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Interpretation of a 3D seismic survey located on the western margin of the Northern North Sea Basin demonstrates how the propagation, overlap and linkage of two north–south-striking, en echelon normal fault segments exerted a powerful influence over prospective subtle stratigraphic traps. The relay ramp that formed between the segments appears to have focused sediment dispersal, controlled reservoir distribution and aided post-depositional petroleum migration. Integration of electrical well log data, root-mean-square (RMS)-amplitude analysis and biostratigraphy with seismic interpretation demonstrates that a series of elongate, linear, sand-prone (reservoir) channel complexes characterize the depositional slopes generated by fault growth. The combination of synsedimentary rotation of bedding due to fault propagation and associated footwall uplift led to erosion and truncation of a laterally extensive, older channelized system (Lower Sequence), the downdip parts of which extend beyond the relay ramp. Its subsequent drape by transgressive shales created the subtle stratigraphic trap that now hosts the Cladhan Field, with charge occurring because the sandstones belonging to the Lower Sequence extend as far as the active kitchen in the neighbouring (hanging-wall) depocentre situated downdip and to the east. In contrast, the exploration of a younger, Upper Sequence of sandstones has proven to be disappointing due to their more restricted distribution, lack of access to charge, and occurrence of faults that offset and breach the thin end of the stratigraphic wedge. The implication is that partially breached relay ramps not only provide a preferential site for synrift clastic reservoirs to develop but also form important migration pathways through which oil passed from a petroleum kitchen into a trap. Thematic collection: This article is part of the Under-explored plays and frontier basins of the UK continental shelf collection available at: http://www.lyellcollection.org/cc/under-explored-plays-and-frontier-basins-of-the-uk-continental-shelf

Following observations of mobilized boulder trail populations from Cerberus Fossae, Mars, that have been interpreted as possible evidence of large magnitude marsquakes rupturing for distances of ~207 km along exposed active faults, additional boulder trail populations were measured along shorter faults within the region of Grjota Valles (50-150 km length) to test the hypotheses that (1) these faults are also candidate locations for marsquakes, and (2) that marsquake magnitude might be smaller, limited by fault dimensions available for rupture. For a region containing two en echelon graben, boulder trail data define two anomalies with maxima in (a) boulder trails per kilometer, and (b) maximum width of boulder trails, one that is ~116 km in length along strike and the other ~70 km in length along strike. Values for the maxima are 45 trails per km and 5 m mean trail width for the 70 km long anomaly, and 115 trails per km with 5.3 m mean trail width for the 116 km long anomaly, above background values measured elsewhere along these faults of zero trails per kilometer with zero boulder trail widths. If combined with published data from Cerberus Fossae with a ~207 km long anomaly in boulder trails per km (125 trails per km maxima) and maximum mean boulder trail width (8.5 m maximum trail width), the 3 datasets suggest correlations between the (a) along-strike length of boulder trail anomalies, (b) boulder trails per km and (c) maximum boulder trail width. If interpreted as due to single marsquakes, and if the dimensions of these anomalies are a proxy for rupture length, when combined, one interpretation of this is that boulders have been mobilized by marsquakes and that the marsquake magnitude is proportional to the along strike length of the anomalies. In other words, the data suggest that marsquake magnitude, if that is the cause of the anomalies, is limited by fault length as expected for terrestrial seismically active faults. Such findings suggest that the Martian surface may have been shaken, in the very recent past, by large magnitude marsquakes. We discuss this in terms of the seismicity of Mars.

The strike-slip fault systems in the central Tarim Basin, China, afford an exceptional opportunity to document the structural characteristics and evolution process of small displacement intracratonic strike-slip faults using three-dimensional seismic reflection data. These strike-slip faults display subvertical segments at depth and en echelon normal fault zones where relatively shallow. Fault segmentation and flower structures can be commonly observed in plan view and cross-section view, respectively. Consistent with the notion that segment coalescence is the fundamental process for fault evolution, the mean segment length of representative strike-slip faults examined in this study is positively correlated to the measured fault offset. The width of the en echelon normal fault zone is positively correlated with the estimated maximum overburden thickness. The integrated data sets suggest that the evolution of the conjugate fault array followed a sequential evolution process instead of forming simultaneously. The switch in slip direction of the master fault of the conjugate fault array is attributed to the change of stress orientation. Regarding individual strike-slip faults, increase in displacement induces the formation of faults with lower fault-array angles linking initially formed en echelon normal faults. In cross sections, throughgoing fault surfaces can also form, connecting the lower subvertical fault segment and the upper en echelon normal faults. The presented data sets and evolution models established in this study can be used as tools to better predict the structural attributes of subsurface strike-slip fault systems with important consequences for reservoir formation and hydrocarbon accumulation in the Tarim Basin in particular, and in ancient marine basins in general.

The near-surface of London is faulted, although the locations, architecture and tectonic origins of the faults are broadly unknown. This presents serious issues for geotechnical engineering in London and has implications for our structural understanding of the London Basin. The region is a product of Alpine compression, yet it is unclear whether these major faults are new Alpine shears or reactivated basement faults. We assess the plausibility of Alpine reactivation and inheritance of basement faults in London through three investigations: analysing structures in the near-surface; mechanically assessing the feasibility of basement fault reactivation under Alpine stress conditions; and comparing inheritance mechanisms with observations in London and the Thames Estuary. Three major en echelon fault sets are identified. These appear to have compartmentalized London9s near-surface geology and are all interpreted to be products of Alpine reactivation of the underlying basement faults. Fault interaction and linkage is evidenced by complex zones of intense faulting identified by tunnelling projects. The role of new structural development in accommodating Alpine compression is considered to have been comparatively minor. The lack of major faulting in the basin9s interior may reflect the competence of the underlying Anglo–Brabant Massif in restricting Alpine strains to its margins. Thematic collection: This article is part of the Geology of London and its implications for ground engineering collection available at: https://www.lyellcollection.org/cc/london-basin

Modern sand transport pathways in a multiple-sand-ridge system are elusive and have rarely been studied in recent years. We report herein a field with four en echelon linear sand ridges offshore of Hainan Island in the Beibu Gulf and describe the distribution and morphology of these sand ridges in detail for the first time. Dune crest comparisons and seismic profiles are also interpreted to assess sand transport over the dunes. Based on a Delft 3D model, regional tidal currents and tide-induced bedload transport in the multiple-sand-ridge system were simulated to provide insights into the related bedload transport paths. The results show that bedload transport and residual flows are mostly directed to the south on the east side of the sand ridges and to the north on the west side, and these differences coincide with dune asymmetries and migrations. Cross-ridge transport is weak and mostly converges on the crests of sand ridges from the two flanks. The bedload transport is unbalanced on either flank of the sand ridges, thus leading to the asymmetry and crest kinks of the sand ridges. A distinct correlation is not observed between the net cross-ridge sand transport and sand ridge asymmetry. In this system, cross-swale transport is significant between the neighboring sand ridges and helps construct the bedload transport circulation in the swales. Sand accretion also occurs in the swales and benefits from the southwestward bedload transport from the north end of sand ridge 3. Distinct evidence has not been found for closed bedload transport circulation around sand ridges in this multiple-sand-ridge system, although limited clockwise current vortices develop on several sand ridges. Additional field observations and simulations are required to characterize the suspended load transport in a multiple-sand-ridge system, and the effects of local sand supply conditions on sand transport also need further evaluation. Copyright (c) 2018 John Wiley & Sons, Ltd.

The mechanism and evolution of fault linkage is important in the growth and development of large faults. Here we investigate the role of coseismic stress changes in shaping the hard-links between parallel normal fault segments (or faults), by comparing numerical models of the Coulomb stress change from simulated earthquakes on two en echelon fault segments to natural observations of hard-linked fault geometry. We consider three simplified linking fault geometries: 1) fault bend; 2) breached relay ramp; and 3) strike-slip transform fault. We consider scenarios where either one or both segments rupture and vary the distance between segment tips. Fault bends and breached relay ramps are favoured where segments underlap, or when the strike-perpendicular distance between overlapping segments is less than 20% of their total length, matching all documented examples. Transform fault linkage geometries are preferred when overlapping segments are laterally offset at larger distances. Few transform faults exist in continental extensional settings, and our model suggests that propagating faults or fault segments may first link through fault bends or breached ramps before reaching sufficient overlap for a transform fault to develop. Our results suggest that Coulomb stresses arising from multi-segment ruptures or repeated earthquakes are consistent with natural observations of the geometry of hard-links between parallel normal fault segments

Upper Permian (Zechstein Supergroup) evaporites have a major control on structural styles and prospectivity in the UK Southern North Sea (SNS). They form the regional super-seal for the main Rotliegend Group (Leman Sandstone Formation) reservoir play fairway immediately beneath. The evaporites have highly variable thicknesses due to the syndepositional basin architecture, differential loading and post-depositional deformation through diapirism and salt withdrawal. The halokinetic activity leads to touchdown (welding) of the supra-salt section onto the sub-salt strata and the development of narrow (up to 15 km-wide) graben systems. The interpretation and depth conversion of well-calibrated, high-quality, 3D post-stack time-migrated (PSTM) seismic data along the southwestern margin of the basin show that a NW–SE-striking elongate extensional Dowsing Graben System transects the area. The graben is defined by a series of large, overlapping, en echelon listric growth faults, with oblique secondary planar faults, which sole-out on two main (deep and shallow) decollement levels in the Zechstein Supergroup and the Middle Triassic Rot Halite Member. Whilst its initial formation was related to Mesozoic extension, the graben system also displays a contractional overprint resulting from regional compression and structural inversion during the Cenozoic. Detailed mapping of the Zechstein Supergroup has revealed that the evolution of the extensional system was influenced by the ESE–WNW-striking anhydrite–carbonate Zechstein shelf-margin. The occurrence of variable-thickness, low-velocity sediments within the graben impacts seismic imaging and depth conversion, leading to prospective structures being overlooked; something that has implications for prospectivity in the SNS and other evaporite basins where similar graben occur.

Calderas represent morphological depressions several kilometers in diameter, and the unloaded crustal stresses they produce can form rapidly (e.g. Pinatubo, 1990) or slowly (e.g. Hawaii, 2018). Active calderas are known as sites of persistent magma intrusions, and yet the dynamics of their shallow plumbing system is not well constrained. We use scaled laboratory experiments to study how experimental intrusions are created beneath a caldera by injecting dyed water (magma analogue) into the base of an elastic gelatin solid (crust analogue) with a cylindrical cavity in its surface to mimic a caldera-like topography. The evolving dike geometry and stress field were qualitatively determined using polarized light, and digital image correlation allowed the incremental and total strain to be quantified by tracking passive-tracer particles in the gelatin that fluoresced in a thin 2D vertical laser sheet. Our results show that the unloaded stress field from a caldera can cause a divergence of vertical dikes, and leads to circumferential dikes and cone sheets. When the caldera was large the initially vertical dike became arrested, then grew laterally via circumferentially-propagating en echelon segments; these eventually joined to complete a cone sheet that was parallel to, but extended outside and beneath, the large caldera. When the caldera was small, a circumferential dike erupted, producing a short fissure which was outside, but parallel to, the caldera. We suggest that the distinct curved geometry, velocity, strain and stress characteristics of circumferential dikes and cone sheets can be used to interpret the origin and growth of post-caldera magmatism and the likelihood of eruption in caldera systems.

In microseismicity analyses, reliable focal mechanisms can typically be obtained for only a small subset of located events. We address this limitation here, presenting a framework for determining robust focal mechanisms for entire populations of very small events. To achieve this, we resolve relative P- and S-wave polarities between pairs of waveforms using their signed correlation coefficients - a byproduct of previously performed precise earthquake relocation. We then use cluster analysis to group events with similar patterns of polarities across the network. Finally, we apply a standard mechanism inversion to the grouped data, using either catalog or correlation-derived P-wave polarity datasets. This approach has great potential for enhancing analyses of spatially concentrated microseismicity such as earthquake swarms, mainshock-aftershock sequences, and industrial reservoir stimulation or injection-induced seismic sequences. To demonstrate its utility, we apply this technique to the 2014 Long Valley Caldera earthquake swarm. In our analysis, 85% of events (7212 out of 8494 located by Shelly et al. [2016]) fall within five well-constrained mechanism clusters, more than 12 times the number with network-determined mechanisms. Of the earthquakes we characterize, 3023 (42%) of have magnitudes smaller than 0.0. We find that mechanism variations are strongly associated with corresponding hypocentral structure, yet mechanism heterogeneity also occurs where it cannot be resolved by hypocentral patterns, often confined to small-magnitude events. Small (5-20°) rotations between mechanism orientations and earthquake location trends persist when we apply 3D velocity models and might reflect a geometry of en echelon, interlinked shear and dilational faulting.

Here we present the results of the inversion of a new geodetic data set covering the 2012 Emilia seismic sequence and the following 1 year of postseismic deformation. Modeling of the geodetic data together with the use of a catalog of 3-D relocated aftershocks allows us to constrain the rupture geometries and the coseismic and postseismic slip distributions for the two main events (Mw 6.1 and 6.0) of the sequence and to explore how these thrust events have interacted with each other. Dislocation modeling reveals that the first event ruptured a slip patch located in the center of the Middle Ferrara thrust with up to 1 m of reverse slip. The modeling of the second event, located about 15 km to the southwest, indicates a main patch with up to 60 cm of slip initiated in the deeper and flatter portion of the Mirandola thrust and progressively propagated postseismically toward the top section of the rupture plane, where most of the aftershocks and afterslip occurred. Our results also indicate that between the two main events, a third thrust segment was activated releasing a pulse of aseismic slip equivalent to a Mw 5.8 event. Coulomb stress changes suggest that the aseismic event was likely triggered by the preceding main shock and that the aseismic slip event probably brought the second fault closer to failure. Our findings show significant correlations between static stress changes and seismicity and suggest that stress interaction between earthquakes plays a significant role among continental en echelon thrusts.

Whether an earthquake, blasting, or pile driving caused non-structural "cosmetic" wall cracks or extended pre-existing fractures is often based on judgment: the structure presumably vibrated therefore new cracks formed and existing fractures were extended. The proposed two- part primarily post-event fracture mapping and analysis of intensity data is designed to test the static as opposed to vibratory cause of wall fractures assuming: (1) tension fractures form perpendicular to the principal tensile stress (σ3) direction; (2) during structure vibrations the local stress field rotates relative to the static stress field; (3) stress reversals, common during earthquakes, can cause tension fractures at all corners of rectangular doors and windows; (4) progressively younger en echelon tension fractures, originating at the corner of a rectangular wall opening, trend toward horizontal due to the transient rotation of the principal stresses where as fractures from non-vibrational causes trend toward vertical; (5) crack extension occurs when two fractures are linked by hook-shaped fractures; and (6) site-specific intensity observations and local intensity data provide qualitative data regarding the timing and amplitude of wall strains and the potential for co-seismic damage. Co-seismic tension fractures can therefore be differentiated from pre-existing cracks suggesting pre-blasting and pile driving crack mapping, building condition surveys and co-seismic ground and structure motions recordings may not be required if the building has: (1) no prior exposure to potentially damaging vibrations; and (2) a reconnaissance inspection indicates there are no pre-existing horizontal or sub-horizontal wall fractures.

Southern Calabria and the NE corner of Sicily (Italy) were struck in 1783 and 1908 A.D. by two of the most catastrophic earthquakes ever in European history. Although it is generally acknowledged that the seisms were yielded by normal faults rupturing the upper crust of the Calabria-Peloritani terrane, no consensus exists on seismogenic source location and orientation. Here we report on a high-resolution low-altitude aeromagnetic survey of southern Calabria and Messina Straits. In southern Calabria we document a broad weakly positive (5–10 nT) anomaly zone interrupted by three en echelon SW-NE null to negative magnetic anomaly corridors. Euler deconvolution and magnetic modeling show that the anomaly pattern is produced by a 1–1.5 km thick crustal “layer” located within 3 km depth. This layer is offset by a 25 km long NE trending fault that corresponds to the Armo normal fault, recently inferred to be the source for the 1908 earthquake. Few kilometers to the south, we also document a subparallel and previously unrecognized fault, entering the Messina Straits and likely joining the Armo fault at depth. Further east, we model a 40 km long normal fault, probably extending northeastward for additional 40 km, running along the south Calabria axis from Aspromonte to the Serre mountains and partly following the 18 km long surface rupture witnessed by Deodat de Dolomieu after the 1783 earthquake. Thus, aeromagnetic data suggest that the sources of the 1783 and 1908 earthquakes are en echelon faults belonging to the same NW dipping normal fault system straddling the whole southern Calabria.

Earthquake swarms represent a sudden increase in seismicity that may indicate a heterogeneous fault-zone, the involvement of crustal fluids and/or slow fault slip. Swarms sometimes precede major earthquake ruptures. An earthquake swarm occurred in October 2015 near San Ramon, California in an extensional right step-over region between the northern Calaveras Fault and the Concord–Mt. Diablo fault zone, which has hosted ten major swarms since 1970. The 2015 San Ramon swarm is examined here from 11 October through 18 November using template matching analysis. The relocated seismicity catalog contains ∼4000 events with magnitudes between − 0.2 M d 3.6 . The swarm illuminated three sub-parallel, southwest striking and northwest dipping fault segments of km-scale dimension and thickness of up to 200 m. The segments contain coexisting populations of different focal-mechanisms, suggesting a complex fault zone structure with several sets of en echelon fault orientations. The migration of events along the three planar structures indicates a complex fluid and faulting interaction processes. We searched for correlations between seismic activity and tidal stresses and found some suggestive features, but nothing that we can be confident is statistically significant.

The Wharton Basin in the Indian Ocean is one of the most extensively deforming ocean basins, as confirmed by the occurrence of several very large earthquakes starting from January 12, 2012 with Mw 7.2 followed by the great earthquakes of April 11, 2012 with Mw 8.6 and Mw 8.2. Although the Mw 7.2 and Mw 8.2 earthquakes seem to have ruptured the re-activated N–S striking fracture zones, the largest event (Mw 8.6) required the rupturing of several faults, oblique to each other, in a very complex manner. In order to understand the nature of deformation in these earthquakes rupture zones, we recently acquired 90 000 km2 of bathymetry, 11 400 km of sub-bottom profiling, gravity and magnetic data covering the rupture areas of the 2012 earthquakes east of the Ninety-East Ridge, in the northwestern Wharton Basin. These new data reveal six N8°E striking re-activated fracture zones (F5b, F6a, f6b, F7a, F7b and F8), where the fracture zone F6a can be followed for over 400 km and seems to be most active. The epicenters of the Mw 8.6 and Mw 8.2 earthquakes lie on the fracture zones F6a and F7b, respectively. The newly observed fracture F5b in the east is short, and has an extensional basin at its southern tip. The fracture zone F8 defines the eastern boundary of the Ninety-East Ridge. The presence of en echelon faults and pull-apart basins indicate left-lateral motion along these fracture zones. In between these fracture zones, we observe pervasive 290° striking right-lateral shear zones at 4–8 km intervals; one of which has cut through a seamount that might have ruptured during the Mw 8.6 earthquake. We also observe another N20°E striking left-lateral shear zones in the vicinity of F7b and F8, which is coincident with the strike of one of the nodal planes of the Mw 8.6 focal mechanism. These N20°E striking shear zones are interpreted as R Riedel shears and the N290°E striking shear zones as R′ Riedel shears. These shear zones are formed by a series of N335°E striking en echelon normal faults. Our data also show the presence of N65°E striking thrust faults east of the Ninety-East Ridge, orthogonal to the regional principal direction of compression. Furthermore, extensive bending-related faulting is also observed close to the Sumatra trench with normal faults also striking at N335°E, similar to the normal faults that form the shear zones. Normal faults with a similar orientation are also present at the southern tip of F5b. We explain all these observations with a single coherent model of deformation in the Wharton Basin, where a dominant part of the regional NW–SE compressional stress is accommodated along the N8°E re-activated fracture zones, and the rest is distributed along shear zones, thrust and normal faults between these fracture zones. The thrust and normal faults are orthogonal to each other and define the direction of principal compressive and extensive stresses in the region whereas the two shear zone systems form a conjugate pair. NRF (Natl Research Foundation, S’pore) MOE (Min. of Education, S’pore) Published version

New observations from an active orogenic wedge help link the seismotectonic behavior of individual faults to wedge deformation rates and patterns over multiple timescales. We provide the first detailed constraints on the distribution and timing of deformation at the front of the Andean orogenic wedge in southern Bolivia, where a recent study suggests that great (Mw > 8) earthquakes could rupture the master fault underlying the wedge. We use stratigraphic relationships across fault-related folds and elastic dislocation modeling of seismic reflection horizons to obtain probabilistic estimates of wedge-front fault ages and slip rates. Our analyses reveal that at least half of the previously determined GPS-based wedge-loading and Quaternary whole-wedge shortening rates are absorbed across a 20–40 km wide wedge-front zone consisting of 1–4 en echelon and partially to fully overlapping faults and folds associated with blind thrust faults. The difference between our slip rates and the geodetic/geologic observations combined with evidence for activity across internal wedge structures supports the notion that nonsteady state mass balance conditions coupled with elevated erosional efficiency result in distributed wedge deformation. The orogenic wedge in southern Bolivia behaves in a similar fashion to the Taiwanese and Himalayan ranges; slip accumulates at downdip locations along the master fault and is released incrementally by earthquakes that rupture the wedge-front fault zone. The faults and folds comprising this zone pose a major source of seismic hazard. Accumulating slip is also released in the wedge interior and older, internal wedge faults must be considered in any future assessment of regional earthquake risk.

International audience; The ferocity of volcanic eruptions – their penchant for either effusive or explosive behaviour – is to a large extent a matter of the ease with which volatiles are able to escape the volcanic system. Of particular importance are the mechanisms by which permeable networks within magma are fabricated and how they permit gas escape, thereby diffusing possibly calamitous explosions. Here, we present a series of experiments that confirms sample-scale fracture propagation and permeability development during shearing viscous flow of initially impermeable, bubble-bearing (2×10−4 s−1). At moderate shear strain rates (2×10−4 s−13) via en échelon Mode I fractures produced by repeated fracture events. At high shear strain rates (γ˙>4.5×10−4 s−1), permeability develops shortly after the onset of inelastic deformation and is, again, established through a series of en échelon Mode I fractures. Critically, strain is not immediately localized on Mode I fractures, making them long-lived and efficient outgassing channels that are ideally oriented for directing volatiles from the central conduit upward and outward toward the conduit rim. Indeed, Mode I fracture arrays may prove necessary for dissipating gas overpressures in the central regions of the magma column, which are considered difficult to outgas. These experiments highlight mechanisms that are likely active along conduit margins and constrain previously postulated processes under truly applicable conditions.

[1] We detect and precisely locate over 9500 aftershocks that occurred in the Yuha Desert region during a 2 month period following the 4 April 2010 Mw 7.2 El Mayor-Cucapah (EMC) earthquake. Events are relocated using a series of absolute and relative relocation procedures that include Hypoinverse, Velest, and hypoDD. Location errors are reduced to ∼40 m horizontally and ∼120 m vertically. Aftershock locations reveal a complex pattern of faulting with en echelon fault segments trending toward the northwest, approximately parallel to the North American-Pacific plate boundary and en echelon, conjugate features trending to the northeast. The relocated seismicity is highly correlated with published surface mapping of faults that experienced triggered surface slip in response to the EMC main shock. Aftershocks occurred between 2 km and 11 km depths, consistent with previous studies of seismogenic thickness in the region. Three-dimensional analysis reveals individual and intersecting fault planes that are limited in their along-strike length. These fault planes remain distinct structures at depth, indicative of conjugate faulting, and do not appear to coalesce onto a throughgoing fault segment. We observe a complex spatiotemporal migration of aftershocks, with seismicity that jumps between individual fault segments that are active for only a few days to weeks. Aftershock rates are roughly consistent with the expected earthquake production rates of Dieterich (1994). The conjugate pattern of faulting and nonuniform aftershock migration patterns suggest that strain in the Yuha Desert is being accommodated in a complex manner.

We use GPS, bathymetric/structural, and seismic data to define the pattern of present deformation along the northern half of the Azores plateau, where the Nubia–Eurasia plate boundary terminates at the axis of the Mid-Atlantic Ridge (MAR). New and existing campaign GPS velocities from the Azores islands reveal extension oblique to a series of en echelon volcanic ridges occupied by Terceira, S. Jorge, Pico, and Faial islands. In a frame of reference defined by 69 continuous GPS stations on the Eurasia plate, Terceira Island moves 2 ± 1 mm / yr away from Eurasia, consistent with the islandʼs location within the Terceira Rift and plate boundary structure. The volcanic ridges south of the Terceira Rift move toward WSW at progressively faster rates, reaching a maximum of 3.5 ± 0.5 mm / yr ( 2 - σ ) for the Pico/Faial volcanic ridge. The hypothesis that the Terceira Rift accommodates all Nubia–Eurasia plate motion is rejected at high confidence level based on the motions of sites on S. Jorge Island just west of Terceira Rift. All of the islands move relative to the Nubia plate, with Pico Island exhibiting the slowest motion, only 1 ± 0.5 mm / yr ( 2 - σ ). Detailed bathymetry from the interior of the hypothesized Azores microplate reveals faults that crosscut young MAR seafloor fabric. These observations and the GPS evidence for distributed deformation described above argue against the existence of a rigid or semi-rigid Azores microplate, and instead suggest that Nubia–Eurasia plate motion is accommodated by extension across a ∼140-km-wide zone east of the MAR axis, most likely bounded to the north by the northern shoulder of the Terceira Rift. The MAR spreading rate along the western end of the Azores deformation zone (∼38.5°N–39.5°N) is intermediate between the Eurasia–North America rate measured at 39.5°N and the Nubia–North America rate measured at 38.5°N, consistent with the joint conclusions that the Nubia–Eurasia boundary is broad where it intersects the MAR, and the Azores Triple Junction is diffuse rather than discrete.

In terms of neotectonics, the Timan Range and Kanin Peninsula represent a spacious mountainous structure 1150 km long and 80–160 km wide extend� ing in the northwesterly direction from Mount Poly� udov Kamen to Cape Kanin Nos in the Kanin Penin� sula. It consists of isolated en echelon arranged horst� like uplifts with Upper Precambrian rocks cropping out in their arch parts. In the southwest the Timan Range borders the Pechora Plate that surrounds the Karelian part of the East European Platform in the northeast. The boundary between the Pechora Plate and Mezen megablock of the East European Platform

Hough Transform analysis is used as an objective means to constrain volcano distribution in the central Sunda Arc, Indonesia. Most volcanoes in the arc define four en echelon, linear segments, each of 500–700 km length. Javan volcanoes that do not lie on these segments either (i) formed at an early stage in the history of the arc and erupted products that are petrologically and geochemically distinct from typical arc magma, or (ii) lie along other mapped structures. The en echelon distribution of volcanoes in the central Sunda Arc is best explained as originating from two possible sources. First, interaction with the subducting Indo-Australian Plate may induce stress in the arc lithosphere generating pathways for magma to exploit. Second, downward flexure of the arc lithosphere, as a result of mantle flow or loading by the arc, would also establish arc-normal tension towards the base of the lithosphere, where magma is supplied to volcanic systems. To the west and east of the central Sunda Arc deviations from the distribution of long, en echelon, linear segments can be understood as responses to specific stress fields in the arc lithosphere of Sumatra and eastern Nusa Tenggara, respectively. Control of volcano distribution by arc lithosphere explains why there are large variations in the depth from volcanoes to the zone of slab seismicity in the central Sunda Arc, where there is little variation in slab geometry or the rate of plate convergence.

We use interferometric synthetic aperture radar (InSAR), body wave seismology, satellite imagery, and field observations to constrain the fault parameters of the Mw 7.1 2011 Van (Eastern Turkey) reverse-slip earthquake, in the Turkish-Iranian plateau. Distributed slip models from elastic dislocation modeling of the InSAR surface displacements from ENVISAT and COSMO-SkyMed interferograms indicate up to 9 m of reverse and oblique slip on a pair of en echelon NW 40°-54° dipping fault planes which have surface extensions projecting to just 10 km north of the city of Van. The slip remained buried and is relatively deep, with a centroid depth of 14 km, and the rupture reaching only within 8-9 km of the surface, consistent with the lack of significant ground rupture. The up-dip extension of this modeled WSW striking fault plane coincides with field observations of weak ground deformation seen on the western of the two fault segments and has a dip consistent with that seen at the surface in fault gouge exposed in Quaternary sediments. No significant coseismic slip is found in the upper 8 km of the crust above the main slip patches, except for a small region on the eastern segment potentially resulting from the Mw 5.9 aftershock on the same day. We perform extensive resolution tests on the data to confirm the robustness of the observed slip deficit in the shallow crust. We resolve a steep gradient in displacement at the point where the planes of the two fault segments ends are inferred to abut at depth, possibly exerting some structural control on rupture extent. ©2013. American Geophysical Union. All Rights Reserved.

[1] The transpressive Alpine Fault is the boundary between the Pacific and Australian plates across the South Island of New Zealand. Earthquakes on the Alpine Fault and related structures pose a serious risk to many urban centers, including the city of Christchurch. Although it is a major feature on satellite images, the Alpine Fault is a difficult target for surface studies along much of its length; it mostly traverses densely forested and mountainous terrain and where it occurs in the lowlands it is usually covered by recent sediments. To investigate the Alpine Fault at a rare accessible location (Inchbonnie), we have acquired high-resolution seismic reflection data along five 380–1200 m long lines. Images produced from these data reveal a glacially overdeepened valley containing a thick sequence of diverse glacigenic sediments that have been disrupted by three en echelon strands of the principal Alpine Fault and several secondary fault strands. Based on their seismic facies, the sedimentary sequence is interpreted to comprise basal lacustrine beds overlain successively by alluvial-colluvial deposits that possibly include the remnants of large landslides, deltaic-fan units, and braided river gravels. Whereas the principal Alpine Fault strands disrupt the entire post-glacial sedimentary section and likely offset basement at depths up to 400 m, most of the secondary faults either merge with the principal fault strands at shallow depths or are surficial features limited to the sedimentary section.

Experimental modeling is used to study the geometry and evolution of structures and related secondary faults along releasing bends and offsets and restraining bends on strike-slip faults. The controls of the relative positions of adjacent strike-slip faults on the geometry of the structures and the difference in geometries between bends and offsets are investigated. A new method of laser scanning is used to map the geometry and evolution of the structures and related faults. The models show that oblique releasing bends connecting approaching faults result in spindle-shaped basins, whereas transverse bends result in more S-shaped or rhomboidal basins. Offsets result in the distribution of strain over a wider area and a larger number of faults compared with preexisting bends, which result in fewer well-defined basin-bounding faults. Secondary faults include R, R, and Y Riedel shears near the main strike-slip faults and oblique normal faults in the center of the basin. Fault patterns exhibit en echelon geometries with a progressive step down into the deepest parts of the basin. Symmetric, asymmetric, and double basins may form in any of the structural settings, depending on the slip distribution among faults on the basin margins. For restraining bends, oblique (45) bends connecting approaching faults result in spindle-shaped uplifts, whereas transverse or oblique (135) bends connecting overlapping faults result in more rhomboidal or rectangular uplifts. The fold trends are at increasingly higher angles with the strike faults for transverse and oblique (135) bends. Secondary faults include en echelon reverse faults, which typically form along the steep limbs of asymmetric uplifts, normal faults, which are transverse or oblique to the axis of the structure, and R, R, and Y Riedel shears near the main strike-slip faults. The aspect ratios of the basins and uplifts increase with increasing displacement on the strike-slip faults. The results of these models can be used to interpret the structural and fault geometries in surface and subsurface structures formed along strike-slip faults.

We have captured a remarkable sequence of microearthquakes showing progressive melt intrusion of a dyke moving upward from a sill at 18 km depth in the mid-crust of the northern volcanic rift zone in Iceland. Two-thirds of the earth's crust is created at mid-ocean rifts. Two-thirds of that crust is formed by intrusion and freezing before it erupts of molten rock generated within the underlying mantle. Here we show seismicity accompanying melt intrusion from 17.5 to 13.5 km depth along a dyke dipping at 50° in the mid-crust of the Icelandic rift zone. Although the crust at these depths is normally aseismic, high strain rates as melt intrudes generate microearthquakes up to magnitude 2.2. Moment tensor solutions show dominantly double-couple failure, with fault mechanisms sometimes flipping between normal and reverse faulting within minutes in the same location, but breaking along fault planes with the same orientations. We suggest several possible reasons for the flipping fault mechanisms: the breakage of solidified plugs of basalt within the dyke itself as more melt intrudes; intrusion along sub-parallel fractures or dykelet fingers into the local stress field created near the tip of a propagating dyke; or movement on small jogs or offsets between adjacent en echelon dykes. Although the faulting is caused ultimately by melt movement, there is no resolvable volumetric component in the moment tensor solutions. The inferred fault planes from microearthquakes align precisely with the overall plane of the dyke delineated by hypocentres. Melt injection occurs in bursts propagating at 2–3 m/min along channels c. 0.2 m thick, producing swarms of microearthquakes lasting several hours. Intervening quiescent periods last tens to hundreds of hours.

Tectonic-stretching models have been previously proposed to explain the process of continental break-up through the example of the Asal Rift, Djibouti, one of the few places where the early stages of seafloor spreading can be observed. In these models, deformation is distributed starting at the base of a shallow seismogenic zone, in which sub-vertical normal faults are responsible for subsidence whereas cracks accommodate extension. Alternative models suggest that extension results from localised magma intrusion, with normal faults accommodating extension and subsidence only above the maximum reach of the magma column. In these magmatic rifting models, or so-called magmatic intrusion models, normal faults have dips of 45–55° and root into dikes. Vertical profiles of normal fault scarps from levelling campaign in the Asal Rift, where normal faults seem sub-vertical at surface level, have been analysed to discuss the creation and evolution of normal faults in massive fractured rocks (basalt lava flows), using mechanical and kinematics concepts. We show that the studied normal fault planes actually have an average dip ranging between 45° and 65° and are characterised by an irregular stepped form. We suggest that these normal fault scarps correspond to sub-vertical en echelon structures, and that, at greater depth, these scarps combine and give birth to dipping normal faults. The results of our analysis are compatible with the magmatic intrusion models instead of tectonic-stretching models. The geometry of faulting between the Fieale volcano and Lake Asal in the Asal Rift can be simply related to the depth of diking, which in turn can be related to magma supply. This new view supports the magmatic intrusion model of early stages of continental breaking.

Hundreds of narrow, linear ridge segments are found in the transition zone between the Elysium Rise and the Utopia basin, occurring as both single and multiple ridges. The ridges are distinctive because of their very linear, steep-sided nature, their often sharp ridge crest (which sometimes is fractured), their association with stubby flows, their continuity over long distances and their cross-cutting of different terrain. The linear ridges are interpreted to be single dikes and dike swarms, either emplaced as normal dikes or as dikes emplaced subglacially feeding an explosive or effusive eruption. Five dike swarms are identified, having lengths ranging from 10–45 km and being between 1–7 km wide, while single ridges are up to 20 km long and 100–500 m wide. In the areas of dike swarms, crustal dilatation is estimated to vary from 15–60%. Dikes emplaced en echelon suggest that variations in the local stress field caused rotation during dike emplacement and dikes crosscutting flow units imply that dike emplacement can account for some of the observed linear fractures in the area. The ridges both modify and constrain Early Amazonian flows and flood plain deposits suggesting intense dike emplacement in the Early Amazonian. The association with different stages of inverted craters, as well as some features of ice-related origin (possible ice-cauldron and tindar-like features), indicate that the dikes may have been exposed due to eolian erosion and loss of volatile rich units subsequent to their emplacement.

Crustal deformation models of the Tibetan plateau are assessed by investigating the nature of Neogene deformation along the India–Asia suture zone through geologic mapping in south-central Tibet (84°30′E). Our mapping shows that the suture zone is dominated by a system of 3 to 4 ENE-striking, south-dipping thrust faults, rather than strike-slip faults as predicted by models calling upon eastward extrusion of the Tibetan plateau. Faults along the suture zone are not active, as they are cut by a system of NNW-striking oblique slip normal faults, referred to herein as the Lopukangri fault system. Fault-slip data from the Lopukangri fault system shows that the mean slip direction of its hanging wall is N36W. We estimate the net slip on the Lopukangri fault by restoring components of the thrust system. We estimate that the fault has accommodated ∼ 7 km of right-slip and ∼ 8 km of normal dip-slip, yielding a net slip of ∼ 10.5 km, and 6 km of horizontal east–west extension. The Lopukangri fault system is active and geomorphic offsets indicate right separations and westside-down dip-separation. The mapview curviplanar geometry and geomorphic expression of the Lopukangri fault system is similar to faults and rift basins to its east and west. These extensional faults are en echelon in map view and encompass a region that is 200 km long (east–west) and 95 km wide (north–south). Assuming our results for the Lopukangri fault are applicable to the entire system, we estimate a maximum of 18% extension across the zone. All active faults in the system terminate southward adjacent to the India–Asia suture zone. Because the individual rift geometries are similar and suggest a common kinematic relationship, we propose that the extensional system formed as a trailing extensional imbricate fan at the southern termination of the central Tibet conjugate fault zone. Alternatively, the extensional system may terminate to the north and represent a group of isolated crustal tears. Both kinematic interpretations imply a semi-smooth north–south variation in the magnitude of east–west extension in southern Tibet, with higher magnitudes in the north along the Bangong–Nujiang suture zone than in the south along the India–Asia suture zone. Our results from southern Tibet show that deformation between southern Tibet and the Himalayas is broad (95 km wide) and best described as a continuum possibly since the Late Miocene. Conversely, the structural boundary between western Tibet and the Himalayas, which is defined by the Karakoram fault, is presently a discrete boundary, and probably has been since the Middle Miocene. We think this variation in the displacement gradient and age of these structural boundaries within the interior of Tibet is best explained by the fault patterns and strain history describing wholesale E–W stretching and N–S shortening of the Tibetan crust.

Geological structure of the active foreland fold and thrust belt of Papua New Guinea has been interpreted using high-quality seismic-reflection data. Three en echelon anticlines, the Strickland, Cecilia and Wai Asi, are located along the frontal margin of the Papuan Fold Belt. All three are foreland-vergent and cut by hinterland-dipping thrust faults that sole into a common detachment beneath the Oligocene to Miocene Darai Limestone. Two of the anticlines are linked by a right-lateral transfer zone. Folding occurs primarily in the upper 2000 m of strata, which consist of Darai Limestone overlain by Miocene to Quaternary siliciclastic sedimentary rocks. Beneath the Darai Limestone lies the less-competent shaly Ieru Formation, which exhibits disharmonic folding and variable bed thickness. Seismic-reflection data clearly show that the Plio-Pleistocene upper Era Beds are deformed to the same extent as the underlying Darai Limestone, demonstrating that most of the observed deformation has occurred during the L...

article i nfo Extensional rifts are characterized by significant lateral variations in crustal rheology as lithospheric material cools and advects away from the rift axes. Nevertheless, most models used for modeling of crustal movements consist of horizontal layers with uniform properties. This paper explores the role spatial variation of rheological properties play, in modifying the style of surface deformation at rifts, by comparing finite element model predictions to inferred inter-rifting surface deformation in Iceland's Northern Volcanic Zone. Crustal deformation has been observed by satellite radar interferometry (InSAR) and GPS measurements. Extension is observed across the entire northern rift zone, whereas subsidence occurs in two distinct areas, corresponding to mapped fissure swarms, where major rifting took place in 1975-1984 and 1874-1876, respectively. Our models indicate that the observed inter-rifting plate spreading deformation field is controlled by rheological variations within the en echelon arrangement of fissure segments, and that a central ridge axes model does not apply. Uniform stretching across a plate boundary zone, where fissure swarms are weaker than the surrounding crust, and reflect the surface expression of the rift, reproduces the characteristics of the deformation field. The most realistic fissure swarm structure consists of a wedge of

The kinematics and architecture of orogenic systems along the leading edges of accreting terranes may be heavily influenced by climate, but little research has been devoted to the long-term effects of glacial erosion on orogenesis. Here we use low-temperature apatite and zircon (U–Th)/He and fission-track thermochronometry, along with subsidiary structural relationships and seismicity, to develop a new architectural model of the St. Elias orogen in southern Alaska, which is one of the best examples of a glaciated orogenic wedge worldwide. These data illustrate that the orogen consists of a deformational backstop on the leeward flank and a rapidly deforming and eroding, thin-skinned fold and thrust belt on the windward flank. A structure beneath the Bagley ice field separates these distinct deformational domains, which we propose is a backthrust that makes the orogen doubly-vergent. Thermochronometry within the orogenic wedge suggests that denudation and deformation are strongly influenced by glacial erosion. Long-term exhumation, at rates of up to 4 mm/yr, is concentrated within a narrow zone along the windward flank, where glacier equilibrium lines intersect the orogenic wedge. The onset of enhanced glaciation also coincided with a marked acceleration in exhumation across the orogenic wedge, accelerated backthrust motion, and a major shift in deformation away from the North American–Yakutat terrane suture (Chugach St. Elias fault). We propose that accelerated glacial erosion forced the redistribution of strain along the backthrust and an en echelon array of forethrusts that lie beneath the zone of heaviest glaciation, which in turn are systematically truncated by the backthrust. This focusing of deformation matches predictions from analytical models of orogenic wedges and implies a high degree of coupling between climate and tectonics in this glacially-dominated orogen.

Nd isotope data are presented for granitoid orthogneisses from the Central Metasedimentary Belt (CMB) of the Grenville Province in order to map the extent of juvenile Grenvillian-age crust within this orogenic belt that is composed mostly of older crustal terranes. The data reveal a 150 km-wide belt of juvenile crust in Ontario, but this belt contains a block of pre-Grenvillian crust (containing the Elzevir pluton) which yields an estimated crustal formation age of 1.5 Ga. The recognition of an older block within the CMB has profound implications for its structure and tectonic evolution, because it implies that juvenile Grenvillian crust, apparently forming a wide NE–SW belt, is in fact distributed in two narrower segments with approximately N–S strike. We suggest that the CMB comprises an en echelon series of ensimatic rift segments, created by back-arc spreading behind a continental margin arc. These rift segments extend southwards (in the subsurface) into the northeastern Unites States. The rift segments contain abundant marble outcrops, consistent with marine incursion into the rift zone, and these deposits also continue northwards into a ‘Marble domain’ of the CMB in Quebec. However, crustal formation ages in the latter domain are largely pre-Grenvillian, implying that the Quebec rift segment was ensialic. Hence, we interpret the CMB in Ontario and Quebec as the northern termination of a failed back-arc rift zone.

Gas production from the Barnett Shale relies on hydraulic fracture stimulation. Natural opening-mode fractures reactivate during stimulation and enhance efficiency by widening the treatment zone. Knowledge of both the present-day maximum horizontal stress, which controls the direction of hydraulic fracture propagation, and the geometry of the natural fracture system, which we discuss here, is therefore necessary for effective hydraulic fracture treatment design. We characterized natural fractures in four Barnett Shale cores in terms of orientation, size, and sealing properties. We measured a mechanical rock property, the subcritical crack index, which governs fracture pattern development. Natural fractures are common, narrow (0.05 mm; 0.002 in.), sealed with calcite, and present in en echelon arrays. Individual fractures have high length/width aspect ratios (1000:1). They are steep (75), and the dominant trend is west-northwest. Other sets trend north-south. The narrow fractures are sealed and cannot contribute to reservoir storage or enhance permeability, but the population may follow a power-law size distribution where the largest fractures are open. The subcritical crack index for the Barnett Shale is high, indicating fracture clustering, and we suggest that large open fractures exist in clusters spaced several hundred feet apart. These fracture clusters may enhance permeability locally, but they may be problematic for hydraulic fracture treatments. The smaller sealed fractures act as planes of weakness and reactivate during hydraulic fracture treatments. Because the maximum horizontal stress trends northeast-southwest and is nearly normal to the dominant natural fractures, reactivation widens the treatment zone along multiple strands.

Oil and gas reservoirs of the Ordovician Trenton–Black River interval in the Appalachian Basin are commonly associated with fault-related hydrothermal dolomites. However, relationships between porosity development and fault geometry in these fields are poorly documented. In this article, we integrate three-dimensional (3-D) seismic and wire-line data from the Trenton–Black River interval at Saybrook field in northeastern Ohio to study relationships between faulting and porosity development there. Faults were mapped using a combination of amplitude and coherency versions of the seismic data, and a 3-D porosity volume was generated for the Trenton–Black River interval by integrating attributes derived from the seismic data with log-based measures of porosity. The productive trend in the Trenton–Black River interval at Saybrook is controlled by a 3.4-mi (5.5-km)-long, northwest-southeast–oriented basement fault that was probably reactivated during the Taconic orogeny (i.e., Late Ordovician). Strike-slip movement along the fault generated en echelon synthetic shear faults that branch at least 1350 ft (411.5 m) upward into the Trenton–Black River interval. The best porosity is developed in areas between overlapping synthetic shear faults. Antithetic shear faults probably formed at these locations and, when combined with minor dip-slip movement, created conduits for subsequent porosity-generating fluids. Circular collapse structures associated with localized extension between overlapping shear faults are the primary drilling targets, and horizontal wells running parallel to the strike of the fault would have the best chances of intercepting good porosity development.

Existing models of brittle shear failure are unable to account for three-dimensional deformation involving the development of polymodal sets of fractures. Motivated by field observations of contemporaneous arrays of quadrimodal faults and deformation bands, we use an idealised micromechanical model to explain how brittle shear fractures can form oblique to all three remote principal stresses. We model tensile microcracks as finite ellipsoidal voids, subjected to small opening strains, in a linear isotropic elastic matrix. The geometry of the tensile stress lobes around the ends of an isolated microcrack promotes the en echelon interaction of neighbouring cracks with respect to the prescribed crack orientation. Coalescence of these interacting crack arrays into a through-going composite fracture surface leads to a brittle shear failure plane oriented obliquely to all three coordinate axes and all three remote principal stresses. Experimental evidence supports the idea that composite shear fractures can propagate in-plane through the coalescence of many constituent tensile microcracks. Our new model, based on the 3-D geometry of the elastic stress field around a mode I crack, can explain the oblique orientations of polymodal faults formed in a triaxially compressive stress field.

Out-of-sequence thrusts (OST) play an important role in the thickening of accretionary prisms, formation of forearc basins, and tsunami generation in subduction zones. By detailed investigations of geological and paleogeothermal structures, an ancient seismogenic OST is identified in the Shimanto accretionary complex, southwest Japan. This OST consists of a mappable en echelon fault system developed at a late stage of the accretionary process. The estimated accumulated displacement of 2.5–8.5 km and formation depth of 2.5–5.5 km, based on a thermal analysis, indicates much higher fault activities sense of slip with a sinistral slip, and the large displacement was distributed among multiple small faults each with a small offset. Most of the faults in the en echelon fault system recorded repeated brittle failure, and one fault preserves pseudotachylyte, a fault rock indicative of seismic faulting. This is the first report of pseudotachylyte along an OST in an accretionary prism. Repeated fracturing in the same narrow shear zones (each only a few millimeters thick) suggests the OSTs perform as major shear localized zone in the accretionary prism. Shallow estimated formation depth and consequent low normal stress also support the hypothesis by a weak coupling along this fault zone. These findings are in accord with a thrust activity in the shallow portion of the accretionary prism being associated with rupture propagation from the deep seismogenic region. This OST showed high activity and repeated faulting in the shallow portion of the accretionary prism, comparable to that of the submarine tsunamigenic OST in the present Nankai trough.

A series of experiments are described where dyed water (a magma analogue) was intruded into solid gelatine (a crustal analogue) to investigate the formation of sills. We considered a layered gelatine system with contrasting adjacent layers. By varying the density and rigidity of the gelatine we found that experimental sills form when the upper layer is more rigid than the lower layer, with intrusion occurring in a plane directly below the interface. Experimental dykes were observed to propagate to the surface when the Young's Modulus ratio of upper to lower gelatine layers was less than one. Experimental dyke arrest occurred when the upper layer was more rigid and the interface was strong. Two varieties of experimental sill formed when the upper layer was more rigid than the lower layer and the interface was sufficiently weak. The form of the intrusion depends on the balance of driving pressures and the Young's Modulus ratio of contrasting adjacent layers. When the rigidity ratio is high and there is a large driving pressure the experimental feeder dyke completely converts to propagate as a sill. However, when the rigidity ratio and driving pressure are both close to one a dyke–sill hybrid forms. Under these conditions the experimental sill formation is accompanied by contemporaneous dyke intrusion into the overlying more rigid layer. During sill propagation deformation structures such as faults and en echelon fractures are formed into the lower layer. Experimental sill propagation dynamics are controlled by viscous dissipation along the length of the sill; causing acceleration with increasing length. Our study suggests that rigidity contrasts may play a major role in the location of sills and development of igneous complexes. In ancient cratonic areas the Moho is a suitable site for the preferential formation of sills with higher rigidity continental crust overlying weaker mantle. Mantle plumes impacting ancient continents provide a situation in which large sills can form to fractionate prior to eruption of flood basalts. The boundary between the upper and lower crust (Conrad discontinuity) may provide a preferential focus for the emplacement of sheets of silicic magma at continental arcs where the lower crust is weakened by prolonged heating and possible hydration.

The Owen Conglomerate comprises coarse-grained siliciclastics that were deposited in response to Late Cambrian extension. The identification of normal faults that host thickened accumulations of siliciclastics is used here to support interpretation of syn-fill extension. Local mapping and section construction have identified a series of north-trending, en echelon, segmented normal faults that exhibit changes in along-strike polarity. The Late Cambrian faults are adjacent to sedimentary packages that define half-graben geometries, with an unconformity that defines basal contacts with underlying Mt Read Volcanics and onlap geometries onto the opposing basin margin. Faults that were active during deposition of the Owen Conglomerate were subsequently reversed during D1 Middle Devonian deformation, with reverse displacement controlling the development of inversion structures defined by north-trending fold structures. Pervasive northwest-trending D2 deformation extensively overprints earlier deformation feature...

Analog models were used to investigate the influence of cross-trending basement faults on inverted rift systems similar to those located in the Makassar Straits. Multilayer models composed of sand, silicone putties, and a silicone-plasticine mixture were constructed so that laterally offset rift basins would form on either side of a preexisting basement fault. Transfer zones comprising a series of en echelon normal faults that connect downward with the basement fault formed to link the laterally offset rift basins during the extension phase. Subsequent oblique inversion of the models resulted in a general progression starting with broad folding, followed by the development of small-scale inversion anticlines associated with reactivated rift faults, to thrust faulting, and finally, to the formation of cross-trending wrench faults. Rift fault reactivation and wrench fault development were prominent in the transfer zones because of the orientation of the constituent faults (oblique to the shortening direction) and their connection with the underlying basement fault. The addition of an intrasedimentary detachment layer, which represents a thick overpressured shale present in the vicinity of the economically important Mahakam delta, allowed gravitational collapse and extensional faulting to occur on structural highs. The cross-trending basement fault was again the locus for inversion structures, both above and below the detachment layer in the latter case, which may have influenced the distribution of petroleum in the region.

Pink Mountain Anticline stands out in front of the foothills of northeastern British Columbia (57°N, 123°W). Geologic mapping and prestack depth-migrated seismic sections show that it is localized above and west of a northwest-trending subsurface normal fault. Along with isopach maps these data demonstrate episodic normal movement on the fault during deposition of the Carboniferous Stoddart Group, Triassic Montney Formation and possibly the Jurassic–Cretaceous Monteith–Gething formations. West of this fault, during Laramide compression, a pair of backthrusts nucleated on either side of a minor east–west trending Carboniferous fault and propagated across it in an en echelon pattern. One backthrust ramped laterally across the area and separated the Pink Mountain and Spruce Mountain structures, both of which are contained within a 30+ km long pop-up structure above the Besa River Formation decollement. Glomerspirella fossils confirm the existence of the Upper Jurassic Upper Fernie Formation and Upper Jurassic to Lower Cretaceous Monteith Formation at Pink Mountain.

Modern three-dimensional seismic imaging enables a basic geometric description and kinematic interpretation of part of the Fish Creek Slide, a massive (4000 km2; 1500 mi2) slope failure beneath Alaska's North Slope. The Fish Creek Slide is divided into a dominantly extensional organized slide zone in the west and a mostly contractional disorganized slide zone in the east. In the organized slide zone, a series of six organized slide blocks detached from the lower slope along steep escarpments during an inferred Albian earthquake focused at depth on a preexisting southeast-striking fault. Blocks were transported eastward into the basin as failure progressed catastrophically yet systematically by northwestward, en echelon, upslope, footwall collapse. During slide evolution, the primary flat detachment switched from the highly radioactive zone down stratigraphic section to the Lower Cretaceous unconformity. Although the seismic interpretation is hampered by a variety of slide-induced signal problems, several significant, previously unrecognized elements of the Fish Creek Slide are described: shear zones related to displacement variations in the slide, duplex structures in the slide blocks, postslide erosional features, and axial structural lows in slide blocks.

The North Fiji transform zone, a 1500 km long and 200 km wide transform segment of the Pacific–Australia plate boundary, is one of the major transform fault systems of the Earth. New data collected during the ALAUFI cruise (March 2000) on board the R/V L’Atalante make it possible to define more accurately the geometry and kinematics of this transform plate boundary. Three spreading centers or extensional zones (the North Cikobia spreading center, the Futuna spreading center and the southeast Futuna volcanic zone) and a strike-slip fault zone (the Futuna transform fault) have been discovered over a distance of 500 km along the eastern North Fiji transform zone, from the north of the Fiji platform to the east of the Futuna archipelago. The Futuna transform fault oriented 100° has been mapped over a distance of 250 km. It must be considered to be an important tectonic element of the transform plate boundary. Pure strike-slip as well as transpression and transtension motions are responsible for the complex morphology of this feature. The uplifted Futuna–Alofi ridge represents a major compressional relay along the Futuna transform fault. The Futuna spreading center trending 20–30° is composed of a series of en echelon left-stepping spreading segments. It represents a 200 km long extensional relay between the Futuna transform fault and the western part of the North Fiji transform zone, the Fiji transform fault, which bounds the Fiji platform to the north. The opening rate at the Futuna spreading center is estimated at 4 cm/yr. Although the North Cikobia spreading center and the southeast Futuna volcanic zone have been only partly mapped, bathymetric and reflectivity data clearly reveal that active extension also takes place along these two features. A spreading rate of 2 cm/yr is inferred at the North Cikobia spreading center. Therefore, the North Fiji transform zone appears to be composed of two main overlapping transform segments relayed by parallel extensional zones. The three active extensional zones have an ENE–WSW to NNE–SSW orientation, while compressive features along the Futuna transform fault are NW–SE to NNW–SSE oriented, in accordance with the present-day left-lateral transform motion along this part of the Pacific–Australia plate boundary.

The development of volcanic elongate ridges (VERs) could be controlled in many cases by complex processes related to interaction between mantle plumes and spreading centers. We report here measured ages for the Foundation Chain, SE Pacific, showing that volcanism along morphologically distinct VERs can develop occasionally as rapidly formed continuous lines of coeval volcanism extending from a region of intraplate volcanism to the Pacific–Antarctic spreading center. But a significantly more dominant trend is for coeval, yet structurally disconnected, segments of Foundation Chain VERs to develop in a series of en echelon, NE–SW elongate ‘zones’ of coeval hotspot volcanism. These elongate zones developed at intervals of approximately 1 Myr while maintaining a basically steady state orientation and size as the Pacific–Antarctic spreading center migrated continually closer to the Foundation plume hotspot. We infer from this new information that Foundation Chain VER development was controlled by an interplay between short-lived, local factors (e.g., location of nearest spreading center segment, lithospheric stress) and long-lived attributes of the Foundation plume hotspot (e.g., size, orientation, periodicity).

The geological structures associated with the site of the 55 million m 3 Karameh embankment dam constructed in the Jordan Valley and the tectonic effects on dam foundation and reservoir margins were reviewed. The dam crosses the strike-slip fault of the Jordan Valley Rift Zone. Trace evidence of the fault indicates a displacement of 8 to 15 m over a rupture length of some 130 km, which probably took place several centuries ago. Earthquakes with Richter magnitudes as great as 7.8 have occurred along the Jordan Valley Fault. Deterministic studies by Tapponnier (1992) indicated that the dam design should incorporate the possibility of a 7.8 event, a maximum horizontal rupture displacement on the fault of 10 m and a design peak ground acceleration (PGA) of 0.74 g at the site of the dam. These values are consistent with those which would be used in the USA for a similar case. However, the dam was actually designed by a consultant and constructed for a PGA of about a quarter of this value, based on seismic hazard analysis following guidelines of the International Committee on Large Dams (ICOLD) (1989). Moreover, the dam was designed for displacements of 6 m horizontal and 2 m vertically. Liquefiable sand layers were found in the dam foundation. A PGA of 0.50 g will trigger liquefaction of the sand layers in the dam foundation which would be expected to result in a crest settlement of 4.4 m. Slope stability analysis indicated deep failure planes in the foundation zone. The excavation of loose materials from under the dam foundation has not precluded the possibility of liquefaction occurring under the expected earthquake. Field mapping of geological features during the dam foundation excavation and construction revealed that: a) the most likely location of the Jordan Valley fault is in the area where the Wadi Mallaha stream crosses the dam axis, b) zones of en echelon type open fissures have been defined in the laminates sub-parallel to the Jordan Valley Fault Zone, c) at the Wadi Mallaha stream bed a parallel zone of faulting and warping of the Lisan Formation was identified, and d) the alignment is clearly confirmed by the exposure immediately upstream of the core at Ch 1375. The main wrench fault zone crosses the embankment footprint (upstream to downstream approximately) and reaches the surface around Ch 1375. The critical safety elements of the embankment are the core, the downstream fine filter, the chimney drain and the drainage blanket. To resist large earthquake events safely, the following safety measures should be implemented: 1. A freeboard of 7.0 m instead of the 5.0 m constructed. 2. The foundation of the dam should be stabilized against liquefaction. 3. The embankment internal zoning should be designed to accommodate damage resulting from earthquake events with a magnitude of 7.8. 4. The foundation needs relief measures downstream to lower the pore pressure. This paper describes the measures taken during construction as overall defense against future fault movements through a wide plastic core, an extensive upstream blanket, a 5.0-m thick downstream chimney filter and drain zones, a 5-m freeboard and an upstream crack stopper zone which may be critical for normal faults with a lateral extension component. The geological determination of the main wrench fault alignment resulted in the addition of an extra 2-m width to each of the already wide chimney filter and drain zones. In order to reduce potential seepage, local cut-off trenches or slush grouting were used for treatment of any open fissures at the upstream edge of the external blanket and the right bank ridge. The scale and scope of this dam and inherent engineering geological hazards are unprecedented. The design is considered deficient. This paper documents serious safety issues with the dam. The constructed dam presents serious safety risks and represents a case history of a disaster waiting to happen.

Deep-tow sidescan sonar data acquired along 240 km of the Mid-Atlantic Ridge (MAR) between 35°40′N and 38°N have been combined with new bathymetric compilations and used to establish the recent tectonic and volcanic history of six second-order segments and their bounding non-transform offsets (NTOs). The segments show a range of volcanic and tectonic types, but in general the northernmost segments (i.e. those with greater influence from the Azorean hotspot) are shallower and more volcanically robust than those to the south, with hydrothermal activity in segment centres (for example, Menez Gwen and Lucky Strike). Nonetheless, this generalisation requires some modification due to temporal variations in the balance between magmatic supply and tectonic dismemberment. The NTOs are broad right-stepping discontinuities, locally up to 25 km wide, and accommodate offsets between 10 and 50 km. The discontinuities are mostly sediment-floored, and link the spreading segment tips with a range of structures. These include locally dense arrays of en echelon extensional normal faults, short lengths of linear strike–slip fault strands occasionally cutting basement blocks apparently stranded within the offsets. Basement blocks within the offsets are cross-cut by complexes of intersecting faults, suggesting that deformation is distributed across the zone. The pervasive faulting taking place at the NTOs favours fluid circulation and associated hydrothermal activity, as at the Rainbow Site at 36°17′N.

We investigate arrays of en echelon dike segments and their associated deformation in porous sandstone to infer the segmentation mechanism and the state of stress during dike emplacement. The en echelon arrays are interpreted as breakdown segments of planar parent dikes that propagated from greater depth under mixed-mode conditions. Typically, an array consists of either continuous nonoverlapping stepped segments (offset smaller than segment thickness) or overlapping connected segments (offset larger than segment thickness). The deformation associated with the nonoverlapping stepped segment arrays consists of newly documented fan-like patterns of deformation bands (lamellae of crushed detrital quartz grains), whereas the overlapping connected segment arrays consists of net-like patterns of deformation bands. Thus the patterns of deformation are related to offset geometry and are likely to be diagnostic of the states of stress. We simulated the stress and deformation fields around interacting breakdown segments by applying a continuum damage mechanics model. The simulation results mainly illustrate the stress dependence of the damage distribution and the sensitivity of the damage distribution to the geometry of the segment offset and the mutuality of segment propagation. By changing the applied stress and by controlling the segment tip growth, symmetric and asymmetric distributions of damage were produced. We describe which aspects of the generated damage zones satisfactorily correlate with field observations. Damage mechanics simulations are useful tools for studying the state of stress during dike emplacement.

A rigid element method of restoration (UNFOLD) is used to restore competent folded and faulted layers of the Ventura and Los Angeles basins to their initial horizontal state. Comparison of initial (undeformed) state with present (deformed state) allows one to estimate both the finite crustal deformation and its associated horizontal displacement field (relative to an arbitrary fixed line). Including data from the Santa Barbara Channel basin, the total finite displacement field for the western Transverse Ranges and vicinity (within the Pacific plate) is inferred from this map restoration and is modeled as a double fan closure. This model implies a 10° clockwise rotation of the northeen boundary of the western Transverse Ranges and a 5° counterclockwise rotation of its northeast boundary. Lateral variation of the deformation reveals the heterogeneity of the subsurface deformation. Most of the major thrusts appear to initiate as en echelon structures along the left-lateral northern margin and the right-lateral northeastern margin of the studied area. The resulting deformation and displacement values closely match those derived by other geological methods (section balancing techniques or fault slip measurements) and by geophysical methods (geodetic, paleomagnetic, and focal mechanism data). Map restoration thus is a method that can independently quantify both local and regional deformation including folds and faults. This method also reveals the zones where problems of compatibility appear with the available geological and geophysical data and thus where the next studies might be focused.

A model for the compressive failure of rocks via the process of shear faulting is presented. The model addresses the progressive growth of damage that leads to the formation of a critical fault nucleus which grows unstably in its own plane by fracturing the grain boundaries in an increasingly rapid succession. The model uses a two-parameter Weibull-type shear strength distribution for defining the nucleation of initial damage, followed by the use of stress enhancement factors for addressing the increased probability of failure in the vicinity of already cracked grain boundaries. These factors essentially involve surface averaging of enhanced stresses in the neighboring grains with the appropriate strength distribution as the weighting function. As the stress is further increased, similar correlated fracturing events get preferentially aligned to the crack cluster, resulting in en echelon of cracks. This crack cluster is modeled as an elliptical inhomogeneity within which the cracks interact and lead to material pulverization, the effect of which, mechanistically, is to lower the shear modulus compared with the uncracked material on the outside. The shear stress concentration resulting from this moduli mismatch is calculated and used to compute the stress enhancement factors for defining the nucleation of additional cracking events near the crack cluster. Eventually, the size of the crack cluster becomes sufficiently large that it carries a stress concentration high enough to fracture all grain boundary elements in from of it in an increasingly rapid succession. The stress associated with this event is taken as the failure stress. Since the model allows other cracking events to occur within the material volume in accordance with the assumed strength distribution function, formation of other competing but subcritical shear faults naturally occurs. Besides the faulting stress for a prescribed confinement, the model is able to predict the angle of the shear fault fairly well. The model is applied to a variety of rock types, including granite, eclogite, gabbro, aplite, rock salt, sandstone, dunite, limestone, and marble, and the predicted failure envelopes compare rather well with the failure data available in the literature.