|6 May 2009
GSA Release No. 09-25
Director of Education, Communication, & Outreach
April 2009 Lithosphere Media Highlights
Boulder, CO, USA - The second issue of GSA's newest journal, Lithosphere, investigates mantle cooling and craton thickness; hillslope steepness and erosion; the formation of the Ligurian Tethys oceanic basin; heat flow anomalies associated with the Rio Grande rift; seismic data networks in the north-central Apennines; the relationship between the number of faults in Earth's crust and the amount of tectonic stretching; how uplift of the ocean floor may affect sea-level rise; and the behavior of Earth's tectonic plates.
Highlights are provided below. Representatives of the media may obtain complementary copies of articles by contacting Christa Stratton at . Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to LITHOSPHERE in articles published. Contact Christa Stratton for additional information or assistance.
Non-media requests for articles may be directed to GSA Sales and Service, .
Does the mantle control the maximum thickness of cratons?
C.M. Cooper, Washington State University, School of Earth and Environmental Science, P.O. Box 642812, Pullman, WA 99164, USA; and Clinton P. Conrad. Pages 67-72.
Cooper and Conrad present an explanation for the geochemical evidence of a maximum cratonic thickness. They base this on the interplay between a craton's rheological and thermal structure and the deforming asthenosphere. In addition, they also explore the implications on Earth's cooling history that maintaining a near constant cratonic thickness over billions of years requires.
Quantifying the climatic and tectonic controls on hillslope steepness and erosion rate
Jon D. Pelletier, University of Arizona, Geosciences, 1040 E. Fourth St., Tucson, AZ 85749, USA; and Craig Rasmussen. Pages 73-80.
Existing models that quantify the steepness of hillslopes and determine how much soil covers the bedrock do not adequately include the effects of climate and rock type. In this paper, Pelletier and Rasmussen apply a new model for the climatic and rock-type-controls on rock weathering to estimating the steepness, thickness of soil cover, and erosion rates of hillslopes at any location based on simple input data.
Evolution of the lithospheric mantle in an extensional setting: Insights from ophiolitic peridotites
Giovanni B. Piccardo et al., Dipartimento per lo Studio del Territorio e delle sue Risorse, Universita di Genova, Corso Europa 26, I-16132 Genova, Italy. Pages 81-87.
This paper describes the main mechanisms that led to extension and breakup of the Europe and Adria continents during Jurassic times and the formation of an oceanic basin, the Ligurian Tethys, between them. Fundamental information has been deduced by the study of the ophiolitic sequences; i.e., the associations of rocks cropping out in the orogenic terranes of the Alpine and Apennine mountain belts of northwestern Italy, which originally represented the basement rocks of the Jurassic oceanic basin. Continental extension and rifting in the Ligurian domain was induced by far-field tectonic forces and was characterized by the interplay of tectonic and magmatic processes. Extension and thinning of the continental lithosphere (i.e., continental crust and uppermost mantle) induced adiabatic upwelling and decompression melting of the asthenosphere (i.e., the convective mantle). Mid-oceanic-ridge basalt (MORB) melts were formed by the melting asthenospheric mantle and percolated by porous flow through the extending lithospheric mantle and were trapped therein by interstitial crystallization. The interplay of tectonic and magmatic processes caused a transition from distributed continental deformation (i.e., the ultra-slow continental extension) to localized oceanic spreading (i.e., the opening of the oceanic basin) and the peculiar structure and composition of the passive continental margins formed after breakup of the Europe and Adria continents. This paper investigates some relevant processes that operate during formation of the oceanic basins.
Heat-flow anomalies crossing New Mexico along La Ristra seismic profile
Marshall Allan Reiter, New Mexico Institute of Mining and Technology, New Mexico Bureau of Geology and Mineral Resources, 801 LeRoy Place, Socorro, NM 87801-4681, USA. Pages 88-94.
Heat flow measurements along La Ristra seismic profile crossing New Mexico are interpreted to indicate an important thermal source associated with the Rio Grande rift at mid-step depth of about 65 km. This depth is coincident with the mid-depth of the most significant seismic anomaly. The heat flow study suggests higher temperatures as well as partial melting may cause the slow seismic velocities. A second thermal step is interpreted at about 33 km depth under the Rio Grande rift, a depth near the Moho and midway between the top of the upper mantle anomalous seismic velocity region and the top of the crustal anomalous seismic velocity region. A wedge of the Colorado Plateau occurs between the Rio Grande rift and the Jemez Lineament with heat flow values between the approximate mean of the Colorado Plateau in New Mexico and values typical of the Rio Grande rift. This intermediate heat flow is consistent with the shallower Moho derived from seismic studies and suggests the crustal integrity of this part of the plateau has resisted widespread upward movement of heat sources. The eastern boundary of the Rio Grande rift appears to be rather sharp from the heat flow data, but the seismic analyses suggest velocity anomalies for a considerable distance into the Great Plains. The proximity of the southern Rio Grande rift to La Ristra may influence the data, or perhaps recent deep temperature increases are not noticed in near surface temperature gradients.
Deep geometry and rheology of an orogenic wedge developing above a continental subduction zone: Seismological evidence from the northern-central Apennines (Italy)
Claudio Chiarabba et al., Istituto Nazionale di Geofisica e Vulcanologia, CNT, Via di Vigna Murata 605, Roma 00143, Italy. Pages 95-104.
Data from high-density seismic networks deployed between 2000 and 2007 in the north-central Apennines (Italy) yield unprecedented images of an active orogenic wedge. Earthquake foci from the northern Apennines define a Benioff zone deepening westward from the Adriatic foreland down to about 60 km depth below the chain. The seismicity shows that only the lowermost approx. 10 km of the Adriatic foreland crust is subducted, whereas the uppermost approx. 20 km is incorporated into the orogenic wedge. Farther west, an aseismic mantle with markedly negative P-wave-velocity (Vp) anomalies is interpreted as asthenosphere flowing toward an Adriatic slab in retrograde motion. Three crustal layers with different Vp and seismicity characteristics are imaged below the northern Apennines: an uppermost 10-km-thick fast layer affected by extensional faulting, a slow layer with diffuse seismicity down to about 15 km depth, and a lowermost fast and aseismic layer resting directly above the asthenosphere. We interpret the latter layer as having formed by anhydrous crust undergoing granulitization, whereas trapped CO2 (either from the underlying granulites or from the subducting Adriatic crust) is inferred to have been responsible for both low Vp and diffuse seismicity in the middle crust. Trapped CO2 is released along the easternmost normal fault systems breaking the Apennine upper crust, consistent with geochemical and seismotectonic evidence. Compressive earthquakes at 20-25 km depth along the external front suggest offscraping of the subducting foreland crust and show that asthenospheric flow represents the primary source of ongoing shortening along the belt front.
Fault frequency and strain
Alan Morris et al., Southwest Research Institute, Dept. of Earth, Material, and Planetary Sciences, 6220 Culebra Road, San Antonio, TX 78238, USA. Pages 105-109.
Earth’s crust is the source of hydrocarbons and groundwater -- essential fluids for modern human habitation -- and provides sites for disposal of waste products such as carbon dioxide and high level radioactive waste. The rock layers that make up Earth's crust contain geologic structures that influence our ability to extract resources or sequester waste. Geologic faults, whether active or inactive, are among the most common and important of geologic structures, and the identification and characterization of faults is essential to many industries, including exploration for oil and gas, groundwater management, waste disposal, and natural hazard assessment. This paper explores the relationship between the number of faults in a portion of Earth's crust and the amount of tectonic stretching that has occurred in the area of study. Morris et al. measured faults in south-central and west Texas, and they show that the number of faults correlates directly with tectonic stretching. Their results provide a method for estimating the number of faults in areas where only the largest faults are observed, but where smaller, unseen faults also influence resource extraction and fluid flow through Earth's crust. They expect that their results can also be applied to parts of the crust that have experienced other forms of tectonic activity.
Influence of dynamic topography on sea level and its rate of change
Clinton P. Conrad, University of Hawaii, Dept. of Geology and Geophysics, 1680 East-West Road, Honolulu, HI 96822, USA; and Laurent Husson. Pages 110-120.
Sea level changes considerably over geologic time. At the beginning of the Cenozoic (65 million years ago, at the end of the age of dinosaurs), sea level was 100 to 200 m higher than it is today, and the interior of North America was flooded. Some of this sea level change may be caused by the slow deflection of Earth's surface by the dynamics of Earth's interior. These surface deflections are known as "dynamic topography" and can raise or lower Earth's surface by up to a kilometer over regions thousands of kilometers wide. Uplift of the ocean floor will decrease the volume of the ocean basins, thus causing sea level rise everywhere. Using a computer model of Earth's interior dynamics, the authors found that the ocean floor is currently being deflected upward by an average of about 100 m, which raises sea level. This sea level offset is currently increasing, causing sea level rise at rates of up to 1 m per million years. Unlike currently observed sea-level change (which is much faster), this dynamic offset of sea level produces sea-level rise that is sustained over geologic time. The magnitude of this change may be large enough to offset ice sheet formation and other climatic and tectonic processes that are thought to produce the 100-200 m of sea level drop that has occurred during the Cenozoic. Therefore, surface deflections caused by the dynamics of Earth's interior have a first-order influence on Earth's sea level and help determine which portions of the continents are submerged.
On the relation between trench migration, seafloor age, and the strength of the subducting lithosphere
Erika Di Giuseppe et al., Universita degli Studi Roma Tre, Dipartimento di Scienze Geologiche, Lgo. S. Leonardo Murialdo 1, 00146 Rome, Italy. Pages 121-128.
The subduction process takes place when one tectonic plate moves beneath another one and sinks within the Earth's mantle. It is largely known that the behavior of the subducting plate is affected by a number of parameters in competition with each other. Moreover, a plate's properties may vary with time: a plate gets old and thickens. This paper highlights the effects of plate ageing on plate behavior and kinematics and provides an important contribution to the understanding of subduction zones by combining three-dimensional numerical calculations with natural data of subduction zone kinematics.
View the complete table of contents and review abstracts for the current issue of Lithosphere at http://lithosphere.gsapubs.org/current.dtl.