September/October Geological Society of America Bulletin media highlights

September 25, 2007

Boulder, CO, USA -- Geology topics of interest include: Loma Prieta earthquake aftershocks (1989-1990) reveal the fault zone had the strength of normal crust instead of being abnormally weak; reconstruction of the Chicxulub ejecta plume; and analysis of the Araguainha peak-ring impact crater in central Brazil.

Response of bankfull flood magnitudes to Holocene climate change, Uinta Mountains, northeastern Utah
Eric C. Carson et al., San Jacinto College Geology Department, 5800 Uvalde Road, Houston, TX 77049. Pages 1066-1078.

Keywords: Holocene, floods, climate, sub-alpine, Uinta Mountains

In many settings, including the sub-alpine northern Uinta Mountains of northeastern Utah, the magnitude of the most frequently occurring flood determines the cross-sectional size of stream channels. Prior to human occupation, long-term climatic changes provided the primary control on changes on the flood magnitudes (and therefore channel sizes). This paper reconstructs the dimensions of ancient stream channels that were abandoned and have been preserved on shallow gradient floodplains. Radiocarbon dating of channel-bottom sediments from the relict channels provides dating control for the ages of channel abandonment. The authors compare the channel sizes--and, by extension, flood magnitudes--for 30 relict channels to the flood magnitudes observed today in modern channels to quantify the variations in flood over the past approximately 10,000 years. Their data indicate that, in this mountainous setting, floods were typically 10 to 15% larger than modern ones from 8500 to 5000 calendar years before present, and from 2800 to 1000 calendar years before present. Conversely, floods were as much as 15 to 20% smaller than modern from 5000 to 2800 calendar years before present, and from 1000 calendar years before present to near present. These results compare well with other regional data evaluating climatic conditions during this period, and highlight the significance of snowpack accumulation and release on flood magnitudes and water budgets.

The magmatic development of an intra-oceanic arc: High-precision U-Pb zircon and whole-rock isotopic analyses from the accreted Talkeetna arc, south-central Alaska
Matthew Rioux et al., Department of Earth, Atmospheric and Planetary Sciences
77 Massachusetts Ave., Room 54-1124, Cambridge, MA 02139, USA. Pages 1168-1184.

Keywords: island arc, arc plutonism, Talkeetna, Chugach, Alaska, zircon.

The Chugach and Talkeetna Mountains in south-central Alaska expose a rare cross section from the crust-mantle boundary to subaerial volcanic rocks of an accreted intra-oceanic volcanic chain. These exposures allow for direct observation and sampling of rocks formed up to ~30 km below the Earth's surface and provide an opportunity to study how intra-oceanic volcanoes, such as the modern Aleutian volcanic chain, grow and evolve through time. In this article, the authors document the development of the volcanic arc over ~50 million years in the Jurassic period. New high-precision dating demonstrates that plutonic rocks in the Chugach Mountains formed between 202.1 and 181.4 million years ago. The magmatic activity then shifted northward to form the plutonic rocks that now comprise the central and eastern Talkeetna Mountains. It is estimated that this occurred between 177.5 to 152.7 million years ago. The youngest plutonic activity is coeval with evidence for compressive tectonic forces and uplift of the plutonic rocks, likely reflecting a collision between the Talkeetna volcanic arc and either another intra-oceanic volcanic chain or the North American margin. The data represent the first high-precision ages from this area and provide important new constraints on the development of this archetypal crustal cross section.

Structure, Deformation, and Strength of the Loma Prieta Fault, Northern California,U.S.A., as Inferred from the 1989-1990 Loma Prieta Aftershock Sequence
Robert J. Twiss and Jeffrey R. Unruh, Geology Department, University of California at Davis, Davis, California 95616, USA. Pages 1079-1106.

Keywords: Loma Prieta earthquake, aftershocks, inversion, strain, partitioning, strength, brittle deformation, fault zones, rotation

This paper analyzes over a thousand aftershocks of the 1989 Loma Prieta earthquake, which occurred along the San Andreas Fault system in northern California. It determines details of the fault structure from aftershock alignments at different scales in the fault zone, and the authors used the slip directions on local groups of aftershock faults to map the variations in deformation throughout the fault zone. The main fault is about 60 km long, and is defined by three segments that form a 20° bend, shaped like an open 's'. Aftershock slip was dominantly in a horizontal, west-side-north sense on the Southern Segment, a mixture of west-side north and west-side-up on the Central Segment, and a complex distribution of slip directions characterized by west-side-north with a large to dominant component of west-side-up on the Northern Segment. These three segments of the fault do not reach the surface, but are overlain by a Shallow Zone comprising another set of faults that are distributed along the main trend of the three fault segments but are oriented at an angle to them. The slip on faults in the Shallow Zone is dominantly west-side-up. These zones of the fault all have a substructure of smaller faults at length scales of several kilometers to several tens of meters on which the deformation actually accumulates. The patterns of slip and the relation between the orientations of these smaller faults and the deformation, can be interpreted to show that the Loma Prieta fault zone had the strength of normal crust when the earthquake and its aftershocks occurred. This conclusion contradicts numerous previous studies, which inferred that the fault was abnormally weak. The detailed analysis of the aftershock deformation has also provided evidence for a newly recognized mechanism of brittle deformation by which a three-dimensional deformation is split into two two-dimensional deformations, both of which occur within the same volume of rock at the same time. This provides a new model for understanding this type of brittle deformation in the Earth's crust. Constraints on the rotations of fault blocks in the shear zone also can be inferred from the seismic data, a result that supports a model we have proposed previously but that is not predicted by classical seismology.

Detrital zircon U/Pb geochronology of southern Guerrero and western Mixteca arc successions (southern Mexico): New insights for the tectonic evolution of southwestern North America during Late Mesozoic
Oscar Talavera-Mendoza et al., Universidad Autónoma de Guerrero, Unidad Académica Ciencias de la Tierra, A.P. 197, Taxco, Guerrero 40200, Mexico. Pages 1052-1065.

Keywords: U-Pb detrital zircon, Late Jurassic-Cretaceous arc successions, Guerrero terrane, Mixteca terrane, southern Mexico, southwestern North America, Caribbean plate.

This paper deals with the zircon provenance of southern Guerrero and western Mixteca terranes arc successions of southern Mexico. Data presented provide important new insights on the tectonomagmatic and sedimentary evolution of the southwestern realm of North America during Late Mesozoic time and help to visualize the transition of Mesozoic arc series of Mexico and those of western Unites States and the Caribbean area.

Late Holocene sediment accumulation on the northern California shelf: Oceanic, fluvial and anthropogenic influences
Christopher K. Sommerfield et al, University of Delaware, College of Marine Studies, 700 Pilottown Rd., Lewes, Delaware 19958, USA. Pages 1120-1134.

Keywords: continental shelf sedimentation, fluvial systems, northern California, late Holocene, 14C geochronology, marine sedimentology

It is well established that erosion rates within mountainous river basins of north coastal California have generally increased as a result of agriculture and timber harvesting since the time of settlement by Europeans in the early 1800s. However, effects of human activities on sediment accumulation offshore are poorly documented. In addition to landscape disturbances, extreme river floods related to natural climatic factors have a major bearing on landocean sediment movement in the region. In this paper, we used the continental shelf sedimentary record to identify changes in sediment properties and accumulation rates that have taken place from late Holocene (past 5000 years) to historical times. Sediment cores from the shelf archive variations in the sedimentary system related to infilling of river mouths with rising sea level during the past several millennia, as well as evidence of massive, flood-generated sediment discharges of the Eel River. Superimposed on this record is confirmation of accelerated sediment delivery to the coast after the early 1800s, particularly after about 1950, most likely due to land-use practices in conjunction with the effects of extreme floods in 1955 and 1964. Owing to sediment dispersal processes in the coastal ocean, the marine record of human impacts in the watershed is localized, incomplete and limited to the most rapidly accumulating (>2 mm/yr) shelf sites.

Tectonic significance of Cretaceous to Tertiary magmatic and structural evolution of the northern margin of the Xolapa Complex, Terra Colorada area, southern Mexico
L.A. Solari et al., Centro de Geociencias, UNAM, Campus Juriquilla, 76230 Querétaro, Qro., México. Pages 1265-1279.

Keywords: southern Mexico, U-Pb geochronology, Xolapa Complex, Cenozoic tectonics, arc magmatism

The Tierra Colorada area lies along the northern limit of the Xolapa Complex, where it is juxtaposed against the Mixteco (Paleozoic) and Guerrero (Mesozoic) terranes of southern Mexico, just north of Acapulco. This article presents combined structural and geochronological data from Tierra Colorada area that show evidence of four deformational events and several episodes of arc magmatism during Mesozoic and Cenozoic. Their interpretations can help to speculate over the Mesozoic to Cenozoic tectonics of southern Mexico and the Caribbean.

Pressure-temperature-time evolution of Paleozoic high-pressure rocks of the Acatlán Complex (southern Mexico): Implications for the evolution of the Iapetus and Rheic oceans
Ricardo Vega-Granillo et al., Departamento de Geología, Universidad de Sonora, Rosales y Encinas S/N, Hermosillo, Sonora, México, 83000. Pages 1249-1264.

Keywords: eclogite, blueschist, Paleozoic, Appalachian, Acatlán Complex, Mexico

This paper describes three distinctive events of high-pressure metamorphism from the Acatlán Complex of southern Mexico. These events point to the closing of two different oceanic tracts and a continental collision, which occurred during the Paleozoic Era. Because of correlative events that have only been described in the Appalachian-Caledonian orogen, we propose that the Acatlán Complex formed part of that chain, at least until the end of the Paleozoic Era. It is hoped that this finding will lead to a more comprehensive understanding of the Gondwana-Laurentia interactions preceding the assembly of the Pangea supercontinent.

Petrology of the Mount Whitney Intrusive Suite, eastern Sierra Nevada, California: Implications for the emplacement and differentiation of composite felsic intrusions
William H. Hirt, Department of Natural Sciences, College of the Siskiyous, 800 College Avenue, Weed, CA 96094, USA. Pages 1185-1200.

Keywords: Sierra Nevada batholith, Mount Whitney Intrusive Suite, pluton, crystal fractionation, emplacement, thermal modeling.

The Mount Whitney Intrusive Suite (MWIS) is a group of three nested granite bodies that were formed when a large volume of magma was emplaced into the crust of western North America between 88 and 83 million years ago. This magma rose along steep fractures that opened at a "step over" between two right lateral faults, and spread horizontally to form a tabular intrusion that is probably thickest near its center. Later uplift and erosion have exposed the interior of the suite, and reveal variations in the textures and compositions of the granites which demonstrate that the younger bodies: (1) contain larger contributions of material from old crustal rocks; and (2) underwent more extensive differentiation and coarsening to produce large alkali-feldspar crystals. A model of the suite's emplacement and thermal history indicates that successive magma inputs would have cooled more slowly as it grew, and thus have been better able to mix with one another and undergo later in situ differentiation and textural coarsening. Agreement between the model's predictions and trends among the suite's members suggest that warming of the upper crust accounts for much of its textural and compositional diversity. Melts were also segregated into sheets high in the center of the suite when dense, crystal-rich layers broke away from the roofs of magma reservoirs in the two younger bodies.

Interaction of tectonic, sedimentary and diapiric processes in the origin of chaotic sediments: and example from the Messinian of the Torino Hill (Tertiary Piedmont Basin, NW Italy)
Francesco Dela Pierre, et al., Università degli Studi di Torino, Dipartimento di Scienze della Terra and CNR, Istituto di Geoscienze e Georisorse, Torino, 10125, Italy. Pages 1107-1119.

Keywords: Chaotic deposits, tectonic sedimentary and diapiric processes, Messinian, Tertiary Piedmont Basin.

The origin of chaotic rock bodies or mélanges is commonly attributed to tectonic disruption and mixing of originally coherent sequences, gravitational submarine down-slope movements and shale diapirism, caused by the rising towards the sea floor of overpressured, fluid-permeated fine grained sediments. The recognition of the role played by each of these processes in the geological record is problematic, due to the strong facies convergence of their products and to the fact that these mechanisms can coexist and interact in a complex way. In this paper the relative contribution of tectonic faulting, gravitational and diapiric processes in the genesis of a Messinian composite chaotic unit have been evaluated. It appears that these mechanisms impacted on each other. Faulting was responsible for the disruption of the originally coherent stratigraphic succession, favored the failure of mechanically weakened sediments and encouraged shale diapirism through the creation of mechanical discontinuities working as conduits for the rising of overpressured sediments. Loading provided by deposition of gravitative chaotic sediments could have contributed (together with the circulation of methane-rich fluids in the sedimentary column) to diapir intrusion, which in turn caused a partial reorganization of the dismembered sediments produced by the other two processes.

Eruptive and structural history of Teide Volcano and rift zones of Tenerife, Canary Islands
J. Carracedo, CSIC IPNA Dept., Volcanological Station of the Canary Islands, Santa Cruz de Tenerife, Spain, Volcanology, Avda. Fco Sanchez 3, La Laguna, Tenerife 38206, Spain. Pages 1027-1051.

Keywords: radioisotopic ages, volcanic history, structural history, rift volcanism, nested central volcanoes, Teide Volcano, Tenerife, Canary Islands.

Teide Volcano is the third highest volcanic feature on Earth (3718 m a.s.l., >7 km high). The Teide and Pico Viejo stratocones and the Northwest and Northeast Rifts are products of the latest eruptive phase of the island of Tenerife, initiated with the lateral collapse of its northern flank that formed the Las Cañadas Caldera and the Icod-La Guancha Valley ca. 200 ka. The eruptive and structural evolution of this volcanic complex has been reconstructed after detailed geological mapping and radioisotopic dating of the significant eruptive events. A set of 54 new 14C and K/Ar ages provides precise age control of the recent eruptive history of Tenerife, particularly Teide Volcano, the third-highest volcanic feature on Earth (3718 m above sea level, >7 km high), and unique in terms of its intraplate setting. The development of the Teide-Pico Viejo Volcanoes may be related to the activity of the Northwest and Northeast Rifts. Volcanic and intrusive activity along both rift zones may have played an important role in activating the gravitational landslide and in the subsequent growth, nested within the collapse embayment, of an increasingly higher central volcano with progressively differentiated magmas. The coeval growth of the central volcano with sustained activity along the rifts led to a clear bimodal distribution in composition of eruptive products, with the basaltic eruptions in the distal part of the rifts and phonolitic and more explosive eruptions in the central area, where the differentiated stratocones developed. Current volcanic hazard in Tenerife is considered to be moderate, because eruptive frequency is low, explosivity is modest, and the eruptive activity of the Teide stratocone seems to have declined over the past 30 k.y., with only one eruption in this period (1150 yr B.P.).

Subsurface seepage of seawater across a barrier: A source of water and salt to peripheral salt basins
Jeffrey A. Nunn and Nicholas B. Harris, Department of Geology and Geophysics, Louisiana State University, Baton Rouge, Louisiana, 70803, USA. Pages 1201-1217.

Keywords: South Atlantic, subsurface seepage, rift lakes, salt, salt basins, water flux, seepage, Messinian Salinity Crisis

Many of the great Phanerozoic salt basins, such as the Messinian salt deposits of the Mediterranean Sea, share a common paleogeography, in which a deep lake or sea is separated from the ocean by a narrow barrier. Marine transgressions over the barrier have been proposed as the origin for saline water in these basins. This paper tests an alternate hypothesis, that subsurface seepage of seawater through the barrier was a significant source of water, influencing both water level and water composition. Investigated are three settings, a modern analog in the Gregory Rift of northern Kenya, the Messinian Mediterranean basin and the Early Cretaceous Aptian salt basins of the South Atlantic. Results indicate that seepage of seawater through a barrier to a peripheral basin ranges from the dominant component of lake water to insignificant depending on the climate, size of the peripheral basin, and permeability of the barrier. The flux of salt is significant over the full range of parameters considered in this study, producing brines in the peripheral lake/sea within 0.5 my in most cases.

Reconstruction of the Chicxulub ejecta plume from its deposits in drill core Yaxcopoil-1
A. Wittmann et al, Museum of Natural History, Mineralogy, Invalidenstrasse 43 Berlin, Germany 10115, Germany. Pages 1151-1167.

Keywords: Chicxulub crater, suevite, impactite, meteorite crater, Yaxcopoil-1, ejecta

This paper discusses how a 100-m-thick section of melt rich debris in a drill core through the Chicxulub impact crater reveals evidence for the collapse of the crater and the evolution of its ejecta plume. Detailed petrographic analyses allows the reconstruction of a timeframe, physical and chemical conditions, and atmospheric interactions during the formation of this large impact crater that caused the mass extinction at the end of the Cretaceous.

Insights into the morphology, geometry and post-impact erosion of the Araguainha peak-ring structure, central Brazil
C. Lana et al., Department of Geology, University of Stellenbosch, Private Bag X1, Matieland 7620, South Africa. Pages 1135-1150.

Keywords: impact craters, Paraná Basin, Araguainha, peak ring, crater collapse

The 40-km-wide Araguainha structure is the largest and oldest peak-ring impact crater in South America. The crater was excavated in flat lying sediments of the intracratonic Paraná Basin. This paper demonstrates that Araguainha is one of the best preserved and well-exposed peak-ring craters, similar in many aspects to peak-ring craters on the Moon. It also proposes that the impact took place on a shallow estuarine environment, at approximately 245 million years ago. The energy released upon the impact was sufficient to excavate more than 2 km of the target rocks within the Paraná Basin, forming an initial transient cavity of 20-25 km in diameter. Much of the present morphological and structural features at Araguainha relates to gravitational collapse of this transient cavity, shortly after the impact event.

Use of U-Pb geochronology to identify successive, spatially-overlapping tectonic episodes during Silurian-Devonian orogenesis in south-central Maine, USA
Christopher Gerbi et al., Department of Earth Sciences, University of Maine, Orono, Maine 04469, USA. Pages 1218-1231.

Keywords: Acadian, Neo-Acadian, Appalachians, geochronology, U-Pb, transpression, strain partitioning

During oblique collision of two continental masses, the two components of deformation - contractional and strike-slip - may be accommodated along either single or multiple structures. In south-central Maine, both styles of deformation are evident in the same geographic area. U-Pb dating of metamorphic minerals that formed during the metamorphism associated with deformation indicates that rather than being related to a single deformational event, the two styles of deformation occurred during separate collisions separated by approximately 30 million years.

Exhumation of Ultrahigh-Pressure Rocks Beneath the Hornelen Segment of the Nordfjord-Sogn Detachment Zone, Western Norway
Scott Johnston et al., Department of Earth Science, University of California, Santa Barbara, CA 93106-9630, USA. Pages 1232-1248.

Keywords: ultrahigh-pressure rocks, exhumation, Nordfjord-Sogn Detachment Zone, low-angle detachment, western Norway, Hornelen Region

Recent work in Norway investigating an ancient fault suggests a multi-stage process for the demise of the world's largest mountain ranges. From approximately 500-400 million years ago as the North American and European plates collided, a mountain range, comparable to the Himalaya today, was formed with high peaks that were supported by continental crust buried greater than 100 km deep within the earth. In this paper, the authors nvestigate the processes by which this mountain range collapsed over a period of 10 million years, and the deepest roots of the mountains were exhumed into the upper crust. Data presented in this paper establishes that the Nordfjord-Sogn Detachment Zone, a normal fault exposed along the west coast of Norway, was a broad fault zone that initiated at a depth of 30-40 km, and was active from approximately 410-400 million years ago. This new data supports a multi-stage model for the exhumation of deep crustal rocks with initial exhumation (from greater than 100 km to approximately 50 km depth) occurring along poorly defined structures followed by subsequent exhumation (into the upper crust less than 10 km deep) along large normal faults like the Nordfjord-Sogn Detachment Zone.
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