|18 March 2014
GSA Release No. 14-19
Detail of the Ediacaran trace fossil Archaeonassa from Sete Lagoas Formation, Bambuí Group, central-eastern Brazil. See related article by Warren et al., http://dx.doi.org/10.1130/G35304.1.
New from Geology: Fossils, Earthquakes, Gold, and Sea-Bed Landslides
Articles published online ahead of print 17 Mar. 2014
Boulder, Colorado, USA – Geology papers published 17 Mar. 2014 cover (1) modeling of seabed turbidity currents; (2) a large earthquake at Lake Vättern, Switzerland, about 11,500 years ago; (3) genesis of high-grade gold at the Porgera gold deposit, Papua New Guinea; (4) discovery of the Ediacaran guide fossil Cloudina sp. and the depositional age of the Bambuí Group; (5) earthquakes along the fossil Moho in Alpine Corsica; and (6) using LiDAR to better understand New Zealand's Alpine Fault.
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Processes and products of turbidity currents entering soft muddy substrates
Jaco H. Baas et al., School of Ocean Sciences, Bangor University, Menai Bridge, Ynys Môn LL59 5AB, UK. Published online 17 Mar. 2014, http://dx.doi.org/10.1130/G35296.1.
New laboratory experiments reveal that cohesionless underwater landslides, called turbidity currents, are able to enter cohesive, "sticky," muddy seabeds without losing their shape. These intrabed currents are driven by bed shear stress exceeding bed cohesive strength and by flow density exceeding bed density. The flows produce unique deposits, called turbidites, with internal mud layers, mixed cohesive-noncohesive sediment layers, and various soft-sediment deformation structures. A depositional model for intrabed turbidites is proposed, comprising from base to top: (I1) sand-bearing mud, with an eroded base, dispersed mud, and mud clasts; (I2) muddy sand from the intrabed portion of the turbidity current; (I3) sandy mud with a speckled appearance; and (I4) mud-poor sand from the suprabed portion of the flow. Complete I1–I4 turbidites are inferred to dominate locations in nature where the currents mix with the bed and deep erosional scours form, filled with deformed or chaotic sand-mud mixtures. Further downflow, base-missing I2-I4 and I4 sequences signify gradual deceleration, loss of erosivity and termination of intrabed flow.
Major earthquake at the Pleistocene-Holocene transition in Lake Vättern, southern Sweden
Martin Jakobsson et al., Department of Geological Sciences, Stockholm University, 106 91 Stockholm, Sweden. Published online 17 Mar. 2014, http://dx.doi.org/10.1130/G35499.1. Open Access.
Lake Vättern is located in southern Sweden and is the second largest lake in the country. Geophysical mapping and geological coring show striking deformation structures in the soft sediment accumulated on the lake floor. The bottom sediments have drastically been bent downwards in response to vertical tectonic displacements below the lake floor. The vertical displacements reach in places as much as 13 m. The new geological and geophysical data suggest that an earthquake took place in Lake Vättern with an estimated seismic moment magnitudes of about 7.5. Analysis of pollen in sediment infilling of the mapped collapse structures suggest that this major seismic event occurred at about 11,500 years before present. Authors Martin Jakobsson and colleagues suggest that this seismic event is related to the rapid release of ice-sheet load of the area following the deglaciation.
Linking high-grade gold mineralization to earthquake-induced fault-valve processes in the Porgera gold deposit, Papua New Guinea
Eleanor C. Peterson and John A. Mavrogenes, Research School of Earth Sciences, Australian National University, Canberra, Australian Capital Territory 0200, Australia. Published online 17 Mar. 2014, http://dx.doi.org/10.1130/G35286.1.
Previous studies indicate that earthquake induced ruptures and associated "fault-valve" processes contribute to gold mineralization in some hydrothermal systems, though a comprehensive geochemical record of these processes has been lacking. The Porgera Gold Deposit (Papua New Guinea) provides an opportunity to examine the complex processes surrounding high-grade gold mineralization in structurally controlled hydrothermal systems. New high-resolution trace element and sulfur isotope data obtained by in situ analysis of pyrite from Porgera reveal a temporal evolution in fluid chemistry. The data uncover repeated geochemically distinct zones within individual pyrite crystals present in the highest grade gold event, with fluctuations observed on the micron scale. Results provide the first clear geochemical documentation of rapid, complete switching in ore-forming processes, preserved in individual pyrite crystals throughout an evolving hydrothermal system. It is postulated that geochemical variations are attributable to rapid gold deposition during pressure release due to fault failure followed by switching to more metal-poor fluids from adjacent sediments as the rupture seals. This study demonstrates the usefulness of applying contemporary analytical techniques to pyrite in hydrothermal systems to gain new insights into ore genesis; revealing that deposition of high-grade ore at Porgera resulted from multiple events in a fluctuating ore-forming environment.
The puzzle assembled: Ediacaran guide fossil Cloudina reveals an old proto-Gondwana seaway
L.V. Warren et al., Departamento de Geologia Aplicada, Instituto de Geociências e Ciências Exatas, Universidade Estadual Paulista, Avenida 24A, 1515, Rio Claro 13506-900, Brazil. Published online 17 Mar. 2014, http://dx.doi.org/10.1130/G35304.1.
During the Ediacaran period, the Clymene ocean separated the Laurentia to the northeast, Amazonia and Río Apa cratons to the southeast, and several landmasses to the west, forming the Proto-Gondwana supercontinent. However, no clear evidence about the existence of Ediacaran epeiric seas over those landmasses has been found so far. In this paper, L.V. Warren and colleagues report and discuss the discovery of the Ediacaran guide fossil Cloudina sp. associated with other metazoan body and trace fossils in the Bambuí Group (central-eastern Brazil). This new finding helps to solve an old geologic puzzle of the Gondwana supercontinent -- the depositional age of the Bambuí Group. The paleogeographic position of Cloudina-bearing successions in Brazil, Antarctica, Namibia, and Argentina suggest a new scenario of ocean connectivity among coeval intracratonic basins of South America, Africa, and Antarctica at the end of Neoproterozoic.
Large subduction earthquakes along the fossil Moho in Alpine Corsica
T.B. Andersen et al., Centre of Earth Evolution and Dynamics, Department of Geosciences, University of Oslo, P.O. Box 1047, Blindern 0316 Oslo, Norway. Published online 17 Mar. 2014, http://dx.doi.org/10.1130/G35345.1.
This paper reports the discovery of rocks formed by large earthquakes along the crust-mantle boundary (moho) in an ancient ocean basin of Alpine Corsica. The Piemonte-Ligura basin closed by subduction during the Alpine orogeny, and the earthquakes took place during the subduction. The exhumed earthquake rocks formed at depths of approximately 50 km and are known as intermediate depth earthquakes, common in present-day subduction zones. The shear heating that developed on the faults resulted in melting of several tons of the mantle peridotite per square meter along the fault zone. The quenched melts (pseudotachylyte) show that the faulting was rapid and associated with earthquakes. The discoveries of exhumed subduction earthquakes allow T.B. Anderson of the University of Oslo and colleagues to study phenomena associated with the abrupt energy release associated with the earthquake faulting. These observations enable interpretations based on the direct study from individual mineral grains to the outcrop scale of the mechanism(s) that triggers earthquake faulting at such depths in subduction zones. Based on their observations of the deformation products of the large earthquakes, they suggest that they were triggered by a self-localizing shear heating process rather than by weakening related to the release of free water in the rocks by dehydration of water-bearing minerals.
LiDAR reveals uniform Alpine fault offsets and bimodal plate boundary rupture behavior, New Zealand
Gregory P. De Pascale et al., Department of Geological Sciences, University of Canterbury, Christchurch 8140, New Zealand. Published online 17 Mar. 2014, http://dx.doi.org/10.1130/G35100.1.
Understanding the behavior of plate boundary faults and the recurrence of major earthquakes along these faults is critical for understanding seismic hazards. Using light detection and ranging (LiDAR) data combined with field mapping, the authors from the University of Canterbury, New Zealand, measured tectonic offsets along surface ruptures in the dense rainforest of New Zealand’s Plate Boundary Alpine Fault. They found evidence for repeated 7-m increments of dextral slip during each of the last three Alpine Fault earthquakes. By comparing these slip increments with the slip-rate and their newly compiled approximately one-thousand-year Alpine Fault paleoseismic and offset records, the authors provide evidence for at least two modes of plate boundary fault behavior. One mode, where the fault has full ruptures, that is long surface ruptures greater than 300 km, and large to great earthquakes, and a second mode with partial ruptures that are perhaps width-limited (and smaller magnitude but still damaging earthquakes). These combined data suggest that plate boundary faults may not behave as characteristically as previous models suggest and indicates a higher frequency of Alpine Fault earthquakes while also expressing caution about attributing off-fault "shaking" paleoseismic data to any specific fault in a plate boundary setting.