T51A-0431 0800h
Finite vs Instantaneous Deformation of the Indo-Australian Plate.
The Indian Ocean is well known for its diffuse plate boundary between India and Australia, involving "intraplate" deformation distributed over a broad area. N-S shortening of the oceanic lithosphere reflects an exceptionally high level of stress across the Indo-Australia plate resulting from the Himalayan collision. Bengal sedimentation shows that deformation has occurred during the past 7.5 Myrs. Two wavelengths of deformation are superimposed in the Central Indian Basin: seismic profiles revealed a regular thrust fault spacing of $\sim7$ km and long wavelength ($\sim200$ km) undulations of the basement. We analyze new seismic reflection profiles collected during the Andaman 2000 Cruise near NinetyEast Ridge ($\sim5 \deg S, 88 \deg E$). This highly deformed region shows many more thrust faults (fault spacing $\sim 2 km$) and shorter undulation wavelength ($\sim 100 $km), possibly in relation with the proximity of NinetyEast Ridge and/or the presence of a fossile spreading ridge. Reflectors in the sediments may indicate an earlier initiation of deformation, around 12 Ma. We compare the style and amplitude of the observed finite deformation recorded in the seismic profiles to instantaneous deformation obtained by kinematic modeling of India/Australia intraplate motion. Using Haines and Holt method, the horizontal velocity field is obtained from a combination of recent GPS data and the seismic moment release of 128 earthquakes (M$>$5). Our analysis confirms that NinetyEast Ridge is a major rheological discontinuity acting as the true plate boundary between India and Australia.
T51A-0432 0800h
Geometric and Fractal Characteristics of Veins in San Joaquin, Queretaro, Mexico
This work was made in the most eastern part of the Fold and Fault Mexican Belt of the Sierra Madre Oriental, where a sea Mesozoic sequence is exposed and formed by three lithostratigraphic units with different mechanical characteristics: the upper and lower mechanically incompetents, and the middle one mechanically competent. From the geometrical point of view, four vein systems were identified: the oldest one parallel to the stratification; two other more recent and contemporaneous with general orientations N75°E, 70°SE and N25°E, 70°SE, which are symmetrical to the youngest one with orientation N50°E, 80°SE. All the system orientations are compatibles with the tectonic transport of the area. In the other hand, Fractal Geometry application allows describing vein characteristics that conventional techniques does not: as the filled space measurement and the power laws that rule veins. The fractal analysis consisted on various stages: the first one was made with developed software, which estimates the fractal dimension of an image basing on box-counting method. All the vein systems images were studied by site, and it was found that the fractal dimension varied from 1.2 to 1.8, and the largest values were associated with the most competent lithology and not with the most competent unit. With the analysis for each vein system the fractal dimension varied from 1.1 to 1.82, and it was concluded that the values tend to decrease as the systems are younger, and the intermediate systems have the larger variation because they were affected by the latest vein system.
T51A-0433 0800h
Growth of Syntectonic Fibrous Veins in Shale, Ouachitas Orogen, Arkansas
Tectonic fibrous quartz and quartz-calcite veins in the Lower Ordovician Mazarn Formation display veinlets (5 to 25 $\mu$m wide) separated by 15 $\mu$m wide host segments parallel to the vein edge throughout the vein length, their number increase in fine host laminae and disappear in coarse host laminae. At the vein tip, fibers: a) are only quartz, b) range in width between 20 and 120 $\mu$m, c) discontinuously show host bands parallel to the vein edge. At the tip, the vein edge is irregular. At the center of the vein the vein all is smooth and, a) fibers are composed of both calcite and quartz, b) quartz fiber widths range between 30 and 300 $\mu$m, c) wide fibers ($>$100 $\mu$m) show inclusion trails parallel to the fiber long axis and, d) 10% of the fibers span the width of the vein. Vein fibers widen with distance from the vein tip. Cathodoluminescence (CL) in quartz fibers show growth bands parallel to vein-host interface. Width of growth bands is similar to width of veinlets. Growth bands disappear towards the center of fibers. Calcite in the host is found in distinct coarse-grain laminae and is absent in fine-grain laminae. Fluid inclusion data shows two thermal episodes of growth, one at around 210 $\deg$C and a second one at 130 $\deg$C. These observations suggest that the mechanisms of growth at the tips and central part of the vein are different. Specifically, at the tips vein growth takes place by cracking followed by competitive crystal growth (higher temperature diffusion-controlled process). Fiber width at this stage is determined by the initial crack aperture. Veins widen by addition of material at vein-host interface. At center of vein fibers widen by a combination continued precipitation on fiber walls and replacement of calcite by quartz.
T51A-0434 0800h
Strain Localization and 3-Dimensional Distribution of Strain Gradient Inferred From X-ray CT Imaging
Strain localization develops from bifurcation and discontinuity in the strain field. In the laboratory a spectrum of localization modes have been observed in relation to dilatant and compactive failure of a porous rock. To investigate the evolution of strain localization it is desirable to map out nondestructively the spatial distribution of the strain field and its gradient in 3 dimensions. In this study micro-CT imaging was used to address this question. Serial sections of X-ray radiographs were acquired on intact and deformed samples of porous sandstones. The X-ray attenuation data reflect the spatial variation of local density, which in a rock may be related to preexisting heterogeneities like sedimentary laminae or to stress-induced porosity changes. While the former can be readily resolved from CT imaging, the latter often results in relatively weak contrasts in X-ray attenuation that require further analysis to resolve. We developed a technique to infer the strain gradient from the X-ray attenuation data, which allows us to map out in fine detail the strain localization features. The average radial gradient of volumetric strain (or porosity change) is inferred from the mean attenuation gradient at a voxel, which in this study is evaluated from the mean of the difference of attenuation values between that at the voxel and its 26 nearest neighbors (weighted by the distances separating them). One of our Diemelstadt sandstone samples was triaxially compressed at 150MPa pressure, and on the exterior cylindrical surface of the failed sample several compaction bands oriented subperpendicular to the maximum principal stress were visible. From the 3D visualization of mean attenuation gradient, we infer that these compaction bands had indeed propagated from the exterior surface into the interior of the sample. These bands can be identified with relatively thin structures with anomalously low strain gradients, implying the development of appreciable homogenization of the porosity distribution within the compaction bands. The data can also be used to characterize the tortuosity and geometric complexity of the strain localization features.
T51A-0435 INVITED 0800h
Controls on Plume Spacing and Plume Population in 3-D High Rayleigh Number Thermal Convection
Dynamics of mantle plumes are important for understanding intra-plate volcanism and heat transfer in the mantle. Using 3D numerical models and scaling analyses, we investigated the controls of convective vigor or Ra on the dynamics of thermal plumes in isoviscous and basal heating thermal convection. We examined Ra-dependence of plume population, plume spacing, plume vertical velocity, and plume radius. We found that plume population does not increase with Ra monotonically. At relatively small Ra ($<$10$^{6}$), plume population is insensitive to Ra. For 3x10$^{6}$$<$Ra$<$3x10$^{7}$, plume population scales as Ra$^{0.31}$ and plume spacing $\sim$ Ra$^{-0.16}$ $\sim$ $\delta$$^{1/2}$, where $\delta$ is the thermal boundary layer thickness. However, for larger Ra ($\sim$ 10$^{8}$) plume population and plume spacing become insensitive to Ra again. This indicates that the box depth poses a limit on plume spacing and plume population. We demonstrated from both scaling analyses and numerical experiments that the scaling exponents for plume population, n, heat flux, $\beta$, and average velocity on the bottom boundary, v, satisfy n = 4$\beta$ - 2v. Our scaling analyses also suggest that vertical velocity in upwelling plumes V$_{up}$ $\sim$ Ra$^{2(1-n+\beta/2)/3}$ and that plume radius R$_{up}$ $\sim$ Ra$^{2(\beta-1-n/2)/3}$, differing from the scalings for the bottom boundary velocity and boundary layer thickness.
T51A-0436 INVITED 0800h
Predictions of Fault Spacing at the Scale of the Lithosphere From Analytical, Numerical, and Analogue Studies
The location of individual faults is influenced by various local phenomena such as deformation history and heterogeneity in the deforming rock masses. Nevertheless, large-scale instabilities attempt to impose a deformation length scale or wavelength on the deforming rocks. At the scale of the lithosphere, one such instability that has been recognized for many years is the buckling/necking instability. This instability occurs in lithosphere models containing vertical strength contrasts and undergoing horizontal shortening or extension. Stress heterogeneities due to the buckling/necking instability can influence fault distribution. More recently, it has been found that if the rheology of a layer obeys a strain-rate weakening law, a superposed faulting instability develops, immediately producing a network of regularly-spaced faults. The fault spacing expected from the faulting instability is smaller than for necking-controlled faulting. Hence, in field studies it is ambiguous how to interpret structures that display a characteristic fault spacing. Focusing on tensile environments, we will compare the expected spacing that arises from the faulting and necking instabilities to the fault patterns obtained in numerical and analogue models of extension in an idealized layered structure.
T51A-0437 0800h
Two-phase damage and tectonic plate generation
The two-phase theory for compaction and damage employs a nonequilibrium relation between interfacial surface energy, pressure, and viscous deformation, thereby providing a model for damage (void generation and microcracking) and a continuum description of weakening, failure, and shear localization. Here we examine the application of this theory to the problem of generating plate-like behavior from convective-type divergent (poloidal) motion through a source-sink formulation. We extend the previous damage theory to consider two possible damage effects: (1) growth and nucleation of voids associated with dilation of the host matrix, and (2) increasing fineness (i.e., reducing coarseness) of the mixture by, for example, grainsize reduction. Void-generating damage is found to be poor at plate generation because of the predominance of dilational motion that is adverse to the development of plate-like flow. Fineness-generating damage is found to be very efficient at generating plate-like behavior if we assume that the matrix viscosity is a simple function of grain/void size, as is typical for diffusion creep. The implied grainsize reduction mechanism is different than that of dynamic recrystallization, and appears highly capable of generating the requisite shear-localization for forming tectonic plates from mantle flow.
T51A-0438 0800h
Computational Approach to Coupled Structural and Thermal Restoration of Salt-Bearing Sedimentary Basins
The coupled structural and thermal evolution of salt-bearing sedimentary basins is one of the important processes occurring in these basins, because the highly mobile and buoyant salt deforms (sometimes greatly) its sedimentary overburden. Moreover, the temperature and maturity distribution in the basins will be significantly affected by salt tectonics, as salt has a higher thermal conductivity than other sedimentary rocks. The scientific goal of this ongoing project is to develop an innovative computational methodology (based on reasonable physical assumptions) for the thermo-structural restoration of salt-bearing basins in order to understand the interplay between geodynamic, geothermal, and tectonic processes involved in the evolution of the basins. The developed methodology will be applied to the Pricaspian basin (the northern Caspian Sea region), which plays an important role in hydrocarbon explorations and exploitations. We intend to study the present-day geothermal structure of the Pricaspian basin based on the results of available and new geothermal measurements and to restore a thermo-mechanical evolution of the basin. The methodology can be employed to analyze the thermo-structural evolution of other sedimentary basins as well.
T51A-0439 0800h
Filtering Stratigraphic Patterns Associated With Internally Generated- Versus Externally Driven-Processes: An Example from the Late Pliocene Fisk Basin, Gulf of Mexico
The stratigraphy of a basin fill can be viewed as a time sequence of surfaces providing a long view of earth-surface dynamics. These surfaces and the strata between them record external forcings such as tectonism, climate and sea level that interacted with and were overprinted by internally generated fluctuations in the depositional systems themselves. Accurate reconstructions of these external forcings therefore require decoding the stratigraphic record into its externally and internally generated signals. Here we present such a deconvolution for the Late Pliocene stratigraphy of the Fisk Basin, located on the upper continental slope about 350 km southwest of New Orleans, LA. We separate signals by systematically comparing spatial patterns of sedimentation against the pattern of basin subsidence. At short time intervals these patterns are different and capture the internally generated (autogenic) variability in the transport/deposition system. At long time intervals these fluctuations average out and the two patterns become one in the same, defining the averaging window required to approximate steady state topography. Though our analysis is for a submarine basin, the technique can be applied to any basin fill having the necessary data set. Fisk Basin is 20 km wide and 30 km long and entirely imaged by a high-quality 3D seismic grid. Multiple well penetrations provide excellent lithologic and biostratigraphic age control for the approximately 1.5 km thick Late Pliocene section. Nine surfaces mapped basin wide subdivide this section into 8 intervals recording the interplay between basin subsidence and sedimentation over 1.2 Ma. Similarity between patterns of sedimentation and subsidence, measured as the standard deviation of their ratio, decays exponentially with increasing time interval. The e-folding length for this decay is 0.47 Ma. This value indicates that autogenic fluctuations average out at a time scale that is long relative to characteristic times for known variability in external forcings. This overlap in scale complicates the separation of apparent trends in stratigraphic properties such as bed thickness and stacking patterns into their stochastic (autogenic) and deterministic (externally derived) components.
T51A-0440 0800h
Mechanical Dauphine Twinning in Quartz on a Regional Scale: Texture of r and z Rhombs in Mylonites from Neutron Diffraction
Preferred orientation in quartz is routinely used to determine the deformation history of rocks. Most studies use only the c-axis distribution because it has been conventionally measured with the U-stage and has been discussed in terms of slip system activities. With diffraction methods, a complete orientation distribution function (ODF) can be calculated. From the ODF, the preferred orientation of any lattice direction can be determined, including overlapping peaks such as the r and z rhombs. We have used neutron diffraction, combined with Rietveld texture analysis, to determine preferred orientation of quartz in mylonites from the Santa Rosa mylonite zone in southern California. We observe that the r and z planes do not have the same textures, with r-rhombs showing a maximum normal to the foliation and z-rhombs a minimum. These patterns are consistent over a regional scale. Simple modeling shows that the difference in texture between positive and negative rhombs can be attributed to mechanical Dauphine twinning, which is imposed on an initial deformation texture produced by slip through compression. The r and z planes have different compliances and Dauphine twinning acts to maximize stored elastic strain energy. Reversing the sense of twinning produces identical textures in the two rhombs. Dauphine twinning therefore occurs concurrently with regional deformation and is not an expression of late-stage deformation governed by local stresses. Due to the orientation dependent nature of the twinning, it may be possible to infer stress direction from texture patterns.
T51A-0441 0800h
Anomalous Cleavage-Bedding relation: flexure folding of non-parallel planes in NW India
Failure to put a relative time stamp on cleavages or comparable metamorphic bandings in multiply deformed terrains may lead to apparently irresolvable geometric situations. Secondary metamorphic banding within upright non-plunging folds in impure marble of Proterozoic Delhi Supergroup shows deviations from the time-tested cleavage-bedding acute angle relations in locating fold closures. In adjacent beds from the same limb of macroscopic folds the cleavage-bedding relations indicate opposing directions of closures, suggesting that each neighboring pairs of beds belong to two different limbs of isoclinal folds. The confusion reaches its highest where within the same bed, in the same limb of a macroscopic fold, the secondary bandings show contrasting cleavage-bedding acute angle relation (1) at the same bedding surface, and (2) at the two different bounding surfaces of the bed. A model involving change in orientation and length of early metamorphic banding in the limbs of late flexure folds on beds explains the apparent anomaly. This change is a function of the pre-flexure angle between metamorphic banding and the bedding. The sense of shear being different in the two limbs of a flexure, the metamorphic banding would develop contrasting geometries in the two limbs. As the magnitude of shear changes continuously along a limb, the new geometry of early secondary banding will also vary continuously in the same limb. The model suggests that even in isoclinal folds the examination of cleavage-bedding relation in only one limb may not always lead to a correct correlation of the metamorphic banding as the axial planar structure of the fold. In areas with strongly cross-laminated rocks flexure folding of beds would lead to selective folding of cross laminations within individual beds that would resemble, and can be mistaken as, soft-sediment deformation folds. Keywords: Metamorphic banding, Cleavage, Acute angle relation, Fold closure, Superposed Deformation, Flexure, Soft sediment deformation.
T51A-0442 0800h
Origin of Breccia Deposit Within the Solund Devonian Basin, Western Norway
The Solund basin of western Norway developed in the hanging-wall of a large extensional detachment zone in the lower Devonian. The basin consists of an approximately 10km thick succession of primarily conglomeratic sediments, but also includes an anomalous brecciated body composed of gabbro, rhyolite and metamorphosed quartzite of uncertain origin near the basal deposits of the basin. The body has been previously interpreted as an in-situ intrusive and volcanic rocks, as a thrust slice, and as a landslide. Previous U/Pb zircon dating of glassy felsic rocks within the body (439$\pm$3 Ma) suggest a lowermost Silurian age of the volcanic protolith. Detailed field mapping combined with microtexture analysis and laboratory modeling is currently used to determine the origin of this body and investigate the physical mechanisms that control the brecciation process. Based on the basal structure, the geometry of the body, and an unconformable upper contact with the conglomerate we present evidence to support the landslide interpretation. The unit is characterized by a large degree of brecciation, including "jigsaw breccias", particularly common within the foliated metamorphic rocks of the body. These breccias contain fragments ranging from microscopic grains to intact blocks up to 30m in length. The breccias are clast supported and the clasts have rotated around multiple axes, with little apparent space for these rotations to occur. The metamorphic rocks exhibit a higher degree of brecciation than the more massive gabbroic to granitoid rocks in contact with them. This apparent intrusive relationship, seen at many localities within the body, suggests that the majority of the jigsaw brecciation occurred prior to emplacement, with the body remaining quite coherent and preserving internal relationships through the emplacement event. A simple brecciation mechanism does not fully explain the observed morphology and arrangement of the breccia. We are currently addressing this problem and will present preliminary results from our analysis.
T51A-0443 0800h
Structural Analysis of Serpentinite in the Jikkoku Pass Area, Northwestern Kanto Mountains, Central Japan
Serpentinite is a product made by hydrothermal alteration of ultramafic rocks such as peridotite. It has been understood that serpentinite along a fault or a plate boundary plays a role as a lubricant for rheological properties. However, it is unknown how serpentinite actually acts through its deformation processes. The study area is located on the Jikkoku Pass area, northwestern Kanto Mountains, central Japan. Serpentinite bodies are intermittently distributed along fault boundaries. In this study, we divided serpentinite into four deformation stages (D1-D4) based on meso- and microstructures. 1) D1 stage: It is represented by block-in-matrix structure. Block sizes are various, and their longest axis is about 1 m. Shapes of blocks are almost oblate type and rounded to subangular. The long axes of blocks nearly show a same orientation, and planar structures are nearly parallel to them. Inner structures of blocks are entirely deformed, and mesh texture as a pseudomorph of olivine and euhedral chromian spinels are observed. Serpentines consist mainly of lizardite and chrysotile. Fine-gained serpentines of various sizes are randomly scattered in the matrix. 2) D2 stage: It is represented by alternation in portions of different colors and textures. Colors consist mainly of dark green and yellowish green. Intervals of this alternation vary from 1 to 3 mm. In portions of dark green, needle-shaped antigorites have a preferred orientation. Antigorites are remarkably fine-grained in some cases, and are similar to characteristics of ultramylonite. Portions of yellowish green include opaque minerals such as magnetite, forming planar structures. A chromian spinel occurs as a porphyroclast (rounded shape) together with fine-grained recrystallized antigorites. 3) D3 stage: It is represented by fracture foliation. It is formed by foliation as fracture planes that are penetratively developed with a few hundreds of Em intervals. Besides, intervals of this foliation also vary on meso-scale. Fracture planes are often filled up with chrysotile. Lens-shaped grains from 100 Em to 5 mm are frequently observed. D3 foliation is recognized in the matrix of D1 tectonite. In this case, blocks of D1 tectonite are extremely flattened and rounded. 4) D4 stage: It is represented by cataclastic texture. Cataclastic textures are locally (a few meters on scale) observed in serpentinite deformed in previous stage or in non-deformed serpentinite. In fault zone, foliated cataclasite (gouge) is observed, and is often characterized by S-C structure. Foliation or cracks between microbreccias are filled up with chrysotile and/or carbonate minerals. Thus, it is clear that deformation facies of serpentinite in the study area show ductile-brittle deformation. Serpentines are composed mainly of lizardite and chrysotile in non-deformed serpentinite (inclusive of a block of D1 tectonite). In deformed serpentinite, serpentines vary from antigorite (high temperature type) to chrysotile (low temperature type) in relation to a difference of deformation facies. Strikes of D1-D3 foliation coincide nearly with an elongate direction of serpentinite body. It is inferred that deformation styles of serpentinite were constrained by a mode of serpentinite body. D4 stage seems to be the deformation not related to serpentinite emplacement causing D1-D3 stages. Deformation degree of serpentinite is independent of a distance from fault boundaries, and frequently varies on meso-scale. Hence, we can conclude that serpentinite in the study area records a deformation caused by solid intrusion as well as faulting.