T33C-1474
Non-linear Surface Interpolation of Large Deflection Fold Shapes
Knowledge of the continuous geometry of geologic folds is essential for predictions about associated deformation, specifically fracturing and hence alterations of fluid flow. Data on the geometry of kilometer-scale folds is becoming available through airborne laser swath mapping (ALSM), which, in general, delivers sparse and non-continuous information on the geometry of particular strata. The raw data must be interpolated to reveal the geometry of individual strata over the entire fold. Interpolations of geologic folds are often carried out assuming a smooth resultant surface but physical processes such as faulting and fracturing may preclude such geometries. A first order physical analog is that of small deflection plate bending in which the middle surface of a thin plate is used as a proxy for the surface to interpolate. The application to geologic folding assumes that layer boundaries deform similar to the middle surfaces of that layer. In the small deflection plate bending analysis lateral forces, shear tractions on the top and bottom surfaces of the thin plate, and in-plane strains in the middle surface are assumed to be negligible. The physical process of folding multiple sedimentary layers may involve some, if not all of these, complications. A physical process based interpolation technique that incorporates more of these complications may thus improve the quality of the more elementary surface interpolations. We present a interpolation technique that omits the conditions of small deflection and zero strains in the middle plane of the layer and apply the interpolation at Raplee monocline, southwestern UT. We solve the non-linear large deflection plate bending problem using the finite element method, use observed elevations as displacement boundary conditions, and predict elevations throughout the geologic fold where data is absent. The resulting surface reveals a smoothly varying geometry that compares well with previous interpolations. Additionally, we calculate strains in the plane of the surface and compare regions of high strains to areas of high fracture intensity mapped in the field. The method can improve on existing interpolations techniques by addressing the physical process more adequately and predicting regions of high strain along the geologic fold.
T33C-1475
The Evolution of the Perdido Fold Belt, Northwestern Gulf of Mexico - Insights from Numerical Modeling
The Perdido Fold Belt (PFB) in the northwestern Gulf of Mexico is a large deep-water salt-tectonic structure. It is located above the seaward limit of the Jurassic Louann salt and lies within the compressional domain of a salt tectonic continental margin system. The PFB is well documented in several geophysical surveys and deep-water hydrocarbon discoveries associated with this structure have recently been made. The region consists of up to 8 symmetric, ~15 km wide and 60 km long folds in exceptionally thick (4.6 km) overburden sediments. Folding is estimated to have occurred synchronously within only ~6 Ma and the entire fold belt is tilted seawards. The conditions that led to the given geometry, timing and extent of the fold belt including the role of the landward located Sigsbee salt canopy are poorly understood. This work investigates the formation of the PFB using 2D finite-element models in which frictional-plastic sediments overly a viscous salt layer. The models comprise the sediments of a passive margin from shelf to deep water to account for the dynamical interaction of gravity gliding and spreading caused by shelf progradation. The model experiments also include flexural isostasy, loading by the overlying water column and sediment compaction. Effects of pore fluid pressures are included by parametric calculations of an effective internal angle of friction. An analytical stability analysis and numerical experiments reveal that gravity spreading alone can fold a 4.6 km thick layer of frictional-plastic sediments in a setting like the western Gulf of Mexico. To obtain failure along the margin, the overburden is required to have a strength of only moderately overpressured, wet sand. Sediment compaction, which increases density, increases the translational velocity of the unstable overburden and results in earlier and faster formation of the fold belt. It also enhances the evolution of buoyancy driven structures. Additional numerical model results show the role of an adjacent salt nappe on the evolution of the PFB and the inflation of the detachment layer. Furthermore, the influence of salt taper and basement steps on the timing and extent of the fold belt is shown which helps constrain the initial salt thickness.
T33C-1476
The Role of Transverse Faults in Accommodating Lateral Propagation of Faults and Folds: Evidence From Geomorphic and Structural Analysis of Active Folding in the Camarillo Fold Belt, Ventura County, California
Length of faults and fault propagation folds in contractional environments are generally thought to increases with displacement in response to fault tip migration. However, only a few studies have documented fault tip migration and lateral growth of hangingwall anticlines. Here, we present several examples in which the interplay of discrete structural domains and transverse faults leads to punctuated fault growth and lateral propagation of hangingwall folds. Folds of the Camarillo Fold Belt, including the South Mountain anticline, are the topographic expression of north-south shortening in the western Transverse Ranges. The inception of tectonic uplift and folding is likely no older than 500 ka and may have begun as recent as 50 ky on the westernmost folds in the belt. Folds are developed above discrete fault segments that strike east-west and range in length from 4 to 50 km with displacement from 100's to 1000's of meters. Fault segment boundaries are characterized by north-south oriented transverse faults that are either exposed at the surface or inferred from subsurface data. At least one of the transverse faults is inherited from an earlier Miocene extensional phase in which it appears to have limited the growth of the ancestral normal dip-slip Simi fault. Adjacent structural domains are separated by transverse faults, and characterized by plunging folds at their margins, distinct changes in the topographic expression and structural geometry of folds, and the relative timing of deformation. Topographic and structural analysis of folding suggests anticlines within discrete structural domains developed as doubly-plunging fault propagation folds that amplified with time. However, fold and fault lengths were constrained by the presence of transverse faults and remained constant through time. Following a period of amplification, new faults and folds become active in the adjacent domain to the west and begin accumulating strain, resulting in composite folds that young to the west. Structural domains along composite fault zones all young to the west and typically step southward with time. These observations suggest that transverse faults initially impede fault tip migration until a critical strain gradient across the transverse fault is achieved. At which time the main fault breaches the transverse fault, resulting in westward lengthening of the main fault and lateral propagation of the fold(s) into the adjacent structural domain.
T33C-1477
Can trishear explain strain distribution in syn-tecontic sedimentary growth faults: evidence from deep water Niger Delta?
This study uses a 3D seismic reflection volume, courtesy of CGG Veritas, in the deepwater fold and thrust belt of the Niger delta to investigate variations in strain distribution between pre-tectonic and syn-tectonic sedimentary strata. Two en-echelon reverse faults and associated pre and syn-tectonic strata are evident. Despite opposing polarity both faults have similar fault geometry, limb angle (30°) and a reduction of fault displacement from ca. 700 m to zero in the zone of en-echlon overlap. Furthermore, in the pre-tectonic strata both faults are characterized by an abrupt termination of seismic reflections along a narrow zone (ca. 100 m) with little or no deformation within hangingwall or footwalls. In contrast, within the syn-tectonic package variations in reflection characeter suggest heterogenous strain and significant deformation of both hangingwall and footwall within a triangular zone. The application of trishear reproduces the observed fault and stratal geometries and variations in strain distribution associated wtih the stratigraphic variations. In addition, trishear successfully results in a line balance section
T33C-1478
Pore Pressure Development in Sub-Décollement Sediments in Subduction Zones: Insights From Laboratory Data and Numerical Modeling
Pore pressure in subduction zones is a primary control on fault strength and sliding stability. Rapid loading of sediments by tectonic loading and burial generally outpaces fluid diffusion, resulting in the generation of excess pore pressure. The limited drainage allows only a small amount of consolidation and deformation relative to a completely drained section. In addition to the development of excess pore pressure, it is important to understand the distribution of porosity reduction, because it partly controls deformation style. The sub-décollement (underthrust) sediments are especially important because they directly influence the frictional properties and stress state in the décollement zone. Previous studies have documented elevated sub- décollement fluid overpressures at several margins, which serve to reduce the frictional strength along the wedge base. Here, we use numerical methods to solve the one-dimensional equation for fluid diffusion through porous media, to analyze the development of overpressures within subducting sediments beneath a highly permeable décollement that acts as a drainage boundary. Our model allows the compressional and hydrological properties of consolidating sediments to vary in time and space in response to drainage and porosity reduction (e.g., Gamage and Screaton, 2006). We define sediment hydraulic and mechanical properties through extensive geotechnical testing of sediments gathered during ODP Legs 190 (Nankai margin) and 205 (Costa Rican margin). We use constant rate of strain (CRS) consolidation experiments to define coefficient of consolidation (cv), coefficient of volume compressibility (mv) and permeability (k), at effective stresses from 0 - 90 MPa, corresponding to porosities of 12 - 68 %. Permeabilities derived from the CRS tests are supplemented by flow through tests in which a hydraulic gradient is imposed across a sample. The experimental results define a log-linear relationship between permeability and porosity ( φ). All permeability and porosity data fall within an upper bound defined by logk= 8.8261 φ - 20.7370 and lower bound defined by logk= 6.2952 φ - 20.9633. We have used our model to study the buildup of pore pressures in the Nankai Trough, an actively accreting prism where the décollement is located approximately 600 mbsf at the trench, and at the non-accretionary Costa Rica margin. At both margins, we compare our modeled pore pressures and porosities with those observed by drilling within a few kilometers of the trench, and with values inferred from geophysical data up to ~20 km from the trench. Simulated excess pore pressures for the Nankai Through range from about 5.5 - 8 MPa at 20 km from the trench. These values correspond to the permeability bounds reported above and differ substantially from values of ~15 MPa inferred from seismic velocities 20 km landward of the trench (Tobin and Saffer, 2006) suggesting that décollement permeability and overpressure in the fault zone play an important role in affecting excess pore pressure in the sub-décollement zone. In both cases, we observe that drainage allows the zone of minimum effective stress to migrate down section, which may partly control downstepping of the décollement. Ongoing work includes continued testing of samples to further constrain the compressional and hydrological properties of subducting sediments, and incorporation of fluid production from clay dehydration, and evaluation of the effects of finite décollement permeability or overpressure into our models.
T33C-1479
Where has all the strain gone? Mis-match between extension and contraction during gravity tectonics, SW Arican continental margin
Many continental margins comprise coupled deformation of the sedimentary sequence with growth strata in the near shore and toe-thrust compression in the distal portion Intrinsically, strain should balance between these two domain such that compression equals extension, however, many reflection seismic studies suggest that extension is greater than extension. Caution must be used in many of these examples because of poor imaging of toe-thrust geometry, and hence, significant uncertainty in the calculated strain. This study presents a 200 km long transect across the Namibian passive margin of the South Atlantic that images over 4 km of Cretaceous and tertiary post-rift strata. The seismic data permit good matches across faults and a low uncertainty in the geometric interpretation. These fault geometries derive robust line-length restorations of the stratal reflectors. Nevertheless there is a significant discrepancy between the stratal shortening (9km) in the toe thrust domain and the minimum layer extension (> 35km ) in the correlative updip extensional domain. The role of distributed strain, out-of-plane motion and missing contraction (geometric uncertainty) are explored in the context of the seismic dataset.
T33C-1480
Depth to Detachment Estimates for the Tanos and West Tanos Faults, Hagen Embayment, New Mexico
The Hagen embayment is part of the Albuquerque basin, one of the four large basins in the northern portion of the north-northeast trending Rio Grande rift. While the Albuquerque basin has been studied in the search for hydrocarbons, little is known about the relatively shallow Hagen embayment, which consists of several down to the northwest, northeast-striking normal faults, including the Tanos and West Tanos faults. The strike of these and other faults in the Hagen embayment are suggestive of transverse structures such that the embayment acts as an accommodation zone. In an attempt to resolve the subsurface geometry of the Tanos and West Tanos faults and to constrain the structural context of the Hagen embayment, geologic models were constructed along three transects using well data, existing cross-sections, regional geologic information, and gravity data that were collected over several years of the SAGE (Summer of Applied Geophysical Experience) program. A three- dimensional depth-to-Precambrian crystalline basement inverse model created during SAGE 2006 was used to plot the basement contact for the subsurface models. Given the change in thickness of the middle to late Miocene Blackshare Formation along the transects, greater bedding dips in the hanging walls of the Tanos and West Tanos faults than in their footwalls and a decrease in bedding dips upsection, it is hypothesized that the Tanos and West Tanos faults are growth faults. Depth to detachment levels are calculated and used to try to constrain the fault geometry and their broader context. The results of subsurface modeling and depth to detachment calculations suggest that the Tanos and West Tanos faults detach either at the Precambrian crystalline basement contact (approximately 2.5 km below the surface) or at a shallower level (approximately 1.15 km depth). The acquisition of more geophysical data in the Hagen embayment will help elucidate the geometry of the Tanos and West Tanos faults, and allow us to determine the role the embayment played in the development of the Rio Grande rift.
T33C-1481
Sedimentary and structural history of the basal member of the Abiquiu Formation: Implications for Cenozoic landscape evolution of the northern New Mexico
Structural, stratigraphic, and sedimentalogic data from the the basal member of the Oligocene-lower Miocene Abiquiu Formation provides information on the late Cenozoic deformation history and landscape evolution of north-central New Mexico. The study area lies along the western margin of the Rio Grande rift. The basal member of the Abiquiu Formation lies unconformably on Mesozoic strata and locally the Cenozoic El Rito Formation. RTK GPS mapping of the unconformity shows that it is planar and regionally strikes N70E and dips 17SE. Within the study area the basal member of the Abiquiu Formation is a 125 m thick sequence of interbedded boulder, pebble, cobble conglomerate and coarse-grained sandstone. Bedding ranges from 10 to 50 cm thick and bedforms are planar to slightly undulatory at their base. Clasts within conglomerate beds are matrix supported, but are locally imbricated. Paleoflow measurements from the base of the basal member yield a mean paleoflow direction of 326 degrees. Clasts composition consists of a variety quartzites and foliated granite that are in general subrounded and highly spherical. These bedforms and sedimentary textures suggest that the environment of deposition is sheet flood dominated alluvial fan. Provenance and paleoflow data from the basal member of the Abiquiu Formation indicate that the paleogeography of the western margin of the Rio Grande rift in north-central New Mexico underwent a significant change during the Miocene. At the time of deposition the region presently defining the western margin of the Rio Grande rift was a topographic low and receiving detritus from the south-southeast. Sometime after the Early Miocene, the landscape was inverted, whereby the western margin of the rift was uplifted resulting in southeastward titling of the Abiquiu Formation, while regions to the southeast subsided. Regional southeast tilting of the Abiquiu Formation cannot be readily explained by slip along faults delineating the western margin of the Rio Grande rift. Rather, regional tilting likely results from basin-scale tilting due to slip along faults defining the eastern margin of the Rio Grande rift.
T33C-1482
Geometric Model of Conjugate Faulting in the Gyeongsang Basin, Southeastern Korea
A conjugate fault complex comprised of the NNE-trending dextral Yangsan fault system and the WNW-trending sinistral Gaum fault system cuts the Gyeongsang Basin on the SE margin of the Korean Peninsula. These conjugate faults may have formed contemporaneously in a compressional stress system accompanied by a 12° block rotation and separation as calculated from a geometric model using angles between conjugate fault sets, widths of fault domains, and measured fault displacements. Paleomagnetic show that the angle between declinations in both diverging block sets increases with age, suggestive of progressive block rotation. Assuming a progressive overall rotation of the entire Gyeongsang Basin (or the Korean Peninsula) with respect to the Eurasian continent simultaneously with block rotation within the Gyeongsang Basin, the paleomagnetic data correspond to the geometric model. This model accounts for Yucheon Group volcanism and subsequent caldera subsidence in the Uiseong sub-basin, and volcanism in the Miryang sub-basin produced by regional tension.
T33C-1483
The Age and Tectonic Significance of the Paleoproterozoic Great Falls Tectonic Zone, Southwestern Laurentia
The Great Falls tectonic zone (GFTZ) separates the Archean Wyoming province from the Hearne/Medicine Hat craton. Its northeastern limit is a high angle intersection with the Trans-Hudson orogen and marks the site of the Williston basin. Its southwestern extent probably reaches the Neoproterozoic margin of Laurentia. Though long recognized as a fundamental boundary in the accretionary history of Laurentia, its overall structure and developmental history are not well constrained. Limited exposures. Sm-Nd systematics and U-Pb ages zircons from metasedimentary rocks in the Little Belt Mountains (MT) and xenoliths provide strong evidence for consumption of juvenile oceanic crust and development of a continental magmatic arc at ~1.86 Ga on Paleoproterozoic to Archean crust of the Medicine Hat/Hearne blocks, rather than Wyoming crust. U-Pb ages (magmatic and xenocrystic zircons) and cooling ages (40Ar/39Ar) of exposed crust, drill cores, and xenoliths suggest that the eastern GFTZ beneath the Williston Basin has a similar history to crust formed in the central GFTZ and Trans-Hudson zones. The western GFTZ has a more complex crustal structure and metamorphic history involving Paleoproterozoic tectonic accretion and reworking of Archean crust (~1.77 Ga) as well as magmatic additions both younger (to ~1.65 Ga) and older (to ~2.45 Ga) than this event. This complexity strongly suggests the western GFTZ does not reflect a single collision following arc formation, but more likely indicates that the western GFTZ records collisions between multiple terranes, which may or may not include the Medicine Hat/Hearne block. The spatial coincidence of this complex collisional zone and the formation of the Belt basin provides an analog to the formation of the Williston basin of the eastern GFTZ. Tectonically, the formation of these major basins along the GFTZ suggests it originally developed with a strong transcurrent component that predisposed the GFTZ crust to basin development.
T33C-1484
Deformation of Rock Mass Caused by Strike-Slip Faulting: 3D Analysis of Analogue Models by Helical X-ray Computed Tomography
Strike-slip fault zones are induced experimentally in artificial rock subjected to strike-slip displacement along basement fault. The purpose is to investigate in three dimensions, the geometries and sequence of development of structural elements comprising the fault zones by use of a helical X-ray CT scanner. 860 mm long, 310 mm wide, 25 mm high artificial rocks were made by mixing sand, plaster and water. The basement fault was displaced up to 100 mm at a displacement rate of 0.1mm/sec. The deformation of the artificial rocks with increasing basement displacement was observed as follows. 1) En echelon fractures corresponding to the Riedel shears are observed at the surface of the artificial rock. These Riedel structures contain within them similar Riedels on a smaller scale (Riedel within Riedel structures). The length of the first and second order Riedel fractures is of the order of 100 mm and 10 mm, respectively. In three dimensions, each fracture has helicoidal shape. 2) Fractures corresponding to the first and second order P-shears form at the junctions between two first and second order Riedel shears, and serve to connect the Riedel shears. The combination of displacement along the Riedel and P-shears leads to the formation of the principal displacement shears including first and second order jogs and pull-aparts. 3) New shears (outer shears) branch off from Riedel and P- shears in compressional jogs and propagate aside from the fault zone that consists of Riedel and P-shears. The outer shears do not join the basement fault directly and develop near the surface of the artificial rock. The region among the Riedel shear, P-shear and outer shear is an up-squeezed block (push-up), which undergo rotation with increasing displacement. The push-up structures tend to be limited to shallow part of the artificial rock. The lower artificial rock on the one side of basement fault adheres to one on the other side in the compressional jogs. 4) As slip proceed, wear erode the second order jogs and produce the progressively more continuous and smoother principal displacement shear plane, and the real area of contact on the shear plane decrease. 5) With decreasing distance between adjacent first order compressional jogs (protrusions on surface), the real area of contact on the shear plane increase. Such evolution of the shears and its associated structures in the fault model tests agrees well with that of strike-slip fault systems and its associated geomorphic structures.
T33C-1485
Quasi-static Analysis of Strike Fault Growth in Layered Media
We study the effects of structural inhomogeneity on the growth of strike-slip faults. A layered medium is considered, made up of an upper layer bounded by a free surface and welded to a lower half-space with different elastic property. Mode III crack is employed as a mathematical model of strike-slip fault, which is nucleated in the lower half-space and then propagate toward the interface. We adopt FEM-β, newly proposed analysis method for failure, to simulate the quasi-statistic crack growth governed by the stress distribution in layered media. Our results show that along planar traces across interfaces a compliant upper layer has significant effects on promoting/suppressing crack growth before/after its extension into the layer and vise versa for a rigid one. This proposes a possibility that surface breaks due to strike faulting could be arrested by deposit layers at the topmost part of the Earth's crust.
T33C-1486
Modeling Propagating Discontinuities in Finite Elements with Linear Displacement Jumps
Finite elements with embedded discontinuities have been developed over the course of a more than a decade. Upon detection of a failure condition, a surface can be added in these elements at an orientation consistent with the critical angle of the failure condition, and at a location consistent with the propagating fracture. Displacements may then occur along this surface according to a post-localization constitutive law. Hence, the element may be used to track propagating deformation bands and fractures through a body. Until recently, however, the embedded displacement jump in the element was limited in that it had to be constant in that element, disallowing a jump in the strain and stress fields on opposite sides of the discontinuity. In many fault systems, however, there is relative tension on one side of the fault in the direction parallel to the fault. This is especially true at the ends of the fault, where the behavior can significantly affect the propagation, and lead to secondary cracking and other structures. We enhance the kinematics of the element to account for linear displacement jumps, thus allowing for stress and strain jump across the element, while maintaining traction continuity. This enhancement allows for more realistic capturing of the stresses along a fault, and hence more realistic propagation. It is also a preliminary step in modeling secondary cracking (e.g. splay cracks) in the system. We apply this element to some example problems from the literature.
T33C-1487
Stabilized low-order finite elements for simulating coupled solid deformation and fluid flow in fault zones
In fluid-saturated sediments and sedimentary rocks, we recognize that the presence of the pore-fluid can serve to impose an incompressibilty constraint on the solid matrix in the limit of undrained conditions or fast loading rates. While such situations are commonly encountered in practice, they pose a challenge for the numerical analyst. From a finite element point of view, incompressibility constraints will often lead to non-physical oscillations in the pressure field unless special care is taken to use stable combinations of pressure and displacement interpolations. Unfortunately, many seemingly natural combinations of mixed interpolations---e.g. linear interpolations for both displacements and pressure---are unstable. The relatively high computational burden associated with standard stable elements has limited the widespread adoption of fully-coupled geomechanical models, especially for large three-dimensional simulations. In this work we explore a stabilizing modification of the coupled balance equations that allows for the use of low-order mixed elements while avoiding non-physical pressure oscillations. The improved efficiency of this technique is a step toward making large-scale, fully-coupled three-dimensional simulations feasible. We demonstrate the efficacy of the technique for simulating coupled solid deformation and fluid flow in fault zones.
T33C-1488
Empirical Relationships Among Magnitude and Surface Rupture Characteristics of Strike-Slip Faults: Effect of Fault (System) Geometry and Observation Location, Dervided From Numerical Modeling
In order to determine the magnitude of pre-historic earthquakes, surface rupture length, average and maximum surface displacement are utilized, assuming that an earthquake of a specific size will cause surface features of correlated size. The well known Wells and Coppersmith (1994) paper and other studies defined empirical relationships between these and other parameters, based on historic events with independently known magnitude and rupture characteristics. However, these relationships show relatively large standard deviations and they are based only on a small number of events. To improve these first-order empirical relationships, the observation location relative to the rupture extent within the regional tectonic framework should be accounted for. This however cannot be done based on natural seismicity because of the limited size of datasets on large earthquakes. We have developed the numerical model FIMozFric, based on derivations by Okada (1992) to create synthetic seismic records for a given fault or fault system under the influence of either slip- or stress boundary conditions. Our model features A) the introduction of an upper and lower aseismic zone, B) a simple Coulomb friction law, C) bulk parameters simulating fault heterogeneity, and D) a fault interaction algorithm handling the large number of fault patches (typically 5,000-10,000). The joint implementation of these features produces well behaved synthetic seismic catalogs and realistic relationships among magnitude and surface rupture characteristics which are well within the error of the results by Wells and Coppersmith (1994). Furthermore, we use the synthetic seismic records to show that the relationships between magntiude and rupture characteristics are a function of the observation location within the regional tectonic framework. The model presented here can to provide paleoseismologists with a tool to improve magnitude estimates from surface rupture characteristics, by incorporating the regional and local structural context which can be determined in the field: Assuming a paleoseismologist measures the offset along a fault caused by an earthquake, our model can be used to determine the probability distribution of magnitudes which are capable of producing the observed offset, accounting for regional tectonic setting and observation location.
T33C-1489
A Lateral Tensile Fracturing Model for Listric Fault
The new discovery of a major seismic fault of the great 1976 Tangshan earthquake suggests a lateral tensile fracturing process at the seismic source. The fault is in listric shape but can not be explained with the prevailing model of listric fault. A double-couple of forces without moment is demonstrated to be applicable to simulate the source mechanism. Based on fracture mechanics, laboratory experiments as well as numerical simulations, the model is against the assumption of stick-slip on existing fault as the cause of the earthquake but not in conflict with seismological observations. Global statistics of CMT solutions of great earthquakes raises significant support to the idea that lateral tensile fracturing might account for not only the Tangshan earthquake but also others.
T33C-1490
An extended finite element algorithm for cracks with rate- and state-dependent coefficient of friction
The coefficient of friction between two contacting rough surfaces sliding past each other is known to vary with slip speed and a state variable reflecting the maturity of contact. For rocks this rate- and state-dependence of the coefficient of friction commences as soon as a crack forms during the period of slip weakening. In this work we report the performance of a recently developed numerical algorithm based on the extended finite element method for simulating the process of slip weakening and frictional sliding at different speeds on rocks with a well-defined frictional interface. Rate-dependence of the coefficient of friction leads to a first-order ordinary differential equation for balance of momentum, which we integrate numerically with the generalized trapezoidal method. We use the extended finite element algorithm to simulate some of the laboratory experiments performed by Dieterich and co-workers, as well as investigate the feasibility of using the rate-dependence of the coefficient of friction as a regularization to the problem of frictional crack propagation.
T33C-1491
Slip zone and Temperature of Faulting in the 1999 Chi-Chi Taiwan Earthquake
The activity of Chelungpu fault, induced the 1999 destructive Chi-Chi earthquake with magnitude Mw = 7.7, caused a total surface rupture of about 80-90 km long and the largest measured vertical offsets reaching as much as 5-9 m in west-central Taiwan. It, thus, provide us an opportunity to examine the fresh materials occurred by a large displacement faulting. The purpose, thus, of the Taiwan Chelungpu-fault Drilling Project (TCDP) is to obtain physical samples of the slip zone of 1999 Chi-Chi earthquake in the deep to make progress in understanding the weakening mechanism, energy budget and stress states of large displacement seismic faulting. We have finished drilling the TCDP hole-A and hole-B in the end of 2004 and May, 2005, respectively. At least six major fault zones have been identified in the cores. Based on several lines of evidence, including (1) the surface trail of rupture, regional geology and shallow seismic reflection; (2) thermal anomaly; (3) well logging data showing sharply decreasing resistivity and permeability, the lowest density, Vp and Vs, and high Vp/Vs and poisson's ratio; (4) principal stress rotations near the fault zone; (5) isotropic gouge layer in the fault core; and (6) pseudotachylyte found in the depth of 1111 m, the slip zone of 1999 Chi-Chi earthquake may be located in the black materials of fault zone FZ1111 around the depth of 1111.26 m. It is 2 cm in thickness which is located at 12 cm thick ultracataclasites with slickenline. The XRD and TEM analyses show that the black ultracataclasite consists of amorphous phase, pseudotachylyte and rich in smectite, but poor or scarce in other clay minerals, i.e. illite, chlorite and kaolinite. The decomposed experiments of thermal gravitational analyzer (TGA) on clay minerals indicate that the temperature of slip zone by frictional heating may reach as high as 950 ˘J during the faulting.
T33C-1492
Clay-clast Aggregates: A New Textural Evidence For Seismic Fault Sliding ?
To identify the particle dynamic processes responsible of slip-weakening in clay-rich seismic slip zones, several rotary-shear experiments were conducted at coseismic slip-rates (equivalent to 0.09, 0.9 and 1.3 m/s) at a fixed normal stress of 0.6 MPa, on a natural clay-rich gouge for saturated and for non-saturated initial conditions. The representative mechanical behavior of the simulated faults show a reproducible slip-weakening behavior, whatever initial moisture conditions. Total achieved displacements are comprised between 4 and 64 m. Detailed examination of gouge obtained at the residual friction stage in saturated and non-saturated initial conditions allows to define two types of microstructure implying two deformation regimes: a rolling regime with formation of clay-clast aggregates, and a sliding regime with formation of a foliated layer localized along the gouge-wall-rock interfaces. The observed slip-weakening behavior of simulated faults appears to be related to a decrease of the proportion of grain rolling to grain sliding with increasing slip displacement. Elevation of pore fluid pressure in the first meters of slip displacement, which represents the major contribution to the total energy budget experimentally produced, is thought to create the excess space necessary for grain rolling and subsequent formation of clay- clast aggregates. Spherical aggregates have already been reported from natural environments involving rapid slip along a seismogenic fault, landslide sole discontinuity, or sole of pyroclastic flows for which such aggregates are known to be formed by accretion of a cortex around a central spherical or ellipsoidal clast in an expanded fluidized granular flow. Therefore, the occurrence of clay-clast aggregates in natural clayey fault gouges could constitute a new potential textural evidence for thermal pressurization and consequently for past-seismic fault sliding.
T33C-1493
Sub-/Seismic Structure and Deformation Prediction across different scales between 1D well data and 3D reflection seismics
The evolution of a sedimentary basin is mostly affected by deformation. Large-scale, subsurface deformation is typically identified by seismic data, small-scale fractures by well data. However, faulting at the medium sub- seismic scale plays an important role, e.g. in reservoirs: large individual reservoirs can be disrupted by faults enhancing fluid flow, or producing compartmentalized deposits due to cementation of fractures. Thus, between both scales, seismic and well data, we lack a deeper understanding of how deformation scales in the sub- seismic region. To start tackling this problem, a 3D reflection seismic data set in the North German Basin was analysed with respect to structure and faults in great detail, calibrated by well data. This led to the determination of magnitude and distribution of deformation and its accumulation in space and time on the seismic scale. The structural interpretation unravels the kinematics in the North German Basin with extensional events during basin initiation and later inversion. For further quantitative deformation and fracture prediction on the sub-seismic scale, two different approaches are introduced. An increased resolution of subtle tectonic lineaments is achieved by coherency processing yielding together with geostatistic tools the distribution of low- and high-strain zones in the region. Independently, the distribution and quantification of the strain magnitude is predicted from geometrical 3D retro-deformation of the identified structures. For the fault structure analysed, it shows major-strain magnitudes between 5-15% up to 1.5 km away from a fault trace, and variable deviations orientation of associated extensional fractures. The small scale is represented by FMI data from borehole measurements, showing main fault directions and densities. These well data allow the validation of our sub-seismic deformation analyses. In summary, the good correlation of results across the different scales makes the prediction of small- scale faults/fractures possible. The temporal component of faulting will be gained in the future by analogue models. The suggested geomechanical workflow is applicable to reflection seismic data, but requires the 3D coverage of a region as basic principle. It yields in great detail both the tectonic history of a region as well as predictions for the genesis of structures below the resolution of reflection seismics.
T33C-1494
Displacement-Length Scaling Relations for Geologic Structural Discontinuities and Implications for Near-Tip Processes
Displacement-length ( D-L) scaling relations for faults and other common geologic structures provide a window into the mechanics of brittle strain localization. We present and synthesize the displacement-length data for the various types of geologic structural discontinuities, including faults, joints, veins, igneous dikes, deformation bands, and compaction bands. Neglecting the influence of short-range mechanical interaction and other effects, the scaling relations of geologic structural discontinuities define two groups. The first group, having a slope of n = 1 and therefore a linear dependence of maximum displacement and discontinuity length ( Dmax = γ L), comprises faults and shear deformation bands. These shearing-mode structures grow under conditions of constant driving stress, such as shear stress drop, with the magnitude of near-tip stress on the same order as the rock's yield strength in shear. The second group, having a slope of n = 0.5 and therefore a square-root dependence of maximum displacement and discontinuity length ( Dmax = α L0.5), comprises hydrothermal veins, igneous dikes, cataclastic (compactional/shear) deformation bands, and compaction bands. These opening- and closing-mode structures grow under conditions of constant fracture toughness, implying driving stresses that scale inversely with L and significant amplification of near-tip stress within a zone of small-scale yielding about the discontinuity tip. Scaling exponents of n = 1 imply that rock shear strengths are much less than the tensile strength. On the other hand, grain cracking, and thus control of propagation by the mode-I fracture toughness, appear responsible for scaling with n = 0.5. http://mines.unr.edu/geo-eng/schultz
T33C-1495
Magnitude of contractional strain accommodated across stylolites in micritic limestone
In fault zones, stylolites result from water-assisted pressure solution and participate in mass-transfer and volume loss in the rock as an important deformation mechanism in faults involving carbonate rocks. Although it is well known that the orientations of stylolite surfaces and the associated teeth track the direction of the local stress state, making them reliable paleostress indicators, the amount of contractional strain accommodated by a stylolite remains difficult to assess given its dependence on the properties and chemical heterogeneity of the rock and the paucity of appropriately constrained field examples. We analyse a population of fault-related stylolites by comparing the amplitudes of stylolitic teeth and spikes to independent measures of thinning in the rock, and investigate the scaling relations between along strike trace length (L) and the maximum (Dmax) and average (Davg) amplitudes of teeth. The studied stylolites occur in a 212-mm-thick limestone layer dragged into the damage zone of the Gubbio normal fault zone in Central Italy. Layer thinning was assessed independently from the layer geometry, with a maximum value of ~23% nearest the fault. 28 mm of thinning is related to 24 stylolites whose lengths range from 6.4 to 146 mm and average amplitudes from 0.1 to 1.3 mm. The average and maximum amplitudes of teeth generally increase with stylolite length, with Davg = 0.0011L0.17, r2 = 0.57, consistent with stylolites being analogous mechanically to anticracks and compaction bands that propagate to greater lengths as contractional strains increase along them. The stylolites increase in number and amplitude into the most thinned area, correlating with increasing contractional strain accommodated by the layer. Although the maximum amplitude scales with stylolite length, its location along the stylolite appears random, consistent with recent work that associates maxima with processes at the grain scale, such as the positions and surface energies of less soluble grains. In contrast, the average amplitude appears related to continuum-scale factors such as rock stiffness. By summing lengths and average amplitudes, the contractional strain accommodated by the stylolites increases from zero far from the fault to ~5% in the area of greatest layer thinning. The average amplitude of stylolites visible in outcrop provides a measure of the minimum magnitude of contractional strain in the rock, although other mechanisms such as grain-scale dissolution appear necessary to account for the remaining layer thinning.
T33C-1496
Preferential Acceleration of Pressure Solution Creep of Shear Zones in Shale of Subduction Zone
We have estimated activation energy of natural pressure solution deformation in shales of the Shimanto accretionary complex in southwestern Shikoku Island, Japan. Pressure solution deformation affected two types of shales: shear-dominated (type S) and compaction-dominated (type C) shales. Additionally, within each type of shale, pressure solution seams (PSS) density, indicative of the intensity of the deformation, is highly variable in relation with a relatively wide range of paleotemperature [Kawabata et al., 2007]. Correlating the PSS density and the paleotemperature with an Arrhenius expression, we extracted low activation energies of 18 kJ/mol for type S and 45 kJ/mol for type C. The good agreement between these values and the activation energy of diffusion controlled pressure solution creep (approximately 35kJ/mol), which is sum of activation energy of diffusion coefficient through grain boundary fluid (15kJ/mol) [Nakashima, 1995] and equilibrium constant of silica (20kJ/mol) [Rimstidt and Barnes, 1980], as well as our microstructural observations, show that diffusion is indeed the controlling process of pressure solution deformation. Difference in activation energy between Type S and Type C is probably caused by grain boundary structure, which can be either wetted (connected) or non-wetted (non-connected) [Nakashima, 1995]. Shear deformation, active in type S shales, creates more wetted (connected) grain boundary than compaction in type C shales, resulting in smaller activation energy. Such interplay between shear deformation and the fluid phase structure promotes the localization of deformation within shear zones deforming by pressure solution.
T33C-1497
Coupling between brittle fracture and anticrack-vein pressure solution at asperities along a small-displacement thrust fault in limestone
A significant fraction of displacement on a fault may take place by aseismic creep, but specific mechanisms of creep are not well-documented. In addition, seismic events are often nucleated at asperities, with associated damage tending to smooth the fault. Alternatively, aseismic creep processes such as pressure solution dissolve material on one side of asperities and precipitate material on the opposite side, thereby maintaining fault waviness. Depending on the tectonically-imposed displacement rate, slip by aseismic creep will take place if asperity roughness is sufficiently small, and roughness will be preserved. To study these processes, we mapped a wavy fault zone in limestone. Our observations are consistent with creep by pressure solution mechanisms and asperity reduction by brittle failure. The decimeter-scale thrust fault studied is in a complexly-faulted tabular region near the axial surface of a kilometer-scale syncline in Ordovician carbonates, Valley & Ridge Province, State College, Pennsylvania. The fault was selected because of excellent exposure, known displacement, and the presence of asperities. Dissolution and precipitation, with transport across asperities, may be inferred from local concentration of calcite precipitation at the fault surface. This is the first mechanism associated with slip across an asperity. Brecciation is also observed, which acts to smooth the fault, whereas the pressure solution mechanisms allow slip without smoothing. The latter requires small fault creep rate, whereas brecciation implies that pressure solution could not accommodate the slip rate required by the tectonic loading rate. Within the wall rock, anticracks (tectonic solution seams) & veins are strongly concentrated at undulations with wavelengths of 10s of cm to a few meters and amplitudes of approximately 10 cm. If maximum dissolution is approximately 0.03 anticrack length, as measured elsewhere, vein precipitation roughly balances dissolution locally. This local strain of a few percent is associated with slip of about 1 m. Estimates of asperity wavelength and amplitude, localization of pressure solution, relative proportions of brecciation and mass transport across asperities have been used to constrain a mechanical model of the coupled processes. Condition for a transition between aseismic slip and seismic, brittle failure can be established.
T33C-1498
Muscovite, Illite and Organic Material Drive Quartz Pressure Solution and Stylolite Development in Plio-Pleistocene, Jurassic and Ordovician Sandstones
Muscovite and illite are integral drivers of pressure solution in Ordovician (Bromide), Jurassic (Rhum Field, North Sea) and Plio-Pleistocene (Tulare Formation) sandstones. In all cases, pressure solution was not found between clean quartz grains, but was found at mica-quartz interfaces and between illite-coated quartz grains. Preliminary cathodoluminescence observations of the Ordovician Bromide Sandstone of Oklahoma revealed highly sutured quartz grain contacts in millimeter-scale zones of illite-coated quartz grains. Away from the illite- rich zones where quartz grains lack illite coatings, no interpenetration of quartz grains was observed. This variation in quartz pressure solution demonstrates that a) illite is necessary to promote pressure solution and b) the localization of pressure solution is due to primary differences in sandstone mineralogy. A cathodoluminescence study of pressure solution and stylolite development in Jurassic sandstones from a North Sea core also revealed that no pressure solution occurred between clean quartz grains. However, quartz dissolution, including dissolution of overgrowths, did occur along mica-, organic- and clay-rich stylolites, and between quartz grains with thin clay coatings. Microprobe and XRD analysis confirmed that illite and muscovite are the clays within both the stylolite and bulk rock, thus no preferential chemical alteration of the clay occurs in the stylolite seam. Therefore, in these samples, muscovite or illite and/or organic material are necessary for dissolution of quartz and stylolitization. Furthermore, observations show that stylolites begin as originally flat clay/organic-rich laminae (~3% organic) and acquire stylolite morphology as varied dissolution of quartz grains occurs along the stylolite interface. Shallow pressure solution is observed in core samples of the Plio-Pleistocene Tulare Formation from less than ~600 meters depth. Point-count analysis shows that mica-rich areas have 2.5 times more flattened grain contacts relative to mica-poor areas of the sandstones. These relationships hold in the presence of biotite or muscovite. Interpenetration of illite-coated quartz grains was also observed. Clay and mica enhance quartz dissolution at all depths and should be considered when modeling quartz sandstone compaction.
T33C-1499
Coupling between pressure solution and fracturing processes discussed from indenter experiments
Pressure solution is a mechanism competing with cataclasis during sediment deformation. For example, both mechanisms are well documented in fault zones where they interact to make sedimentary rocks behave in both brittle and viscous manners. Cataclasis is associated with earthquake rupture whereas pressure solution accommodates post-seismic creep and sealing processes. In basins, sedimentary grains deform both by pressure solution and cataclastic deformation and are responsible for sediment compaction and porosity loss. As a consequence, the coupling between pressure solution and fracturing processes is a major issue, which we have studied experimentally. Indenter technique is a good technique for pressure solution studies since it allows controlling the distance of mass transfer, a crucial parameter in pressure solution constitutive laws. We have performed pressure solution indenter experiments on various kinds of single crystals leading to contrasting effects of the fracturing process on the kinetics of pressure solution creep. Indenting of quartz crystals leads to various hole shapes under the indenter. Cylindrical holes with a diameter equal to the indenter diameter are obtained at low stress (25-50 MPa), whereas hole larger than the indenter diameter are obtained at higher stresses (100-300 MPa). Reverse crown-shaped fractures below the indenter are associated with such a hole enlarging process. Successive fracture sets are created, then partially healed during the progressive indenting. However, displacement rates showed an exponential dependence on the stress values, as predicted theoretically. So the development of such fractures does not seem to significantly increase the kinetics of pressure solution. Conversely, indenting halite crystal in presence of brine solution led to different fracturing effects. At low stress no fracturing could be observed and the diameter of the hole was equal to the diameter of the indenter. However, near halite yield stress value, radial fractures developed and the rate of pressure solution kinetics was increased by a factor ten. We interpreted radial fracturing to have augmented the rate of diffusive mass transfer along the contact between the indenter and halite by a short cut of diffusion through the free-fluid filled fractures. We discussed the applications of these results to the modelling of rheological and permeability evolution of sedimentary rocks.
T33C-1500
Solid-solid phase transformation: Roughening of stylolites
Sedimentary rocks under uniaxial compression often react by changing the texture during compaction or cementation, which is accompanied by the formation of stylolites spanning the grain contacts or the rocks along surfaces normal to the applied stress. Many field observations corroborate a common feature of stylolites, namely that they are rough interfaces that contain insoluble minerals. Stylolites are outstanding examples of interfacial patterns developed in out-of-equilibrium systems. We study the roughening of stylolites within a model of a moving interface boundary between two stressed solids. The set up of our model consists of two dissimilar elastic bodies that are separated by a sharp interface and subjected to uniform compression in the direction perpendicular to the interface profile. Based on the balance laws of force and energy, we derive the jump conditions for a moving interface driven by a phase transformation process, i.e. the solid phase with higher energy (more porous) is removed and replaced by the same amount of less porous solid phase. An initially flat interface perturbed with small irregularities develops grooves or finger like structures, which align with the principal direction of compaction. The system is dissipative and approaches asymptotically the equilibrium configuration between the two phases. Our numerical investigations reveal several issues: 1) a morphological instability of the solid-solid interface does develop; 2) the instability is driven by the porosity jump across the interface; 3) the energy concentration at the tip of the fingers may influence the development of cracks perpendicular to the stylolites planes, as observed in nature.
T33C-1501
Discrete Compaction Bands in Porous Sandstone: Stress Scaling and Energetics of Propagation
Compaction bands are a kinematic end-member of strain localization in porous rock, forming thin tabular structures normal to the maximum compressive stress with negligible shear offset. They are associated with significant porosity loss and permeability reduction. In the field where they were first identified, such localized features typically have thickness on the order of 1-10 mm and trace lengths on the order of 1-10 m. Recent investigations in the laboratory have detailed the development of comparable discrete compaction bands in several sandstones with porosities ranging from 23% to 25% at stress states in the transitional regime from brittle faulting to cataclastic. In many respects the geometric attributes of discrete compaction bands as observed in the laboratory are qualitatively similar to compaction bands that have been documented in the field. Nevertheless, there are at least two discrepancies between the laboratory and field structures. First, the stress level inferred for compaction band development in the field are lower than that required in the laboratory. Second, the dimensions as well as the damage intensity measured in the field are also appreciably lower than those in the laboratory deformed samples. If the laboratory measurements can be realistically extrapolated to the geologic setting, the mechanical basis for the growth of the band, stress state and damage intensity should first be established. In this study we address these questions by compiling field observations and laboratory data on the stresses associated with the compaction band failure mode and the geometric attributes of the bands. To complement existing laboratory data on discrete compaction bands in the Bentheim sandstone, we conducted a suite of experiments on the Diemelstadt and Bleurswiller arkosic sandstones. The propagation and geometric attributes of the bands produced in the deformed samples were characterized using acoustic emission data, X- ray CT images and conventional microscopy techniques. Synthesis of field and laboratory data for five sandstones over length scales of 10-3-10 m shows that the thickness and length of compaction bands seem to obey a quadratic scaling relation where the thickness scales approximately with the square root of the band length. In a recent analysis Rudnicki (2007) formulated several models for compaction band propagation, and in particular he proposed a combined anti-crack/dislocation model which would indeed result in the quadratic scaling that we identified in the field and laboratory data. On the basis of this fracture mechanics model, we explore the mechanical interpretation for the broad range of stresses associated with compaction band propagation on the field and laboratory settings and obtained a scaling relation in which the stress level is inversely proportional to compaction band thickness. Thus for the laboratory bands, higher propagation stresses are predicted, as is the potential for significant damage, while for compaction bands in the field, lower stresses and less damage are expected. Together the laboratory and field data constrain the critical strain energy release rate in the model to be on the order of 2-40 kJ/m2, comparable with lab measurements of the compaction energy.
T33C-1502
Deformation Bands: Strain Localization Structures in Highly Porous Sandstone
Deformation bands are the most common strain localization feature found in deformed porous sandstones and sediments, including Quaternary deposits, soft gravity slides and tectonically affected sandstones in hydrocarbon reservoirs and aquifers. They occur as various types of tabular deformation zones where grain reorganization occurs by grain sliding, rotation and/or fracture during overall dilation, shearing, and/or compaction. These structures form in rocks and sediments where porosity exceeds approximately 15 percent, where the pore space allows for a more flexible grain reorganization that that seen in non- and low-porosity rocks. Deformation bands with a significant component of shear are most common and typically accommodate shear offsets of millimeters to centimeters. They can occur as single structures or cluster zones, and are the main deformation element of fault damage zones in porous rocks. Factors such as porosity, mineralogy, grain size and shape, lithification, state of stress and burial depth control the type of deformation band formed. The different types are controlled by deformation mechanisms: cataclasis, rigid grain reorganization (granular flow) and cementation and/or dissolution (wet diffusion). Most bands show a reduction in porosity and permeability, usually between 0 and 3 orders of magnitude. Of the different types, phyllosilicate bands and most notably cataclastic deformation bands show the largest reduction in permeability, and thus have the greatest potential to influence fluid flow. This is particularly so where the bands occur in clusters, and if dissolution accompanies the cataclasis. Disaggregation bands, where non-cataclastic, granular flow is the dominant mechanism, show less influence on fluid flow unless assisted by chemical compaction or cementation.
T33C-1503
Numerical Modeling of Shallow Sediment Deformation Under Complex States of Stress
The accurate numerical simulation of the deformation of shallow geomaterials under complex states of stress is dependent on both the input of realistic sediment parameters as well as the application of appropriate parameters and approximations within the numerical model itself. With this understanding, the aim of the current study is to model the elasto-plastic deformation of sand and clay in response to constant and changing states of stress using the finite element method. The importance and influence of various constitutive equation parameters, boundary conditions, and model geometries are analyzed and applied to a real world example. The particular model presented simulates the long-term deformations associated with the construction of a shallow tunnel in London, UK using a 2D plane strain approximation. The predicted deformations are then compared to the observed surface displacement data of Standing et al. (1996).
T33C-1504
Quantifying strain beneath glaciers: fabric analyses at macro and micro-scales
There has been much discussion in recent years regarding the role and significance of subglacial deformation in the dynamics of contemporary and former ice masses. It has been proposed that the presence of deformable substrates beneath an ice mass exerts fundamental controls over ice dynamics, and may have a critical influence over the stability of ice masses. However, to unravel the clearly complex relationship between ice dynamics and subglacial deformation, an effective means of assessing the magnitude of deformation from field evidence is required. Particle arrangement (fabric) in sediments is thought to provide evidence of the physical conditions operating at the time of deposition, and has been traditionally considered in glacial sediments to yield information regarding former ice-flow directions and processes of till deposition/deformation, and indications of consequent glacier dynamics. Developments have focused on the application of statistical methods (vector and eigenvector analysis) to develop envelopes of characteristic fabric characteristics as the basis for interpretation. However, the value of traditional vector based approaches for reconstruction of subglacial stress patterns has been questioned in recent years, as a consequence of the ambiguity of fabric shape envelopes' indicating specific subglacial conditions. This study reports the findings from a range of field-based experiments that have investigated the fundamental processes by which subglacial fabrics develop. Studies at macro- and micro-scales of tills recently exposed at two sites in Iceland are used to develop a model by which patterns of particle orientation as a function of particle size reflect the magnitude and duration of sediment strain during subglacial deformation. We demonstrate that it is possible to identify the relative strain history of subglacial tills from different locations, and identify the presence of accreting till bodies and zones of net erosion of the deforming bed. This model is applied to a multiple till sequence in Central Scotland, to identify three distinctly different glacial dynamics regimes during the last glacial cycle, and to illustrate how such data may be applied to understanding the dynamics of past glaciation.
T33C-1505
Diatomite of McKinney Bay, Lake Tahoe
Lake Tahoe's McKinney Bay was formed by at least one landslide occurring in a geologic sequence of continuous stratified deposits of thinly bedded mudstones, sandstones, and diatomite. Unfortunately, little is known about the strength capabilities and fracture properties of diatomite. This study is focused on a better understanding of the strength of the diatomite beds within the section in order to more accurately distinguish the mode of failure. We hypothesize that failure is either 1) initiated within the diatomite; or 2) initiated in weak layers below the diatomite Determining the precise mode of failure is important because several smaller landslides have occurred in recent times at the head of McKinney Bay. Moreover, beaches at the head of McKinney Bay are narrow; perhaps an indication of continuous failure. Some failure debris is offset by faulting; causing some concern that failure is related to seismicity. Rock fracture experimentation and fracture orientation analysis will be used to determine if failure is currently active. Other, qualitative study will determine if fissures noted on the ground surface are related to slope failure. Ideally, this study will prove to be significant to property owners in the McKinney Bay area, the Truckee River dam at Tahoe City (several miles north of McKinney Bay), and the state of California.
T33C-1506
Texture development in naturally deformed granite, Seine metaconglomerate, Ontario
We present lattice-preferred orientation (LPO) data and microstructural observations from granite conglomerate clasts deformed at greenschist facies conditions in the Archean Seine River - Rainy Lake shear zone region, Ontario, Canada. Samples for analysis were selected from a strain gradient recognized with detailed three- dimensional strain analysis, allowing us to interpret our results in the context of progressive deformation. Granite clasts are composed dominantly of quartz and albite, with lesser amounts of orthoclase and mica. Mica abundance increases slightly during deformation as feldspar reacts with metamorphic fluids. Mica shape- preferred orientation intensifies and grain linkages increase during deformation though a combination of intracrystalline strain and dissolution-precipitation processes. Quartz LPO intensity increases at low to moderate strain with progressive deformation, but plateaus at moderate to high strain. Microstructural observations including undulose extinction and subgrain formation indicate that dislocation creep was the dominant deformation mechanism at low strain. At higher strains, the prevalence of smaller, undistorted grains indicates that recrystallization was an important recovery mechanism. Feldspar LPO undergoes similar intensity variation from low to high strain, but the magnitude of variation is less than that for quartz. Microstructures in feldspar grains include through-going fractures and undulose extinction at all magnitudes of strain. We infer both dislocation creep and fracturing deformation mechanisms for feldspar. Quartz c-axes are sub-parallel to macroscopic lineation at low strain and form weak girdles sub-parallel to foliation at higher strain. Asymmetry of the quartz LPO is consistent with dextral shear in the region. In contrast, feldspar crystallographic axes maintain a stable orientation oblique to macroscopic fabric throughout deformation. Our results suggest that quartz and mica were the dominant minerals accommodating strain throughout deformation. Feldspar was not entirely rigid, but accommodated little strain. The consistency of LPO observed in both quartz and feldspar at moderate to high strain demonstrates that intensity does not continually increase. Rather, some minerals may develop a steady-state LPO in granite deformed at greenschist facies conditions.
T33C-1507
Dehydration Kinetics of Volterra Gypsum: Experiments and Overview
Dehydration reactions are often envisaged as a triggering mechanism for seismicity in rocks under tectonic loads due to the reduction in effective pressure during the release of fluids that may eventually produce mechanical embrittlement. Understanding of metamorphic transformation in deforming rocks is even more important in fault zones where periods of seismic slip are reported. Dehydration of gypsum under controlled conditions, in laboratory experiments and in numerical models, provides information on deformation processes operating in seismically active regions and may be of help in understanding their cyclicity and their evolution. Two series of simple heating experiments of Volterra gypsum samples at room pressure, using intact and powdered specimens, provide reference data for further experiments under confining and differential stress during dehydration. Heating experiments were run at constant temperature between 80 degC and 140 °C in intact specimens and at 86 °C and 97 °C using powders with five different grain size fractions: <0.063, 0.063-0.125, 0.125-0.25, 0.25-0.5 and >0.5 mm. The complete dehydration of 1 mol of gypsum produces 1 mol of anhydrite and two moles of water generating a porosity of about 38% and implying a weight loss of 21% upon removal of water. The progressive loss of weight during dehydration was used as the method to estimate the progress of the reaction. The reaction is characterized by an initial stage under 10% reaction were reaction rate accelerates, which is followed by a linear stage for about 50 to 70% of the reaction and a final third stage with decelerating reaction rates. All tests run above 85 °C reached about 90% reaction. Those below 85 °C seem to converge to a lower final fraction (75%) suggesting partial dehydration, very likely to bassanite. The temperature dependence of the linear rates indicates in an Arrhenius plot that the full dehydration of gypsum has an activation enthalpy of 96 kJ/mol. The two temperatures tested with powdered specimens are consistent with this activation enthalpy, although the higher intercepts with the y-axis indicate that reaction rates are about one order of magnitude faster. This is interpreted in relation to the very large initial porosity of the un-compacted specimens (>45% porosity). There is an additional increase in rates in powdered specimens as the grain size decreases, however, the difference is marginal despite the eight-times difference in grain size. At laboratory deformation rates, gypsum behaves in a brittle-ductile mode by a mixture of plasticity (twinning) and cataclasis in discrete and short-lived bands. A first series of deformation tests on dry gypsum have also been run to set a reference for mechanical behaviour to be compared in the future with behaviour during syn-tectonic dehydration. The deformation tests were run at room temperature, at strain rates of 2x10-5s-1 and confining pressures of 13, 50, 100, 146, 190 MPa. Tests were run in a triaxial rig using 20 mm diameter by 45 mm length specimens. Stress-strain curves show well-defined yield points and an almost straight plastic behaviour with a slight strain hardening component, similar to previous work. Stress-strain curves have minor, episodic and short-lived stress drops that have been related with the development of cataclastic bands (Milsch and Scholz, 2005). The grain fracturing associated with the generation of these cataclastic bands during experimental deformation of gypsum will have an effect in the dehydration kinetics by providing fined-grained gypsum and thus high surface area to speed up the reaction.
T33C-1508
The Acoustic Signature of Woodford Shale and Upscale Relationship from Nano-Scale Mechanical Properties and Mineralogy
The complex composition of shale, the most encountered and problematic lithology in the Earth's crust, has puzzled many researchers attempting to find the key for understanding their micro- and macro-scale acoustic and mechanical signatures. Recent advances in nano-technology, in particular the progress of the Atomic Force Microscope (AFM) base indentation technique, have made it possible to mechanically study porous material at a nano scale (10-9 m) and consequently have allowed linking shale mechanical properties to intrinsic micro- and macro-properties such as porosity, packing density, and mineralogy. Based on more than 20,000 nano- indentation tests conducted on a number of shales with varying physical properties, a GeoGenomeTM model was developed to upscale macroscopic shale mechanical parameters from mineralogy composition, porosity, and packing density. In this work, the mechanical properties such as the elastic stiffness coefficients, Cij, and the anisotropic Biot's Pore Pressure Coefficients, αij, of the Woodford shale, were acquired using sonic log data and Ultra-Sonic Pulse Velocity (UPV) measurements conducted on preserved retrieved shale core samples from a 200-ft well drilled in the Woodford formation, in Oklahoma. Furthermore, the dependency of the Cij and αij, on applied stresses and the relationship between the dynamic moduli and the quasi-static moduli were also investigated using an array of piezoelectric crystals mounted around the samples while subjecting the samples to different applied stress states using a series of tri-axial tests. X-Ray Diffraction (XRD) and mercury injection tests were also performed on the retrieved core samples to obtain mineralogy composition and porosity of the shale at different depths. Comparison of the simulated mechanical and poromechanical properties and stiffness coefficients using the Quantitative GeoGenomeTM Mineralogy Simulator (QGGMSTM) with field and acoustic lab measurements showed excellent agreement both qualitatively and quantitatively. The results from field sonic and petrophysical logs, laboratory UPV measurements, and the QGGMSTM model show that despite a relatively high quartz and pyrite content as revealed by the XRD, the Woodford shale does possess clear transverse isotropic macro-mechanical characteristics as reflected through the Thomsen parameters.