T43C-1332 1340h
Westward-directed Grenvillian thrusting on the western margin of Fennoscandia: evidence from syn-kinematic pegmatites.
Magmatism is often described as being syn-kinematic where one or more increments of intrusion punctuate deformation with successive generations of injections being progressively deformed. Recent studies have also demonstrated that there is a strong link between sites of concentrated magmatism and crustal deformation zones. Pegmatite formation in the Mesoproterozoic of south Norway has always been considered as post-kinematic in nature relative to Sveconorwegian (Grenvillian) deformation (1.13Ga to ~0.85Ga) during accretion of the SW margin of Baltica. We present structural data demonstrating that the pegmatites are kinematically related to fold geometries associated with peak metamorphism and form an integral part of the deformation episode associated with terrane accretion. Undeformed pegmatites are emplaced in sub-horizontal fractures suggesting that the maximum compressive stress was sub-horizontal. The pegmatites display a systematic deformation pattern that is consistent with deformation in the limbs of the isoclinal folds in the country rock into which they intrude. The sense of shear of deformation kinematics on the pegmatites reverse across the isoclinal fold limbs suggesting that the pegmatites are syn-deformational and that they have been injected into fractures intrinsically linked to the fold development. Pegmatites are also deformed into asymmetric anticlinal folds above thrust structures and are cut by thrust structures. We also present data which demonstrates that the style of deformation changes with proximity to the major terrane-bounding thrust structure and that the pegmatites demonstrate classic imbricate style geometries on a regional scale related to regional transpression. This evidence suggests that the pegmatites are syn-deformational and were injected into thrust-related fractures and that the pegmatites are structurally related to Sveconorwegian fold geometries associated with peak metamorphism at approximately 1.14Ga. Deformation was progressive and incremental with longer periods of ductile deformation at low strain rate punctuated by shorter periods of fracturing and pegmatite injection at high strain rate. The pegmatites also, therefore, delineate the orogenic event responsible for overthrusting of the Bamble Terrane with the underlying Telemark Terrane during crustal accretion on the western margin of Fennoscandia. We also present preliminary Re-Os data from some of these pegmatites to date the exact timing of thrusting. This work therefore implies an intimate spatial and temporal relationship between deformation and magmatism during crustal accretion on the western margin of Fennoscandia.
T43C-1333 1340h
Morphostructural Analysis of an Escape Tectonic Zone : the North-Western Venezuelan Andes
Lateral motions in active collisional areas have been mainly described in the Asian and European belts. The most common view is that part of the intracontinental convergence between to plates is accommodated by major strike-slip faults bounding a lithospheric block that laterally moves away. This process, privileging horizontal compressional forces at boundaries of the moving block corresponds to an extrusion or escape of the continental landmass. Diffuse extensional deformation within the block implies that buoyancy forces, leading eventually to an extensional collapse, must be taken into account. Escape of the block is perpendicular to the convergence direction of the orogen and can only occur if a free lateral boundary exists. Our study concerns the analysis of the deformation in the northwestern part of the Venezuelan Andes, located in the northern part of South America. It is based on analysis of satellite and Digital Elevation Model imagery, complemented by field structural observations. We shall describe new tectonic features advocating that part of the belt is subjected to lateral escape tectonics during the Plio-Quaternary. The zone displays major active faults forming crustal blocks having a triangular shape pattern, and which are moving toward the north-east. The triangular corners are composed of smaller pluri-kilometre size crustal blocks bounded by normal faults. A major result is that in an area characterized by on-going intracontinental convergence, extension and strike-slip deformations predominate. In order to better constrain the depth of the deformation, we have generated a 3-D geologic model displaying the seismicity the area showing that the earthquakes are mainly restricted to the crust. The type and distribution of the deformation are not consitent with the behaviour of a simple rigid extrusion. The Venezuelan Andes are rather composed of crustal blocks that are tilted and move relative to each others. This is consistent with extensional crustal deformation related to the escape of a main triangular block (Trujillo block) towards the Caribbean plate that constitutes a free boundary towards which the Venezuelan belt landmass can easily flow.
T43C-1334 1340h
Rootless Mountains and Gravity Lows in the Sangre de Cristo Mountains, Southern Colorado-Northern New Mexico
Gravity lows over large portions of the Sangre de Cristo Mountains of the southern Rocky Mountains are a geophysical curiosity. Two very low gravity anomalies in the continental United States are found in southern Colorado, in the San Juan Mountains and in the Colorado Mineral belt. Gravity modeling implies that these gravity lows may be attributed to granitic batholiths emplaced at a shallow depth. However, low gravity anomalies along the Sangre de Cristo Mountains cannot be attributed to subsurface batholiths. The Sangre de Cristo Mountains are largely composed of Proterozoic basement and Paleozoic sedimentary rocks. Exposed and uplifted, this presumably dense, Proterozoic basement in the Sangre de Cristo Mountains should be associated with gravity highs, but this is not the case. In this study, we focused on two gravity lows in northern New Mexico-southern Colorado. One is centered over the Sangre de Cristo Mountains in Colorado and northernmost New Mexico, and the other is located near Mora, New Mexico. The northern low can be attributed to Precambrian rocks being thrust over less dense Paleozoic rocks resulting in a rootless basement. In the Mora area, the low is attributed to unusually low-density Precambrian granitic rocks (the 1.68 Ga Guadalupita pluton) underlying a thick sequence.
T43C-1335 1340h
Crustal Structure of the Tarim Basin and Northern Edge of the Tibetan Plateau
We have studied the crustal velocity structure of the northeastern margin of the Tibetan plateau in northwest China using seismic wide-angle reflection/refraction data. The 1200-km-long profile starts at the northern margin of the Tarim basin, crosses the eastern Tarim basin, Altyn Tagh Range and Qaidam basin and ends at the eastern margin of the Qaidam basin. The crust beneath the sedimentary fill of the Tarim basin shows a typical platform/shield crust with an upper (6.1 km/s), middle (6.6 km/s), and lower crust (7.25 km/s) composed of felsic, intermediate and mafic rocks, respectively. In contrast, the crustal velocity structure beneath the Altyn Tagh Range and Qaidam basin suggests that high-velocity ($>$6.9 km/s) mafic lower-crustal material is missing. The Altyn Tagh crust consists of a felsic upper (6.2 km/s), intermediate middle (6.4 km/s), and intermediate-to-mafic lower crust (6.75 km/s); and the Qaidam crust has a felsic upper (6.2 km/s), intermediate middle (6.4 km/s) and lower crust (6.6 km/s). Along the profile, crustal thickness smoothly increases with a thickness of $\sim$50 km beneath the Tarim basin, $\sim$55 km beneath the Atyn Tagh Range and $\sim$60 km beneath the Qaidam basin. The Altyn Tagh crust is underlain by a wedge-shaped area with velocities of $\sim$7.7 km/s which may indicate a crust-mantle mix (i.e. mafic to ultramafic composition). The wedge reaches down to a maximum depth of 90 km below the East Altyn Tagh strike-slip fault suggesting intracontinental subduction of the Tarim mantle (with some involvement of lower Tarim crust) beneath the Altyn Tagh Range.
T43C-1336 1340h
Contemporary Crustal Deformation Around Southeast Borderland of Tibetan Plateau
We derive horizontal velocity field in the southeast borderland of Tibetan Plateau using GPS data collected from the Crustal Motion Observation Network of China between 1998 and 2001. Our result shows a complex spectrum of deformation field in the region. The crust is fragmented into tectonic blocks of various sizes, separated by strike-slip and transtensional faults. Being driven mainly by eastward extrusion and gravitational buoyancy force from the west and resisted by stable south China block to the east, the region south and west of the Xianshuihe-Xiaojiang fault system turns from eastward to southward motion, resulting in a cluster of clockwise rotation for its internal sub-blocks. Most noticeable regional deformation includes: 9-10 mm/yr left-slip across the Xianshuihe fault, $\sim$7 mm/yr left-slip across the Anninghe-Zemuhe-Xiaojiang fault, $\sim$4 mm/yr NW trending right-slip shear deformation around the south segment of the Lancang River fault, $\sim$3 mm/yr left-slip across the Lijiang fault, and 5-6 mm/yr right-slip across a deformation zone located $\sim$150 km northwest of and in parallel with the Longmen Shan fault. There is no noticeable deformation across the southern segment of the Red River fault; instead, the fault seems to be cut orthogonally by a shear zone which is continued south-southeastward from the Xiaojiang fault with $\sim$6 mm/yr left-slip across. Our results overall are more consistent with predictions of dynamic models assuming a weak lithosphere, particularly the lower crust, than that assuming a strong lithosphere of Tibet.
T43C-1337 1340h
Shortening calculations at the western end of the Indo-Asian collision zone: Implications for the evolution of the Tibetan orogenic belt
One of the most striking aspects of the Indo-Asian collision zone is the asymmetry of thickened and elevated crust north of the Indian continent, increasing in north-south width from $\sim$500 km in the Pamir/Western syntaxsis region, to $\sim$1,500 km along the central and eastern Tibetan Plateau. Determining whether this asymmetry is the result of east-west variation in the magnitude of convergence between India and Asia, or other processes and boundary conditions, is critical in understanding the evolution of the orogenic belt. To address this, we calculated the amount of shortening for the western end of the Indo-Asian collision zone by area balancing a cross-section along a north-south line at 73$\deg$E. Crustal thicknesses were interpreted to be $\sim$40 km thick for the Indian plate, increasing to $\sim$70 km beneath the Pamir-Karakoram-Kohistan region based on previously published studies. Our results yield a minimum of 735 km of north-south shortening partitioned between four processes: 1) 295 km of northward underthrusting of the Indian continent beneath the southern margin of Asia based on $\sim$200 km of upper crustal shortening within India (e.g. DiPietro and Pogue, 2004), and a minimum of 95 km of Early Eocene subduction along the Main Mantle Thrust (based on Early Eocene coesite bearing eclogites). 2) $\sim$130 km of internal shortening and crustal thickening within the Karakoram and Pamir, assuming a pre-Tertiary crustal thickness of 35 km. 3) ~280 km of southward subduction of Asian lithosphere (Tarim) beneath the Pamir to a depth of ~200 km. 4) ~30 km of crustal shortening and thickening in the Tian Shan. Results of this study have two primary implications: 1) 295 km of underthrusting suggests that Indian lithosphere has been inserted beneath the Pamir/Western syntaxsis region as far north as the southern Pamirs (37.5$\deg$N). Therefore, $\sim$2/3 of the thickened crust beneath the Pamirs/Western Syntaxsis region can be explained by underplating of Indian lithosphere, suggesting only $\sim$130 km of internal north-south shortening. 2) The total magnitude of shortening at the western end of the collision zone is similar to estimates of the amount of underthrusting of India beneath the Tibetan Plateau along the central Himalayas ($\sim$700 km, DeCelles et al., 2002). Assuming Cenozoic shortening north of the Indus Suture Zone of several hundred km, these results indicate a westward decrease in the total amount of convergence between India and Asia.
T43C-1338 1340h
An integrated approach for determination of nature of the crust in the Laxmi Basin, western continental margin of India
The nature of the crust in the Laxmi Basin, northwest continental margin of India is an uncertain issue and more importantly this is limiting our understanding of the evolution of the lithosphere in the Arabian Sea. The aspect has become a key issue principally from the point of paleogeographic reconstructions of the western Indian Ocean. In order to determine the nature of the crust below the Laxmi Basin we have analysed two new geophysical datasets in combination with previously published datasets, thereafter gravity and magnetic anomalies were modelled with the constraints from seismic reflection and refraction results. The sediments derived mostly from the Indus cone are generally thicker on west of the Laxmi Ridge than that in the Laxmi Basin. The basement in the Laxmi Basin includes numerous intrusive structures and faulted blocks, thus the basement topography becomes highly irregular and shallower compared to that of the Western Basin. The ship-borne and satellite gravity anomalies had enabled to map the regional extent of the structures in the basin especially the gravity lows associated with the Laxmi and Panikkar ridges. Intrusive structures mapped in the Laxmi Basin are seen coinciding with significant magnetic anomalies, but these anomalies were earlier considered as pre-Tertiary spreading-related anomalies. Free-air gravity anomalies of the Laxmi Ridge and Laxmi Basin are in general anomalous as they are associated with negative and positive gravity anomalies respectively. Within the basin the gravity anomaly is gradually rising toward north, reaches 40 mGal more, indicating the accretion of additional magmatic material toward north within crust. The velocity structures of the Laxmi Ridge, Laxmi Basin, Western Basin, Seychelles Bank and Indian continental shield reveal the Moho boundary only below the Seychelles Bank, Western Basin and Indian subcontinent. An anomalous velocity layer, 7.15 to 7.19 km/s is observed in the lower crust of the Laxmi Basin and Laxmi Ridge. The velocities are contrastingly deviating from the normal velocity structure of both continental and oceanic crust. On close observation of velocity structure we found that 6.2 - 6.3 km/s layer is conspicuously present in the upper crust of all the geological provinces except below the Western Basin. Instead the Western Basin bears about 6.7 km/s velocity layer, which is considered as a normal velocity for layer 3 of the oceanic crust. As the velocity layer 6.2 - 6.3 km/s in general represents the upper crustal rocks (granitic gneisses) of the continental crust, the similar velocity layer below the Laxmi Basin may be attributed to upper crustal rocks of the continental crust. The gravity and magnetic model studies have revealed that the Laxmi Basin consists of 11-14 km stretched continental crust, in which magmatic bodies have been emplaced, while the Panikkar Ridge in the middle of the basin has relatively less disturbed stretched continental crust. The seaward dipping reflectors (SDRs) on western margin of the Laxmi Ridge and sharp changes in regional gravity and magnetic fields across it lead to place the ocean-continent boundary on west of the Laxmi Ridge. The Reunion hotspot, when the basin was passing over it, had emplaced the volcanic material in the form of dykes and sills within existing stretched continental crust. We, therefore, believe that the Laxmi Basin basically consists of continental crust, which subsequently got modified by extensive stretching and volcanic outpourings of the Reunion hotspot.
T43C-1339 1340h
Crustal Structure Across Coast Shear Zoone in SE Alaska and Western British Columbia: Extension of ACCRETE Wide-Angle Results to 3-D
The accreted terranes and the continental arc in southeastern Alaska and western British Columbia represent one of the best areas to study crustal structures and processes for the growth of continental crust. In particular, the structure and formation of the Coast Shear Zone (CSZ) and the Coast Mountains Batholith (CMB) is critical for understanding these processes. In 1994, the multidisciplinary ACCRETE project was carried out in this area targeting the structural contrasts across the CSZ. Seismic profiles consist of closely spaced marine air gun shot lines into 3C recording stations on land yielding densely spaced receiver gathers. Present research extends the previous ACCRETE interpretation of the crustal structure across the CSZ into 3D using two additional wide-angle lines. Line 1255, which is perpendicular to the CSZ, extends for ~80 km to the west of the CSZ, and line 1256 extends for ~200 km on the west of the CSZ, approximately parallel to the CSZ. With the basically 2-D data from line 1255 and 3-D data from line 1256 with 4 consecutive stations, a 1-D velocity model is determined to the west of the CSZ. Thereafter, several groups of 1-D models are derived to constrain the variations of the upper crustal structure across and along the strike of CSZ. A 2-D model is then obtained from Moho reflections to constrain the variations of Moho depth across the CSZ. To the west of the CSZ, the crust is modeled as consisting of 4 layers with velocities increasing from ~6.0 km/s in the upper crust to 6.8 km/s near the Moho depth of ~27 km. A shallow Tertiary graben is identified by ~300 ms advances in both the first arrivals and the reflections at the west end of shot line 1255 and the south end of shot line 1256. The observed variations of the Moho depth are in agreement with those obtained previously along the main ACCRETE transect (~27 km to the west and ~32 km to the east of the CSZ), and show that the Moho ramp is characteristic of the CSZ. The upper mantle velocity is ~7.7-7.8 km/s indicating high mantle temperature.
T43C-1340 1340h
Mapping Geology and Structure in Hyper-Rugged Terrain Using ASTER Remote Sensing Data: A Case Study From Northern Pakistan
Extreme topography in mountainous terrane introduces radiometric distortion in remote sensing data. This effect influences the ability to correctly map lithology based on unique spectra. In this study we utilize several topographic correction regimes to accommodate spectral variation caused by topography, including the Cosine-correction and the Minnaert-correction methods. The study area is located in Chitral, northern Pakistan. In this area, the world's three greatest mountain ranges, the Himalayas (represented by Kohistan terrane), the Karakoram, and the Hindukush terrane merge together. The Himalayas are separated from the Karakoram terrane by the Shyok Suture. Whereas, the Karakoram plate is separated from the Hindukush terrane by the Reshun Fault. The area is extremely rugged; local relief is more than 2,500 meters. Many peaks are higher than 5,000 m and Tirch Mir, 12 km north of the mapped area, is 7,702 m high. ASTER imagery and a digital elevation model developed from the ASTER data is used to first apply the topographic correction then perform image analysis techniques, such as relative band-depth, to characterize the lithology. High pass filtering and textural analysis are also used to help delineate the structures in the region. This study aims to update the most current geologic map, produced by Calkins et al. (1981) in order to fully understand the interaction between the Kohistan, Karakroam and Hindukush along the Shyok Suture and the Reshun Fault. We utilized the published geological map and our field knowledge of the area to classify the ASTER data with spectral angle mapper (SAM) and Mixture Tuned Matched Filtering (MTMF).
T43C-1341 1340h
Some Key Issues in Contemporary Continental Tectonics: An Overview
Plate tectonics theory, grounded on the assumption of nearly rigidity of plate interior, is elucidated as deformation concentrated on the narrowly defined plate boundaries; however, the current flooding datum of space-based geodetic observations have shown that continental deformation is not confined to the nominated plate boundaries, but even expanding to the inland area thousand kilometers far away from which. To understand this diffuse continental deformation is of fundamental importance to modern earth science and geological hazards mitigation. Despite of much advancement of our knowledge on continental deformation in the past few decades, many aspects of it still remain uncertain. Here we will focus on three controversial but interesting topics in the latest literatures on continental tectonics as follows: continental deformation pattern derived mainly from kinematic constraints, the rheology and strength of the continental lithosphere based on the studies on earthquake activities and experiment-based rock mechanism extrapolation, especially contrast between the effective elastic thickness (Te) and seismogenic thickness (Ts). Suffered from the long history of tectonic alternation with accretion, collision and magmatism, the continental lithosphere is characteristiced by remarked heterogeneities in composition, structure and mechanical properties, including some pre-existing tectonic inheritances as the weak zones. In a conclusion, we propose that thermo-rheological heterogeneity in continental lithosphere and the pre-existing tectonic inheritance are the key factors controlling its deformation pattern and rate, under the applied forces derived from the far-field plate boundary, along with the gravitational potential energy within continent due to the existence of density contrast. More attention should be paid on the enhancement of data on kinematics and lithospheric structure from observations of GPS, quaternary fault slip rate, and seismology, respectively; combining these constraints and more realistic rheology with advanced computation geodynamic modeling will no doubt lead to our better understanding on the continental tectonics.
T43C-1342 1340h
Intra-Lithospheric Tectonics: Iceberg Model for Stabilization of Archean Cratons
Thick buoyant mantle lithosphere roots appear to have developed as part of Archean cratons contemporaneously with formation of the crust. However, the genesis of post-tectonic, crustally derived granites requires appreciable late heating, remelting, and fractionation events that imply significant input of heat into the crust and removal of residual mafic-ultramafic lower crustal material. It has been postulated that the heating and removal may be explained by Rayleigh-Taylor (RT)-type instability of portions of the lithosphere or full mantle lithosphere delamination. We suggest that these processes based on modern analogues were less likely in Archean cratons owing to the low density of mantle lithosphere and its apparent durability since craton formation. Rather, we introduce an `iceberg model' of lithosphere keel inversion, in which mafic lower crust enters the eclogite field upon post-orogenic cooling, triggering rollover of a lithosphere cell that is lighter than asthenosphere but internally gravitationally unstable. A series of parameterized numerical experiments were conducted to quantify the dynamical evolution of the crust and mantle in a model Archean setting. The models demonstrate that intra-lithospheric rollover may occur where sufficient eclogitic crust overlies relatively buoyant mantle lithosphere. This is assisted by traction along the base of the lithosphere which may be caused either by underlying subduction, or viscous drag by mantle flow across the deep lithospheric root. The lithospheric rollover leads to a rapid pulse of elevated temperature in the lower crust when $\sim$1350$^{\circ}$C basal lithosphere/asthenosphere is juxtaposed with the new Moho during overturn. The dense eclogitic crust is removed from the lithosphere after inversion of the lithospheric root and descent into the mantle. These events are consistent with the observed enigmatic episodes of late, deep-crustal metamorphism, melting, and widespread granite blooms in some Archean cratons. We show that viscous RT instability or ``dripping'' of eclogite yields little thermal impact on the lower crust since the upward return flow of hot lithosphere material tends to be considerably more distributed and diffuse.
T43C-1343 1340h
The Stress Field and Structure-Forming Processes Caused by Density and Topography Variations in the Pacific Continent-Ocean Transition Zone.
The subduction is considered as one of the most important external forces in modeling structure and activity of the transition zone from continent to ocean, while much less attention is paid to body forces acting inside this zone. Analytic solutions of the following problems have been obtained and analyzed: an elastic layer of variable thickness on an inviscid Newtonian liquid (2D problem); a viscous layer of variable thickness on an inviscid Newtonian liquid (2D problem); thin elastic plate on an inviscid Newtonian liquid (3D problem). Other analytic solutions were applied to study structure and geodynamics of a transition zone (Mindlin and Boussinesque problems) for arbitrary variations of density and surface loads (3D problems). The source of stresses and movements in the above problems is the volumetric forces created by the gravitational field of the Earth. No other forces were taken into consideration. Computer programs were written for calculations of stress field and displacements for the problems listed above. Formulas, relating the outer gravitational field anomalies and stress field components generated by density and topography variations of these layers were obtained. Stress distribution for variations of density and topography typical for a zone of transition, as well as for deep sections of the Earth's crust and the upper mantle in the Eastern part of the Asian continent, the Sea of Japan, and the Philippine Sea have been calculated and analyzed on a base of the above solutions. The common features of those stress fields, typical for all models, were observed. They are: (1) tension in the continental part of the model, (2) compression in the oceanic part of the model and (3) there is a narrow subvertical zone of high shear stresses acting downward under the continental part of the model. Can it be a seismofocal zone? The slope of this zone depends on the degree of isostatic compensation of the crust. The magnitudes of the stresses are high enough to produce fractures in the Earth's crust, forming deep faults and initiating a series of tectonic events. Thus, density and topography variations of the Pacific continent-ocean transition zone are responsible for major structure forming processes within this area. On the other hand, magnitudes of horizontal and vertical rates of displacements produced by density and topography variations of the transition zone are rather low to fit the observed lithospheric plates kinematics. A number of geotectonic models is considered by taking into account the influences of external factors on the crust and upper mantle of the transition zone to receive quantitative conformity of the theoretical to the observable data.
T43C-1344 1340h
The Chicxulub Impact - Connection to K/T Boundary Event Based on Petrophysical and Paleomagnetic Investigations
Establishing relationships between the Chicxulub impact crater and the K/T boundary event has been one of the main reasons for Chicxulub Scientific Drilling what was carried out in framework of the International Continental Drilling Program (ICDP). Drilling-site (Hacienda Yaxcopoil) was chosen on the basis of previous work achieved by drillings of the oil exploratory program by Petroleos Mexicanos and more recently by the National University of Mexico (UNAM). The Yaxcopoil-1 borehole is 1511 m deep, and sampling started from 404 m. Drillcore runs through 3 intervals: the post-impact layer, the impact layer and the pre-impact target rock. Samples of this study were gathered in collaboration of many institutions (Helsinki University, UNAM, Vrije University of Amsterdam and Humboldt University of Berlin) and cover all these intervals. Standard petrophysical and paleomagnetic measurements (including magnetostratigraphy) were carried out in order to clarify some of the dating issues. Results show that paleomagnetic measurements of the drillcore, coupled with petrophysical data provide a good tool of isolating various units including the impact layer, the K/T boundary- and the post-impact sequences. NRM and susceptibility values of samples from pre- and post-impact layers show that most samples are very weakly magnetized, with exception of the interval from 790 m to 900 m that includes the K/T boundary layer. Also our data show that magnetostratigraphy of the impact layer is quite complex due to possible hydrothermal remagnetization process. Additionally these data reveal that the Chicxulub impact event took probably place within the magnetic chron 29R and therefore can be related to K/T boundary event.
T43C-1345 1340h
State of Stress and Structure of Volcanic Shield Penetrated by the HSDP Deep Core Hole
As part of the Hawaii Scientific Drilling Project (HSDP), a pilot hole was drilled in 1993 to a depth of 1056 meters below sea level (mbsl) and a deeper hole was drilled to 3098 mbsl in 1999. Complementary borehole deviation and acoustic televiewer logs were obtained in the deep hole that provide fundamental information regarding the structure and the state of stress that exist within a volcanic shield. Fracture planes observed in the Mauna Kea basalts between 1829 and 2896 mbsl display a fairly uniform strike whose direction transitions smoothly into the fracture orientations observed in the shallow pilot hole. The combined directional data from both holes provides a continuous record of fracture strike with depth and also with age to 540 ka. It depicts a clockwise rotation through the surficial Mauna Loa basalts that stabilizes to a constant heading in the underlying Mauna Kea rocks. This information corresponds to the direction of lava flows and the location of volcanic sources relative to the drill site. The deviation log mimics this behavior because the drill bit methodically drifts in a direction perpendicular to the dominant fracture strike; the wellbore trajectory, in effect, becomes a surrogate for the evolution of fracture orientation and, thus, for the direction of basalt deposition with time. Breakouts observed in the televiewer log identify the orientations of the maximum (S$_{HMAX}$) and minimum (S$_{hmin}$) horizontal principal stresses to be north-south and east-west, respectively. The additional appearance of ovals cut into the borehole walls implies that S$_{HMAX}$ $>$$>$ S$_{hmin}$ at this site, and examination of local topographic conditions supports this conclusion. A sharp break in onshore-offshore slope reduces stress east-west and a prevailing north-south slope associated with the emergence of Kilauea increases stress north-south. These two effects combine to amplify the differences in the magnitudes of the horizontal principal stresses. Consequently, breakouts are extensive and appear over about 30 percent of the open hole.
T43C-1346 1340h
Deep Coring in the Valles Caldera, Northern New Mexico to Obtain a Long-Term Paleoclimatic Record
The 22-km diameter Valles caldera in the Jemez Mountains of northern New Mexico contains a thick sequence of lacustrine sediments and hydromagmatic deposits that date from the inception of the caldera (c.a. 1.25 Myr). Geologic mapping shows that lakes formed in the caldera immediately after its formation and existed for some period of time before the caldera wall was breached and the lake drained to the SW through San Diego canyon. Another substantial lake formed during the mid-Pleistocene in the SE caldera moat (Valle Grande) when a post-caldera eruption (c.a. 520 kyr) filled the drainage to S.D. canyon. To determine their paleoclimatic significance, the deposits of this ancient lake were cored in May, 2004 (GLAD 5). Hole VC-3 achieved a total depth of 81 m recovering a complete section of ~75 m of lacustrine mud and silts and gravels. Recovery of lacustrine mud/silt was close to 100 percent while the recovery of gravels encountered at the top and bottom of the sequence was considerably less. The core is currently archived at the National Lacustrine Core Facility (LacCore) at the University of Minnesota. Preliminary analyses show considerable down-core variability in parameters including magnetic susceptibility, gamma-ray density and sedimentary facies. The base of the core consists of pumiceous-rhyolitic sands and gravels intercalated with indurated muds, which grades up into variably laminated and bioturbated lacustrine mud and silty mud. In the lower lacustrine sequence, several turbidites interrupt the laminated mud sequences and in some sections, thick diatomites (up to 5 cm) occur and are indicative of surface eutrophication in the lake. Higher in the core, thin sand lenses indicate periods of enhanced runoff into the lake, and occasional rhyolitic dropstones are observed. Much of the laminated silty clay is rich in diatoms, although both the density and diversity of diatoms are highly variable. In the middle of the core, a brecciated, diatom poor facies correlates with high magnetic susceptibility. Rapid facies changes and intervals with well-developed mudcracks indicate multiple lake level changes over the lake history that probably spans tens of thousands of years over the mid-Pleistocene. Future work on the core will include pollen, diatom, stable isotope and other geochemical and geophysical analyses.
T43C-1347 1340h
Overview of results from deep drilling in the Siljan Ring impact struct
A meteorite impact in Devonian time, about 360 million years ago, resulted in the formation of the 52 km wide Siljan ring impact structure. As part of the Swedish Deep Gas Drilling project two deep boreholes to nearly 7 km depth were drilled within the crater rim (Gravberg-1 and Stenberg-1). Several shallower boreholes were also drilled. Both deep boreholes penetrate mainly granitic rock to total depth. Prior to drilling, extensive geophysical surveying was carried out in the area. These surface investigation together with data from the boreholes provide a large amount of information on the geological conditions in the upper crystalline crust in the area. Main results are (1) high amplitude reflections from the granitic bedrock correspond to sub-horizontal dolerite sills, (2) the upper 1-1.5 km of crust is highly fractured with more competent rock below, (3) hydraulic conductivity is low at about 10$^{-9}$ - 10$^{-10}$ m/s, but not insignificant, (4) highly saline fluids (salinity of 10-15%) are present below 6 km, (5) isotope data on calcite indicate groundwater may percolate to great depth, (6) the temperature gradient is nearly constant to 7 km at 16 degrees/km, and (7) the stress field is anisotropic with the maximum horizontal and vertical stresses being about equal and the minimum horizontal stress being about 2/3 of the vertical. Although the rock is relatively intact below 1.5 km, fracture zones are present at varying intervals down to total depth in both boreholes. In the deeper parts of the Gravberg-1 borehole, relatively large concentrations of helium and nitrogen are present. The isotopic signature and concentration levels of the helium indicate that the water at these depths has been stagnant on the order of hundreds of millions of years.
T43C-1348 1340h
AHC (Active Heave Compensation) - 800 Drilling on the Atlantic (New Jersey) Margin
The New Jersey continental shelf, an old, stable passive margin, has been a focus of latest Pleistocene-Holocene sea-level studies for decades, because eustasy is a major driving force in the production of the surficial stratigraphic record there. This margin is also geographically proximal to diverse oceanographic resources - laboratories, ports and ships - so hypothesis-testing using a "natural laboratory" approach has been suitable for data acquisition, analysis and interpretation. The Office of Naval Research has taken advantage of this shelf's characteristics to support collection and interpretation of a huge and diverse suite of geophysical data off New Jersey since the late 1980's - MCS profiles at multiple frequencies, deep-towed boomer and chirp profiles, multibeam bathymetry/ backscatter control, and most recently sediment samples using a lake-drilling system owned and operated by Drilling, Observation and Sampling of the Earth's Continental Crust (DOSECC), Inc., modified with active heave compensation for deployment off the Woods Hole Oceanographic Institution research vessel Knorr. The goal is to understand how the diverse interaction of depositional and erosional processes culminates in the preserved stratigraphic record. A first test of the AHC-800 drilling system took place in November 2001 in Block Island Sound off southern New England, with the following results: 1) some success was achieved in sampling mud, 2) fall weather was a limiting factor (heave compensation limits of 2.44 m in 8 s were often exceeded), 3) the vessel's dynamic positioning (DP) system was not always capable of maintaining station in shifting winds; precise navigation (beyond differential GPS) was required, and 4) the need for automated drillpipe handling to increase efficiency was recognized. A second test took place on the New Jersey shelf in September-October 2002, using updated software, automated pipe handling, and differential GPS navigation supplemented by a POS/MV 320 (inertial navigation) installation; this test included extensive calibration and tuning of the Knorr's DP system. Three sites were occupied: Site 1 in 129 m of water to a sub-seafloor depth of 5 m, Site 2 in 79 m of water to a sub-seafloor depth of 13 m, and Site 3 in 75 m of water to a sub-seafloor depth of 8 m. Sediment recovery at all three sites was excellent. Deep-towed chirp sonar profiles were correlated to recovered lithologies, and to gamma ray (multi-sensor track, MST) logs, using a derived reference compressional wave velocity of 1750 m/s. Lessons learned from the 2002 test included the following: 1) weather remains a factor, 2) despite improvements in navigation and DP, open shelf sampling is at the limit of the Knorr's capabilities, and 3) hydraulic piston coring, advancing by half (approximately 1.5 m) strokes, appears to be the most successful sampling tool in the intercalated unconsolidated muds and sands of the latest Pleistocene-Holocene section. The need for ground truth of the extensive geophysical control on the New Jersey shelf remains as strong as ever. We remain convinced that the AHC-800 is the right drilling system to use, given optimal weather, navigation and dynamic positioning. We envision further sampling off New Jersey, with support from a variety of sources, in either 2006 or 2007.
T43C-1349 1340h
Compositional Variations in the Continental Lithosphere Constrained by Non-Geochemical Data
A substantial part of the present knowledge on mantle composition is based on geochemical studies of locally abundant mantle-derived xenoliths. However, because xenoliths provide random, uneven, and in many areas only a sparse sampling of the Earth's deep interior, it is challenging to see if geochemical constraints on global-scale compositional variations in the mantle are consistent with modern geophysical data. Furthermore, large-scale compositional variations in the mantle reflected in seismic tomography models and mantle gravity anomalies are substantially masked by temperature anomalies. New data on the thermal regime of stable continental lithosphere (Artemieva and Mooney, 2001) allow separation of thermal and non-thermal effects in global geophysical models. A global anelastic seismic tomography model based on Rayleigh waves (Billien et al., 2000) was analyzed jointly with the new thermal model to outline regions where Qs anomalies cannot be explained by T anomalies alone. Bouguer gravity data (Kaban et al., 2003) corrected for the effect of thermal expansion were used to constrain density anomalies in subcrustal lithosphere of the continents. The results show that large variations in lithospheric composition are well correlated with regional variations in lithospheric thickness.