T41A-01
Poly-phase Deformation Recorded in the Core of the Coast Plutonic Complex, Western British Columbia
The Coast Plutonic Complex of western British Columbia constitutes the largest batholith within the North American Cordillera. The field area for this study is Mt. Gamsby, an unexplored region above the Kitlope River, east of the Coast Shear Zone and at the southern end of the Central Gneiss Complex. The dominant lithologies on Mt. Gamsby include amphibolite and metasedimentary gneiss, gabbro-diorite, and orthogneiss. The amphibolite gneiss contains alternating amphibolite and felsic layers, with chlorite and epidote pervasive in some regions and garnet rare. This unit is commonly migmatized and contains various folds, boudins, and shear zones. The metasedimentary gneiss contains quartz, k-spar, graphite, chlorite, and perhaps cordierite, but appears to lack muscovite and aluminosilicates. The gabbro-diorite is salt and pepper in color and contains ca. 50% pyroxene and plagioclase. The orthogneiss is light in color and plagioclase-rich, with a texture varying from coarse-grained and undeformed to mylonitic. In some regions, this unit contains abundant mafic enclaves. At least four deformational events (D$_{1-4}$) are observed. The second generation of folding, F$_{2}$, is dominant in the area and resulted in the production of a large synform during sinistral shearing. The S$_{1}$ foliation is observed only in the amphibolite gneiss and is orthogonal to S$_{2}$, creating mushroom- type fold interference patterns. S$_{2}$ foliations strike NW-SE and dip steeply to the SW, suggesting SW-NE directed shortening. L$_{2}$ lineations developed on S$_{2}$ plunge shallowly to the NW and SE, implying strike-slip motion. Although both dextral and sinistral motions are indicated by shear band data, sinistral motion is dominant. The average right and left lateral shear band orientation is nearly identical to S$_{2}$, suggesting that right and left lateral shearing were synchronous. Foliations within the orthogneiss are parallel to the axes of S$_{2}$ folds and boudins in the amphibolite gneiss, suggesting that emplacement of orthogneiss was concurrent with S$_{2}$ deformation of the amphibolite gneiss. Tectonic strains calculated by the Rf-$\phi$ method using mafic enclaves in the orthogneiss vary from 4 to 10 within an area $<$1 km$^{2}$, suggesting strong strain gradients during D$_{2}$. S$_{3}$ foliations strike WNW-ESE to E-W and dip shallowly to the south, suggesting NNE-SSW to N-S shortening. L$_{3}$ lineations plunge shallowly to the SW and SE, and are associated low-angle shear bands with greenschist facies mineral assemblages which overprint higher temperature assemblages. Deformation phase D$_{4}$ is characterized by low temperature, brittle deformation as shown by discrete fault surfaces with abundant chlorite. The following tectonic history can be determined based on structural observations. Amphibolite and immature sedimentary material formed from supracrustal (e.g. basalt flows?) and intrusive protoliths. These units were then intruded by the gabbro- diorite, which was deformed by right and left lateral shear zones sometime after crystallization. Both the amphibolite and gabbro-diorite were intruded by the orthogneiss, which was emplaced as sills during right and left lateral shearing and F$_{2}$ folding. This geologic history is similar to that preserved in other parts of the Coast Plutonic Complex where dextral transpression and sinistral transtension are documented. The localization of low angle normal shear zones with greenschist facies mineral assemblages suggests extension occurred during cooling of the arc.
T41A-02
Nature of the Crust in the Laxmi Basin, Western Continental Margin of India
The nature of the crust in the Laxmi Basin, western margin of India is an uncertain issue; more importantly this has implications on paleo-geographic reconstructions of the western Indian Ocean. We have analysed three geophysical datasets and modelled gravity and magnetic anomalies for determining nature of the crust. Basement of the Laxmi Basin includes numerous highs, which make the basement uneven and shallower compared to the Western Basin. The Laxmi Basin is characterised by a broad gravity high and a narrower prominent gravity low within it, while within the basin the broad anomaly gradually increases towards north. The Panikkar Ridge is associated with the gravity low, which is comparable, at least in sign, to known negative gravity anomaly of Laxmi Ridge. Intrusive structures mapped in the Laxmi Basin coincide with significant magnetic anomalies, which were earlier interpreted as seafloor-spreading anomalies. Model studies reveal that the Laxmi Basin consists of ~14 km thick stretched continental crust, in which magmatic bodies have been emplaced, whereas Panikkar Ridge remains less altered stretched continental crust. The crust of the Laxmi Basin is mostly thinner than crust under Laxmi Ridge and continental margin. In addition to the rift-drift related stretching of the continental margin the Laxmi Basin possibly has undergone extra stretching in E-W direction during the pre-Tertiary period. At ~68 Ma Deccan volcanism on western India may have disrupted the initial conditions that were leading to onset of spreading in the basin. Subsequently the Réunion hotspot had emplaced the volcanic material within the stretched thinned continental crust. We interpret the Laxmi Basin as a failed rift, undergone stretching following intraplate kinematics prior to Deccan volcanism. Key words: Laxmi Basin, Laxmi Ridge, Panikkar Ridge, stretched continental crust, Deccan volcanism, northwest continental margin of India
T41A-03
Perigee: Zero, a Unified Theory of Cultural Heritage and Geological History
Perigee: Zero proposes a unified theory to solve enigmas haunting our cultural heritage and the Earth's geological history. We maintain the theory is simple and provable. We document the terraforming of the Earth by the cratering and accretive action of impacting comets and resulting ejecta, during events occurring at regular intervals over the past 15,000 years. Visual evidence of geomorphed landscape is presented using the perspective of high-resolution satellite imaging and DEM data. Correlation of these images with physical and historic evidence has proven supportive. Sections of the lithosphere have been lofted thousands of kilometers. Excised trenches have filled with water, or left as barren desert. Blankets of ejecta have overlain the continents and their inhabitants. The atmosphere was disrupted by the influx of dust and energy. Composed of hydrated silica, the comets have added a significant percentage of the water and unusually pure silicate (as sand and clay) now present on earth. Human history is immersed in these events. Oral and written works comprising much of the world's cultural heritage provide us with accounts of catastrophic damage visited upon Earth and its inhabitants. Those accounts describe mountains being moved, large expanses of the earth being swept clean - or replaced with open ocean, civilizations buried, years of darkness, the world flooded. Our forefathers invoked deities from the heavens, or perhaps giant kangaroos. We invoke impactors from the Taurid Complex, a simple and natural process, as most of its progenitor's mass may be missing. The damage to the earth is interpreted as the result of hundreds of grazing and nearly tangential impacts, suggesting decaying orbits of earth-captured cosmic bodies as a plausible solution. The events are seen occurring when the low point of the orbit -the perigee- reaches the earth's surface; in effect Perigee: Zero.
<a href='http://perigee-zero.org/treatise'>http://perigee-zero.org/treatise</a>
T41A-04
Analysis of Mineral Inclusions in Zircon From the Eclogite-bearing Ashe Metamorphic Suite, North Carolina: Implications for Exhumation History
Eclogite is typically exhumed by either entrainment of large blocks in an accretionary m\'{e}lange or by extensional unroofing of a regional eclogite-facies terrane. The Ashe Metamorphic Suite (AMS) in the Blue Ridge Mountains of North Carolina contains kilometer-scale eclogite blocks within what appears to be an accretionary wedge that has been metamorphosed to amphibolite facies. The eclogite-facies mineral lineation in the separate blocks are roughly parallel to each other, suggesting that these rocks were exhumed as a coherent eclogite-facies terrane rather than individual blocks in an accretionary m\'{e}lange, where upward flow would disorient blocks. If the eclogite blocks were exhumed within a coherent eclogite-facies terrane, the intervening amphibolite-facies rocks would have once been at eclogite facies. To test this hypothesis we analyzed mineral inclusions in zircons from the intervening metasedimentary and metaigneous rocks using optical petrology, SEM-EDS and Raman spectroscopy. Zircons formed during peak metamorphism are frequently preserved during subsequent metamorphic events. Mineral inclusions in these zircons would preserve a record of peak metamorphism that is no longer evident in the bulk mineralogy. Previous work showed that zircons from eclogite and retrogressed eclogite contain inclusions of eclogite-facies minerals, including omphacite, rutile, and garnet. If the amphibolite-facies rocks surrounding the eclogite had experienced eclogite facies, some of the zircons from these rocks should preserve eclogite-facies minerals. In our work so far, we have not identified any eclogite-facies minerals in zircon from the AMS. Thus, there is no evidence that the AMS metasedimentary and metaigneous rocks ever reached eclogite facies. An exhumation model for the AMS must explain the alignment of large eclogite blocks, as well as, the absence of eclogite-facies metamorphism in the intervening metasedimentary and metaigneous rocks. Thus, the process by which the AMS eclogite blocks were exhumed was more complex than either of the typical exhumation models. Modification of preexisting models may explain these observations: 1) exhumation as a coherent eclogite-facies terrane followed by complete amphibolite facies recrystallization of portions of the terrane, possibly through channelized fluid flow, or 2) exhumation in an accretionary m\'{e}lange under unique conditions such that extremely large eclogite blocks (kilometer-scale) are transported without significant disruption of their orientation. Continuing research of the metasedimentary and metaigneous rocks of the AMS will further test these two models.
T41A-05
Crustal discontinuities of the Northern Andes interpreted from satellite, airborne and terrestrial gravity data
The tectonic analysis of northwestern South America shows a complex tectonic setting with regional crustal discontinuities that can be detected from satellite and terrestrial gravity data sets. The advantage of using gravity data at high altitudes, for instance 20 km , relays on reducing the terrain gravity effects while keeping the ability to measure the root gravity effects related to the crustal - mantle interface. By applying an inverse modeling of point source solution we can estimate the depth to gravity MOHO for the area. It allows us to account for the regional components of the gravity anomalies that can be removed from total gravity fields obtaining the residual gravity components, enhancing the location of causative bodies at the shallow crust. Thickness of the continental crust varies from 30 km to 47 showing that the mountain ranges are partially isostatic compensated by continental roots. A thicker continental crust provides a better scenario for magmatic segregation of parental magmas to occur. Those magmas are generated at the converging continental margin, and ascend through the continental crust where being contaminated by crustal elements. FAGA at regional and local scales show the mass distribution into the crust adding root and topographic effects. FAGA maps are useful to outline upper crustal discontinuities and gravity zones. Bouguer anomaly maps show density mass distribution of the subsurface after removing the topographic effects from FAGA. These maps are also useful to outline crustal discontinuities at depth. Those discontinuities detected at FAGA and Bouguer anomaly maps show a vertical extent of the crustal discontinuities. Regional profiles of terrain correlated FAGA (TCFAGA) and gravity Moho are useful to model the subduction zone of the pacific and the geometry of oceanic and continental crusts. The oceanic subducting slab has an initial dip angle of 10o in the first 70 km depth. Deeper and steeper portions of the subducting slab are expected, at depths greater than 100 km, but are not detected / modeled by the gravity data
T41A-06
Evidence for Fluid Flow Along the Husavik Flatey Fault, Northern Iceland.
Geophysical data collected offshore Northern Iceland (2001, 2003) imaged large pockmarks along the Husavik Flatey Fault system (HFF). The distribution of pockmarks observed in the bathymetry and side-scan is predominantly along the footwall block within close proximity of the fault scarp ($\sim$200 m). Our study area is located in Skjálfandí Fjord, offshore Northern Iceland, between the onshore expression of the Neovolcanic Zone and the southern extent of Kolbeinsey Ridge. There are numerous asymmetric pockmarks in the study area (50-250 m water depth) that suggest either a structural control or a current control on their formation and evolution. The HFF runs east west across the basin, and the character of the fault changes significantly between the western (15 m throw) and eastern sections (0-2 m throw), which are separated by a topographic high ($\sim$40 m high). The sidescan data along the western portion of the HFF imaged large elongate compound pockmarks at a high-angle to the fault (10-50 m maximum length). Conversely few pockmarks are observed along the eastern section of the HFF. The CHIRP subbottom data across the western segment of the HFF imaged relic pockmarks, attesting to the persistent nature of pockmarks in this area. These large pockmarks near the HFF appear to record fluid expulsion, and their location suggests a structural control Using TowCam CTD data and bottom photographs to define the distribution of biota along the fault, we attempted to determine if the fluid venting is active in this region. TowCam photos collected along the fault imaged small single and compound pockmarks (cm's in diameter) along the seafloor, which appear to be biogenic in origin with evidence of recent sediment excavation. Other pockmarks near the fault may record fluid expulsion and the CTD data will be presented that places important constraints on active fluid expulsion.
T41A-07
Tectonic activity maps of the Earth: Final report
Tectonic activity maps of the Earth: Final report Lowman, Paul.D.,Jr. (Paul.D.Lowman@nasa.gov), Goddard Space Flight Center (Code 698),Greenbelt, MD 20771 United States This paper presents final versions of a series of global tectonic and volcanic activity maps whose compilation began in 1977 as part of the NASA Crustal Dynamics Project. They show tectonic and volcanic features active within the last one million years, a period long enough to be truly representative of present activity but short enough that structures produced by tectonism and volcanism are still geomorphically recognizable. The primary maps have been prepared with equatorial van der Grinten and Robinson projections, both centered on the prime meridian to avoid splitting land masses. Polar maps have also been prepared showing Arctic and Antarctic tectonism, volcanism,and seismicity. Robinson projection maps are in two versions, one with a digital topographic base, the other schematic. Supporting maps on matching scales and projections include global seismicity (200,855 epicenters, 1963-1998), satellite-derived gravity maps, and VLBI site motions. Sea- floor spreading rates are from NUVEL-1. The maps have been widely used in scientific and educational publications, and have been frequently accessed on the Internet. They bring out several aspects of global tectonic and volcanic activity not shown well on conventional maps. These include intraplate tectonism, seismicity , and volcanism. They show the unrealistic nature of most "plate maps" in continental areas, plate boundaries being generally broad and diffuse, and many supposed plates (e.g., Anatolian) being pervaded by seismic activity. True continental margins, the sialic/mafic crust boundaries, are far more irregular than generally shown; isolated fragments of sialic crust (e.g. Seychelles) are also brought out by the maps. Problems unsolved by classic plate tectonic theory, such as the fact that the African and Antarctic plates are surrounded by spreading centers,are illustrated. Intraplate volcanism is more widespread, viewed on a million year time scale, than generally realized, suggesting the importance of mantle plumes. The maps show anomalies based on choice of reference frames; for example, space geodesy sites in western Europe appear to be moving northeast if the Pacific Plate is taken as the reference, whereas a more geologically realistic motion, to the southeast, results if stable North America is used.
T41A-08
Permian-Triassic Mascon in Antarctica
A prominent positive GRACE satellite-measured free-air gravity anomaly over regionally depressed subglacial topography may identify a mascon centered on (70oS, 120oE) between the Gamburtsev and Transantarctic Mountains of East Antarctica. Being more than twice the size of the Chicxulub crater, the inferred Wilkes Land impact crater is a strong candidate for a Gondwana source of the greatest extinction of life at the end of the Permian. Its ring structure intersects the coastline and thus may have strongly influenced the rifting of East Antarctica from Australia that resulted in the enigmatic lack of conjugate crustal thinning on the Australian block.