Journal of Structural Geology

Published by Elsevier
Online ISSN: 0191-8141
Publications
Article
During ODP Leg 170, five sites were drilled and sampled off the Costa Rica Pacific margin. Two of them, site 1040 and 1043, yielded material from a wedge of deformed sediments, the main décollement zone, and the underthrusting sedimentary sequence of the subducting Cocos plate. Detailed mesoscopic and microscopic analyses of the deformation features characteristic of each domain help to define four different structural/hydrologic regimes. Unexpectedly, the wedge of deformed hemipelagic sediments does not represent an accretionary wedge, because little or no transfer of material from the subducting plate has occurred. The deformed sedimentary wedge records periods of tectonic bulk strain, in which fluid pressure intermittently rises and induces fracturing, alternating with periods of gravitational bulk strain. The last stage of this alternating strain regime is represented by the growth of thick, sub-horizontal shear zones. The sediments caught up in the décollement zone show distinctly different deformation features, reflecting different mechanical behaviour and a different response to fluid flow, but again transient pulses of increased pore pressure are interpreted as the driving mechanism of dewatering. The underthrust pelagic and hemipelagic section is also characterised by dewatering features, but only in the uppermost part. The basal sediments of this section, in contact with underlying gabbro intrusions, have a completely different structural history and hydrologic regime. Structures here are interpreted as related to near ridge processes, and the hydrologic system is not linked to the upper sediment-dewatering regime, but rather seems to have been fed by a seawater source.
 
Article
Detailed analysis of ODP Leg 110 cores provides evidence for a complex structural evolution of the frontal thrust system in the northern Barbados Accretionary Prism. Initial east-directed in-sequence thrusts are biostratigraphically documented, and are overprinted towards the west by out-of-sequence overthrusts, largescale folds and penetrative deformation of the accreted rocks. This structural progression is interpreted to reflect the need for secondary shape adjustments of the accretionary prism in order to maintain a critical taper. At present the basal décollement is located within Lower Miocene strata. Accreted Eocene sediments suggest either fossil frontal off-scraping at a lower stratigraphic level or duplex formation and underplating.The present localization of the basal detachment is provoked by a zone of initial high porosity in the Lower Miocene radiolarian-rich mudstones. Shearing is accompanied by partial porosity breakdown in a 30–40 m wide zone of intense scaly fabrics and stratal disruption that constitutes the Atlantic-Caribbean plate boundary at present. Large displacements are taken up by a relatively narrow zone of contact strain. Initially the preferred orientation of clay minerals (incipient scaly fabrics) may be a flattening fabric, but with progressive rotational flow the scaly planes may act as discrete microshears and provide small-scale flow partitioning. Alternatively, scaly fabrics may be interpreted as brittle microshears. Mud-filled veins are present in the accreted rocks as well as at the oceanic reference site east of the deformation front. Carbonate-filled veins are restricted to the accreted rocks. This indicates persistence of high fluid pressures, but a fundamental change in the type of fluid-rock interaction during frontal accretion tectonics.
 
Article
Toes of accretionary prisms record the initial deformation of wet sediments during accretion at a convergent plate margin. ODP Leg 131 succeeded in coring the toe of the Nankai prism, SW Japan, together with the basal décollement, the underthrust sediments and the basaltic ocean basement. An unprecedented structural geological inventory has been compiled. Gravitational deformation is important in the slope-apron deposits, but tectonic stresses, too, are transmitted to these levels. Core-scale deformation bands, due to heterogeneous bulk-shortening of the prism, are exceptionally well-developed at Nankai, and range through kink-bands, shear zones and faults. Anomalous frequencies indicate localized high-strain zones. Stress-inversion analysis gives a principal compression close to the plate convergence vector.The frontal thrust of the prism is a 26 m thick zone of breccia/scaly fabric, with a 309 m net slip. It may well be currently active, and accommodating almost half of the present convergence strain. The basal décollement, a 19 m thick breccia zone, separates the deforming, overconsolidated sediments of the prism from the virtually undeformed materials below. These are highly underconsolidated and probably overpressured. Anomalous fluid pressures localize several of the prism structures. Unlike some other prisms, drainage of the Nankai prism is dominantly pervasive rather than controlled by major structures.
 
Article
The 9th September, 1349, earthquake was one of the most catastrophic events experienced along the Apennines. At least three main shocks struck a vast area of the Molise–Latium–Abruzzi regions, and damage was even sustained by the distant monumental buildings of Rome. The southern-most shock (Mw ∼ 6.7) occurred at the border between southern Latium and western Molise, razing to the ground the towns of Isernia, Venafro and Cassino, amongst others, and devastating Montecassino Abbey. As with other Medieval catastrophic sequences (e.g., in December 1456, Mw ∼ 6.5–7.0), this earthquake has not yet been associated to any seismogenic source; thus, it still represents a thorn in the flesh of earthquake geologists. We have here carried out a reappraisal of the effects of this earthquake, and through an interpretation of aerial photographs and a field survey, we have carried out paleoseismological analyses across a poorly known, ∼N130 normal fault that crosses the Molise–Campania border. This structure showed repeated surface ruptures during the late Holocene, the last one of which fits excellently with the 1349 event, both in terms of the dating and the rupture dimension. On the other hand, archaeoseismic analyses have also indicated the faulting of an Augustean aqueduct. The amount of apparent offset of the aqueduct across the fault traces shows that there were other surface faulting event(s) during the Roman–High Middle-Age period. Therefore, in trying to ascertain whether these events were associated with known (potentially of 346 AD or 848 AD), or unknown earthquakes in the area, it became possible that this ∼20-km-long fault (here named the Aquae Iuliae fault) is characterized by high slip rates and a short recurrence time. This is in agreement with both the similar behaviour of the neighbouring northern Matese fault system, and with recent GPS analyses showing unexpectedly high extension rates in this area.
 
Article
The Roum fault is the westernmost branch within the Lebanese restraining bend of the Dead Sea Transform Fault. This strike-slip fault extends for about 35 km from north of the Hula basin to the Awali river, and shows left-lateral strike-slip displacements (manifested as offset streams) and vertical movements. Recent seismic records indicate its seismogenic potential as the source of the double shock of 16 March 1956 (Ms 4.8, 5.1) earthquake. We studied the Roum fault using combined field investigations in geomorphology, structural geology, and palaeoseismology. Fresh fault scarps and pressure ridges visible along the fault trace attest to recent coseismic ruptures. A palaeoseismic trench investigation exposed a complex fault zone with several rupture strands and a minimum of four faulting episodes in the last ∼10,000 years, the most recent event being post 84–239 AD. According to historical records, the 1 January 1837 (Ms 7.1) earthquake, which induced severe damage in the region, is the most likely candidate. Our results assign a slip-rate of 0.86–1.05 mm/year along the Roum fault, which shows that it accommodates about 14% of the total predicted strike-slip motion within the Lebanese restraining bend, and it should be considered a potential seismogenic fault for seismic hazard estimates in Lebanon.
 
Article
Thomson & Tait (Treatise on Natural Philosophy, Vol. 1, 1867) described the kinematics of simple shear in terms of a construction. This construction is redescribed in a geological context. It provides a simple method of determining the orientation and amount of strain in a shear zone, and is an excellent aid to teaching the geometry and equations of simple shear.
 
Article
The Sub-Himalayan fold-thrust belt consists of deformed late Cenozoic and older deposits south of the Main Boundary thrust (MBT). In Pakistan, east of the Indus River, the Sub-Himalaya comprises the Potwar Plateau and the Salt Range, which is thrust southward over the Jhelum River floodplain along the Salt Range thrust. Although an estimated 9–14 mm a−1 shortening has been taken up on the Salt Range thrust during the last 2 Ma, the range-front scarp does not show signs of recent faulting. Shortening may be shifting southward to the Lilla overpressured anticline, which rises from the Jhelum floodplain as a fault-propagation fold. Farther east, shortening is partitioned among several anticlines underlain by foreland- and hinterland-dipping blind thrusts. Southeast of the main deformation zone, the Pabbi Hills overpressured anticline is best explained as a fault-propagation fold. Throughout the Potwar Plateau and Salt Range, thrusts and folds rise from a basal décollement horizon in Eocambrian evaporites. The Pakistani part of the décollement horizon could generate large earthquakes only if these evaporites die out northward at seismogenic depths.In India and Nepal, the Sub-Himalaya is narrower, reflecting the absence of evaporites and a steeper slope of the basement towards the hinterland. The southern boundary of the Sub-Himalaya is the Himalayan Front fault, discontinuous because part of the shortening is expressed at the surface by folding. Broad, alluvial synclinal valleys (dun valleys) are bounded on the south by rising barrier anticlines of Siwalik molasse. The 1905 Kangra earthquake (M8) produced uplift on the Mohand anticline and the Dehra Dun Valley, suggesting that this earthquake occurred on a décollement horizon above basement, downdip from the fold. If so, the Kangra event is the largest known earthquake on a blind thrust expressed at the surface as a fold.
 
Article
Active faulting and seismic properties are re-investigated in the eastern precinct of the city of Thessaloniki (Northern Greece), which was seriously affected by two large earthquakes during the 20th century and severe damage was done by the 1759 event. It is suggested that the earthquake fault associated with the occurrence of the latest destructive 1978 Thessaloniki earthquake continues westwards to the 20-km-long Thessaloniki–Gerakarou Fault Zone (TGFZ), which extends from the Gerakarou village to the city of Thessaloniki. This fault zone exhibits a constant dip to the N and is characterised by a complicated geometry comprised of inherited 100°-trending faults that form multi-level branching (tree-like fault geometry) along with NNE- to NE-trending faults. The TGFZ is compatible with the contemporary regional N–S extensional stress field that tends to modify the pre-existing NW–SE tectonic fabric prevailing in the mountainous region of Thessaloniki. Both the 1978 earthquake fault and TGFZ belong to a ca. 65-km-long E–W-trending rupture fault system that runs through the southern part of the Mygdonia graben from the Strymonikos gulf to Thessaloniki. This fault system, here called Thessaloniki–Rentina Fault System (TRFS), consists of two 17–20-km-long left-stepping 100°-trending main fault strands that form underlapping steps bridged by 8–10-km-long ENE–WSW faults. The occurrence of large (M≧6.0) historical earthquakes (in 620, 677 and 700 A.D.) demonstrates repeated activation, and therefore the possible reactivation of the westernmost segment, the TGFZ, could be a major threat to the city of Thessaloniki. Changes in the Coulomb failure function (ΔCFF) due to the occurrence of the 1978 earthquake calculated out in this paper indicate that the TGFZ has been brought closer to failure, a convincing argument for future seismic hazard along the TGFZ.
 
Article
The Imperial anticline, about 50 km west of Norman Wells, Northwest Territories, Canada is a fault-bend fold which overlies an unusual thrust ramp, totally dictated by stratigraphy and not at all like those in text books. The lower-flat/ramp/upper-flat geometry of the underlying thrust fault is decreed by the stratigraphic configuration, which is that of a southward dipping, gently flexed, Neoproterozoic Mackenzie Mountains Supergroup (MMS) succession truncated unconformably by a flat-lying Phanerozoic succession. The lower flat and ramp are controlled by a bedding-parallel glide zone in the flexed MMS, whereas the upper flat is controlled by another bedding-parallel glide zone in salt beds in the lower part of the flat-lying Phanerozoic.This study provides new insight into a type of stratigraphically dictated thrust ramp that may well be present in other thrust belts. Angular truncations at unconformities are to be expected at craton margins and stratigraphic configurations similar to the one documented here may be the cause for other large, low-angle thrust ramps.Understanding the Imperial anticline and its underlying thrust permits a logical linkage of an inferred deep detachment in Proterozoic strata beneath the Mackenzie Mountains with the much shallower detachment in Upper Cambrian salt beds beneath the northern Franklin Mountains. Represented on the seismic lines are at least 1.5 km of basal MMS strata that do not outcrop in Mackenzie Mountains. They probably correlate with the Pinguicula Group that is exposed in the Wernecke Mountains about 250 km to the west.
 
Article
We document the structural context of the 1999 Chi-Chi earthquake (Mw=7.6) in western Taiwan, which is one of the best-instrumented thrust-belt earthquakes. The main surface break and large slip (3–10 m) is on two segments of the shallow otherwise aseismic bedding-parallel Chelungpu–Sanyi thrust system, which shows nearly classic ramp-flat geometry with shallow detachments (1–6 km) in the Pliocene Chinshui Shale and Mio-Pliocene Kueichulin/Tungkeng Formations. However, rupture is complex, involving at least six faults, including a previously unknown deeper thrust (8–10 km) on which the rupture began. We compare the coseismic displacements with a new 3D map of the Chelungpu–Sanyi system. The displacements are spatially and temporally heterogeneous and well correlated with discrete geometric segments of the 3D shape of the fault system. Geodetic displacement vectors are statistically parallel to the nearest adjacent fault segment and are parallel to large-scale oblique fault corrugations. The displacement magnitudes are heterogeneous at several scales, which requires in the long term other non-Chi-Chi events or significant aseismic deformation. The Chelungpu thrust has a total displacement of ∼14 km but the area of largest Chi-Chi slip (∼10 m) is on a newly propagated North Chelungpu Chinshui detachment (∼0.3 km total slip) which shows abnormally smooth rupture dynamics.
 
Article
The northern segment of the Chelungpu Fault shows an unusually large co-seismic displacement from the event of the Mw 7.6 Chi-Chi earthquake in western Taiwan. Part of the northern segment near the Fengyuan City provides an excellent opportunity for characterizing active thrust-related structures due to a dense geodetic-benchmark network. We reproduced co-seismic deformation patterns of a small segment of this Chelungpu Fault using 924 geodetic benchmarks. According to the estimated displacement vectors, we identified secondary deformations, such as local rigid-block rotation and significant shortening within the hanging wall. The data set also allows us to determine accurately a 3D model of the thrust fault geometry in the shallow subsurface by assuming simple relations between the fault slip, and the horizontal and vertical displacements at the surface. The predicted thrust geometry is in good agreement with borehole data derived from two drilling sites close to the study area. The successful prediction supports our assumptions of rigid displacement and control of displacement in the hanging wall by the fault geometry being useful first approximations.
 
Article
Surface ruptures associated with the 1999 Mw=7.6 Chi-Chi earthquake in central western Taiwan have been characterised by mapping along the northern fault-segment. The earthquake occurred on the reactivated Chelungpu fault in the frontal portion of the thin-skinned Taiwan fold-and-thrust belt. The N–S trending Chelungpu fault is a 90-km-long major west-verging thrust, which principally slips within, and parallel to, bedding of the Pliocene Chinshui Shale. In the northern segment of the earthquake fault trace, that we name the Shihkang–Shangchi fault zone, the surface ruptures turn to an E–W strike and produce a series of thrust-and-backthrust pop-ups, about 15 km long, forming several discontinuous subsegments distributed within a broad regional Pliocene syncline.
 
Symbols used in the text.
(a) The granite wall at Erd osmecske. (b) Digitized fracture network of the wall. 
Fracture length distribution for the Erd} osmecske granite wall at meso-scale. Misfit is caused by diminishing detection for small values and censoring at large values. Inset: Histogram of fracture lengths prior to logarithmic transformation.
Article
To simulate a suitable fracture network for hydrogeological modelling, input statistical data of the individual faults, as well as fracture sets, should be determined first using either 2D sections or 1D scanlines. Although the accuracy of this measurement is fundamental, exact determination is rather problematic and is usually possible only at a particular scale. This paper introduces a coupled method for computing length exponent (E) and spatial density (Dc), the two most essential parameters for modelling fracture networks. To calculate the length exponent, data sets of at least two independent imaging processes are needed. Utilizing different sensitivity thresholds of the two methods and the well-known analytical form of a fracture length distribution function, its parameters can be calculated. To estimate the spatial density of fracture centres in 3D, the series of intersections should be analysed as a fractional Brownian motion and then calibrated with virtual wells simulated with optional modelling software. The method makes fracture intensity logging possible along scanlines. Based on these approaches, there is no need to import fracture parameters from the outcrop survey or from other parts of the reservoir, because all geometric information of the fracture system refers to the rock body under examination. Using site-specific parameters makes fracture network modelling more reliable.
 
Article
A major earthquake (M=6.6) occurred on 21 June 2000, in South Iceland. This paper presents an unusual example of left-lateral strike-slip displacement recorded in a newly asphalted car park surface through a mechanically consistent pattern of open fissures and pressure ridges resulting from simple shear and rotation. Measurement of these features allows accurate reconstruction of the local deformation. The behaviour of the asphalt layer resembles that of analogue physical models, especially in terms of rotations induced by shear deformation. It is finally shown that through a wide range of scales some basic patterns associating rotation and opposite senses of strike-slip exist in the South Iceland Seismic Zone.
 
Article
Field investigations reveal that the surface rupture of the 2008 Mw 7.9 Wenchuan earthquake, China, occurred along a pre-existing shear zone in the Longmen Shan Thrust Belt. Structural analyses of the coseismic fault zone and fault rocks show that i) the main coseismic shear zone consists of a fault core that includes a narrow fault gouge zone of <15 cm in width (generally 1–2 cm) and a fault breccia zone of <∼3 m in width, and a wide damage zone of >5 m in width that is composed of cataclastic rocks including fractures and subsidiary faults; ii) the foliations developed in the fault core and damage zones indicate a dominantly thrust slip sense, consistent with that indicated by the coseismic surface rupture; and iii) coseismic slip was largely localized to within a narrow fault gouge zone of <2–3 mm in width. The structural characteristics of the coseismic shear zone and cataclastic rocks indicate that the location of coseismic slip zone associated with the 2008 Wenchuan earthquake was controlled by a pre-existing shear zone and that the main active fault of the Longmen Shan Thrust Belt has moved as a thrust since the formation of cataclastic rocks along the fault during the late Miocene or early Pliocene.
 
Assembly of allochthons within allochthons of the Martinsburg / Dauphin foreland. Read top to bottom. Dates are generalized from figure 2 and expanded on figure 4. Pre-Taconic Late Cambrian and Early Ordovician deep-water units became allochthons embedded in Mid-Ordovician deep-water, offshore clastic matrices. With the start of Phase I, these were moved onto and across the Early to Mid-Ordovician platform, some picking up piggy-back cover enroute. These became embedded in new Mid-to Late Ordovician matrices before final emplacement as the two contrasting, major allochthonous sheets comprising this area's Dauphin Formation. After the final emplacement, the sheets were conformably covered by thick Martinsburg flysch. (After Wise and Ganis, fieldtrip guidebook, NE GSA, March 2006). The discrimination of the various major allochthons was based on work in Dauphin, Lebanon, and western Berks counties, Pennsylvania, or the western part of the Dauphin/Martinsburg foreland segment. Other minor allochthons may be identified with further detailed mapping and the allochthon emplacement history in the eastern part of the Dauphin/ Martinsburg foreland segment needs confirmation.
Dating Taconic events. This figure, an enlargement of the bottom of figure 3, uses the more precise dates of figure 2 to expand the interpretations with details not possible in the limited space of figure 3.
Schematic cross-section of the Martinsburg/Dauphin foreland along line A-B of figure 1. Faded geometry is above present land surface. The Alleghenian-age Yellow Breeches thrust juxtaposes two strongly overturned to recumbent Taconic anticlinoria. On the north, diagenetic to anchizone metamorphic grade Martinsburg Formation plus two major allochthonous sheets of the Dauphin Formation are repeated across a strongly overturned anticlinorium. The anticlinorium is broken by a relatively small Alleghenian-age thurst that brings up core rocks of the Cumberland Valley carbonate sequence, a western facies. On the south, the Mesozoic basin separates the more-than-recumbent lower limb of a greenschist-grade anticlinorium, into a Cocalico North and Cocalico South portion. Reconnaissance shows Dauphin-like allochthons in both Cocalico areas but details necessary to map a distinct Dauphin-like Formation are not yet available. Here, the core rocks are part of the Lebanon Valley carbonate sequence, an eastern and southern facies. Some type of facies change is required beneath the master thrust.
Graptolites preserved as external molds, tectonically distorted to varying degrees, from the Cocalico North Formation. This figure represents the first clearly documented assessment of graptolite fossils from the Cocalico (see also note *10 in Appendix). It is part of the primary evidence for this paper's correlations of the Martinsburg and Cocalico. Location is roadcut on Chambers Street, Steelton, Pennsylvania (40° 13 30.85; 76° 49 26.80). Each outline has a 1mm bar for scale. (A) a climacograptid, U.S. N.M. 535000; (B) Orthograptus cf. calcaratus (Lapworth), U.S.N.M. 535001; (C) an orthograptid, highly stretched, U.S.N.M. 535002; (D) a glossograptid, U.S.N.M. 535004; (E) Hallograptus sp., U.S.N.M. 535003. U.S.N.M. is United States National Museum, Washington, D.C.
Article
In Pennsylvania, the Taconic Orogeny lasted from ∼461 to ∼443 Ma as Cambro-Ordovician slope deposits were deformed into mountains edging the Laurentian craton at the same time that materials from an adjacent deep-water basin were being transported ∼50 –70 km across a carbonate platform into foreland basins. This paper focuses on shelf-edge hinterland features, mostly the Martic Zone as a folded, stack of imbricate thrust sheets of slope materials that corresponds to Vermont's Taconic Mountains and Southern Quebec's zone of Taconic allochthons. Work of the last century is summarized, corrected, and combined with a new ∼450 Ma radiometric date and fluid inclusion data from the Pequea Mine within the Martic Zone. These and abundant new graptolite and conodont dates in the foreland paint a revised Pennsylvania picture differing from the northern Taconic areas. Differences are: (1) transport of very large allochthonous masses of deep-water material, the Dauphin Formation, far across the carbonate platform, and (2) deformation migrating progressively across that platform during a ∼15 –20 m.y. period, incorporating it and its foreland cover into alpine-scale, recumbent folds and thrusts. The scenario has many analogies to Italy's modern Apennine Mountains minus the Latian volcanics.
 
Article
Displacement distributions along fault surfaces are a record of the processes of fault nucleation, slip, linkage, and propagation. Because several disparate processes can collectively influence the fault displacements, the relative contributions of these processes can be challenging, but are therefore necessary, to decipher. Continued advances in the mechanics of discontinuous slip surfaces in rock masses, when combined with appropriate field studies, should determine the roles of cohesive end zones, fault interaction, and propagation direction in shaping the displacement distributions. Studies of faults on other planetary surfaces provide a window into the role of a broad range of environmental conditions that can influence the faulting process. More inroads need to be made into traditional strain-based classes in structural geology so that mechanically sound concepts of fault analysis can become better utilized in the curriculum and by the non-specialist geological community.
 
Article
Near the eastern end of the Tonale fault zone, a segment of the Periadriatic fault system in the Italian Alps, the Adamello intrusion produced a syn-kinematic contact aureole. A temperature gradient from ∼250 to ∼700 °C was determined across the Tonale fault zone using critical syn-kinematic mineral assemblages from the metasedimentary host rocks surrounding deformed quartz veins. Deformed quartz veins sampled along this temperature gradient display a transition from cataclasites to mylonites (frictional–viscous transition) at 280±30 °C. Within the mylonites, zones characterized by different dynamic recrystallization mechanisms were defined: Bulging recrystallization (BLG) was dominant between ∼280 and ∼400 °C, subgrain rotation recrystallization (SGR) in the ∼400–500 °C interval, and the transition to dominant grain boundary migration recrystallization (GBM) occurred at ∼500 °C. The microstructures associated with the three recrystallization mechanisms and the transitions between them can be correlated with experimentally derived dislocation creep regimes. Bulk texture X-ray goniometry and computer-automated analysis of preferred [c]-axis orientations of porphyroclasts and recrystallized grains are used to quantify textural differences that correspond to the observed microstructural changes. Within the BLG- and SGR zones, porphyroclasts show predominantly single [c]-axis maxima. At the transition from the SGR- to the GBM zone, the texture of recrystallized grains indicates a change from [c]-axis girdles, diagnostic of multiple slip systems, to a single maximum in Y. Within the GBM zone, above 630±30 °C, the textures also include submaxima, which are indicative of combined basal 〈a〉- and prism [c] slip.
 
Article
The results of a detailed microstructural and microchemical study of a slate from the Chewton area, Victoria, Australia are reported in which it was found that cleavage developed from microcrenulations. Phyllosilicates in P-domains differed in their chemistry from those in Q-domains and were concentrated in the P-domains by localized metamorphic reactions which were aided by mechanical rotation and solution processes. Wide, dark cleavage lamellae, often taken as evidence for the passive concentration of insoluble minerals, appear to have had a similar origin. The P-domains exhibited two types of microstructure, one which was consistent with annealing and the other with deformation within the domain. Cataclasites had formed along some P-domains, suggesting that they had become preferential sites for late phase brittle deformation.
 
Article
Fault geometry is a primary control on hanging wall deformation. In order to examine their geometrical relationships, a positive inversion analogue experiment was conducted using a rigid fault surface of listric geometry. The hanging wall deformation observed on a representative vertical section was examined with conventional 2D geometric models, and was restored to its pre-inversion phase with two techniques. These results suggest that the deformation can be best approximated by inclined simple shearing (ISS). The ISS model can determine the inclination of the apparent shear plane and the amount of apparent horizontal shortening, which is equivalent to that calculated with the conventional depth-to-detachment method. This estimated apparent shortening was generally smaller than the actual amount of the experiments, probably due to tectonic compaction.
 
Article
A mathematical model for rigid inclusions with a slipping interface immersed in a general 2D homogeneous deformation is developed. Under bulk pure shear inclusions are expected to rapidly approach the stretching axis when compared to the behaviour of inclusions with no slip at the interface. The derived model predicts synthetic and antithetic motion into a stable orientation under simple shear, and thereafter the inclusion makes an antithetic angle with the shear direction. Under simple shear rotation rates can be higher or lower than those of no-slip inclusions, depending on orientation. A direct relationship between object inclination to the shear direction and the vorticity of the bulk flow is predicted. The model compares well with published analogue and numerical experiments.
 
Article
Jeffery's equations ascribe a theoretical cyclic nature to the shape fabric of non-interacting rigid particles immersed in a viscous fluid undergoing simple shear flow. This theoretical behaviour is confirmed at ‘low’ shear strains (γ < 6) by two-dimensional experiments in a torsion apparatus, inducing shape fabric development of particles evenly distributed on the surface of a silicon fluid and at low particle concentrations (13–14% in area). For larger shear strains however (6 < γ < 20), the shape fabric orientation tends to remain close to the shear plane, its magnitude remains at low values and the cyclicity of the fabric disappears. This is due to interactions between particles, forming tiling features with variable shape ratios. Interactions rapidly increase in number for γ > 5 (first experiment: 134 identical particles) or γ > 1 (second experiment: 178 particles with two size classes), then become stable at 17% (first experiment) and at more than 50% (second experiment) of the population of particles. Due to the contribution of the tiled particles, the shape fabric becomes asymmetrical in its orientation distribution, with a maximum lying above the shear plane. The latter result provides a new shear sense indicator, in addition to the statistical determination of the tiled features. The study also suggests that crystalline fabrics in magmas could be acquired at high melt fractions, i.e. early in the crystallization history of the magma.
 
Article
A solution for the deformation of a non-rigid viscous elliptical inclusion in a matrix of differing viscosity is developed for the case of a general 2D deformation. A Newtonian rheology is assumed and velocities and stresses are equated at the boundary. An important parameter is the viscosity ratio given by the ratio of the external to the internal viscosities. The dynamics of the behaviour of such inclusions is examined for the cases of pure and simple shear and variable viscosity ratio. In general less viscous inclusions tend to accumulate finite strain more rapidly than more rigid inclusions. Large discordancies between the internal finite strain ellipse orientation and the bulk external finite strain ellipse are to be expected. It is also found that the kinematics of deformation inside an inclusion can often be one of super shear (i.e. kinematic vorticity number, Wk, greater than one) even though the external bulk kinematics is one of pure or simple shear (Wk=0 or 1). Objects tend to continuously rotate (the viscosity ratio must be less than 0.5) or asymptotically rotate (i.e. tend to ultimately align parallel to a fixed direction). This solution has many applications, some of which are briefly considered.
 
Article
In the Nankai accretionary prism and its associated slope sediments early (pre-lithification) mechanical modification of mud induces preferred alignments of elongate or platy particles and the loss of intergranular porosity. Generic types of particle alignment include: 1. particles having long axes aligned in the plane of bedding, most likely as a consequence of burial compaction; 2. diverse bioturbation structures including alignments parallel to burrow walls, burrows filled with obliquely aligned phyllosilicates, and blotchy disruption of bedding; and, 3. planar deformation bands showing parallel alignments of both silt- and clay-size particles.Subtle compositional contrast between deformation bands and host rocks is consistent with loss of intergranular micropores within bands and supports the dominance of mechanical over chemical processes in their formation. Field-emission SEM imaging of Ar-ion-milled cross-sections shows that collapse of larger (>2 μm) pores, many localized at the margins of silt-size particles, reduces porosity within the bands by about 5 percent compared to the adjacent host rock. Despite the clear role of shear, evidence for particle comminution is equivocal.These observations on mechanical processes in early diagenesis provide useful context for interpretation of pore types and fabric anisotropies in mudrocks across a wide range of subsurface conditions.
 
Article
Historical and archaeological data are used to test geological claims that, in the fourth to sixth centuries AD, the Eastern Mediterranean experienced an unusual clustering of destructive earthquakes (the ‘Early Byzantine Tectonic Paroxsym’). A review of historical accounts of a notable earthquake at this time, that of 21 July AD 365, indicates that this event destroyed nearly all the towns in Crete and was followed by a tsunami which devastated the Nile Delta. The AD 365 event was also probably responsible for reported or observed destruction in ancient towns of west Cyprus and Libya. This earthquake is most likely to be identified with a Hellenic Arc subduction-zone event of ‘great’ (M>8) magnitude, as testified by up to 9 m of uplift in western Crete dated by previous geological studies to around this time. Historical and archaeological data also support the hypothesis that the fourth to sixth centuries AD was a period of abnormally high seismicity in the Eastern Mediterranean. The high seismicity rates of this period may reflect a reactivation of all plate boundaries in the region (Dead Sea Transform, East Anatolian Fault, North Anatolian Fault, Hellenic Arc, Cyprus Arc Fault).
 
Article
40Ar/39Ar dating methods have been applied to rocks across the Caledonian orogen in southwestern Norway. In the eastern nappe area, K-feldspar thermochronological modeling, microfabric characteristics and conodont color alteration together indicate that temperatures were not above the closure temperature of muscovite for any significant period of time. Two groups of muscovite and biotite ages from this area (415–408 for most samples with top-to-the-SE fabrics and 402–394 for samples with top-to-the-NW fabrics) are therefore interpreted as ages of contractional (thrusting) and extensional (hinterland-directed nappe translation) deformation, respectively. In the west (hinterland), peak Caledonian temperatures were higher, and 40Ar/39Ar plateau ages are generally interpreted as cooling ages. The western basement and lower nappes cooled rapidly through ∼500°C (basement) at ∼404 Ma and 350°C (basement and lower nappes) shortly after, i.e. during extensional top-to-the-NW transport of the orogenic wedge. In addition, tectonostratigraphically higher nappes in the hinterland show evidence of earlier cooling, probably following Ordovician orogenic activity prior to the main collisional stage. The new 40Ar/39Ar data conform to kinematic observations that contraction and extension in the Caledonian nappe region were sequential, and that the change from contraction (convergence) to extension (divergence) was quick (between 408 and 402 Ma).
 
Article
The ability to quantify feldspar microstructure using the electron backscatter diffraction (EBSD) method has direct application in the study of rock deformation and strain kinematics. However, automated EBSD analysis of low symmetry phases, such as feldspar, has previously proven difficult. Here, we successfully apply the EBSD method to a number of granitic feldspars and develop automated phase and orientation mapping to discriminate K-feldspar and plagioclase, and quantify orientation variations within individual K-feldspar grains. These results represent the first automated quantitative mapping of orientation microstructure in K-feldspar. We use the method to evaluate the relationship between microstructure and 40Ar/39Ar age, a controversial problem in thermochronology. In a granitic K-feldspar from central Australia, the range of observed orientation domains matches the small to intermediate and largest domain sizes predicted from multiple-diffusion domain modeling. In situ ultra-violet laser microprobe analyses show that the youngest ages from the 40Ar/39Ar age spectra are recorded by grain mosaic K-feldspars with diameter around 10–50 μm. These K-feldspars are the smallest coherent microstructural features observed on scales of >1 μm. Large 250–1000 μm diameter microstructurally simple grains record the oldest ages observed in the age spectrum. These results suggest a first order relationship between K-feldspar microstructure and 40Ar/39Ar age and demonstrate a microstructural control on multidomain diffusion.
 
Article
To advance our understanding of the kinematics of the Xuefengshan tectonic belt (XFSTB) and the Mesozoic tectonics of the South China Block (SCB), this paper presents new data and interpretations from our structural studies and thermochronological dating. The XFSTB is characterized by greenschist-facies metamorphism and development of S and S–L tectonic fabrics. An asymmetric positive flower structural pattern is composed of NW–WNW- and SE–ESE-dipping cleavages, faults and shear zones. Kinematic indicators indicate a dominant top-to-WNW-thrusting with a sinistral strike-slip component on the ESE-dipping shear zones and top-to-ESE-back-thrusting on the WNW-dipping shear zones. The quartz c-axis orientations of mylonitic rocks exhibit monoclinic point-maximum asymmetry, indicative of a sinistral shear sense under the 〈a〉 basal gliding condition. The timing of the major deformation event has been constrained to the middle Triassic to early Jurassic (244–195 Ma) on the basis of 40Ar/39Ar geochronology and other geological observations. The structural characteristics of the XFSTB are probably related to an Indosinian oblique convergent zone, in combination with tectonic wedging and associated back thrusting, all above a low-angle, SE-dipping basal detachment. Oblique northwestward and upward movement along the basal detachment was partitioned into the NW–WNW-directed thrusting, SE–ESE-directed back-thrusting and subsidiary strike-slip movement along the NE- and NNE-trending faults and shear zones. The XFSTB may represent part of a huge structural fan between the Yangtze and Cathaysian blocks, and can be interpreted as a product of the Indosinian intracontinental collision involving a weak zone.
 
Article
Paleozoic to Oligocene metasedimentary rocks present in the Alpi Apuane region of the Northern Apennines, Italy, have been sequentially deformed during a Tertiary progressive deformation. In an attempt to date the individual deformation episodes, over 50 conventional K-Ar and 1140 Ar/39Ar incremental gas release analyses have been carried out on fine grained white micas separated from samples whose structural settings were well known. Mineralogy, X-ray diffractometry, and thin-section analyses indicate that the constituent muscovite and phengite formed under metamorphic conditions of 3–4 kbars and 300–400°C during all deformational phases. Pre-existing micas were variably crenulated during each subsequent deformational phase. Both K-Ar and 40Ar/39Ar analyses were carried out on 0.6-2μm, 2–6 μm and 6–20 μm size separates of the phengitic white mica. Although the K-Ar apparent ages range from 11 to 27 Ma and are consistent with available stratigraphic constraints, the 40Ar/39Ar age spectra display variable internal discordancy. These isotopic data indicate that: (1) both the K-Ar and 40Ar/39Ar total-gas ages decrease as the degree of crenulation increases; (2) the K-Ar and 40Ar/39Ar total-gas ages decrease as grain size decreases; (3) for each sample, characteristics of the 40Ar/39Ar age spectra depend upon grain size, with fine sizes yielding discordant patterns which systematically increase in apparent age from low to high temperature and (4) phengitic micas associated with earliest structures yield generally older ages than micas associated with later structures.
 
Article
40Ar/39Ar dating of syn-kinematic white mica (phengite) and biotite is used to constrain the age of shear zones within the granodiorites of the Grimsel Pass area (Aar Massif, Central Alps). Three sets of steeply dipping shear zones have been mapped. Stage 1 shear zones are very broad (∼200 m wide) zones of biotite-bearing rocks, with a progressive increase in intensity of ductile deformation from rim to core, and have dip-slip kinematics. Subsequent Stage 2 shear zones are narrower, contain phengite-rich mylonites, and show a gradual change from dextral strike-slip to dip-slip kinematics across strike. Strain localisation during the formation of Stage 2 shear zones is interpreted to have been aided by reaction-weakening during extensive fluid–rock interaction. Brittle precursors of Stage 2 shear zones are marked by biotite-rich cataclasites preserved along the northern contact of the overprinting main Stage 2 shear zone.Biotite from a Stage 1 shear zone yields an age of 21.1 ± 0.2 Ma. Phengites in Stage 2 mylonites in the Grimsel Pass area have a very narrow age range (13.8–12.2 Ma), which is interpreted to bracket the duration of ductile deformation during Stage 2 deformation at mid-crustal levels. However, the biotite-rich cataclasites at the margin of a Stage 2 shear zone may have localised fluid flow up to ∼2 Ma prior to the onset of main Stage 2 ductile deformation. Stage 3 shear zones are brittle strike-slip faults containing cataclasites, breccias and clay fault gouges. Earlier studies have established that these late faults formed at <9 Ma and were active until at least 3 Ma.
 
Article
NE-trending, sinistral ductile shear belts in the southern part of the Tan–Lu fault zone, East China are exposed along the eastern edge of the northern Dabie belt and southern Zhangbaling belt. Three hornblende, three phengite and 12 biotite separates from greenschist facies mylonites in the shear belts were dated by the 40Ar/39Ar method in this study. A hornblende plateau age of 143.3 Ma is interpreted as representing the time of sinistral motion on the Zhangbaling part of the Tan–Lu fault zone. Seven biotite samples from the same part yield 40Ar/39Ar cooling ages from 137 to 125 Ma. A phengite plateau age of 138.8 Ma from the eastern edge of the Dabie belt is considered as being close to the time of deformation in the Tan–Lu strike-slip zone. Five biotite 40Ar/39Ar ages from 120 to 110 Ma and one phengite age of 121 Ma from the same area represent cooling ages after the sinistral faulting. It is concluded on the basis of these 40Ar/39Ar ages that the sinistral ductile shear belts in the southern part of the Tan–Lu fault zone were produced in earliest Early Cretaceous. This sinistral motion on the Tan–Lu fault zone is considered to be related to high-speed, highly oblique subduction of the Pacific plate at this time.
 
Article
Understanding the tectonic evolution of orogenic belts and intracratonic areas depends on our ability to determine the age of tectonic features on a variety of scales. This study demonstrates the value of the laser-probe 40Ar/39Ar dating technique, which, if applied to fault-derived pseudotachylites, may be used to directly determine the age of brittle faults. The laser-probe technique affords high spatial resolution, enabling a greater opportunity for discriminating between pseudotachylite matrix, host-rock clasts and alteration products that are often present in varying proportions within pseudotachylites. The laser-probe 40Ar/39Ar technique has been applied to pseudotachylite samples from the Tambach Fault Zone (TFZ), a major NW–SE trending strike-slip fault within the Kenyan part of the Late Proterozoic/Early Palaeozoic Mozambique Belt. The pseudotachylites of the TFZ were previously thought to have formed either (i) at about 530–430 Ma, or (ii) during the Cenozoic evolution of the Kenya Rift. In the latter case, seismic slip on the rift-bounding normal fault would have generated the pseudotachylites, due to the reactivation of old NW–SE trending structures in the basement. Based on our new data, we interpret the pseudotachylite formation age to be 400 Ma. This rules out the possibility that the pseudotachylites are related to the formation of the Kenya Rift. Although the inherited basement faults may have been locally reactivated as transfer faults, reactivation of these structures during rifting did not occur beyond the margins of the Kenya Rift.
 
Article
The geometry and kinematic evolution of small growth faults were analysed from a high-resolution 3D seismic dataset located at the margins of the Levant Basin, in the eastern Mediterranean. The 3D geometry, segmentation history and throw distribution of one particular fault was reconstructed to evaluate and illustrate the changes in dimension and displacement distribution that occurred during the transition from purely blind propagation to propagation at the free surface. The fault is considered to have grown by blind radial propagation of three main segments that hard-linked prior to surface interaction. The fault subsequently reached the seabed and continued to accrue displacement as a syn-sedimentary fault. Most of the fault surface area formed during the blind propagation phase, but most of the displacement was added during the syn-sedimentary phase of the growth history with little increase in surface area. The interaction of the fault with the free surface led to a change in the position of the point of maximum displacement as well as modifying the vertical throw distribution. The amount of displacement added after this transition from blind fault to growth fault is discussed with respect to existing fault-growth models.
 
Article
Growth of a 12 km long, deepwater anticline during the late Pliocene–Recent is documented from 3D seismic reflection data across NW Borneo. The fold is part of a train of folds formed along the slope at the distal margin of the Baram Delta Province. Growth of the anticline involved fold lateral propagation and linkage of two thrusts formed in the anticline forelimb as either break thrusts or as imbricates ramping up from a master detachment (at ∼3 km depth). For the southwestern anticline, the northern tip of the NE–SW striking, SE-dipping thrust passes into an E–W striking oblique termination, which lies in the linkage zone between the two anticlines. The thrust termination is characterised by the following changes passing east towards the fault tip: 1) the fault zone dips steeply to the south, then 2) passes to a vertical segment (inferred to have oblique motion), and 3) furthest east the fault dips northwards with an extensional component of displacement. The fault zone terminates in a transtensional graben. This graben does not fit with a simple pull-apart geometry or simple oblique ramp geometry. In the future if the thrust faults propagate together and link this oblique fault zone may develop into an oblique ramp that acts as a transfer zone between the faults. However at present the oblique fault zone appears to be a region of 3D strain, where deformation at the fault tip, and the gravity effects of plunging folds have affected the shallow, weak sediments and given rise to a complex thrust termination at a early stage of thrust and fold development. The oblique structure may have developed in response to strains imposed by the encroachment of the fold and thrust belt on an uplifted basement or volcanic high that forms a pronounced topographic feature across a narrow part of the thrust front, 14 km NW of the most external (oceanward) fold.
 
Article
The basal units of the allochthonous complexes of NW Iberia represent a fragment of the external edge of Gondwana subducted and subsequently exhumed during the Variscan collision. The structural analysis carried out in orthogneissic massifs of the southern part of one of these, the Malpica–Tui Unit, reveals the generation of recumbent folds and associated axial planar foliation during their exhumation. These folds nucleated in irregular igneous bodies that were initially deformed during the subduction event and show east to southeast vergence. Down-plunge projection of surface data and a series of regularly spaced cross-sections have been used to build 3D models of the two main bodies of orthogneiss, of calc-alkaline and peralkaline compositions respectively. The first is presently a lens-shaped body folded in a recumbent syncline, whereas the peralkaline gneiss, also affected by a train of asymmetric recumbent folds in the south, exhibits in the north a fold-like structure which is interpreted as inherited from its primary geometry, that of a ring dike pluton.
 
Article
Penetrative deformation occurred ca. 70 Ma ago throughout the northern Valhalla complex in Valhalla and Passmore domes and in the Gwillim Creek shear zone, exposed at the deepest structural levels in both domes. Intense strain (ST) in the Gwillim Creek shear zone (domain II) was synchronous with and outlasted deformation (D2) throughout the northern complex (domain I). Upper-amphibolite facies peak mineral assemblages define the predominant foliation. Temperature and pressure results, determined from microdomains with established relationships to reaction textures and microstructures, provide constraints on conditions under which deformation occurred. Deformation was synchronous with and outlasted peak metamorphic conditions at all structural levels. Peak conditions of 825°C and 730 MPa and 850°C and 840 MPa were determined for domains I and II, respectively. This was followed by cooling and retrograde garnet breakdown at conditions of 715°C and 490 MPa and 765°C and 730 MPa in domains I and II, respectively. The faster cooling rate per kilometer of exhumation for domain II relative to domain I is consistent with a model of conductive cooling via thrusting of domain II on to a cold footwall. Metamorphism is interpreted to have resulted from crustal thickening and burial to depths of ca. 25 km based on an inferred clockwise P–T path and the paucity of Late Cretaceous intrusions. Lack of retrograde metamorphism throughout the complex and the high degree of annealing of microstructures indicates that the rocks remained above greenschist-facies conditions until they were exhumed in the Early Tertiary on the Valkyr–Slocan Lake extensional shear zone system.
 
Article
The Knipovich Ridge (73°30′–78°40′N) is an extreme end-member of the mid-ocean ridge spreading system, both in terms of spreading rate (<1.5 cm/yr) and angle between the ridge and the spreading direction (40°–53°). Structural analysis of side-scan sonar images obtained along ∼400 km of the ridge axis reveals systematic relationships between fault population parameters, obliquity, and axial segmentation along the ridge.Fault population characteristics conform to observational and experimental analyses of oblique rifting and spreading systems. For the ridge as a whole, faults in the axial region are short, straight, isolated (not linked into complex fault zones), and exhibit length-scaling relationships characteristic of young and active fault systems. Faults are generally oblique to both the ridge axis and the spreading direction, and orientations vary systematically with angle between ridge and spreading direction.Along-axis analysis reveals the influence of axial segmentation on fault population characteristics. Segment centers are dominated by faults perpendicular and sub-perpendicular to plate motion with longer characteristic length and generally lower fracture density. Conversely, segment ends are dominated by faults striking oblique to plate motion with shorter characteristic length and generally higher fracture density. We infer that faulting in segment centers is strongly influenced by the mechanical effects of dike intrusion perpendicular to plate motion, while faulting in segment ends is controlled by the mechanics of oblique rifting, non-transform discontinuities, and/or accommodation zones. The contrasts between these distinct structural and mechanical settings along the ridge axis are accentuated by the high obliquity and ultra-slow spreading rate of this spreading system.
 
Article
The central part of the Aar Granite (Aar Valley) shows lens-shaped domains of low strain separated by anastomosing domains of high strain. A comparable pattern is found on all scales of observation. Deformation gradients are outlined by a gradual change from isotropic granite to orthogneiss and locally ultramylonites. Ultramylonites are concentrated within conjugate ductile shear zones. Where moderate, the strain is concentrated in very narrow small-scale faults which affect relatively undeformed granite. Within foliated domains, finite strain measurements on deformed xenoliths indicate that (i) strain intensity increases gradually with grain size reduction and (ii) finite-strain ellipsoids are of flattening type irrespective of the degree of mylonitization. The pattern of small-scale faulting, the trends of the regional foliation and the orientation of ductile shear zones are consistent with a bulk strain field of flattening type throughout progressive deformation.
 
Article
Anisotropy of magnetic susceptibility (AMS) was recognized as a feature of minerals in 1899, and petrofabric-compatible AMS fabrics were reported from 1942–1958. Shortly thereafter, cleavage and mineral lineation were associated with the principal axes of the AMS ellipsoid. AMS is describable by a magnitude ellipsoid, somewhat similar in concept to the finite strain ellipsoid, with principal susceptibilities (κMAX, κINT, κMIN) as its axes and their average value being the mean susceptibility (κ). Orientations of the AMS axes usually have a reasonably straightforward structural significance but their magnitudes are more difficult to interpret, being the result of mineral abundances and different mineral-AMS. The strain ellipsoid is dimensionless (i.e., of unit-volume) and readily compared from one outcrop to another but the AMS ellipsoid represents the anisotropy of a physical property. Thus, (κ) determines the relative importance of AMS for different specimens, or compared outcrops, or component AMS subfabrics. AMS provides a petrofabric tool, unlike any other, averaging and sampling the orientation-distribution of all minerals and all subfabrics in a specimen. Sophisticated laboratory techniques may isolate the AMS contributions of certain minerals from one another, and of certain subfabrics (e.g. depositional from tectonic). However, suitable data processing of the basic AMS measurements (κMAX, κINT, κ MIN magnitudes and orientations, and the mean susceptibility, κ) may provide the same information. Thus, AMS provides the structural geologist with a unique tool that may isolate the orientations of subfabrics of different origins (sedimentary, tectonic, tectonic overprints etc.).
 
Article
Shallow dip angles (⩽45°) suggested by field observations of continental extensional faults are not predicted by classical isotropic Mohr-Coulomb-Anderson theory. Earthquake data indicate that normal faults exist in the upper crust with dip angles commonly as shallow as 30°. Some structural evidence suggests brittle-normal faulting with dip angles as shallow as 10°. One explanation of the apparent conflict between theory and structural/seismic observations is that intrinsically weak, shallow-dipping pre-existing faults are preferentially reactivated. Any reduction in frictional strength of the pre-existing structure below that of surrounding rock increases the likelihood that such structures will be first breaking when extension is initiated. Enhanced fluid pore pressure on the pre-existing fault reduces the effective strength and can further enhance shallow fault reactivation. An analytical treatment clarifies the roles of geometry, intact/pre-existing fault strengths and fluid pore pressures. Frictional strength ratios of 3 or greater could account for extremely shallow normal faults (dips 10–20°) without consideration of pore pressures in excess of the least principal stress or of principal stress systems rotated away from the gravity vector. Moderate reduction in friction () with respect to wall rock can reduce the dip to 30° and can account for shallow normal-slip earthquakes.
 
Article
Numerous strike-slip fault zones in granitic rocks of the Mount Abbot quadrangle, California, developed from steeply-dipping, subparallel joints. These joints generally were less than 50 m long and were spaced several centimeters to several meters apart. Some joints subsequently slipped and became small faults. Simple fault zones formed as oblique dilatant fractures (splay fractures) linked non-coplanar faults side-to-side and end-to-end. These simple fault zones are as much as 1 km long and accommodated displacements as great as 10 m. Compound fault zones formed as splay fractures linked small faults and simple fault zones. They are as much as several kilometers long and accommodated displacements as great as 100 m. These zones are distinctly different from ‘Riedel shear zones’ and the-mechanics of their formation are unlikely to be described well by Mohr—Coulomb mechanisms. Simple and compound fault zones are composed of non-coplanar segments that join at steps or bends; splay fracture length determines step widths. The longest splay fractures occur along the longest fault zones, allowing step widths to increase as the length and displacement across the zones increase. These findings are consistent with the structure of some active seismogenic faults, and they provide a mechanically consistent, field-based conceptual model for fault zones that grow in basement rocks from a preexisting set of joints.
 
Article
Using refractory inclusions in quartz grains as a marker for the X finite direction (Xf) of the strain undergone by the quartzites it is shown, in our case, that the shape preferred orientation of the quartz grains (Xq). is perpendicular to Xf within the foliation plane and is a secondary lineation. According to that reinterpreted frame, the lattice preferred orientation patterns become ‘normal’, resulting from the dominant activity of the {m} (a) slip systems in a non-coaxial shear regime, and not from [c] slip. Development of Xq is attributed to grain-growth during a post-tectonic thermal event, with a faster rate of boundary migration parallel to [c] and without substantial change in the lattice fabric. TEM data give support to the latter interpretation but do not demonstrate the absence of [c] slip.This example suggests that a close examination of the significance of the grain-shape lineations is necessary in case of high-temperature deformations. Operation of [c] as a dominant slip direction, identified in experimental deformation of high water-content synthetic quartz crystals, is still to be established in natural quartz tectonites.
 
Article
We have investigated the mechanisms responsible for the evolution of excess pore pressures within and beneath a ramping thrust sheet (i.e. fluid flow, porosity compression, and thermal expansion of water) and the sensitivity of pore pressure to a variety of physical parameters (e.g. permeability, thrust sheet velocity, heat flux). Coupled pore pressure and temperature equations were solved numerically in two dimensions using a generalized hydrostratigraphy of North American thrust belts. Because of the lack of either symmetry or a steady-state in this problem, both deposition and thrust loading stages were simulated. The dominant mechanisms controlling pore pressure evolution were fluid flow and compression of pore space by vertical loading; thermal expansion of the fluids was found to be insignificant in generating excess pore pressures at common thrust loading rates. The results indicate that it is possible to generate high pore pressure to lithostatic pressure ratios (λ) within thrust sheets by depositional loading prior to thrusting. High values of λ are generated and maintained during thrust loading for reasonable assumptions about the conditions thought to have existed in thrust belts. Values of λ were not constant throughout the model. The highest λ values tended to concentrate near the surface of the model and within and below the toe of the thrust sheet. The magnitude and distribution of excess pore pressures and λ values were found to be especially sensitive to variations in permeability. Excess pore pressure generation by compression exceeded pore pressure dissipation by fluid flow for permeabilities less than approximately 10−16 m2; permeabilities greater than approximately 10−16m2 produced hydrostatic pore pressure gradients. The models demonstrate that permeability inhomogeneity due to lithologic variations may exert a strong control on the magnitude and spatial distribution of excess pore pressures within thrust sheets. In addition, these models indicate that it is unlikely that fluid pressure is high everywhere in a moving thrust sheet.
 
Article
In recent years work on extensional fault-controlled structures has illustrated the relevance of spatial heave variations along faults in producing structures such as hangingwall synclines. In this note I mention two models which have been described: in one, the fault heave varies and vertical marker lines remain vertical (the ‘modified Chevron model’), whilst in the other, markers perpendicular to the fault remain perpendicular during movement (the ‘slip-line model’). I show that both these models produce large area changes and cannot be used quantitatively unless these spurious effects are removed. I give here the derivation of models which satisfy the same general criteria but conserve area. Some aspects of the amended models are detailed. Both the modified Chevron model and slip-line model predict strains which increase without limit away from the fault plane, and in the slip-line model infinite strains may be predicted in geologically reasonable scenarios.
 
Article
The Batinah mélange which overlies the late Cretaceous Semail ophiolite in the northern Oman Mountains comprises mostly sedimentary rocks of deep-water facies, alkalic lavas and intrusives, all of continental margin affinities, together with smaller volumes of Semail ophiolitic and metamorphic rocks. Four intergradational textural types of mélange can be recognized. Sheet mélange has large (>1 km) intact sheets either with little intervening matrix or set in other mélange types, and with an organised sheet orientation fabric. Slab mélange is finer textured (>100 m) and more disrupted. Block mélange has smaller (> m) blocks with some matrix and a weak to random block fabric. Clast mélange is matrix-supported rudite with a weak depositional clast fabric. Structural relationships, particularly the absence of tectonic fabrics, the decreasing strength of fragment fabrics with increasing fragmentation, and the abundance of brittle fragmentation, suggest that these mélange types formed by either gravity-driven sedimentary processes or superficial sliding or thrusting of individual rock slabs.In the slab mélange, long sequences can be pieced together, passing up from Upper Triassic mafic sub-marine extrusives and sediments into radiolarian cherts, hemipelagic and redeposited limestones, and terminating in non-calcareous radiolarities with Mn-deposits of early Cretaceous age. Mafic sills are numerous. These sequences can be matched with sub-ophiolite rocks now exposed in fault corridors through the Semail. These sequences become progressively disrupted upwards in the corridors and can be traced continuously into overlying mélange, which then thins away from the corridors.We argue that, during late Cretaceous emplacement over the Arabian margin, active fault corridors split the Semail slab and acted as conduits up which sub-ophiolite rocks were supplied to the ophiolite surface. There the rocks were redisributed by superficial processes.
 
Article
Grabens in Canyonlands National Park, Utah, began extending above a layer of evaporites when the Colorado River cut through the overburden. Two-dimensional finite element models simulate the effects of geometry and rock properties on graben configuration and spacing. Only those models having a progressively increasing slope or no slope mimicked the natural upslope graben propagation. Typical rock properties produced the most realistic fault patterns: an initial friction angle of 31°, a cohesion of 1 MPa, and strain weakening comprising cohesion loss and decrease of friction angle to 26°. A tensile stress limit narrowed the grabens and reproduced the vertical upper portion of the natural faults. The viscous salt resisted overburden spreading and controlled its rate. Modeled grabens spread at typical rates of 1–2 mm a−1 for a salt viscosity of 1×1018 Pa s, and the entire system strained at rates from 6.0×10−14 s−1 to 0.5×1014 s−1. The faults bounding a graben formed nearly simultaneously at the top surface and propagated downward. Salt rose beneath the grabens as reactive diapirs. Overburden adjacent to the canyon flexed as salt was expelled and formed an arching horst and graben. A corresponding horst has been found in the field. The model results scale to larger dimensions, except for the steep upper part of the faults. Reduced dimensions create vertical or no faults.
 
Article
This work is a 2D numerical contribution to the problem of fault and fracture interaction in layered rocks, focusing on fracture aperture. We investigate the influence of an underlying normal fault on the aperture of open fractures in bonded multilayers submitted to vertical shortening and horizontal lengthening. The tests are carried out using the finite element code Franc 2D under plane strain conditions. It is first shown that the presence of a straight normal fault affects the aperture of the above fractures. The fractures located in two very local areas near the upper tip of the fault, one in the hanging wall and one in the footwall, tend to open, whereas the neighboring fractures tend to close. The increases in aperture are systematically greater in the footwall than in the hanging wall. Furthermore, the two areas with increased fracture aperture move towards the footwall when the dip of the fault increases. Second, the case of more complex underlying faults with restraining/releasing bends is studied. These models have similar results to those observed in the case of the straight underlying fault, with two areas of increased fracture aperture. The increases in fracture aperture are comparable with the case of the straight fault in the hanging wall, but are larger in the footwall. The contrasting behaviors of fractures described in the experiments are interpreted as a consequence of changes in the stress field in the central fractured layer caused by the presence of the underlying fault. They may provide a guide to explain fluid flow in fault tip areas. Finally, the case of fracture corridors (swarms of closely spaced fractures) is addressed. It is shown that, whatever the characteristics of the underlying fault, the total aperture of a corridor formed by three equally spaced fractures is equal to 1.41–1.69 times the aperture of a single fracture located at the same place in the fractured layer. This strongly suggests that these structures may act as preferential geological drains, with important consequences in terms of fluid flow.
 
Article
This paper discusses the differences in mechanical properties and kinematics between fold-and-thrust belts detaching on evaporitic décollements and those detaching on stronger detachments. Physical experiments are described that model shortening of a thick brittle cover overlying a weak, viscous décollement to gain a better understanding of these differences. We tested the influence of (1) the décollement layer thickness and (2) the presence of a deformable backstop on the hinterland side and of a décollement pinch-out on the foreland side. Because of the very low shear strength of the viscous décollement, folds and thrusts did not propagate according to a piggy-back sequence but by centripetal, back-and-forth propagation. Additional shortening was accommodated by growth of all existing structures. Fold symmetry and thrust vergence varied between experiments. Models confined between two rigid, vertical endwalls always deformed by symmetric folding and thrusting. In models having a deformable backstop and a foreland décollement pinch-out, forethrusting initially dominated, folds were asymmetric, and fault blocks rotated. In models having a thick décollement layer, folds kept growing asymmetrically. Diapirism also depended on the initial décollement geometry. Diapirs formed only in models having a deformable backstop and were restricted to the hanging wall of a fault-related fold. Finally, the implications of such kinematics on the thermal history are discussed as well as the likelihood of hydrocarbon maturation and preservation.
 
Article
The geometry, kinematics and rates of active extension in Lazio–Abruzzo, Italian Apennines, have been measured in order to gain a better understanding of the spatial and temporal variations in fault growth rates and seismic hazards associated with active normal fault systems. We present fault map traces, throws, throw-rates and slip-directions for 17 parallel, en échelon or end-on active normal faults whose 20–40 km lengths combine to form a soft-linked fault array ca. 155 km in length and ca. 55 km across strike. Throw-rates derived from observations of faulted late-glacial features and Holocene soils show that both maximum throw-rates and throw-rate gradients are greater on centrally-located faults along the strike of the array; total throws and throw gradients show similar spatial variations but with weaker relationships with distance along strike. When summed across strike, throw-rates are increasingly high towards the centre of the array relative to summed throws. We interpret the above to suggest that throw-rates have changed in the recent past (ca. 0.7 Ma) from spatially-random fault growth rates (initiating at 2.5–3.3 Ma) to growth rates that are greater on centrally-located faults. We interpret this as evidence for fault interaction producing throw-rate variations that drive throw profile readjustment on these crustal scale soft-linked faults. The results are used to discuss seismic hazards in the region, which are quantified in a second paper in this issue.
 
Article
Spatial variations in the number of seismic shaking events in a given time period (frequency) implied by measured throw-rates associated with active normal faults in central Italy are investigated through comparison with models of fault growth and the historical record of earthquake shaking for the region. Measured offsets of 18 ka glacial features provide a throw-rate database averaged over a time period much longer than the reliable portion of the historical earthquake record for the region (<ca. 1 ka). Throw-rates are greatest in the centre of the studied fault array and show a six-fold decrease from central to distal faults, implying spatial variations in the frequency of seismic shaking events. We attempt to validate these observations by comparing them with throw-rates predicted by a calculation that takes into account the growth patterns of normal fault systems that exhibit well-known scaling relationships between fault length and throw. We also convert the measured throw-rates into a map of implied frequency of seismic shaking events and attempt to validate the map by comparing it with the probably complete records of seismic shaking since 1349 A.D. that exist for 14 towns in the region. Our results confirm that the throw-rate database is compatible with both the record of historical seismic shaking and what we know about the growth of normal faults. The results imply that knowledge of fault scaling parameters and geological determined deformation rates are valuable tools in seismic hazard assessment.
 
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