Rocks and Fluids: Prediction and Monitoring in Various Geological Settings Including Fault Zones I Posters
Presiding: S J Wilkins, Shell International Exploration and Production; A Levander, Department of Earth Science, Rice University
S41C-01 0830h
The Role of the Palmerville Fault to Mineralization in Northeast Australia: Results from Deformation and Fluid Flow Numerical Modeling
Major fault systems that penetrate deep into the crust are poorly understood but important architectural elements of the Earth. Conceptually and empirically, these structures can provide pathways and foci for both mineralising fluids and magmatism into the upper crust. We investigate the north-to northwest-striking Palmerville Fault, which forms a major structure in northeastern Australia, delineating the boundary between the Palaeozoic Hodgkinson Province and Proterozoic rock assemblages to the west. This terrane-bounding fault played a pivotal role in the evolution of the Hodgkinson Province throughout the Palaeozoic. No mineral occurrences are known along the north-striking section of the Palmerville Fault. Where the strike of the Palmerville Fault changes to northwest (resembling a major jog), the fault system is punctuated by granites and volcanics that host several major gold-copper and skarn deposits. In contrast, a number of north- to northwest-striking fault structures that traverse the Hodgkinson Province and parallel the Palmerville Fault, are associated with orogenic gold-rich regions. In order to understand the processes controlling mineralization in the basin sequence of the Hodgkinson Province, we have integrated spatial data sets, geological, structural and geophysical mapping of major structures, geophysical `worming', and forward modeling of the subsurface structure of the major fault systems. By doing so, we were able to overcome limitations imposed by the limited exposure and relatively low data density in the Hodgkinson Province. Our multi-disciplinary approach allows insight into the nature of the terrane-bounding fault and its significance in the tectonic evolution of the province. Results from this study suggest that the Palmerville Fault represents an eastward-dipping listric normal fault that controlled opening of the Hodgkinson Basin in the Ordovician to Devonian. During basin inversion in the Late Devonian to Carboniferous the fault played a crucial role in gold mineralization in the Hodgkinson Province acting as a major ore fluid conduit. Here, we expand our understanding of the Palmerville Fault by applying deformation and fluid flow numerical modeling scenarios to our fault model. We investigate the role of the Palmerville Fault in mineralization processes throughout the Hodgkinson Province using coupled mechanical and fluid numerical modeling software. A number of fault geometries with varying orientation, dip and crustal depth are considered. Our models provide insights into the loci of dilation and fluid flow with varying geometries, which can then be compared with the Palmerville and adjacent faults in the Hodgkinson Province. Results from our study prove useful in understanding why the major Palmerville Fault itself remained barren, with mineralisation occurring in parallel second-order faults. Furthermore, additional insights are gained on the tectonic evolution of the Hodgkinson Province based on an augmented understanding of the processes that control mineralization in the province. As such, results from our study may be applicable to orogenic gold-rich regions around the world where gold endowment is spatially associated with second-order structures rather than primary fault structures.
S41C-02 0830h
Spectral element modeling of fault-plane reflections arising from fluid pressure distributions
In order to better understand the origin of fault-plane reflections in compacting sedimentary basins, we have numerically modeled the elastic wave equation via the spectral element method (SEM) for several different fault models. Using well log data from the South Eugene Island field, offshore Louisiana, we derive empirical relationships between the elastic parameters (e.g., P-wave velocity and density) and the effective-stress along both normal compaction and unloading paths. These empirical relationships guide the numerical modeling and allow us to investigate how differences in fluid pressure modify the elastic wavefield. We chose to simulate the elastic wave equation via SEM since irregular model geometries can be accommodated and slip boundary conditions at an interface, such as a fault or fracture, are implemented naturally. The method of including a slip interface retains the desirable qualities of SEM in that it is explicit in time and does not require the inversion of a large matrix. We perform a complete numerical study by forward modeling shot gathers over a realistically-sized Earth model using SEM and processing the simulated data to reconstruct post-stack time-migrated images of the kind that are routinely interpreted in the seismic industry. We dip filter the seismic images to highlight the fault-plane reflections prior to making amplitude maps on the fault plane. With these amplitude maps, we compare the reflectivity from the different models to diagnose which contributes most to the observed fault reflectivity. To lend physical meaning to the value of compliance for a slipping fault, we propose an equivalent-layer model under the assumption of weak scattering. This allows us to use the empirical relationships between density, velocity, and effective stress from the South Eugene Island field to relate a slipping interface to an amount of excess pore pressure in a fault zone.
http://www.mines.edu/~rsnieder/thesis_Haney.pdf
S41C-03 0830h
The Features of Seismicity in North Area of Tibet Plateau and Interpretation of Rock Experiment
There are many faults distributed in North area of Tibet plateau, The north border consists of Alkin, Qilian and Haiyuan faults. There are obvious characteristics of earthquake activities: 1).pseudo-periodic of quakes (Ms6); 2).alternative activity of combinative faults; 3).Occurring at the joint area of faults; 4).Speadup-turn rapture of deformation accumulation. Under the press-shear condition, we choose the above three faults to set up experiment sample. The load curve mirrors a pseudo-periodic feature of viseo-slip events. It shows that the time of accumulating the energy for a rapture event is similar under the stable movement of blocks. There is peak value of strain at the joint area of Qilian fault and Haiyuan fault. It is similar to the energy pattern reflected by fault deformation observation. This indicates that, under the constraint of the specific geometric structure and stress direction of the northern edge of the Tibet plateau, the joint area of the Qilian fault and Haiyuan fault is indeed an energy concentrating area. The distribution feature of the strain increment shows that, when it was close to the rupture, strain was in relative critical state. Rupture happens at the peak value area, and it causes a large stress drop that covers almost the whole area of the Haiyuan fault. At the same time, the joint region between Alkin fault and Qilian fault as well as the Qilian fault region that is close to Haiyuan fault form relative increasing region. This indicates that the rupture happens on one fault can influence other connected faults markedly, and increases the risk of ruptures on other faults.
S41C-04 0830h
Analysis of Fracture Alignment and Hydrothermal Flow in Accommodation Zones That Link Facing Half-Grabens, Oregon Basalt Plateau
The Warner Valley, located in the southern Oregon portion of the Basin and Range Province, is composed of igneous units originating from the Steens Mountains volcanism. Normal faults associated with two opposing half grabens which define the valley are linked by a fractured accommodation zone. Within the valley are a series of shallow dilute to evaporatively concentrated hypersaline alkaline lakes that are being investigated as an analog to evaporative paleo-lakes on Mars. Little precipitation falls during the summer months and most of the surface water originates as runoff from snow pack in the winter and early spring. One additional recharge mechanism for the lakes is infill from fracture-controlled springs. Several of these springs are hydrothermal in nature with temperatures ranging from 18° C to 70° C. Integration of remote sensing datasets with field-based data provides a unique opportunity to combine medium to high spatial resolution imaging with ground truth measurements. This permits identification and correlation of the fault and spring locations and verification of the fracture control on the distribution of springs based on a larger data set than is available from field work alone. Landsat and ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer) data, as well as digital elevation models, are used in the analysis. Gradients and absolute values from digital elevation models are used to define the trends of the faults and fractures as well as the fault offsets. Many of the springs are marked by highly vegetated riparian zones that are identified via spectral and textural signatures present in these types of multispectral visible/infrared imagery. A majority of the hot springs are located within the accommodation zone of the opposing half-grabens; this highly fractured area is investigated in the datasets. Maps of vegetation variations within the valley coupled with thermal anomalies, derived from the imagery, serve to verify locations and trends of the faults, fractures, and hydrothermal springs. Results based on the remote sensing data are corroborated by field observations referenced using GPS locations, fault and fracture surface strike and dip measurements, as well as textural field observations.