Lateral Ramp Research Articles

Modelling the spatial distribution of present-day deformation in Nepal: how cylindrical is the Main Himalayan Thrust in Nepal?

SUMMARY The structure of the Himalayas in Nepal is characterized by underthrusting of the Indian lithosphere beneath the chain along the Main Himalayan Thrust (MHT). The MHT is affected by major earthquakes of M > 8 that have ruptured segments several hundred kilometres in length. The study of historical seismicity underlines the existence of a significant seismic gap between the 1905 Kangra and the 1934 Bihar‐Nepal earthquake areas in Western Nepal. This contribution presents a numerical model of the interseismic deformation using the ADELI 2-D finite-element code. A parametric study has been performed to estimate the influence of the topography, the geometry of the flat‐ramp‐flat transition of the MHT and its rheology. The results of these runs are compared along three cross-sections of Eastern, Central and Western Nepal using various recordings of the active deformation of Nepal (neotectonics, microseismicity, GPS velocities, vertical displacement rates deduced from comparative levellings). The results of this study suggest: (1) a small amount of aseismic slip along the southern part of the MHT and (2) an abrupt change in geometry of the MHT between Central and Western Nepal. The stress build-up in Eastern and Central Nepal is produced by coupling of a bend (flat‐ramp transition) of the MHT and a brittle‐ductile transition at the foot of the ramp. In Western Nepal, on the other hand, stress build-up is only induced by a brittle‐ductile transition located along the low-angle part of the MHT. This difference suggests the presence of a lateral ramp on the MHT to allow transition between the Central and Western Nepal geometries of the MHT. This change in MHT geometries between Central and Western Nepal is also clearly marked in the Himalayan relief: an abrupt decrease in altitude and incision between Central and Western Nepal.

Imaging the ramp–décollement geometry of the Chelungpu fault using coseismic GPS displacements from the 1999 Chi-Chi, Taiwan earthquake

We use coseismic GPS data from the 1999 Chi-Chi, Taiwan earthquake to estimate the subsurface shape of the Chelungpu fault that ruptured during the earthquake. Studies prior to the earthquake suggest a ramp–décollement geometry for the Chelungpu fault, yet many finite source inversions using GPS and seismic data assume slip occurred on the down-dip extension of the Chelungpu ramp, rather than on a sub-horizontal décollement. We test whether slip occurred on the décollement or the down-dip extension of the ramp using well-established methods of inverting GPS data for geometry and slip on faults represented as elastic dislocations. We find that a significant portion of the coseismic slip did indeed occur on a sub-horizontal décollement located at ∼8 km depth. The slip on the décollement contributes 21% of the total modeled moment release. We estimate the fault geometry assuming several different models for the distribution of elastic properties in the earth: homogeneous, layered, and layered with lateral material contrast across the fault. It is shown, however, that heterogeneity has little influence on our estimated fault geometry. We also investigate several competing interpretations of deformation within the E/W trending rupture zone at the northern end of the 1999 ground ruptures. We demonstrate that the GPS data require a 22- to 35-km-long lateral ramp at the northern end, contradicting other investigations that propose deformation is concentrated within 10 km of the Chelungpu fault. Lastly, we propose a simple tectonic model for the development of the lateral ramp.

Large‐scale slope failure involving Triassic and Middle Miocene salt and shale in the Gulf of Cádiz (Atlantic Iberian Margin)

AbstractSheets of salt and ductile shale advancing beyond the thrust front of the Gibraltar Arc (Iberian–Moroccan Atlantic continental margin) triggered downslope movements of huge allochthonous masses. These allochthons represent the Cádiz Nappe, which detached from the Gibraltar Arc along low‐angle normal faults and migrated downslope from the Iberian and Moroccan continental margins towards the Atlantic Ocean. Extensional tectonics initiated upslope salt withdrawal and downslope diapirism during large‐scale westward mass wasting from the shelf and upper slope. Low‐angle salt and shale detachments bound by lateral ramps link extensional structures in the shelf to folding, thrusting and sheets of salt and shale in the Gulf of Cádiz. From backstripping analyses carried out on the depocentres of the growth‐fault‐related basins on the shelf, we infer two episodes of rapid subsidence related to extensional collapses; these were from Late Tortonian to Late Messinian (200–400 m Myr−1) and from Early Pliocene to Late Pliocene (100–150 m Myr−1). The extensional events that induced salt movements also affected basement deformation and were, probably, associated with the westward advance of frontal thrusts of the Gibraltar Arc as a result of the convergence between Africa and Eurasia. The complexities of salt and/or shale tectonics in the Gulf of Cádiz result from a combination of the deformations seen at convergent and passive continental margins.

Geomorphic evidence for an emergent active thrust along the edge of the Po Plain: The Broni‐Stradella fault

We present here geomorphic evidence for an emergent active thrust along the northwestern Apennine front about 50 km south of Milano. Fieldwork combined with SPOT image analysis attests to the presence of a ∼35‐km‐long, en échelon, cumulative fault scarp, that cuts E‐W across Quaternary surface deposits from Casteggio to Sarmato. The scarp offsets vertically alluvial fans and terraces emplaced by tributaries of the Po River that flow northward from the Apennines. Incision by such tributaries occurs only south of the scarp. Valleys of smaller streams stand in hanging position. To the northeast, the buried, east dipping Pavia lateral ramp bounds the San Colombano anticline. We interpret the Montebello‐Sarmato scarp to result from the emergence of an active thrust fault dipping south, the Stradella thrust, which splays eastward from that ramp. Total station profiles leveled perpendicular to the Stradella thrust show variable cumulative surface throws ranging from 2 to 25 m. Scarp degradation analysis indicates fairly recent offset of the terrace surfaces (10–100 kyr) suggesting a minimum uplift rate of 0.3 mm/yr. The underlying fault might thus be one of the most active thrust ramps along this segment of the Apennines. N‐S to N15°W shortening is consistent with both the E‐W strike of this thrust ramp and with coeval oblique sinistral motion on the NNE‐SSW trending Pavia lateral ramp. The Vogherese‐Bobbiese earthquake of 1828 (I ≈ IX) south of Casteggio and Broni may have activated one segment of this lateral ramp rather than the Stradella thrust. Other historical events (I > 7) that caused damage in Pavia, Piacenza, Lodi, and Milano in the last 2000 years (290 A.D., 1249 A.D., 1276 A.D., 1473 A.D.), may have occurred on the Broni‐Stradella thrust or on the blind San Colombano and Lodi thrusts farther north. This makes detailed paleoseismological studies and trenching across its trace mandatory.

Kinematic history of the Main Central Thrust zone in the Langtang area, Nepal

This paper describes for the first time evidence for a northeastward brittle extensional movement of the Main Central Thrust (MCT) zone overprinting the southward ductile thrust movement. The MCT zone is the major tectonic boundary between the Lesser Himalayan metasedimentary sequence (LHS) and Higher Himalayan crystalline sequence (HHS) and contributes to the kinematic and tectonic evidence of continent–continent collision in the Neogene. The Langtang area, 50 km north of Kathmandu, is a matter of interest in considering the movement of the Kathmandu Nappe because the general trend of foliation of the HHS and the underlying MCT zone is turned from WNW–ESE to N–S or NNE–SSW. The major tectono-lithostratigraphic units of the MCT zone in the study area are divided into three: the lower and middle units included in the LHS and the upper unit even included in the HHS. Characteristic mesoscopic quartz lenses are developed in the schists of the Lower unit forming asymmetric boudins. This asymmetry indicates an extensional (top-to-the-NE) shear sense. However, a thrust sense of top-to-the-SW is also preserved in microscopic ductile shear bands and mica fish. The extensional shear movement in the Lower unit took place after the major thrust movement in the MCT zone as a negative inversion. The Middle unit contains Ulleri-type augen gneiss (Syabru Bensi augen gneiss) which is widely distributed in the MCT zone in the other parts of the Nepalese Himalaya. Asymmetric clast-tails (or clast-pressure shadows) and asymmetric boudins observed in the Syabru Bensi augen gneiss give a dextral-thrust oblique sense of shear. This dextral component is regarded as movement of a N–S-trending lateral ramp during the overthrusting of the Kathmandu Nappe. The Upper unit above the MCT is commonly made up of kyanite-bearing medium pressure type amphibolite-facies pelitic gneiss. Asymmetric deformational features such as mica fish and shear bands in the pelitic gneisses commonly show a thrust (top-to-the-W) sense of movement. After the emplacement of the Kathmandu Nappe (∼9–6 Ma), brittle extensional shear occurred only in the crystalline schists of the Lower unit presumably associated with layer-parallel slip. This extensional movement in the Lower unit is probably caused by the change in strike of foliations, e.g., from a general trend of E–W in the other areas to N–S as seen in the study area. This is because N–S contraction during continued continent–continent collision produced E–W extension normal to the contraction axis (N–S), thus the relative east-directed movement on normal faults took place only in the N–S trending domain of the MCT zone. Another possibility for the extensional movement is squeezing of the upper part of the LHS between the MCT above and an out-of-sequence thrust or the Main Detachment Fault below.

Large extensional structures developed during emplacement of a crystalline thrust sheet: the Mondoñedo nappe (NW Spain)

The Mondoñedo nappe is a crystalline thrust sheet characterized by large recumbent folds, regional intermediate-pressure metamorphism, synkinematic intrusion of granitoids during nappe emplacement, and an extensional ductile shear zone developed within the nappe during thrusting. A large tectonic window permits the study of the footwall unit, revealing another extensional shear zone contemporaneous with thrusting and a low-pressure metamorphic evolution, in contrast to that of the hanging wall unit. The two main extensional shear zones produced E–W extension parallel to the direction of orogenic shortening and normal to the orogenic structural trend. Furthermore, subordinate N–S longitudinal extension was accommodated by normal faults in the footwall, and some of these faults were used as lateral ramps in late stages of thrusting. The role of the extensional shear zones and faults described is discussed in the context of an evolving orogenic wedge dominated by plate convergence but characterized by large-scale rheological heterogeneities within it. Deep-seated viscous flow, triggered by heat accumulation, seems to account for the horizontal stretching and probable tapering of the orogenic wedge, which was induced by gravitational instabilities due to partial melting and underplating by buoyant continental crust.

Deformation history of the NW salient of the Eastern Ghats Mobile Belt, India

The Eastern Ghats Mobile Belt of India displays fold-thrust belt structure with a prominent salient on its NW margin. The salient consists of Lathore nappe and Turekela klippe that have overthrust the craton to NW. The rocks of the nappes have undergone granulite facies metamorphism and multiple phases of folding corresponding to the Eastern Ghats orogeny, prior to thrusting. As a result the granulites along the thrust are retrograded to amphibolites and the axial traces of the folds have been truncated against the thrust plane. The basal decollement of the fold-thrust belt is represented by the Terrane Boundary Shear Zone that defines the tectonic margin between the craton and the mobile belt. It occurs as a ductile thrust affecting the cratonic basement as well as the mobile belt suggesting that the basement did not behave as a rigid body during thrusting. Thus the study area is comparable with the Caledonide fold-thrust belt. Further, two large lateral ramps namely Khariar and Paikamal lateral ramps have been developed on the decollement at the lithological contact between tonalite–trondhjemite gneisses/granite gneisses and the latetectonic potassic granites of the craton. Fault-bend folds associate with these ramps too. It is suggested that the salient structure of the fold-thrust belt is the combined result of (1) lateral ramps on the decollement and (2) differential displacement along the sole thrust due to lateral variation in detachment strength.

Fault‐related folds and an imbricate thrust system on the northwestern margin of the northern Fossa Magna region, central Japan

Abstract The Nishikubiki Mountains, which are located on the northwestern margin of the northern Fossa Magna region, central Japan, and the area offshore to the north of the mountains are underlain by folded and faulted Neogene and Quaternary sequences. The folds are composed of open, symmetric anticlines or tight, asymmetric anticlines trending north 20–70° east. On the basis of the geometry of the anticlines and growth strata, the symmetric and asymmetric anticlines are interpreted as fault‐bend folds and fault‐propagation folds, respectively. The formation of the anticlines is attributed to the growth of an imbricate thrust system composed of three thrust sheets that developed, from southeast to northwest, mainly in the late Pliocene, early Pleistocene, and middle Pleistocene–Holocene. The horizontal component of the northwestern‐most sheet was estimated to be approximately 1.2 km on the basis of the width of the growth triangle, and the thickness of the sheet at its southeast margin was estimated to be 8.5 km on the basis of area balancing along one of the seismic profiles. The thrust is inferred to extend to a depth of more than 10 km toward the southwest. The three thrust sheets are probably connected by a detachment zone along the boundary between the upper and lower crusts. The anticlines are bounded by the Itoigawa–Shizuoka Tectonic Line (ISTL) to the west and by lateral ramps or tip lines to the northeast. The ISTL possibly continues northward offshore into the Toyama Trough. The structural model proposed in this paper suggests that similar thrust systems are wide spread in the northern Fossa Magna region and that active deformation zones have migrated and switched during the past 2–3 million years along the fold belt.

Ophiolite obduction and the Samail Ophiolite: the behaviour of the underlying margin

Abstract Samail ophiolite emplacement has become a type-example for ophiolite obduction. Despite this, problems and controversy remain with respect to the age of high- P metamorphism, the vergence of structures in deformed rocks beneath the ophiolite, the suprasubduction character of the ophiolite and the P-T conditions of the metamorphic sole. The presence of major, regional-scale NE-facing isoclinal folds and SW- and west-dipping shear zones with top-to-the-NE shear sense in Arabian margin rocks beneath the Samail Ophiolite nappe requires that the margin was not simply passively overridden during obduction of the ophiolite. The development of these folds at 72–76 Ma is at the time the ophiolite is emplaced finally onto the margin (80–70 Ma), accompanied by development of a major shear zone in Saih Hatat (the upper plate-lower plate discontinuity described by earlier workers) at c. 82–80 Ma. Structural scenarios that incorporate these folds and shear zones include lateral escape from a rising buoyant crustal slice along the former subduction interface, back-folding (‘retrocharriage’) associated with major oceanwards-directed underthrusting, or simple underthrusting of the margin by the oceanic realm. Previous models involving craton-directed overthrusting with domal culminations related to deep-seated footwall and lateral ramps are more applicable to the Tertiary structure and Tertiary evolution of the mountain range. Oman ophiolite obduction clearly involves ocean-vergent thrusting within the continental margin platform to slope facies sequences.

Anatomy of an escape tectonic zone: Western Irian Jaya (Indonesia)

The western fold‐and‐thrust belt of New Guinea in Irian Jaya is presently a complex boundary, dominated by the Bird's Head block escape from the collision between the Australian plate and the remnants of volcanic belts carried by the Pacific plate. The escape rate given by geodetic measurements is of the order of 7 cm/yr, and movement is accommodated by a broad shear zone. We analyze this shear pattern using fault slip analyses performed in the field from 1992 to 1996, combined with focal mechanism inversions, moment tensors summations, and surface trace of structures inferred from radar and multispectral satellite images. Time control of the deformation is attained by isotope dating of the recent syntectonic intrusives. The geometry of the macro and microstructures occurred in two stages from the early Pliocene to the Present. The first stage (5 to 2 Ma) is marked by flat‐and‐ramps structures guided by N60°E lateral ramps associated with a N60°E cleavage, affecting the whole of western Irian Jaya. The second stage (2 Ma to Present) shows the collapse of the western fold‐and‐thrust belt and the escape of the Bird's Head and the Lengguru belts along N60°E transtensile faults. In the latter stage, the strike‐slip offset is distributed on the N60°E schistosity zone along which some fracture planes are reactivated as left‐lateral transtensile faults. Shallow earthquakes moment tensors have been inverted for stress and summed to get strain rates to define contrasting structural provinces. The spatial variations in both stress and strain fields from earthquakes and microtectonics show that (1) they are consistent and are assumed to be coeval, (2) they reveal that oblique convergence is partitioned, and (3) they are influenced by the existence of a free boundary. We see no significant rotation of stress axes laterally along the escape zone. Instead, stresses change according to the different orientations of basement structures and thus undergo rapid spatial variations or principal axes permutations. Eastward, in front of the Australian craton, the oblique convergence is partly distributed on reverse and strike‐slip faults. Westward, along the NW part of the craton, we infer the Seram trench is a free boundary which allows material to escape. These results show that the evolution of the escape zone depends on the importance of the geometry of the former margin of Australia (e.g., tilted blocks), which controls the style deformation relative to the regional stress, and the boundary conditions, which control the variation of the state of stress through time.

Magmatic arc fault systems, deformation partitioning and emplacement of granitic complexes in the Coastal Cordillera, north Chilean Andes (25°30′S to 27°00′S)

Three fault systems were responsible for Permian to Late Cretaceous deformation of the overriding plate of the Andean convergent margin in the Coastal Cordillera of northern Chile (25°30′ to 27°00′S). Displacements were linked to crustal growth expressed by the emplacement of a sequence of magmatic arcs. The Tigrillo Fault System, active from Triassic to Early Cretaceous time, was characterized by arc-normal extension with increasing left-oblique extension (transtension) from Early Jurassic to Early Cretaceous time. Stretching of the crust created space for Triassic, Early Jurassic and Early Cretaceous arc basins where epiclastic, volcaniclastic and volcanic sequences accumulated in continental to shallow marine environments. Tabular plutonic complexes were emplaced by roof uplift–floor subsidence that allowed a vertical transfer of material in the crust without significant horizontal extension. The Atacama Fault System was initiated at c. 132 Ma as a (mainly) left strike-slip fault during left-oblique extension of the margin. Elongate, tabular plutonic complexes were emplaced within the Atacama Fault System between c. 132 and c. 106 Ma, again by roof uplift–floor subsidence mechanisms. Ductile–brittle transitions in synplutonic mylonitic rocks of the Atacama Fault System provided the setting for Kiruna-type Fe-apatite, and Fe oxide (with Cu and/or Au) ores. The Chivato Fault System was active as an extensional fault system at the eastern side of the Coastal Cordillera during displacement on the Tigrillo Fault System and later, between c. 125 and c. 93 Ma, as a partitioned left-oblique extensional fault system. In post-Early Cretaceous time the Chivato Fault System was inverted by left-oblique contraction (transpression) when NW-trending transfer faults, some probably reactivated lateral ramps in the Tigrillo Fault System, accommodated clockwise vertical-axis rotations of 35–45°. Contraction inverted the Atacama Fault System and Tigrillo Fault System and was responsible for west-vergent, thin-skinned, fold–thrust deformation in stratified rocks throughout the margin.

Encouraging the extrusion of deep-crustal rocks in collisional zones

AbstractMost unroofing mechanisms invoked for the exhumation of blueschist-plus-eclogite terranes, including corner-flow and extensional collapse of the orogenic wedge, predict steep unroofing paths for the deeply-buried rocks and are applicable only to unroofing from depths within the crust. Many high-P and ultrahigh-P rocks of continental affinity are derived from greater depths than this. Their lack of warming during unroofing, together with indications that they may rest directly on less deeply buried equivalents, are suggestive of shallow unroofing paths similar to those for the subduction-channel model. They are interpreted to have been emplaced by the upward extrusion of coherent slices of continental crust, bounded below by thrust faults and above by normal faults, with unroofing paths essentially reversing the original burial paths.Where continental crust has been subducted into the mantle, upward extrusion is probably driven largely by buoyancy forces, although examples of upward extrusion without subduction into the mantle indicate that buoyancy forces may not be essential. Two features in addition to buoyancy may promote upward extrusion. Slab breakoff may reduce the pull from the descending slab, and subduction-zone geometry may change as a continental margin is dragged into the subduction zone. Both features may promote the extrusion of continental crust at precisely the time at which it has been partially subducted.A close spatial relationship between a lateral ramp and a lobate zone of extruded high-P rocks in the Mesoproterozoic Grenvillian orogen indicates that lateral ramps may be important in localizing extrusion. Lateral ramps disturb the two-dimensional flow, with channelling of material into the region of the lateral ramp as it is extruded. Many exhumed ultrahigh-P terrains are associated with jogs in the trends of orogenic fronts that may reflect the presence of lateral ramps at depth. Ultrahigh-P rocks may be expected to be concentrated at such jogs, and may record the channelling in their deformation history.

Palinspastic restoration of the Anniston transverse zone in the Appalachian thrust belt, Alabama

Transverse zones are cross-strike alignments of cross-strike linking structures (lateral ramps, transverse faults, and displacement-transfer zones) in foreland thrust belts. Cross-strike links constitute one component of a three-dimensional system of fault surfaces, connecting strike-parallel structures (frontal ramps) and detachment flats both geometrically and kinematically. Three-dimensional palinspastic restoration provides the basis to consider causes of cross-strike alignments of cross-strike links into transverse zones, as well as the role of transverse zones in kinematic history. The Anniston transverse zone in the Appalachian thrust belt in Alabama (southeastern United States) exemplifies the types and distribution of cross-strike links within a transverse zone. Palinspastic reconstruction of the Anniston transverse zone relies on matching hanging-wall/footwall pairs of frontal and lateral ramps for geometric balance in three dimensions in palinspastic-map and cross-section views. Cross-strike links within the Anniston transverse zone are systematically distributed across strike, from hinterland to foreland, as domains of lateral ramps, transverse faults, and displacement-transfer zones, in response to variations in depth to basement beneath the thrust belt and variations in thickness of décollement-host weak rocks. Contrasting structural profiles characterize the thrust belt on opposite sides of the Anniston transverse zone as a result of abrupt along-strike changes at the cross-strike alignment of cross-strike links. The Anniston transverse zone is aligned with a northwest-striking basement fault that offsets the boundary faults of the Birmingham basement graben, suggesting kinematic partitioning within the advancing thrust sheets at a stress concentrator. The northwest-striking basement fault separates domains of contrasting structural profiles of basement fault systems, differing elevations of top of basement, and differing thicknesses of the regional décollement-host weak layer in the lower part of the sedimentary succession above basement rocks.

Role of transverse faulting in along-strike termination of Limestone Mountain Culmination, Rocky Mountain thrust-and-fold belt, Alberta, Canada

Surface geology, well data, and ∼100 km of 2D seismic reflection data delineate the 3D geometry of Limestone Mountain Culmination (LMC). The lower lithotectonic package of the culmination is a SE-plunging antiformal stack of four thrust sheets of Paleozoic platform carbonates that is detached from and folds the upper package of NE-verging thrust sheets of Mesozoic siliciclastics, tilting these imbricates toward the foreland. LMC abruptly loses 1.5 km of structural relief along strike in less than 2 km across a NE-striking blind transverse fault that changes from a NW-dipping lateral ramp in the southwest to a steep tear fault in the northeast. It soles into the Brazeau Thrust and merges upward into the roof thrust of LMC. An extension fault that soles into the steepest portion of the transverse fault cuts moderately SW-dipping Mesozoic rocks that drape over the tear fault, thereby juxtaposing Mesozoic siliciclastics against Paleozoic carbonates of the Brazeau sheet. Synchronous thrusting is suggested by preferential imbrication over the forelimb of the antiformal stack, by offset of the roof thrust, and by folded and unfolded thrust segments in the stack. A Precambrian basement structure beneath this transverse fault may have acted as a buttress during the Laramide Orogeny.

Deformation Pattern of the NW Terrane Boundary of the Eastern Ghats Mobile Belt, India: A Tectonic Model and Correlation with Antarctica

Thrusts interspersed with lateral ramp and wrench structures mark the join between Proterozoic Eastern Ghats Mobile Belt and Archaean cratons of the eastern India. The join, referred to as Terrane Boundary Shear Zone, exhibits an arcuate geometry at the NW margin of the Eastern Ghats where the mobile belt shows the presence of northwesterly verging nappes. Based on this, the area has been described to represent the salient part of a fold thrust belt. Juxtaposition of the granulites over the low grade craton is largely attributed to thrusting along the terrane boundary shear zone. As the folds are disoriented and the granulites are retrograded along the thrusts, the terrane boundary shear zone is interpreted to be a retrograde shear zone. The shear zone hosts a number of synkinematic alkali intrusive rocks that constrain the age of thrusting to 1.4 Ga. In the correlative framework of Gondwanaland the terrane boundary shear zone is very likely to be contiguous with the Rayner-Napier boundary of Antarctica.

Strain analysis from calcite e-twins in the Cameros basin, NW Iberian Chain, Spain

Calcite twins and c-axes from sparry grains that fill micro-veins were used to unravel the deformational history of the Cameros Massif, an inverted Mesozoic sedimentary basin located in the northwestern part of the Iberian Chain (Spain). Calcite c-axis and microvein orientations indicate the existence of two extension directions (NW–SE and NE–SW) during the basin formation probably related to the slip along two syn-sedimentary faults. Calcite e-twins were developed only in micro-veins from the frontal ramp area, i.e. in the northeast side of the basin, where the stresses were higher during the basin inversion. Twin analysis yields three different orientations (NW–SE, E–W and NE–SW) for the minimum horizontal extension direction (i.e. Shmax under coaxial conditions), which suggests that the basin inversion was developed through several thrusting episodes, during which the frontal and lateral ramps of each one of these episodes are interchanged. Moreover, the perpendicularity between c-axes and the minimum extension directions suggests that the basin inversion was produced along the same faults that controlled the basin formation. The minimum extension directions deduced from e-twins are also consistent with the shortening directions established previously for the Tertiary evolution of the Iberian Chain, which suggest that the basin inversion involved several tectonic events during the Neogene. These results show the advantage of the combined c-axis and e-twins analysis to unravel deformational and kinematic histories.

Flexural-slip folding in the Meguma Group, Nova Scotia, Canada

Flexural-slip folding has been established for two anticlines in the Meguma Group, Nova Scotia. Flexural slip postdates the main Acadian episode of flexural-flow fold growth and represents a late, brittle-ductile reactivation of the fold belt. Flexural-slip structures are dominated by bedding-parallel movement horizons, but include a linked system of frontal and lateral ramps and conjugate movement horizons. Slip amount calculated from displacement of discordant quartz veins, combined with vein spacing, indicates an average local shear strain of approximately 0.5. However, because flexural-slip occurred when the fold limbs were steep, the shear strain accounts for a small change in limb dip (4–8°). Variation of slip amount with spacing of movement horizons suggests progressive flexural slip was accommodated by continuous initiation of new movement horizons as well as continuing slip on existing movement horizons.The presence of quartz veins along flexural-slip structures together with evidence for ductile deformation coeval with flexural slip suggests that slip episodes were fluid assisted and intermittent, separated by periods of ductile deformation and build up of fluid pressure. Flexural-slip veins in the Ovens Anticline are auriferous and a flexural slip model may be appropriate for other similar Meguma gold deposits.Thrust sheets, up to 10m thick, occurring on a fold limb are interpreted to have originated in flat box fold hinges and to have been placed onto the steep limbs of the chevron folds during the flexural slip episode. Thrusting may have assisted the transformation of a box fold into a chevron.

Braided stream and flood plain architecture: the Rio Vero Formation, Spanish Pyrenees

Early- to middle-Miocene fluvial sandstones of the Rio Vero Formation were studied, in an area around the town of Barbastro, south central Pyrenees Spain. The outstanding quality of outcrops in this area allows a three-dimensional study of architectural elements. Six architectural elements are recognised, described in detail, and interpreted from three key localities. Seven main lithofacies were identified and sub-divided into gravelly, sandy and fine-grained lithofacies. The architectural elements and lithofacies have been combined with a hierarchy of depositional bounding surfaces to fully interpret the evolution of the depositional system at the meso- and macro-scale. Not only the different architectural elements and lithofacies of the complete braided fluvial system, but also the lateral variation of the architectural elements were emphasised in this study. Differential tectonic movements, seasonal climate change, and their effect on vertical and lateral evolution of the area were the main control on basin sedimentation, channel interconnection, palaeocurrent patterns, and consequently the fluvial architecture. The presence of lateral ramp anticlines caused the fluvial system to be laterally restricted, with the main channel-belts being located in the areas of highest subsidence and lowest topography. Intervening topographic highs acted as both flood plains and lateral barriers between the main channel systems. The proposed depositional model comprises broad, low-sinuosity, perennial, but seasonal moderate-energy streams. The sandstone architecture is dominated by channel-fill and sheet sands, and associated simple and more complex bars. Adjacent to the main channel-belts fine-grained sandstones, siltstones and immature paleosols occur. The along-strike relationship between major fluvial systems and their outlets into a foreland basin has important implications for the infill of the basin and the modelling of fluvial systems along mountain belt fronts.

GEOLOGY OF CENTRAL AND EASTERN ELBA ISLAND, ITALY

GEOLOGY OF CENTRAL AND EASTERN ELBA ISLAND, ITALY

The 1968 Inangahua, New Zealand, and 1994 Northridge, California, earthquakes: Implications for northwest Nelson

The 1968 Inangahua earthquake ruptured a west‐dipping, largely blind reverse fault, here called the Rotokohu Fault, that is close to the east‐dipping Lyell Fault at the surface. Most surface rupture was secondary. The Lyell Fault was reactivated west‐side‐up in 1968, in the opposite sense to its long‐term displacement. The west‐dipping Inangahua Fault was also reactivated in the opposite sense to its long‐term offset of west‐side‐up; this rupture may have been related to folding. The southern boundary of secondary surface rupture is a “cross‐basin structure”, a lateral ramp connecting the blind Rotokohu Fault to a blind fault at the western margin of the Grey‐Inangahua Depression. Increase of Coulomb stress at the lateral ramp in 1968 would shorten the time before failure of the continuation of the Rotokohu Fault south of the lateral ramp. In its proximity to another fault with opposite dip, the 1968 earthquake resembles the 1994 Northridge, California, earthquake, in which the blind source fault was beneath another active fault dipping in the opposite direction, uplifting its hanging wall and footwall. The blind, thick‐skinned reverse fault on which the 1968 earthquake occurred is marked at the surface by a monocline with evidence of late Quaternary folding and secondary flexural‐slip faulting. Shortening rates in northwest Nelson are higher in the Murchison Basin than they are farther west, suggesting that the high seismicity from the White Creek Fault to Westport may not be representative of long‐term behaviour and earthquake potential.