacular "Tsangpo Gorge" is may be attributable to tectonic uplift of an antecedent river's headwaters.
Detrital zircon and provenance analysis of Late Cretaceous-Miocene onshore Iranian Makran strata: Implications on the tectonic setting
A. Mohammadi et al., Department of Earth Sciences, ETH Zurich, Sonneggstrasse 5, 8092, Zurich, Switzerland; doi.org/10.1130/B31361.1. This article is online at http://gsabulletin.
We present new U-Pb detrital zircon ages with Hf isotope data combined with Heavy mineral analysis and sandstone framework of Late Cretaceous-Miocene turbiditic sandstones from onshore Iranian Makran. The combined U-Pb ages and Hf isotope data indicate that the protolith rocks belonged to a Middle Jurassic intracontinental rift and a Late Cretaceous-Eocene continental arc. Heavy minerals assemblages imply additional sourcing from ophiolite and blueschists. Results refute opinions that the Makran detritus were supplied from Himalayan sources in a Palaeo-Indus submarine fan delta complex. Instead, the Late Cretaceous-Oligocene erosional scree was presumably transported by two rivers in submarine fans from a nearby complex of continental arc and ophiolites to the north into the Makran Basin. The new ages and geochemical data on sandstones in onshore Makran area provide an important addendum to the understanding of the tectonic history of southwest Asia.
Provenance of detrital K-feldspar in the Jianghan Basin sheds new light on the Plio-Pleistocene evolution of the Yangtze River
Z. Zhang et al., School of Earth Sciences, China University of Geosciences, Wuhan 430074, China; doi.org/10.1130/B31445.1. This article is online at http://gsabulletin.
The Yangtze River is the largest fluvial system draining the Tibetan Plateau, yet its time of formation -- pre-Miocene versus Pleistocene -- has been debated for more than a century. There has been a particular focus on the cutting through of the Three Gorges. In this study, the isotopic composition of Pb, which varies regionally, was measured in K-feldspars from Plio-Pleistocene sediments in the Jianghan Basin to identify their source(s). The Jianghan Basin is located just downstream of the Three Gorges and is thus ideally placed to detect any change in sand supply when the gorges were cut through. The results indicate that feldspars from the eastern Tibetan Plateau were already being delivered to this basin by ca. 3.4 Ma, suggesting that the Three Gorges were eroded prior to the late Pliocene. Our study also sheds light on the factors that control the supply of detrital K-feldspar in large-scale drainage systems.