Simple Topographic Parameter Reveals the Along-Trench Distribution of Frictional Properties on a Shallow Plate Boundary Fault

The critical taper model of a sedimentary wedge best describes the first-order mechanics of a subduction zone wedge. The tapered wedge geometry, which is conventionally defined by two parameters, the slope angle and the basal dip angle, is responsible for the strength of a megathrust. By applying this theoretical model to subduction zones, fault frictional properties and earthquake occurrences can be compared among subduction zones, and within a single subduction zone, the spatial distribution or temporal change of fault strength can be investigated. The slope angle can be accurately estimated from bathymetry data, but the basal dip angle must be inferred from the subsurface structure, and it requires highly accurate depth-converted seismic reflection profiles. Thus, application of the critical taper model is often limited by a lack of a sufficient number of highly accurate profiles, and the spatial distribution of frictional coefficients must be inferred from relatively few data, generally less than a dozen points. To improve this situation, we revisited the theoretical formula of the critical taper model. We found that the effect of the décollement dip angle β on the critical taper model of a sedimentary wedge is negligible when the pore fluid pressure ratio is high or internal friction is small, conditions which are met in many subduction zones. Therefore, this finding allows frictional variation to be approximated by using only the slope angle variation obtained from the bathymetry. We applied this approximation to the Japan Trench as an example of this approximation, and were able to estimate the friction coefficient distribution on the shallow plate boundary fault from 71 data points. We found that the area where the friction coefficient was smaller than the mean corresponded to the segment where a large coseismic shallow rupture occurred during the 2011 Tohoku-oki earthquake (Mw 9.0). This result shows that by approximating tapered wedge geometry using a simple topographic parameter that can be obtained from existing global bathymetry, we can quickly estimate the distribution of frictional properties on a plate boundary fault along a trench and related seismic activity.

Strain-partitioned dextral transpression in the Great Boundary Fault Zone around Chittaurgarh, NW Indian Shield

Delimiting the Aravalli mountain range in the east, the Great Boundary Fault (GBF) occurs as a crustal-scale tectonic lineament in the NW Indian Shield. The structural and tectonic characteristics of the GBF are, as yet, not well-understood. We attempt to fill this gap by using a combination of satellite image processing, high-resolution outcrop mapping and structural analysis around Chittaurgarh. The study area exposes the core and damage zone of the GBF. Three successive phases of folding, F1, F2 and F3, are associated with deformation in the GBF. The large-scale structural characteristics of the GBF core are: (i) a non-coaxial refolding of F1 folds by F2 folds; and (ii) the parallelism between the GBF and F2 axial traces. In addition, numerous metre-scale ductile shear zones cut through the rocks in the GBF core. The damage zone is characterized by the large-scale F1 folds and the mesoscopic-scale strike-slip faults, thrusts and brittle-ductile shear zones. Several lines of evidence, such as the inconsistent overprinting relationship between the strike-slip faults and thrusts, the occurrence of en échelon folds and the palaeostress directions suggest that the GBF is a dextral transpression fault zone. Structural geometry and kinematic indicators imply a wrench- and contraction-dominated deformation in the core and damage zone, respectively. We infer that the GBF is a strain-partitioned dextral transpression zone.

Burial Ages Imply Miocene Uplift of Lu Mountain in East China due to Crustal Shortening

Confined by the eastern and western boundary faults, Lu Mountain has long been considered a block mountain uplifted due to Mesozoic and Cenozoic crustal deformation in East China. However, the formation and evolution of this block mountain are still debated. In this study, the eastern boundary fault is investigated to confirm the tectonic style of the block mountain. In addition, the burial ages of sediments on the fans of the eastern piedmont are measured by 26Al/10Be dating to evaluate the denudation rate. Field evidence indicates the presence of a reverse fault (Xingzi reverse fault) acting as the eastern boundary fault, which demonstrates that the block mountain is not a horst as once thought but an extrusion structure. Corrected 26Al/10Be burial ages show that the sediments on the high-level fans were deposited at approximately 1.1–1.2 Ma, which indicates denudation rates ranging from 0.033 to 0.082 m/kyr. The vertical displacement along the Xingzi reverse fault is estimated to be at least 1,100 m. The hanging wall could have been eroded to its present position within 13–33 Myr at the above denudation rates. Combining our results with regional geological and geomorphological evidence, we suggest that Lu Mountain was mainly uplifted in the Miocene due to crustal compression deformation, which may have been a response to the movement of the Pacific plate.

The Geological Index of the Scottish Caledonides northwest of the Highland Boundary Fault

<p>The quantification and mapping of geodiversity have gained more interest in recent years due to practical application in natural resource management and conservation. The Geological Index (I<sub>Geo</sub>) represents the quantitative expression of geological features and is part of a broader Geodiversity Index (I<sub>Geodiv</sub>), which also includes geomorphological, pedological, paleontological and hydrological elements.</p><p>In Scotland, the area delimited by the Moine Thrust Zone to the northwest and the Highland Boundary Fault to the southeast represents a fragment of the Caledonian orogenic belt that extends across parts of North America, Greenland and Scandinavia. It includes the Highlands, most of the Inner Hebrides and the islands of Orkney and Shetland. The area is underlain by two tectonic blocks – the Northern Highlands Terrane and the Grampian Terrane – separated by a major strike-slip fault, the Great Glen. Both blocks consist of an Archaean-Paleoproterozoic basement covered by the Neoproterozoic metamorphic suites of the Moine and Dalradian Supergroups, together with a series of magmatic intrusions and other rocks of late Precambrian and Phanerozoic age.</p><p>The I<sub>Geo</sub> was obtained from lithostratigraphic and lithodemic units, mapped at group and suite/complex level respectively, major geologic contacts and faults and minor igneous intrusions from the British Geological Survey 1:625k digital datasets. These were reclassified and analyzed using QGIS and ArcGIS software.</p><p>The results show overall medium and high values of I<sub>Geo</sub>, with regional variations and well-individualized areas of very high and very low values. Conspicuous transitions between extremes are observed at the north and south edges of the study area.</p><p>High I<sub>Geo</sub> values occur in five major areas across the mainland: 1). on the north coast, which exhibits small outcrops of varied lithologies; 2). in the northeast Grampian Mountains, where the deformed Dalradian rocks are intruded by the Cairngorms suite of the Newer Granites; 3). along the Great Glen, the meeting place of adjacent tectonic blocks; 4). in the Firth of Lorne area and further inland, where Neoproterozoic and Paleozoic rocks come into contact with more recent Cenozoic rocks of the Hebridean Province; 5). at the southern tip of the Kintyre Peninsula that contains isolated exposures of rocks characteristic of the nearby Midland Valley.</p><p>Low I<sub>Geo</sub> values are encountered in three major areas of the mainland: 1). southeast of the Moine Thrust Zone, an area occupied by the oldest Moine group; 2). in the Pentland Firth area that consists of the Old Red Sandstone Supergroup; 3). in the Firth of Clyde area and further inland, around the main outcrop of the youngest Dalradian group.</p><p>Offshore, the islands of Orkney and Shetland have I<sub>Geo</sub> values at opposite ends of the spectrum. The first are made up of a monotonous sedimentary cover. The latter comprise a mosaic of rocks of Precambrian and early Phanerozoic age.</p>

Do Intraplate and Plate Boundary Fault Systems Evolve in a Similar Way with Repeated Slip Events?

<p>As repeated slip events occur on a fault, energy is partly dissipated through rock fracturing and frictional processes in the fault zone and partly radiated to the surface as seismic energy. Numerous field studies have shown that the core of intraplate faults becomes wider on average with increasing total displacement (and hence slip events). In this study we compile data on the fault core thickness, total displacement and internal structure (e.g., fault core composition, host rock juxtaposition, slip direction, fault type, and/or the number of fault core strands) of plate boundary faults to compare to intraplate faults (within the interior of tectonic plates). Fault core thickness data show that plate boundary faults are anomalously narrow by comparison to intraplate faults of the same displacement and that they remain narrow regardless of how much total displacement they have experienced or the local structure of the fault. By examining the scaling relations between seismic moment, average displacement and surface rupture length for plate boundary and intraplate fault ruptures, we find that for a given value of displacement in an individual earthquake, plate boundary fault earthquakes typically have a greater seismic moment (and hence earthquake magnitude) than intraplate events. We infer that narrow plate boundary faults do not process intact rock as much during seismic events as intraplate faults. Thus, plate boundary faults dissipate less energy than intraplate faults during earthquakes meaning that for a given value of average displacement, more energy is radiated to the surface manifested as higher magnitude earthquakes. By contrast, intraplate faults dissipate more energy and get wider as fault slip increases, generating complex zones of damage in the surrounding rock and propagating through linkage with neighboring structures. The more complex the fault geometry, the more energy has to be consumed at depth during an earthquake and the less energy reaches the surface.</p>