Mapped Fault Zones Research Articles

Strain Rate Distribution in South‐Central Tibet From Two Decades of InSAR and GPS

AbstractThe degree to which deformation and seismicity is focused on major mapped structures remains a key unknown in assessing seismic hazards and testing continental deformation models. Here we combine 208 Global Positioning System (GPS) velocities with 12‐track Interferometric Synthetic Aperture Radar (InSAR) rate maps to form high‐resolution velocity and strain rate fields for south‐central Tibet. Our results show that deformation is not evenly distributed across the region. We find a few zones with high strain rates, most notably the Yutian‐Zhongba strain rate zone. However, the average of the strain rates is similar within and outside the mapped fault zones. In addition, the slip rates are low on all the conjugate strike‐slip faults widespread in central Tibet. The observations are difficult to reconcile with time‐invariant block models or with continuum models that lack mechanisms for strain localization. Our results support arguments that the most robust estimates of seismic hazard should integrate seismicity catalogues, active fault maps, and geodetic strain rate models.

Assessing Aquifer Compartmentalisation in the Daly River Basin, Northern Territory: A Hydrogeophysical Approach

The presence of Neogene fault systems can have a significant impact on hydraulic connectivity of aquifers, juxtaposing otherwise disconnected aquifers, enhancing recharge and/or discharge or acting as barriers to flow and consequently compartmentalising groundwater resources. Previously, regional airborne electromagnetics (AEM) transects allied with groundwater investigations have pointed to the potential for localised compartmentalisation of the Daly River Basin groundwater systems. However, existing data is sparse, and equivocal. In this context, the main aim of the Daly River Basin Project is to determine if compartmentalisation of the aquifers is a significant factor and thus should be explicitly considered in groundwater modelling and water allocation planning. The objectives of the project main goals of the project are to: (1) map Neogene faults through the use of airborne electromagnetic (AEM) and morphotectonic mapping, and (2) assess the permeability and transmissivity of mapped fault zones and their role in potential groundwater system compartmentalisation. Data acquisition includes 3325 line-kilometres of new AEM and airborne magnetics, ground (ground magnetic resonance (GMR)), and borehole geophysics, drilling, groundwater sampling and hydrochemical analysis, geomorphic and morphotectonics mapping. Hydrogeophysical, geomorphic and hydrogeological data will also be used to better understand groundwater-surface water connectivity and the potential for managed aquifer recharge schemes to replenish extracted groundwater resources. The outcomes of this project will inform decisions on water allocations and underpin effective and efficient groundwater use. This paper specifically reports on the ability of AEM and morphotectonics mapping to identify Neogene fault systems in the Daly River Basin.

Imaging shallow gas migration pathways in a mud‐volcano province using an autonomous underwater vehicle (Malta Plateau, Mediterranean Sea)

ABSTRACTData acquired by an autonomous underwater vehicle (AUV) towing a source (1600–3500 Hz) and a horizontal array of hydrophones have been analysed to image discrete, isolated or even a small cluster of scatterers within the sediment, to determine shallow migration paths of hydrocarbons in a mud volcano system of the Malta Plateau. An algorithm based on a semblance function was applied to the acoustic data to highlight scatterers rather than interface reflections. The resulting scatterer map, obtained along the AUV track, generated a pseudo‐three‐dimensional coverage of the study area, with a horizontal and vertical resolution of roughly 3–5 m and 5–10 m, respectively. This map was combined with high‐resolution bathymetric and backscattering seafloor maps obtained from previous explorations. This integrated dataset provides new evidence for the role of fault zones as a preferential path for gas/fluid migration and reveals the intermittent activity of seeping gas. The data show, in particular, that gas bubble slugs, i.e. discontinuous gas columns, rise through Plio‐Quaternary sediments along a complex system of conduits terminating at the surface into quiescent mud volcanoes. The gas flux is facilitated by the regional stress field that results in dilatant conditions on the mapped fault zones.

The Skipjack Island fault zone: An active transcurrent structure within the upper plate of the Cascadia subduction complex

The Skipjack Island fault zone has been mapped in the San Juan Islands between Vancouver Island, Canada, and the Washington State mainland, USA. A decade ago, interpretation of multibeam sonar seafloor imagery revealed that Skipjack Island, an east–west striking sedimentary bedrock outcrop, was a fault-controlled structural feature. A major fault separates Skipjack Island from a deformed sedimentary bedrock outcrop on the seafloor to the north. Recently the Skipjack Island fault zone's morphology and extent have been explored both to the west and east of the island using multibeam echosounder bathymetry, seismic reflection profiles, and sediment cores. The character of the Skipjack Island fault zone is well defined locally by the interpreted seismic-reflection profiles, which show active faults that displace sediments deposited since the Last Glacial Maximum. The central part of the fault zone, near Skipjack Island, appears as a near-vertical structure in the shallow subsurface that has been subjected to left-lateral motion as evidenced by a bedrock exposure on the seabed north of the island where folded strata bend eastward against the fault, the result of drag from fault motion. Interpretations of recent geophysical data suggest an extension of the Skipjack Island fault zone farther to the west, where it trends towards, and possibly connects with, the Fulford Fault on Vancouver Island, and to the east, where it either terminates north of Lummi Island, or continues eastward onto the mainland. The Skipjack Island fault zone is interpreted to be the northern boundary of the San Juan Archipelago with the Devils Mountain fault zone being the southern boundary. Both fault zones represent the longest continuous faults of the San Juan Archipelago. This newly mapped fault zone and its zone of deformation to the north may represent the northern boundary of the Cascadia forearc. Potential sesimicity of the Skipjack Island fault zone and its proximity to the steep unstable northeastern side of Orcas Island and the southern front of the Fraser River Delta poses a geohazads problem in the form of earthquakes, landslides, submarine slides, and tsunamis.

Seismic modelling of fault zones

While faults have long been known as primary pathways for fluid migration in sedimentary basins, recent work highlights the importance of fault zone internal architecture, lateral variation, transmissivity, and impact on migration and trapping. The impacts of fault zone architecture and properties on seismic images are investigated to facilitate accurately mapped fault zones, and to predict subseismic flow properties and sealing potential. A wedge-type fault model with a main fault and a synthetic fault displacing a typical North West Shelf siliciclastic succession is used to replicate the geometrical components of a seismic-scale fault. Elastic properties are derived from rock physics models, which are used in a 2D elastic modelling algorithm to produce realistic marine seismic acquisition geometry. These data were subsequently input into a 2D prestack (one-way wave-equation) migration code to produce an interpretable seismic image. Base-case elastic properties are systematically varied; modelling focuses on gouge properties, fractured fault zone material, the sandstone Vp/Vs relationship, and shale-sand velocity contrast. The workflow from geological model building to elastic property substitution and forward seismic modelling is extremely quick and versatile, allowing testing of a wide range of scenarios. So far this approach has yielded valuable insights into internal fault property prediction and interpretation of the fault zone in traditional post-stack seismic datasets. Implications for processing workflow and attenuation of fault shadows are also expected.

Geologic evidence for multiple slip weakening mechanisms during seismic slip in crystalline rock

We examine exhumed seismogenic faults to investigate the mechanisms that may have achieved dynamic fault weakening during ancient ruptures. Field and microscope observations imply more than one weakening mechanism must have been active during slip events on the faults. Pseudotachylytes that are continuous over the scale of field exposures are indicative of melt lubrication. A fault breccia crosscutting earlier formed cataclasites was mobilized during faulting and possibly represents a pressurized fault rock that resulted from thermal pressurization or elastohydrodynamic lubrication. In some faults, pseudotachylytes are developed in patches several meters long. Cataclasites in which there is no evidence for melting are present immediately along strike from the pseudotachylyte patches. By considering the energy required for melting, we show that the pseudotachylytes must have formed during ruptures larger than the patches, implying that the cataclasites also accommodated seismic slip. The distribution of fault rock types shows that the frictional response to slip during a single event was spatially variable. Reworked pseudotachylytes also indicate that coseismic processes change over time at a point on a fault. These observations emphasize that macroscopic dynamic fault weakening is a function of multiple coeval processes at microscales to mesoscales. Detailed observations of the discontinuous pseudotachylytes show that slip zone thickness is the critical parameter that controls active coseismic processes. The frictional response of a fault to slip is therefore dependent on the internal structure of faults; given the along‐strike heterogeneity of most mapped fault zones, the coexistence of multiple slip weakening mechanisms in a single earthquake will be common.

Analysis of lineaments and their relationship to Neogene fracturing, SE Viti Levu, Fiji

The Fiji archipelago lies along a complex active section of the Pacific and Australian plate boundary zone in the SW Pacific. The availability of a range of remotely sensed imagery and recent field studies have permitted the exploration of the link between lineaments as seen on such imagery and tectonic fractures observed on the ground in SE Viti Levu, the main island of Fiji. Comparisons of statistical orientation peaks as well as the spatial extent of lineaments with field-mapped fractures indicate that the lineaments are largely a reflection of tectonic fractures in the bedrock emphasized on the surface by topography, drainage, and vegetation. Surface lineaments correspond to broad zones of intense fracturing and strongly influence the landform features of the area. It appears that surface fracture sets are open-upward splay zones that merge downward into single planar discontinuities, represented by sparse uninterrupted magnetic lineaments at depth. Fault-correlated lineament sets extend over greater geographical distances than the equivalent mapped fault zones, and these lineament sets represent more complex regional structural patterns than previously known. This study provides an approach to mapping fracture patterns where outcrop is limited and where conventional field-mapping techniques of structural geology are impractical. It highlights the benefits of mapping lineaments from multiple sets of remote-sensing imagery of the surface and bedrock, mapping them at regional and larger scales, as well as linking lineaments to detailed field-mapped structures. Lineament data derived using such an approach can be used effectively to characterize regional structural patterns, which in turn may provide better understanding of the structural and tectonic evolution and seismic hazards of the region. This approach may also find useful application in groundwater, mineral, and oil and gas exploration.

Geochemistry of a spring-dense karst watershed located in a complex structural setting, Appalachian Great Valley, West Virginia, USA

The distribution and chemistry of the springs in the Tuscarora Creek watershed is controlled by both geologic structure and karst dissolution. The watershed is located in eastern West Virginia in the structurally complex Great Valley of the Appalachian Valley and Ridge province. The upper portion of the stream parallels strike along a mapped fault zone and is bordered by clastic rocks that comprise North Mountain. The lower reaches of the stream flow cross-strike through Cambro-Ordovician carbonates. The controlling chemical signature in the spring water is carbonate dissolution. Little evidence was seen for the recharge from adjacent clastic rocks although differences in the Ca/Mg molar ratio between springs indicated the presence of localized spring basins in headwater reaches. Na, Cl and Ca generally increased from upstream to downstream in the cross-strike reaches. Comparison of stream and cumulative spring discharge was consistent with significant groundwater base-flow contribution directly to the creek, particularly in the strike-parallel region. The largest spring in the watershed (>162 L/s) was sampled during and after a large storm event along with the adjacent creek. The creek displayed a typical dilution response with each flood pulse, whereas the spring had only a limited or delayed response. The overall chemical and thermal stability of the spring, relative to the creek, indicated the lack of significant direct hydraulic connection between the two. The conceptual model for the area includes localized flowpaths in the headwater region where the stream flow is parallel to strike and a thrust fault. In addition to the shallow localized flowpaths, a deeper, more regional flowpath likely exists for a large spring further downstream.

A detachment fold model for fault zones in the Late Archean Abitibi greenstone belt

The Abitibi belt is one of the largest and most extensively studied Late Archean greenstone belts. The structural geology of the Abitibi belt consists of one generation of upright to slightly overturned, doubly plunging first-order folds with half-wavelengths of 20–60 km, and E–W-striking, steeply dipping fault zones that are parallel to the fold limbs. Two of the main fault zones are continuous for hundreds of kilometers. Previous tectonic models for the Abitibi belt interpret the fault zones to have formed as extensional growth faults bounding a volcanic-sedimentary basin, which were reactivated as thrusts during subsequent crustal shortening. Other models propose that the fault zones represent tectonic sutures, implying that the Abitibi belt is a collage of exotic terranes. However, distinct geological terranes have not been geologically demonstrated. We propose a new detachment fold model for the deformational history of the southern Abitibi belt, in Ontario, that explains the formation of the fault zones during the single, well-documented folding event that deformed the entire region. The internal structure of the fault zones, documented here with emphasis on the Porcupine–Destor fault zone, consists of isoclinally folded, strongly schistose, highly metamorphosed rock, cross-cut by numerous fault segments. We interpret that the upper crust (greenstones) was folded above a proposed detachment in the lower part of the volcanic stratigraphy. The fault zones would be, in essence, highly evolved detachment anticlines. Ultramafic metavolcanic rock that crops out within the fault zones would represent material from the detachment horizon that was emplaced in the cores of the detachment anticlines. The numerous segments that make up the mapped fault zones would be linked faults that formed within the isoclinal detachment anticlines to accommodate folding of the rheologically complex greenstones. The detachment fold model is compared to the results of analogue experiments designed to investigate crustal-scale folding, using viscous and frictional materials. Detachment folds are produced in the brittle upper crustal analogue on the limbs of folds formed in the ductile middle and lower crust analogues. The experimentally produced structures scale to the structures in the study area and indicate the detachment fold model for the southern Abitibi is mechanically viable.

Geometry, kinematics and scaling properties of faults and fractures as tools for modelling geofluid reservoirs: examples from the Apennines, Italy

Abstract The study of some major fault zones in the Apennines was mostly focused on the acquisition of quantitative data in situ aimed at deriving input parameters for modelling faulted rock volumes. Structural data collected in the Monte Alpi area (southern Italy) and in the central Apennines allowed us to: (i) estimate the fractal dimension characterizing the geometric complexity and size distribution of different fault and fracture patterns; (ii) assess the appropriate parameters defining the overall architecture, anisotropy and related permeability structure of the mapped fault zones; and (iii) constrain the scaling properties of some of the attributes (i.e. length, spacing, map pattern, fracture density, etc.) of both fault-related and regional fracture sets. The results of our work suggest that the above data are appropriate for extrapolating field based information at different scales, and for producing 3D models of fault and fracture networks.

Cinder cone clustering in the TransMexican Volcanic Belt: Implications for structural and petrologic models

Cinder cone distributions have most often been characterized using univariate statistics. Here a new technique to volcano distribution studies, cluster analysis, is applied to cinder cone distribution in the central TransMexican Volcanic Belt (TMVB). A total of 1016 cinder cones are identified over an area of approximately 60,000 km2. Application of cluster analysis reveals structure in cinder cone distribution. Using a search radius parameter of 16 km, 75% of the cinder cones within the central TMVB are found within eight clusters of 45–159 cones each. These clusters are each 2000 to 5000 km2 in area. Only 22 cones are found within clusters of three or fewer cinder cones, indicating that clustering is a pervasive phenomenon. Some petrologic variation is evident among clusters; low‐Mg alkaline cinder cones are found within a single cluster, 360–400 km from the trench. Application of alignment analysis techniques, including the two‐point azimuth method, the Hough transform, and two‐dimensional Fourier analysis demonstrates that cinder cone alignments have common orientations on a regional scale within three clusters, all located in the southernmost part of the Michoacán‐Guanajuato volcanic field in the western portion of the study area. These alignments consist of tens of cinder cones, are 20–50 km long, and are all oriented with azimuths of 020°–040°, parallel to the direction of plate convergence. High‐Mg lavas, which last fractionated at pressures in excess of 8 kbar, are only found associated with these alignments, indicating that magma transport is significantly enhanced in these areas. Although local alignments of three to six cinder cones occur within mapped fault zones that transect the area, regional cinder cone alignment patterns, and the distribution of clusters themselves, do not appear to be affected by the presence of these fault zones. The cluster analysis, the alignment analysis, and some petrologic data support the hypothesis that the TMVB is segmented near 101°W.

Jointly determined focal parameters along a fault in California's northern coast range

The authors recently determined seismic parameters of earthquakes located along a lineation of microearthquakes in the Coast Range Province of northern California (Dehlinger and Bolt, 1984). The lineation closely follows the mapped Bartlett Springs fault zone, which is considered to be the source of seismic activity. This fault strikes N40°W for about 40 km and the mapped fault zone consists of a 2–3 km-wide belt of melange. Earthquake hypocenters indicate that this belt dips steeply to the northeast, with shocks occurring along multiple dislocation planes that together extend downward to depths of about 12 km. Focal mechanisms and associated stress fields were determined for 22 of the best recorded recent shocks along the earthquake lineation. Analyses of these shocks provided a comparison between individually and jointly determined focal mechanisms and of the directions of the corresponding stress axes, for identical shocks along the entire fault length. Thirteen of the shocks exhibited right-lateral, San Andreas type of source motions. Average values of these 13 individually determined focal parameters are within the standard deviation of the same set of jointly determined values. It is thus verified that the probability model algorithm for joint parameters determinations developed by Brillinger et al. (1980) yields reliable values of focal parameters. Moreover, the jointly determined parameters appear to be more reliable than the average-determined ones. The focal mechanisms and associated stress fields along the entire length of the Bartlett Springs fault zone are consistent with shearing motions between the North American and Pacific lithospheric plates that produce displacements along the San Andreas transform fault.

Mapping of Fracture Zones by Helium Emanometry and Possible Relationship of Helium Anomalies to Hydrocarbon Reservoirs in Western Pennsylvania: ABSTRACT

Helium is ubiquitous within the earth's crust. Local helium concentrations in the subsurface are observed in association with ores containing uranium and thorium, oil and gas reservoirs, and thermal fluids. In addition, permeable fracture zones tend to intensify local concentrations of helium because of its relative ease of migration along these conduits. A survey of helium in soil-gas samples was conducted across a previously mapped fault zone and a drilling-defined hydrocarbon reservoir in Greene and Fayette Counties, southwestern Pennsylvania, to demonstrate the utility of such surveys for mapping fracture zones and for locating hydrocarbon reservoirs. In the study area, a northwest-trending fracture zone cuts across both an area of hydrocarbon accumulation and an area thought to be relatively barren of hydrocarbons. The survey indicated that the helium content of the soil was anomalously high in a 30-m wide zone above the mapped fault and fracture system. Similar results were obtained from traverses made across both the accurately mapped fault zone and the hypothesized extensions of that zone. These data support the concept that helium can be used to locate fault and fracture zones where location is precluded by more conventional mapping procedures. Although helium surveys have delineated known hydrocarbon reservoirs in other areas, this preliminary survey, which consisted of a few traverses rather than an extensive grid pattern, failed to produce a significant anomaly above a known reservoir in southwestern Pennsylvania. This may be due to the low de sity of sample distribution and the high average background helium value for these traverses. End_of_Article - Last_Page 1446------------