Boulder, CO, USA - New science published in GSA Bulletin includes evidence from Ellesmere Island that the end-Permian mass extinction may not have been a synchronous, global event; an understanding of how weak faults are formed by weak minerals; a study of the Mull Granite of northwest Scotland; and a proposed revision of 30-year-old model of geomorphic response to climate change based on observations from the hyperarid Nahal Yael watershed in the southern Negev Desert, Israel.
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Evidence for a diachronous Late Permian marine crisis from the Canadian Arctic region
Thomas Algeo et al., Department of Geology, University of Cincinnati, Cincinnati, OH 45221-0013, USA; doi: 10.1130/B30505.1.
Thomas Algeo of the University of Cincinnati and colleagues report on a recently completed high-resolution study of the West Blind Fiord section on Ellesmere Island in the Canadian Arctic. The data indicate that the end-Permian mass extinction, during which ~90% of all marine species disappeared, may not have been a synchronous event at a global scale. Major changes in environmental conditions and the loss of most marine organisms occurred at least 100 thousand years earlier in the boreal Sverdrup Basin, in which the study section is located, than in the equatorial Tethys Ocean, where most earlier studies of the end-Permian mass extinction have been undertaken. Algeo and colleagues hypothesize that early silicic eruptions of the Siberian Traps, the probable cause of the extinction crisis, affected high northern-hemisphere regions first. This study was supported in part by a grant from the U.S. National Science Foundation.
Cyclostratigraphic calibration of the Frasnian (Late Devonian) time-scale (western Alberta, Canada)
David De Vleeschouwer et al., Earth System Sciences and Department of Geology, Vrije Universiteit Brussel, Pleinlaan 2, B -1050 Brussels, Belgium; doi: 10.1130/B30547.1.
Life on Earth underwent major changes during the Devonian: The first fishes crawled out of the water to become four-legged vertebrates. For the first time in Earth's history, plants were growing on land and forests appeared. The Late Devonian mass-extinction event caused the disappearance of three quarters of all species on Earth. In order to understand life on Earth as we know it today, it is most important to thoroughly understand when, and how fast, these changes took place. Unfortunately, the Devonian is currently poorly constrained by absolute ages. In this study, David De Vleeschouwer of Vrije Universiteit in Brussels and colleagues estimated the duration of the Frasnian, the geological interval on the eve of the great Late-Devonian mass-extinction. Through the identification of sixteen rhythmic climate changes, each 405 thousand years long, the duration of the Frasnian was calculated to be 6.5 million years. The observed climate changes are caused by rhythmical changes in the shape of Earth's orbit around the Sun, determining the Earth-Sun distance over the different seasons. Since solar energy is the main driver of Earth's climate system, these rhythms can be traced in the climate record and can be used to better understand the rate at which our distant ancestors evolved.
Localization of deformation triggered by chemo-mechanical feedback processes
Bernhard Grasemann, Dept. for Geodynamics and Sedimentology, University of Vienna, Althanstrasse 14, 1090 Vienna, Austria; and Cornelius Tschegg; doi: 10.1130/B30504.1.
Many natural geologic faults that have accommodated a significant amount of displacement contain very weak minerals and hence are termed "weak faults." Several processes have been described that can significantly weaken tectonic fault zones on a large-scale. Besides the reduction of grain-size, due to crushing of the rocks within the fault, or the injection of fluids into the fault zone, such processes include chemical reactions that alter existing strong minerals into weaker minerals. This paper, by Bernhard Grasemann and Cornelius Tschegg of the University of Vienna, presents an excellent example of such chemical weakening, caused by complex chemo-mechanical feedback processes, leading to the formation of a crustal-scale extensional fault on the island of Serifos (Western Cyclades, Greece). The newly described process involves the injection of a water-rich fluid into dolomitic marbles containing relatively thin quartz layers. This triggered the formation of talc, the softest known mineral, at the boundary of the quartz and marble, facilitating the separation of the quartz layers into discrete blocks (a process called boudinage). This increased the permeability of the rock, allowing an increased influx of water and a massive talc formation, which significantly weakened the rock. This enabled the observed fault to form, with displacement strongly localized in the very weak, talc-rich layers.
Controls on emplacement of the Caledonian Ross of Mull Granite, NW Scotland: Anisotropy of magnetic susceptibility, magmatic, and regional structures
M.S. Petronis et al., Environmental Geology, Natural Resource Management Department, New Mexico Highlands University, PO Box 9000, Las Vegas, New Mexico 87701, USA; doi: 10.1130/B30362.1.
Rock fabric data was measured across the exposed portion of the Caledonian Ross of Mull Granite, Argyllshire, northwest Scotland to investigate the internal architecture of the pluton. Field and petrographic observations support the rock fabric study and reveal a fabric that records a partial tectonic overprint of an emplacement-related magma inflow fabric. The partially preserved inflow pattern indicates a south-to-north emplacement of subhorizontal sheets that coalesced to form a tabular pluton. In the southern-most part of the granite, rock-fabric data are parallel to bedding cleavage orientations preserved in many large (more than 100 meters) Precambrian Moine blocks. The scarcity of Moine blocks in the northern part of the intrusion and the prevalence of randomly oriented stoped blocks suggests that this part of the intrusion is nearer to the ceiling of the pluton. Authors M.S. Petronis of New Mexico Highlands University and colleagues argue that emplacement was not associated with eastward orogenic collapse of the Scandian Moine nappes as proposed by previous workers, but occurred either before orogenic collapse or during a period of compressional reactivation.
Fault-mediated melt ascent in a Neoproterozoic continental flood basalt province, the Franklin sills, Victoria Island, Canada
J.H. Bédard et al., Geological Survey of Canada (GSC-Québec), 490 de la Couronne, Québec, PQ G1K 9A9, Canada; doi: 10.1130/B30450.1.
Large igneous provinces represent immense outpourings of basaltic lava. The Franklin sills of Victoria Island represent the root zones of the Natkusiak flood basalts, a 720-million-year-old Large igneous province. In this study by J.H. Bédard of the Geological Survey of Canada and colleagues, superb preservation of the large igneous province allow magma ascent mechanisms to be understood, and imply that many up-section jogs are related to preexisting faults.
Late Quaternary weathering, erosion, and deposition in Nahal Yael, Israel: An "impact of climatic change on an arid watershed"
Yehouda Enzel et al., The Fredy and Nadine Herrmann Institute of Earth Sciences, The Hebrew University of Jerusalem, Edmond J. Safra Campus, Givat Ram, Jerusalem 91904, Israel; doi: 10.1130/B30538.1.
Yehouda Enzel of The Hebrew University of Jerusalem and colleagues examine a 30-year-old model for the geomorphic response to climate change based on observations from the hyperarid Nahal Yael watershed in the southern Negev Desert, Israel. The change from a semiarid glacial time to extremely arid climates about ten-thousand years ago (ka), were proposed to force vegetation cover reduction that, in turn, increased sediment yield from slopes, and accelerated aggradation of terraces and alluvial fans. New observations and data acquired in Nahal Yael over the 30 years since the original model (a) challenge the occurrence and magnitude of the proposed climate change; hyperarid climates control the region for long time, including in glacial times; and (b) indicate a major 35-20 ka episode of accelerated sediment production on slopes due to increased frequency of wetting-drying cycles by frequent extreme storms and floods between 35-27 ka.; i.e., without lag time, these sediments were formed, transported, and aggraded in depositional landscape components (fluvial terraces and alluvial fans). This intensified sediment production and delivery phase is unrelated to a clear climate transition. The depositional landforms were then rapidly incised between 20 and 18 ka. Since and/or soon after this incision, most material leaving the basin comes from sediments stored in depositional landforms produced earlier and not from bedrock weathering. Enzel and colleagues propose a revision to the earlier model in this and other hyperarid environment. Their revision suggests that the model should include frequent storms and floods responsible for a pulse of intense weathering due to numerous cycles of wetting and drying on slopes and coeval sediment transport to fluvial terraces and alluvial fans. Enzel and colleagues also discuss the common misuse of the model, usually without sufficient data on basin-specific stratigraphic, chronologic, paleoenvironmental, and paleoclimatic information.
Journal
Geological Society of America Bulletin