Showing papers in "Geophysical monograph in 2007"

Atmospheric Composition, Chemistry, and Clouds

Abstract: Venus’ atmosphere has a rich chemistry involving interactions among sulfur, chlorine, nitrogen, hydrogen, and oxygen radicals. The chemical regimes in the atmosphere range from ion-neutral reactions in the ionosphere to photochemistry in the middle atmosphere to thermal equilibrium chemistry and surface-atmosphere reactions in the lower atmosphere. This variety makes Venus an important planet to understand within the context of terrestrial-like planets, both in our own solar system and outside it. The primary chemical cycles are believed known but surprisingly few details about these cycles have been fully verified by concurrence among observations, experiments, and modeling. Good models have been developed that account for many properties of the cloud layers, but the size distribution, shape, and composition of the majority of the aerosol mass are still open issues. This chapter reviews the state of knowledge prior to the Venus Express mission for the composition, chemistry, and clouds of the neutral atmosphere on Venus. Observations by instruments on Venus Express, in combination with ground-based observations, laboratory experiments, and numerical modeling, should answer some of the major open questions regarding the composition, chemistry, and clouds of Venus’ atmosphere.

Geochemistry of primitive lavas of the Central Kamchatka Depression: magma generation at the edge of the Pacific plate

Abstract: New and published major and trace element and isotope (O, Sr, Nd) compositions of the Late Quaternary rocks from the Central Kamchatka Depression (CKD) are used to demonstrate systematic changes in magma genesis along the northern segment of the Kamchatka Arc, above and north of the subducting Pacific slab edge. We envision a number of possible petrologic scenarios for magma generation beneath the CKD and formulate quantitative mass-balance models which lead to three major conclusions departing significantly from previous interpretations of the CKD rocks. First, this study demonstrates that eclogite melts contribute to the composition of virtually all CKD lavas and could be the major agent transferring material from the subducted slab to the mantle wedge, including fluid-mobile elements (e.g., K, Ba). Second, thermal state of the mantle wedge beneath the CKD has primary control on the major composition of primitive magmas, favoring production of low temperature andesitic and dacitic mantle melts toward the slab edge. Third, hydrous slab-fluids might not be required to generate CKD magmatism. Instead, strong enrichment in LILE, high δ 18 O and 87 Sr/ 86 Sr, in some CKD magmas could originate from assimilation of hydrothermally-altered mafic lithosphere. Several concurring factors could facilitate partial melting of the subducting slab beneath the all CKD volcanoes and favor variable modification of the eclogite melts during interaction with the mantle wedge. Large input from slab melting makes CKD magmatism unique in Kamchatka and may contribute to the CKD volcanoes being the most productive arc volcanoes on Earth.

Yakataga Fold-and-Thrust Belt: Structural Geometry and Tectonic Implications of a Small Continental Collision Zone

Abstract: Collision of the Yakutat terrane with southern Alaska created a collisional fold-and-thrust belt along the Pacific―North America plate boundary. This southvergent fold-and-thrust belt formed within continental sedimentary rocks but with the seaward vergence and tectonic position typical of an accretionary wedge. Northward exposure ofprogressively olderrocks reflects that the fold-and-thrust belt forms a southward-tapered orogenic wedge that increases northward in structural relief and depth of erosion. Narrow, sharp anticlines separate wider, flat-bottomed synclines. Relatively steep thrust faults commonly cut the forelimbs of anticlines. Fold shortening and fault displacement both generally increase northward, whereas fault dip generally decreases northward. The coal-bearing lower part of the sedimentary section serves as a detachment for both folds and thrust faults. The folded and faulted sedimentary section defines a regional south dip of about 8°. The structural relief combined with the low magnitude of shortening of the sedimentary section suggest that the underlying basement is structurally thickened. I propose a new interpretation in which this thickening was accommodated by a passive-roof duplex with basement horses that are separated from the overlying folded and thrust-faulted sedimentary cover by a roof thrust with a backthrust sense of motion. Basement horses are ∼7 km thick, based on the thickness between the inferred roof thrust and the top of the basement in offshore seismic reflection data. This thickness is consistent with the depth of the zone of seismicity onshore. The inferred zone of detachment and imbrication of basement corresponds with the area of surface exposure of the fold-and-thrust belt within the Yakutat terrane and with the Wrangell subduction zone and arc farther landward. By contrast, to the west, the crust of the Yakutat terrane has been carried down a subduction zone that extends far landward with a gentle dip, corresponding with a gap in arc magmatism, anomalous topography, and the rupture zone of the 1964 great southern Alaska earthquake. I suggest that, to the east, detachment and imbrication of basement combined with coupling in the fold-and-thrust belt allowed the delaminated dense mantle lithosphere to subduct with a steeper dip than to the west, where buoyant Yakutat terrane crust remains attached to the subducted lithosphere. According to this interpretation, the Wrangell subduction zone is lithosphere of the Yakutat terrane, not Pacific Ocean lithosphere subducted beneath the Yakutat terrane. The Pacific-North America plate boundary would be within the northern deformed part of the Yakutat terrane, not along the boundary between the undeformed southern part of the Yakutat terrane and oceanic crust of the Pacific Ocean. The plate boundary is an evolving zone of distributed deformation in which most of the convergent component has been accommodated within the fold-and-thrust belt south of the northern boundary of the Yakutat terrane, the Chugach-St. Elias thrust fault, and most of the right-lateral component likely has been accommodated on the Bagley Icefield fault just to the north.

Geophysical Structure of the Southern Alps Orogen, South Island, New Zealand

Abstract: The central part of the South Island of New Zealand is a product of the transpressive continental collision of the Pacific and Australian plates during the past 5 million years, prior to which the plate boundary was largely transcurrent for over 10 My. Subduction occurs at the north (west dipping) and south (east dipping) of South Island. The deformation is largely accommodated by the ramping up of the Pacific plate over the Australian plate and near-symmetric mantle shortening. The initial asymmetric crustal deformation may be the result of an initial difference in lithospheric strength or an inherited suture resulting from earlier plate motions. Delamination of the Pacific plate occurs resulting in the uplift and exposure of mid-crustal rocks at the plate boundary fault (Alpine fault) to form a foreland mountain chain. In addition, an asymmetric crustal root (additional 8 - 17 km) is formed, with an underlying mantle downwarp. The crustal root, which thickens southwards, comprises the delaminated lower crust and a thickened overlying middle crust. Lower crust is variable in thickness along the orogen, which may arise from convergence in and lower lithosphere extrusion along the orogen. Low velocity zones in the crust occur adjacent to the plate boundary (Alpine fault) in the Australian and Pacific plates, where they are attributed to fracturing of the upper crust as a result of flexural bending for the Australian plate and to high pressure fluids in the crust derived from prograde metamorphism of the crustal rocks for the Pacific plate.

A Comparison Between the Transpressional Plate Boundaries of South Island, New Zealand, and Southern California, USA : The Alpine and San Andreas Fault Systems

Abstract: There are clear similarities in structure and tectonics between the Alpine Fault system (AF) of New Zealand’s South Island and the San Andreas Fault system (SAF) of southern California, USA. Both systems are transpressional, with similar right slip and convergence rates, similar onset ages (for the current traces), and similar total offsets. There are also notable differences, including the dips of the faults and their plate-tectonic histories. The crustal structure surrounding the AF and SAF was investigated with active and passive seismic sources along transects known as South Island Geophysical Transect (SIGHT) and Los Angeles Region Seismic Experiment (LARSE), respectively. Along the SIGHT transects, the AF appears to dip moderately southeastward (~50 deg.), toward the Pacific plate (PAC), but along the LARSE transects, the SAF dips vertically to steeply northeastward toward the North American plate (NAM). Away from the LARSE transects, the dip of the SAF changes significantly. In both locations, a midcrustal decollement is observed that connects the plate-boundary fault to thrust faults farther south in the PAC. This decollement allows upper crust to escape collision laterally and vertically, but forces the lower crust to form crustal roots, reaching maximum depths of 44 km (South Island) and 36 km (southern California). In both locations, upper-mantle bodies of high P velocity are observed extending from near the Moho to more than 200-km depth. These bodies appear to be confined to the PAC and to represent oblique downwelling of PAC mantle lithosphere along the plate boundaries.

Deformation-induced mechanical instabilities at the core-mantle boundary

Abstract: Our understanding of the core-mantle boundary (CMB) region has improved significantly over the past several years due, in part, to the discovery of the post-perovskite phase. Sesimic data suggest that the CMB region is highly heterogeneous, possibly reflecting chemical and physical interaction between outer core material and the lowermost mantle. In this contribution we present the results of a new mechanism of mass transfer across the CMB and comment on possible repercussions that include the initiation of deep, siderophile-enriched mantle plumes. We view the nature of core-mantle interaction, and the geodynamic and geochemical ramifications, as multiscale processes, both spatially and temporally. Three lengthscales are defined. On the microscale (1-50 km), we describe the effect of loading and subsequent shearing of the CMB region and show how this may drive local flow of outer core fluid upwards into D". We propose that larger scale processes operating on a mesoscale (50-300 km) and macroscale regimes (> 300 km) are linked to the microscale, and suggest ways in which these processes may impact on global mantle dynamics.

Venus Express and Terrestrial Planet Climatology

Abstract: After a delay of more than a decade, the exploration of Venus has resumed through the European Venus Express mission, now in orbit around the planet. The mission payload, its implementation in an elliptical polar orbit, and the science operations planned, all focus on outstanding problems associated with the atmosphere and climate of Venus. Many of these problems, such as understanding the extreme surface warming produced by the carbon dioxide-driven greenhouse effect, and the role of sulfate aerosols in the atmosphere, have resonances with climate-change issues on the Earth and Mars. As data on all three terrestrial planets accumulates, and models of the energy balance and general circulation of their atmospheres improve, it becomes increasingly possible to define and elucidate their behavior in a common, comparative framework. Venus Express seeks to contribute to progress in this area.

Constraints on the Presence or Absence of Post-Perovskite in the Lowermost Mantle From Long-Period Seismology

Abstract: In this study, the ability to detect post-perovskite is examined using three long-period seismology approaches; normal modes, arrival time variations, and seismic tomography Although normal modes provide robust integral constraints on 1D velocity and density profiles of the Earth, their ability to resolve variations in shear velocity and density decreases near the core-mantle boundary (CMB) Therefore, it is possible for post-perovskite to exist globally within 200 km of the CMB without resolution by normal modes Deep-turning long-period S wave arrival times are examined in regions of dense ray coverage and a shift representing an increase in velocity consistent with presence of post-perovskite is observed in localized areas approximately 100 km above the CMB Therefore, post-perovskite is not a ubiquitous feature but may be locally stable within both tomographically slow and fast regions near the CMB To globally determine if regions are cold or iron-enriched enough for post-perovskite to be stable, recent thermo-chemical models are applied to a suite of geotherms and phase transition depths The results suggest that variations in iron content are too small to control the stability of post-perovskite It is demonstrated that only a narrow range of temperatures and phase transition depths can allow post-perovskite to exist in anomalously fast (cold) regions in the lowermost mantle Furthermore, if post-perovskite is a major constituent of fast regions, then the temperature at 2780 km depth ranges from 2400 K - 2700 K, and post-peorvskite does not explain the observed anti-correlation of shear velocity and bulk sound speed near the CMB