September/October 2009 GSA Bulletin highlights

July 31st, 2009

The September-October GSA Bulletin is now online. Studies in this issue trek the globe, detailing geo-phenomena in the Lesser Himalaya of India; the Altai Range of China; the Peruvian Eastern Cordillera; the southern and northern Apennines, Italy; exotic terranes of Alaska; the San Ysidro fault, New Mexico; the Colorado Plateau's CO₂ spikes and fossil gingko; the Santa Clara River and Santa Monica Basin offshore southern California; and the Coast Plutonic Complex and Coast Mountains batholith of British Columbia, Canada.

Related papers:

The Kumaun and Garwhal Lesser Himalaya, India (Part 1): Structure and stratigraphy

Julien Celerier et al., Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia. Pages 1262-1280.

The Kumaun and Garwhal Lesser Himalaya, India (Part 2): Thermal and deformation histories

Julien Celerier et al., Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia. Pages 1281-1297.

The immense and spectacular Himalaya are Earth's most impressive manifestation of plate tectonics. Despite their importance in our understanding of mountain building processes, large swathes of the Himalaya remain poorly understood in terms of their geology and the tectonic processes which built them. Celerier et al. integrate previous studies with new observations and quantitative data-sets to unravel the tectonic processes responsible for building the foothills of the Himalaya - the Lesser Himalaya in northwest India.

Tectonic development of the southern Chinese Altai Range as determined by structural geology, thermobarometry, 40Ar/39Ar thermochronology, and Th/Pb ion-microprobe monazite geochronology

Stephanie M. Briggs et al., Dept. of Earth and Space Sciences, University of California, Los Angeles, California 90095, USA. Pages 1381-1393.

Briggs et al. provide much needed data about the timing and conditions of metamorphism, deformation, and faulting in northwestern China as Asia was being tectonically assembled about 450-150 million years ago. They establish that some of the deformation and faulting is as young as Jurassic in age and potentially connected to regional tectonic events.

Structural and geochemical characteristics of faulted sediments and inferences on the role of water in deformation, Rio Grande Rift, New Mexico

Jonathan Saul Caine, U.S. Geological Survey, P.O. Box 25046, MS 964, Denver, Colorado 80225, USA; and Scott A. Minor. Pages 1325-1340.

The San Ysidro fault is a spectacularly exposed normal fault located in the northwestern Albuquerque Basin of the Rio Grande Rift. This intrabasin fault is representative of many faults that formed in poorly lithified sediments throughout the rift. The fault is exposed over nearly 10 km and accommodates nearly 700 m of dip slip in subhorizontal, siliciclastic sediments. The extent of the exposure facilitates study of along-strike variations in deformation mechanisms, architecture, geochemistry, and permeability. Aeromagnetic data by Caine and Minor indicate that there may be many more unmapped faults with similar lengths to the San Ysidro fault buried within Rio Grande basins. If these buried faults formed by the same processes that formed the San Ysidro fault and have persistent low-permeability cores and cemented mixed zones, they could compartmentalize the basin-fill aquifers more than is currently realized, particularly if pumping stresses continue to increase in response to population growth.

Tectonomagmatic evolution of Western Amazonia: Geochemical characterization and zircon U-Pb geochronologic constraints from the Peruvian Eastern Cordilleran granitoids

Aleksandar Miskovic et al., Dept. of Mineralogy, Earth Sciences Section, University of Geneva, 13 rue des Maraichers, CH-1205 Geneva, Switzerland. Pages 1298-1324.

In this paper, the magmatic evolution of the central proto-Andes is deciphered by combining uranium-lead dating of zircon and geochemical characterization of host granitoids along the 1400-km-long Eastern Cordilleran intrusive belt. The ages of sampled rocks span 1100 million years and reveal seven distinct pulses of magmatism along the proto-Andean cratonic margin of Gondwana. The oldest two magmatic events are related to (1) the collision of Laurentia and Amazonia during the protracted Late Mesoproterozoic Sunsas-Grenville orogeny, and (2) a previously unidentified middle Neoproterozoic magmatic pulse that heralded the opening of the Iapetus Ocean along western Gondwana. Three phases of Paleozoic subduction-related plutonism (early Ordovician, late Ordovician, and Carboniferous) were separated by periods of tectonic dormancy largely manifested in passive margin environments. The Carboniferous arc magmas were emplaced in response to subduction of the proto-Pacific oceanic crust as the supercontinent Pangea was assembled. Magmatism continued with brief lacunae for 100 million years, into the early Mesozoic, albeit with drastically different chemistry, signaling an overall extensive tectonic regime during the Permo-Triassic times, ultimately resulting in the Peruvian Eastern Cordilleran intrusive belt, which remained in a back-arc position throughout 170 million years of the modern Andean orogenic cycle.

Late Proterozoic-Paleozoic evolution of the Arctic Alaska-Chukotka terrane based on U-Pb igneous and detrital zircon ages: Implications for Neoproterozoic paleogeographic reconstructions

Jeffrey M. Amato et al., Dept. of Geological Sciences, New Mexico State University, Las Cruces, New Mexico 88003, USA. Pages 1219-1235.

It has long been known that Alaska is made up of pieces of crust known as exotic terranes that originated elsewhere and eventually were amalgamated into their current positions. Recent work on Seward Peninsula in northwest Alaska by Amato et al. has shown that the Arctic Alaska-Chukotka terrane may have originated far from the other pieces of Alaska, in a region close to the northwestern part of North America near Greenland. The Proterozoic granitic and volcanic rocks in the deepest exposed levels of Seward Peninsula are 870 to 550 million years old, with a majority around 680 million years old. The overlying sedimentary rocks are Paleozoic and have detritus dominated by minerals of similar age. These Neoproterozoic ages are rare in North America but are common to terranes originating in volcanic arc systems that flanked the supercontinent of Gondwana. Our work suggests that the Arctic Alaska-Chukotka terrane originated in a similar arc setting in a configuration near the continent of Baltica (encompassing parts of Norway, Finland, Sweden, northern Europe, and part of northwestern Russia west of the Ural Mountains). The Arctic Alaska-Chukotka terrane must have been transported along the northern coast of Canada to near its present position by Ordovician time based on faunal similarities between it and Siberia. Final transport to its modern position occurred during the opening of the Amerasian basin in Cretaceous time.

Greenhouse crises of the past 300 million years

Gregory J. Retallack, Dept. of Geological Sciences, University of Oregon, Eugene, Oregon 97403, USA. Pages 1441-1453.

Proxies of past CO₂ levels and climate over the past 300 million years now reveal multiple global climate change events in unprecedented detail. Evidence for past CO₂ spike comes from expanded and refined stomatal index data by Retallack of fossil Ginkgo and related leaves. New evidence for synchronous climatic change comes from paleosols in Montana, Utah, and neighboring states. Each CO₂ spike was coeval with unusually clayey, red, and decalcified paleosols that can be traced throughout the Colorado Plateau. Spikes in atmospheric CO₂ also were coeval with increases in paleosol alkali depletion as an indication of high temperature, and spikes in paleosol base depletion and depth to calcic horizons as indications of high precipitation. In the Colorado Plateau, times of warmer climate were also more humid, perhaps due to the greater moisture potential of warmer air. Seasonality of climate did not increase during warm-wet spikes. The Mesozoic greenhouse was not persistently hot with cool spells, but warm with hot flashes. These data furnish power laws predicting the sensitivity and magnitude of change in mean annual temperature and mean annual precipitation due to rising CO₂ in a mid-latitude, mid-continental region. The magnitude of the coming anthropogenic greenhouse pales in comparison with past greenhouse spikes at times of global mass extinctions.

Tectonics of the Mattinata fault, offshore south Gargano (southern Adriatic Sea, Italy): Implications for active deformation and seismotectonics in the foreland of the Southern Apennines

A. Argnani et al., Geologia Marina, Istituto di Scienze Marine-Consiglio Nazionale delle Ricerche, Via Gobetti 101, 40129 Bologna, Italy. Pages 1421-1440.

The relationship between seismicity and deformation in the Southern Apennines of Italy and its Adriatic foreland has recently attracted much attention, mostly focused on the deformation of the Gargano promontory and its surroundings. A recent earthquake, the strike-slip magnitude 5.4 2002 South Giuliano earthquake, destroyed a school and killed about 30 children. This earthquake is thought to be connected to a major east-west fault, the Mattinata Fault, which cuts across the Gargano promontory and has been taken as prototypical to explain some large historical earthquakes, so far poorly understood. The Mattinata Fault extends offshore in the southern Adriatic, but the comprehension of the offshore deformation is hampered by a lack of good quality geophysical data. Argnani et al. present the results of a marine seismic survey purposely intended to cover the offshore extent of the Mattinata Fault. These results indicate that with its remarkable extent and long-lasting geological history, the Mattinata Fault is a unique feature within the Adriatic foreland. Different segments of the Mattinata Fault were reactivated independently and with different behavior through time. These pieces of evidence suggest that some caution should be exercised when extending an idealized South Giuliano-Mattinata earthquake scenario to other poorly known east-west-trending fault zones located in the Adriatic foreland.

Possible causes of arc development in the Apennines, central Italy

Andrea Billi, Dipartimento di Scienze Geologiche, Universita "Roma Tre," Largo S.L. Murialdo, 100146 Rome, Italy; and Mara Monica Tiberti. Pages 1409-1420.

Arcuate mountain belts are among the most ubiquitous but also enigmatic and debated structures within orogenic settings. A beautiful example of an arcuate mountain belt is the northern Apennines, in central-northern Italy. In this belt, the southern portion of the Umbrian Arc, the so-called Olevano-Antrodoco thrust, is one of the most-studied orogenic structures of the world; however, the curvature's cause is still unexplained. Thanks to the recently-acquired CROP-11 deep seismic reflection profile, a thick mid-crustal anticlinorium is imaged right beneath the southern limb of the Umbrian Arc. By integrating the CROP-11 profile with available gravimetric, paleomagnetic, stratigraphic, structural, and topographic data, Billi and Tiberti propose an evolutionary model in which, during late Neogene time, the Olevano-Antrodoco thrust developed in an out-of-sequence fashion and underwent about 16 degrees of clockwise rotation when the thrust ran into and was then raised and folded by the growing anticlinorium (late Messinian-early Pliocene time). This new model suggests a causal link between mid-crustal folding and surficial orogenic curvature that may be a viable mechanism for arc formation elsewhere.

Coarse-grained sediment delivery and distribution in the Holocene Santa Monica Basin, California: Implications for evaluating source-to-sink flux at millennial time scales

Brian W. Romans et al., Dept. of Geological and Environmental Sciences, Stanford University, 450 Serra Mall, Building 320, Stanford, California 94305, USA. Pages 1394-1408.

The history of erosion and transport of sediment from mountainous areas on continents can be evaluated through investigation of the depositional products along the sediment routing system. Deep-marine sedimentary basins commonly represent the ultimate sedimentary sink, especially over geological timescales. Romans et al. document the distribution and timing of sand delivery from the Santa Clara River to the deep-marine Santa Monica Basin offshore southern California for the past 7,000 years. Eleven radiocarbon ages from a 12-meter-thick section of sediment from an Ocean Drilling Program borehole in the basin provide unprecedented constraint on timing of depositional events. An increase in coarse-grained sediment flux to the Santa Monica Basin is coincident with a known increase in frequency and magnitude of El Nino-Southern Oscillation events, which are known to influence sediment discharge of the Santa Clara River. Romans et al. demonstrate the utility of documenting the history of sediment transfer from the continent to the deep sea to help understand past environmental change.

Magmatic evolution of the eastern Coast Plutonic Complex, Bella Coola region, west-central British Columbia

J. Brian Mahoney et al., Dept. of Geology, University of Wisconsin-Eau Claire, 105 Garfield Avenue, Eau Claire, Wisconsin 54702-4004, USA. Pages 1362-1380.

The eastern Coast Plutonic Complex in west-central British Columbia (51-54 degrees north), consists of a broad belt of Jurassic to Eocene granitic to dioritic intrusive rocks that collectively represent more than 140 million years of nearly continuous, subduction-related magmatism. The nature of magmatism fluctuates through time, in response to changing plate dynamics and crustal structure. Pre-Late Cretaceous rocks were produced by episodic subduction-related magmatism characterized by partial melting of preexisting lower arc crust with minimal incorporation of evolved continental material. Crustal thickening in Late Cretaceous time (about 100-90 million years ago), possibly due to underplating of Wrangellia/Alexander, produced a deep crustal root (greater than 40 km) that likely extended into the eclogite transition zone. Mahoney et al. suggest that delamination of this dense crustal root led to voluminous magmatism, extension, and crustal exhumation in Paleocene-Eocene time. The successive intrusive events in the eastern Coast Plutonic Complex may reflect a predictable evolutionary progression common to all continental arc systems.

U-Th-Pb geochronology of the Coast Mountains batholith in north-coastal British Columbia: Constraints on age and tectonic evolution

G. Gehrels et al., Dept. of Geosciences, University of Arizona, Tucson, Arizona 85721, USA. Pages 1341-1361.

Gehrels et al. present 84 new uranium lead (U-Pb) ages of zircons and 41 new U-Pb ages of titanite from plutonic rocks in the Coast Mountains batholith of coastal British Columbia. These ages, together with 229 U-Pb (zircon) and 18 U-Pb (titanite) ages from previous reports, provide important constraints on the spatial and temporal patterns of magmatism along the northern Cordilleran margin from Late Jurassic through early Tertiary time. Magmatism migrated eastward from 120 to 60 million years ago at a rate of 2.0-2.7 km/million years (a pattern and rate similar to other Cordilleran-margin batholiths). Magmatic flux was high (greater than 35-50 cubic kilometers/million years per km) flux at 160-140 million years ago, 120-78 million years ago, and 55-48 million years ago, with magmatic lulls at 140-120 million years ago and 78-55 million years ago. High uranium-thorium values record widespread growth (and/or recrystallization) of metamorphic zircon at 88-76 million years ago and 62-52 million years ago. This magmatic history is consistent with a tectonic model involving formation of a Late Jurassic-earliest Cretaceous magmatic arc along the northern Cordilleran margin; duplication of this arc system in Early Cretaceous time by more than 800 km (perhaps 1000-1200 km) of sinistral motion (bringing the northern portion outboard of the southern portion); high-flux magmatism during mid-Cretaceous orthogonal convergence and terrane accretion; low-flux magmatism during Late Cretaceous-Paleocene dextral transpressional motion; and high-flux Eocene magmatism during rapid exhumation in a regime of regional crustal extension.

Source: Geological Society of America