May GSA Bulletin postings take global geology tour

(Press-News.org) Boulder, Colo., USA – GSA Bulletin papers posted online 3-18 May 2012 cover a variety of locations: the Coast Range basalt province, southwest Washington State, USA; the Faroe Islands of the northeast Atlantic margin; Wairarapa fault, North Island, New Zealand; the eastern Mediterranean Sea offshore of southern Crete; the southern central Andes of Argentina; the Adriatic Carbonate Platform of southwest Slovenia; the Atacama Desert, Chile; Questa caldera, northern New Mexico, USA; the Norwegian Caledonides; and Lake Tahoe, USA.

Petrology of the Grays River Volcanics, Southwest Washington: Plume-Influenced Slab Window Magmatism in the Cascadia Forearc
Christine Chan et al., Geology Dept., University of Puget Sound, Tacoma, WA 98416-1048, USA. Posted online 3 May 2012; doi: 10.1130/B30576.1.

The Grays River Volcanics (GRV) are part of the Coast Range basalt province in southwest Washington State. The GRV consist of ~3,500 m of tholeiitic basalt flows and volcaniclastic rocks that erupted in the Cascadia forearc from 37-42 million years ago. Chemical and isotopic data, combined with migration of the location of magmatism through time, indicate that GRV magmatism was related to subduction of a plume-influenced spreading ridge that produced a northward-migrating slab window. Involvement of a mantle plume source is indicated by oceanic island basalt-like incompatible element enrichments and radiogenic lead isotopic compositions. Authors Christine Chan and colleagues note that these lead isotope data are distinct from most Cascade arc rocks and from Cascadia sediment, but overlap with compositions of other Coast Range basalts. Geochemical differences between the GRV and other Cascadia forearc volcanic units that range from about 55 million years old (Crescent Basalts) to less than 3 million years old (Boring Lavas) were mainly caused by transient changes in tectonic setting (i.e., arrival of mantle plume, ridge subduction) and do not record progressive chemical modification of the mantle wedge



Fault zone evolution in layered basalt sequences: A case study from the Faroe Islands, NE Atlantic margin
R.J. Walker et al., School of Earth and Ocean Sciences, Cardiff University, Main Building, Cardiff, CF10 3AT, UK. Posted online 3 May 2012; doi: 10.1130/B30512.1.

Few studies have focused on the geological characterization of exhumed subsurface faults and fractures within continental flood basalt provinces. Here R.J. Walker and colleagues present field and microstructural observations of basalt-hosted fractures and faults from the Faroe Islands, NE Atlantic margin. Associated fault rocks are formed as a result of various processes, including collapse/infill, abrasion, and implosion, each with a respective decrease in fluid flow potential. Fault rocks resulting from collapse/infill indicate sustained fluid-flow pathways, as they require open, subterranean cavities that are formed faster than mineral precipitation can seal them. Fault rock produced by abrasion or implosion record transient fluid flow, following successive fault movements. Distinction between fault rock types is therefore critically important to modeling subsurface migration and distribution of fluids (e.g., fresh-water aquifers, hydrocarbon accumulations, etc.). Despite having distinctly different fault rock assemblages to sediment-hosted faults, basalt-hosted fault zone thickness and displacement data are indistinguishable from those in sedimentary sequences. This observation suggests that the first-order controls on fault development are the same in layered basalts and sediments; namely, fault surface bifurcation and linkage, asperity removal, and the accommodation of geometrically necessary strains in the wall rocks.



Geometry and scale of fault segmentation and deformational bulging along an active oblique-slip fault (Wairarapa fault, New Zealand)
R.C. Carne and T.A. Little, School of Geography, Environment and Earth Sciences, Victoria University of Wellington, P. O. Box 600, Wellington 6140, New Zealand. Posted online 3 May 2012; doi: 10.1130/B30535.1.

The segmented nature of active strike-slip fault traces and associated surface deformation are well understood, particularly in plan view; however, the factors controlling such segmentation and the three dimensional aspects of their geometry, especially for oblique-slip faults, are less well known. The Wairarapa fault is an active dextral-reverse fault in the North Island, New Zealand. The fault is expressed at the surface of post-Last Glacial Maximum river gravels by a highly segmented trace, where elongate deformational bulges have formed in the stepover region between adjacent overlapping fault strands. R.C. Carne and T.A. Little use newly collected geomorphic and topographic data to investigate the 3-D geometry of fault segmentation in the Wairarapa fault zone. They describe the finite development of deformational bulges occupying contractional stepovers between overlapping splays in the zone, as well as the kinematics of how oblique-slip components are partitioned between the splays and bulges of this segmented, near-surface fault zone. Carne and Little present important additional data on the near-surface geometry of a major strike-slip fault zone, which they believe will further the understanding of strike-slip fault characteristics and kinematics.



Structural decoupling in a convergent forearc setting (southern Crete, eastern Mediterranean)
Eleni Kokinou et al., Technological Educational Institute Crete, 3 Romanou Str. Chalepa, Chania, Crete, GR 73133 Greece. Posted online 3 May 2012; doi: 10.1130/B30492.1.

Eleni Kokinou and colleagues use a multidisciplinary approach to investigate the structure of the southern Cretan margin, which is located in one of the most seismically-active forearc regions in Europe. They use bathymetric, seismic-reflection, and fault plane solution data to identify the main tectonic features on the margin and correlate the evolution of these latter features with the main sedimentary sequences recognized on Crete. In contrast to the majority of forearc settings in the Pacific and Indian Oceans, southern Crete comprises a region of predominant oblique movement above well-defined detachment zones. This work by Kokinou and colleagues shows that structural segmentation at depth is complex, with multiple crustal levels recording contrasting styles of deformation and distinct moment-tensor solutions. This complexity derives from the oblique style of convergence recorded south of Crete, which reactivates distinct crustal levels depending on their rheology and relative degree of metamorphism inherited during Alpine compression.



Thrust belts of the southern Central Andes: Along-strike variations in shortening, topography, crustal geometry, and denudation
Laura Giambiagi et al., CONICET-IANIGLA Centro Regional de Investigaciones Científicas y Tecnológicas, Parque San Martín s/n, 5500 Mendoza, Argentina. Posted online 3 May 2012; doi: 10.1130/B30609.1.

The Andean fold and thrust belts of west-central Argentina (33° and 36°S), above the normal subduction segment, present important along-strike variations in mean topographic uplift, structural elevation, amount and rate of shortening, and crustal root geometry. To analyze the controlling factors of these latitudinal changes, Laura Giambiagi and colleagues compare these parameters and the chronology of deformation along eleven balanced crustal cross-sections across the thrust belts between 70° and 69°W, where the majority of the upper crustal deformation is concentrated, and reconstruct the Moho geometry along the transects. They propose two models of crustal deformation: a 33°40'S model, where the locus of upper-crustal shortening is aligned with respect to the maximum crustal thickness, and a 35°40'S model, where the upper-crustal shortening is uncoupled from the lower-crustal deformation and thickening. This degree of coupling between brittle upper crust and ductile lower crust deformation has a strong influence on mean topographic elevation. In the northern sector of the study area, an initial thick and felsic crust favors the coupling model; while in the southern sector, a thin and mafic lower crust allows the uncoupling model. The results presented by Giambiagi and colleagues indicate that interplate dynamics may control the overall pattern of tectonic shortening; however, they find that local variations in mean topographic elevation, deformation styles and crustal root geometry are not fully explained and are more likely to be due upper-plate lithospheric strength variations.



The Paleocene - Eocene Thermal Maximum (PETM) in shallow-marine successions of the Adriatic Carbonate Platform (SW Slovenia)
Jessica Zamagni et al., Institut für Erd- und Umweltwissenschaften, Universität Potsdam, Karl Liebknecht Str. 24, D-1447, Golm, Potsdam, Germany. Posted online 3 May 2012; doi: 10.1130/B30553.1.

The Paleocene-Eocene Thermal Maximum (PETM; ca. 55 Ma) represents one of the most rapid and extreme warming events in the Cenozoic and is associated with a large, rapid carbon cycle perturbation. Shallow-water stratigraphic sections from the Adriatic Carbonate Platform offer a rare opportunity to learn about the nature and effects of the PETM in this ecosystem. Jessica Zamagni and colleagues use carbon and oxygen isotope stratigraphy, in conjunction with detailed larger benthic foraminiferal biostratigraphy, to establish a high-resolution paleoclimatic record. A prominent negative excursion in delta-13C curves, interpreted here as the carbon isotope excursion during the PETM, has been detected. The strongly 13C-depleted delta-13C record of our shallow-marine carbonates compared to open marine records could result from organic matter oxidation suggesting intensified weathering, runoff, and organic matter flux. Additionally, the earliest Eocene larger benthic foraminiferal evolution is documented in detail. Within an evolutionary scheme controlled by long-term biological processes, Zamagni and colleagues argue that high sea-water temperatures associated with the PETM could have stimulated the rapid specific diversification of the larger benthic foraminifera during the earliest Eocene. Together, these data help elucidate the effects of global warming and associated feedbacks in shallow-water ecosystems, and by inference, could serve as an assessment analog for future changes.



Airborne LiDAR analysis and geochronology of faulted glacial moraines in the Tahoe-Sierra frontal fault zone reveal substantial seismic hazards in the Lake Tahoe region, California-Nevada, USA
James F. Howle et al., U.S. Geological Survey, Carnelian Bay, CA 96140, USA. Posted online 18 May 2012; doi: 10.1130/B30598.1.

New seismic hazard analysis for the Lake Tahoe region of California and Nevada reveals an increase over previous estimates: Results of a new study conducted by the U.S. Geological Survey reveals that faults west of Lake Tahoe, California, represent a substantial seismic hazard to the greater Lake Tahoe region of California and Nevada. The study has utilized a new high-resolution imaging technology, known as bare-earth airborne LiDAR, which is capable of seeing through dense forest cover to reveal active earthquake faults that were not detectable with conventional aerial photography. The faults have offset linear glacial moraines that provide a record of tectonic deformation since the moraines were deposited. The authors developed new three-dimensional techniques to measure the tectonic displacement of moraine crests caused by repeated earthquakes. To calculate the rates of fault movement the authors have dated the faulted moraines deposited by the last two glaciations in the Lake Tahoe basin which occurred approximately 21 and 70 thousand years ago. The study concludes that the faults west of Lake Tahoe, referred to as the Tahoe-Sierra frontal fault zone, represent a two- to three-fold increase in the seismic hazard assessment for the Lake Tahoe basin and could potentially generate earthquakes with magnitudes ranging from 6.3 to 6.9.



Geomorphologic evidence for the late Pliocene onset of hyperaridity in the Atacama Desert
R. Amundson et al., Division of Ecosystem Sciences, 137 Mulford Hall, University of California, Berkeley, California 94720, USA. Posted online 18 May 2012; doi: 10.1130/B30445.1.

R. Amundson and colleagues report their findings that the Atacama Desert, the driest location on Earth, likely entered its present hyperarid condition just over two-million years ago, possibly in response to changes in Pacific Ocean circulation that launched the present La Niña/El Niño cycle -- a change that appears to have significantly reduced rainfall on the northern coast of Chile. Today, rainfall occurs sometimes once a decade, stream systems seldom flow, and the soils are filled with salt. Geological evidence indicates that sometime prior to two-million years ago, major streams flowed through the region, soils were actively removed from hillslopes by erosion, and rivers were able to erode their channels at rates that matched geological uplift. The past 2 million years has been a sustained period of a largely passive accumulation of dust and salt in this vast landscape, which is unable to respond hydrologically to changes in topography caused by tectonic activity. Projected increases in the frequency of El Niño events caused by greenhouse gas emissions may increase rainfall in a region that now appears highly sensitive to even modest changes in climate.



Structure, geochemistry, and tectonic evolution of trench-distal backarc oceanic crust in the western Norwegian Caledonides, Solund-Stavfjord ophiolite (Norway)
Harald Furnes et al., Dept. of Earth Science & Centre for Geobiology, University of Bergen, Norway. Posted online 18 May 2012; doi: 10.1130/B30561.1.

Harald Furnes and colleagues present a study of the Solund-Stavfjord Ophiolite, Norway. The term "ophiolite" is used for ancient oceanic crust that has been brought onto land, commonly during closure of an ocean basin. Ophiolites can be classified into two main types: (1) oceanic crust that formed above a subduction zone; i.e. in a convergent tectonic setting in which one plate moves under another, resulting in the formation of a back-arc basin (e.g., basins in the western Pacific Ocean); and (2) oceanic crust that forms when tectonic plates diverge, resulting in the formation of an ocean basin (e.g., the Red Sea). These two main types can be further subdivided into sub-types related to the specific environment in which the oceanic crust formed and to its stage of development. The two main types of ophiolite and their sub-types can be distinguished from their structural development and the chemical composition of the lavas and dikes (the lava feeders). The Solund-Stavfjord Ophiolite is a variety of the subduction-related type, and formed during closure of the Iapetus Ocean.



The geochronology of volcanic and plutonic rocks at the Questa caldera: Constraints on the origin of caldera-related silicic magmas
Matthew J. Zimmerer et al., New Mexico Bureau of Geology and Mineral Resources Socorro, New Mexico, USA. Posted online 18 May 2012; doi: 10.1130/B30544.1.

Research by Matthew J. Zimmerer and colleagues compares the geochronology of volcanic and plutonic rocks at the Questa caldera, northern New Mexico, USA, to assess different models for caldera magmatism. Results indicate that only small volumes of the exposed intrusions are temporally and chemically linked to the 25.4-million-year-old caldera-forming, rhyolitic Amalia Tuff. These syncaldera intrusions are located at the structurally highest levels of the exposed, subcaldera intrusions and are interpreted to be nonerupted Amalia Tuff equivalent magma. The intrusions emplaced at structurally deeper levels are approx. 100 thousand to 6.1 million years younger than the Amalia Tuff. Therefore, these intrusions are too young to represent the less-differentiated, more mafic parts of the caldera-forming magma chamber. Results of this study suggest that the Amalia Tuff was generated at deeper crustal levels rather than by in-situ differentiation of a large volume magma chamber in the upper crust. The timing of volcanic eruptions and pluton emplacement suggests that Questa caldera magmatism was a dynamic process characterized by repeated emplacement and eruption of compositionally diverse magmas over an approx. 9.2-million-year period.



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