East-west Shortening Research Articles

East–west shortening during north–south convergence, example of the NW Himalayan syntaxis

The northwest terminus of the Himalayan range forms a syntaxis limited by two strike-slip faults, the Chaman Fault to the west, and the Karakorum Fault to the east. In between these faults, smaller-scale syntaxes are observed, as the Nanga Parbat and the Kashmir syntaxes. The fold axial traces of these syntaxes trend nearly north–south, revealing a component of east–west shortening in the geodynamic context of the north–south convergence between India and Asia. We constructed a sandbox with two edges oblique to the convergence to test the influence of the NW Himalayan syntaxis large-scale structural geometry on the construction of a wedge and on its evolution. We used sand and microbeads as standard analogues for brittle crustal layers approximately 30 km thick. We show that the obliquity of the edges can generate an east–west shortening in a north–south convergence. The amount of east–west shortening generated by the oblique edges of the box is up to 40% of the amount of the north–south convergence. This shortening is accommodated by oblique motion on transpressive faults parallel to the edges. The amount of strike-slip motion is up to 60% of the convergence vector. We conclude that the obliquity of the edges is sufficient to generate an east–west shortening in a north–south convergence, but not sufficient to generate a north–south axial-trending fold. One experiment, with a free southern boundary, allowed the development of pure strike-slip faults within the sand layer, conjugated to pure shortening on one north–south fold. The east–west component of the oblique motion generated by the obliquity of the faults bordering the NW Himalayan syntaxis could be absorbed by a north–south fold as the Nanga Parbat if the motion on the border faults is pure strike-slip.

Structural evolution of the Agnew–Wiluna greenstone belt, Eastern Yilgarn Craton and implications for komatiite-hosted Ni sulfide exploration

The Agnew–Wiluna greenstone belt in the Yilgarn Craton of Western Australia is a narrow package of complexly deformed Archean supracrustal rocks that hosts two of the world's largest komatiite-hosted nickel sulfide deposits, the Mt Keith and Perseverance deposits. These deposits and several others in the belt are centred on thick lenses of adcumulate-textured komatiite interpreted to represent areas of channelised magma flow. The large nickel sulfide deposits are located in parts of the belt associated with ca 2720 to 2700 Ma felsic volcanism (e.g. the Leinster and Mt Keith nickel camps). In these areas, felsic to intermediate volcanic rocks are intercalated with syn-volcanic massive sulfides of inferred exhalative origin. While these primary magmatic features are clearly first-order controls on the distribution of Ni sulfide deposits in the belt, several regional-scale deformation events have significantly complicated the interpretation of primary stratigraphic relationships. The earliest recorded deformation events (D1,2,3) resulted in an east–west trending greenstone belt with recumbent isoclinal folds and ductile shear zones. Subsequent west-southwest–east-southeast shortening during the D4 event at ca 2666 Ma involved the refolding of the tectono-stratigraphy to produce belt-scale, north- to north-northwest-trending upright folds, a pervasive axial planar schistosity in all rocks, and the present-day steeply dipping, overturned supracrustal sequences, and emplacement of granitoids in major antiformal fold hinges. Polyphase folding of supracrustal rocks produced Type 2 fold interference patterns with multiple facing reversals at various scales across the belt. West-southwest–east-southeast extension during the D5 event at ca 2665 Ma triggered the development of terrestrial basins (i.e. Scotty Creek and Jones Creek) in areas flanking major antiforms, resulting in the deposition of the Jones Creek Conglomerate. Subsequent west-southwest–east-southeast shortening during the D6 event resulted in the folding of the Jones Creek Conglomerate and formation of gold-bearing veins in the Agnew gold camp. Belt-wide relaxation in east–west shortening during the D7 event caused open, recumbent F7 folding of the steeply dipping stratigraphy. Broadly east–west shortening during the D8 to D10 events resulted in the tightening of existing fold hinges, the dismemberment and displacement of panels of supracrustal rocks by sinistral (e.g. Perseverance shear zone) and then dextral (Waroonga) shear zones. The Agnew–Wiluna belt displays (para)autochthonous associations within the belt, with district-scale heterogeneities caused by primary volcano-sedimentary facies changes combined with polyphase deformation. Importantly, nickel sulfide-bearing sequences identified in nickel camps can potentially be traced to different parts of the belt by unravelling the effects of polyphase deformation.

Structural and thermochronological evidence for Paleogene basement-involved shortening in the axial Eastern Cordillera, Colombia

Although most recent studies regard the northern Andes primarily as a low-shortening inversion orogen, new mapping and thermochronology along the paired basement-involved Floresta massif and Floresta basin in the axial Eastern Cordillera of Colombia suggest major Paleogene shortening in a ramp-flat fold-thrust belt. Field mapping indicates that the hanging wall of the east-directed Soápaga fault system contains a series of upright thrust sheets with flat-on-flat cutoff relationships and a deformed footwall characterized by a complex triangle zone. These geometries necessitate roughly east–west shortening exceeding that of a previously mapped overturned hanging wall anticline and disharmonic footwall folds. Zircon (U–Th)/He (ZHe) ages indicate exhumation-induced cooling of the Soápaga hanging wall through the ∼180 °C closure temperature at 31–25 Ma. This cooling postdated documented shortening to the west and predated shortening to the east, suggesting an eastward progression of Paleogene deformation. Synorogenic Oligocene footwall strata of the Floresta basin contain distal fine-grained sediments and lack growth strata or Oligocene detrital ZHe ages, suggesting relatively high heave along the Soápaga fault system. These results are consistent with a rapidly eastward-propagating, basement-involved fold-thrust belt with ramp-flat structures that accommodated tens of km of shortening. Long-term stasis of the deformation front on the eastern and western flanks of the Eastern Cordillera due to localization of Neogene shortening along Mesozoic rift-bounding normal faults indicates a shift in deformational style in late Oligocene–early Miocene time. This geometric and temporal framework implies: 1) a total shortening in the northern Andes exceeding most current estimates; 2) Paleogene deformation in the Eastern Cordillera marked by rapid advances of the deformation front along a ramp-flat thrust system; and 3) focused Neogene reactivation (inversion) of master rift-bounding faults achieved by earlier crustal thickening related to ramp-flat thrust deformation.

On the origin of the ultradeep East Barents Sea basin

Very large subsidence, with up to 20 km thick sediment layers, is observed in the East Barents Sea basin. Subsidence started in early Paleozoic, accelerated in Permo‐Triassic times, finished during the middle Cretaceous, and was followed by moderate uplift in Cenozoic times. The observed gravity signal suggests that the East Barents Sea is at present in isostatic balance and indicates that a mass excess is required in the lithosphere to produce the observed large subsidence. Several origins have been proposed for the mass excess. We use 1‐D thermokinematic modeling and 2‐D isostatic density models of continental lithosphere to evaluate these competing hypotheses. The crustal density in 2‐D thermokinematic models resulting from pressure‐, temperature‐, and composition‐dependent phase change models is computed along transects crossing the East Barents Sea. The results indicate the following. (1) Extension can only explain the observed subsidence provided that a 10 km thick serpentinized mantle lens beneath the basin center is present. We conclude that this is unlikely given that this highly serpentinized layer should be formed below a sedimentary basin with more than 10 km of sediments and crust at least 10 km thick. (2) Phase changes in a compositionally homogeneous crust do not provide enough mass excess to explain the present‐day basin geometry. (3) Phase change induced densification of a preexisting lower crustal gabbroic body, interpreted as a mafic magmatic underplate, can explain the basin geometry and observed gravity anomalies. The following model is proposed for the formation of the East Barents Sea basin: (1) Devonian rifting and extension related magmatism resulted in moderate thinning of the crust and a mafic underplate below the central basin area explaining initial late Paleozoic subsidence. (2) East‐west shortening during the Permian and Triassic resulted in densification of the previously emplaced mafic underplated body and enhanced subsidence dramatically, explaining the present‐day deep basin geometry.

Clastic deposition, provenance, and sequence of Andean thrusting in the frontal Eastern Cordillera and Llanos foreland basin of Colombia

Sedimentological, provenance, and detrital thermochronological results for basin fill at the modern deformation front of the northern Andes (6°N latitude) provide a long-term, Eocene to Pliocene record of foreland-basin sedimentation along the Eastern Cordillera–Llanos basin boundary in Colombia. Lithofacies assemblages and paleocurrent orientations in the upward-coarsening, ~5-km-thick succession of the Nunchia syncline reveal a systematic shift from craton-derived, shallow-marine distal foreland (back-bulge) accumulation in the Mirador Formation, to orogen-sourced, deltaic, and coastal-influenced sedimentation of the distal to medial foreland (foredeep) in the Carbonera and Leon Formations, to anastomosing fluvial and distributive braided fluvial megafan systems of the proximal foreland (foredeep to wedge-top) basin in the lower and upper Guayabo Formation. These changes in depositional processes and sediment dispersal are supported by up-section variations in detrital zircon U-Pb and (U-Th)/He ages that record exhumation of evolving, compartmentalized sediment source areas in the Eastern Cordillera. The data are interpreted in terms of a progressive eastward advance in fold-and-thrust deformation, with late Eocene–Oligocene deformation in the axial zone of the Eastern Cordillera along the western edge of Floresta basin (Soapaga thrust), early Miocene reactivation (inversion) of the eastern margin of the Mesozoic rift system (Pajarito and Guaicaramo thrusts), and middle–late Miocene propagation of a footwall shortcut fault (Yopal thrust) that created the Nunchia syncline in a wedge-top (piggyback) setting of the eastern foothills along the transition from the Eastern Cordillera to Llanos foreland basin. Collectively, the data presented here for the frontal Eastern Cordillera define a general in-sequence pattern of eastward-advancing fold-and-thrust deformation during Cenozoic east-west shortening in the Colombian Andes.

From embryonic tectonics and geosynclines to modern concepts of rifting, spreading, and Alpine orogenesis: a summary of contributions to the 9th Alpine Workshop (Cogne, 2009)

The 9th Alpine Workshop was held in September of 2009 in the village of Cogne (Italy) amid, the spectacular geology and scenery of the Western Alps. This volume of the International Journal of Earth Science comprises a selection of papers read at the workshop, which in keeping with the workshop’s tradition, encompassed a broad range of themes on the processes and evolution of Alpine-type mountain belts. The workshop itself was dedicated to the memory of Rudolph Trumpy (1921–2009), a key figure in Alpine Geology and a frequent participant of past Alpine workshops. The keynote paper by Sengor and Bernoulli reviews Trumpy’s scientific oevre in the context of his time, especially his landmark paleogeographic reconstructions of Alpine Tethys and his early recognition of the role of strike-slip tectonics in the evolution of the Alps. Beyond these contributions, their paper provides insight into how Trumpy inspired colleagues and students alike to look carefully at rocks and to think critically about the geologic record, even as he acknowledged the need for a deductive approach to further our understanding of orogenesis. One of Trumpy’s many insights was that Alpine Tethys resulted from Mesozoic extension (rather than shortening) and that the structure of the adjacent European and Adriatic continental margins did not coincide with the impressive nappe structures that formed later during Alpine subduction and collision. It is therefore fitting that several papers in this volume deal with this early evolution and its effect on Alpine orogenic structure. Mohn, Manatschal, Massini, and Muntener present a revised interpretation of Austroalpine units that represent the distal Adriatic continental margin as preserved at the junction of Eastern and Central/ Western Alps in eastern Switzerland. In a series of E-W oriented palinspastic reconstructions, the authors show how Early Mesozoic rift-related structures of this margin determined the complex sequence of nappe stacking during late Cretaceous east–west shortening. Further complications arose during Tertiary north–south shortening, yet the original pre-orogenic structure of the passive margin is shown to be remarkably well preserved. In the Western Alps, Loprieno, Bousquet, Bucher, Ceriani, Fugenschuh, and Schmid present new structural and stratigraphic evidence for the existence of Valais oceanic lithosphere in Cretaceous time. Internal Valais units (Versoyen ophiolites and underlying magmatic and sedimentary rocks) are shown to have originated at the ocean–continent transition in contrast with external Valais units that did not have an oceanic lithosphere and probably derived from the European distal continental margin. This interpretation will be controversial to some Alpine geologists, as it refutes previous interpretations of the ‘‘Versoyen ophiolites’’ as a Carboniferous-age complex in tectonic (rather than stratigraphic) contact with post-rift sediments of the internal Valais unit. The Early Mesozoic paleogeography of the eastern part of the European margin of Alpine Tethys has been relatively poorly constrained until now. By careful dating of pre-Mesozoic basement rocks in the western part of the Tauern Window, Vesela, Sollner, Finger, and Gerdes correlate the sub-Penninic ‘‘Zentral Gneiss’’ there with the External basement Massifs in the Western Alps of Switzerland and France. Synsedimentary rhyodacitic and rhyolitic horizons (310–304 Ma) in these units along the Alpine chain are interpreted to mark the onset of sedimentation in a post-Variscan extensional basins, just prior to the onset of Early Perman intracontinental rifting. M. R. Handy C. L. Rosenberg (&) Freie Universitat Berlin, Berlin, Germany e-mail: cla@zedat.fu-berlin.de

Structural controls on the formation of BSR over a diapiric anticline from a dense MCS survey offshore southwestern Taiwan

A dense seismic reflection survey with up to 250-m line-spacing has been conducted in a 15 × 15 km wide area offshore southwestern Taiwan where Bottom Simulating Reflector is highly concentrated and geochemical signals for the presence of gas hydrate are strong. A complex interplay between north–south trending thrust faults and northwest–southeast oblique ramps exists in this region, leading to the formation of 3 plunging anticlines arranged in a relay pattern. Landward in the slope basin, a north–south trending diapiric fold, accompanied by bright reflections and numerous diffractions on the seismic profiles, extends across the entire survey area. This fold is bounded to the west by a minor east-verging back-thrust and assumes a symmetric shape, except at the northern and southern edges of this area, where it actively overrides the anticlines along a west-verging thrust, forming a duplex structure. A clear BSR is observed along 67% of the acquired profiles. The BSR is almost continuous in the slope basin but poorly imaged near the crest of the anticlines. Local geothermal gradient values estimated from BSR sub-bottom depths are low along the western limb and crest of the anticlines ranging from 40 to 50 °C/km, increase toward 50–60 °C/km in the slope basin and 55–65 °C/km along the diapiric fold, and reach maximum values of 70 °C/km at the southern tip of the Good Weather Ridge. Furthermore, the local dips of BSR and sedimentary strata that crosscut the BSR at intersections of any 2 seismic profiles have been computed. The stratigraphic dips indicated a dominant east–west shortening in the study area, but strata near the crest of the plunging anticlines generally strike to southwest almost perpendicular to the direction of plate convergence. The intensity of the estimated bedding-guided fluid and gas flux into the hydrate stability zone is weaker than 2 in the slope basin and the south-central half of the diapiric fold, increases to 7 in the northern half of the diapiric fold and plunging anticlines, and reaches a maximum of 16 at the western frontal thrust system. Rapid sedimentation, active tectonics and fluid migration paths with significant dissolved gas content impact on the mechanism for BSR formation and gas hydrate accumulation. As we begin to integrate the results from these studies, we are able to outline the regional variations, and discuss the importance of structural controls in the mechanisms leading to the gas hydrate emplacements.

Localized pluton deformation and linked focused flow of low-volume fraction residual melt in deforming plagioclase cumulates

Shear-hosted, sub-cm to 50-m-wide, Fe-Ti-Zr-REE (FTZR)–rich melt bands occur exclusively at folded margins of the Bolangir anorthosite pluton (eastern India) transverse to margin-parallel foliation in anorthosite. The N- to NNW-trending FTZR melt bands are oriented at a low angle ( 90% fractionation of anorthosites from crustally contaminated, high-Al gabbro (HAG) parent melt. At the high degree of crystal fractionation (crystal content higher than the particle-locking threshold), the FTZR melts, during last stages of crystallization, were locked within the plagioclase-dominated cumulus pile. The occurrence of FTZR melt bands exclusively at folded—as opposed to straight segments of—pluton margin is attributed to melt-enhanced strain localization in the pluton induced by the FTZR melts locked within anorthosite. Further melt-induced straining led to focusing of the FTZR melts along dilatant shear zones formed synchronously with the east-west shortening deformation; the variable widths of the melt bands correspond to the varying sizes of actively deforming magmatic plagioclase in anorthosite. The extraction and transport of locked-in, low-volume fraction melt documented here suggest long-duration, far-field stresses may segregate and transport large volumes of low-volume fraction melts in planetary interior over considerable distance.

Role of faults and folding in controlling gold mineralisation at Fosterville, Victoria

Structural and fluid-flow evolution at the Fosterville gold deposit in central Victoria was influenced by three distinct stress regimes that each played a role in localising the sulfide-hosted gold deposits. Major gold mineralisation at Fosterville is younger than at the majority of auriferous quartz-reef-hosted deposits in the Bendigo Zone, which formed at around 440 Ma, during regional east–west shortening of the host Ordovician turbidites. In contrast, wall-rock-hosted disseminated, fine-grained auriferous pyrite–arsenopyrite mineralisation at Fosterville post-dated both the development of chevron-shaped upright folds and fold-related faults and the emplacement of Early to Middle Devonian felsic dykes. Here we show that the primary structural control on the distribution of the auriferous sulfides was related to local reactivation of the west-dipping Fosterville Fault and associated low-displacement splay faults that truncate a south-plunging syncline. Analysis of slickenline lineations on slip surfaces indicates most faults initiated as dip-slip, reverse structures during regional east–west compression, and were subsequently reactivated with sinistral–reverse displacement during later northwest–southeast compression, coincident with gold mineralisation. Kinematic analysis of conjugate sets of quartz–carbonate veins that are temporally and spatially associated with the auriferous sulfides at Fosterville also indicates mineralisation occurred during northwest–southeast shortening. A third stress regime, characterised by north–south shortening, is inferred to have been active during the late stages of mineralisation at Fosterville. Late stibnite mineralisation associated with bedding-discordant faults at Fosterville is similar to that associated with many Devonian-age gold deposits in the Melbourne Zone to the east. The absence of quartz reefs, the scarcity of visible free gold in primary ore and the wall-rock-hosted disseminated character of the sulfide ores at Fosterville are atypical of gold deposits in the Bendigo Zone. However, due to the important role of fold and fault relationships in localising mineralisation the geometry and structural setting of the Fosterville deposit is similar to many other goldfields of the Bendigo Zone.

Forearc deformation and tectonic significance of the ultramafic Molucca central ridge, Talaud islands (Indonesia)

Abstract The Molucca Sea is a narrow basin located south of Mindanao (Philippines) and underlined by a north-south ophiolitic ridge. This ridge represents the outer ridge of the Sangihe subduction zone and emerges by uplift in the central part of the basin, in the Talaud Islands. Field studies indicate that forearc sediments rest uncomformably on (i) a dismembered ophiolitic series and (ii) thick melanges. Structural analysis indicates two deformation events, one of which is oriented east-west coaxial with the present state of strain. We interpret the earlier (N20°E) direction as a thrusting event that affected an ophiolitic basement associated with the edge of the Celebes Sea. Thrusting within the oceanic crust and sediments also generated olistostromes (melanges). The style of deformation is characterized by flattened rhombs of peridotites which exists in situ in the upper section of the crustal sequence and were also found inside the melange. Incipient Sangihe subduction around 15 Ma uplifted the deformed crust and buried the melanges beneath the forearc sediments. Recent east-west shortening during subduction of the Snellius Plateau reactivated the melanges within thrusts cutting the forearc series.

Structure of the Isan Orogeny under cover to the east of the Mount Isa Inlier revealed by multiscale edge analysis and forward and inverse modelling of aeromagnetic data

Aeromagnetic data reveals the presence of three north–south orientated, tens of kilometre scale, elliptical structures under cover to the east of the Mount Isa Inlier in Queensland, Australia. The Holy Joe Creek, Boot Creek and Nora Creek structures are typically defined by three narrow, continuous magnetic ridges. Highly magnetically susceptible amphibolite bodies, similar to those exposed in the Inlier, are probably the source of most of these magnetic anomalies. Multiscale edge analysis, 2D forward modelling and 3D inverse modelling methods have been used to resolve the geometry and near surface dip of the structures. These techniques indicate that the structures dip moderately to steeply around their edges. The Holy Joe Creek Structure exhibits sub-vertical to steeply west-dipping sides and a moderate plunge around the northern closure. The Boot Creek and Nora Creek structures are more complex magnetically, but their eastern edges also have sub-vertical to steep westerly dips. Diapirism and domes in an egg-box style (type 1) fold interference pattern are considered as possible origins for the elliptical structures. However, disparity between the shape of the structures and possible diapiric bodies and the lack of complementary basin structures to the domes, makes these two explanations unlikely. A more plausible alternative is that the structures are the result of deformation in a single phase, producing folds with a variable hinge-line plunge. These results indicate that east–west shortening during the D 2 event of the Isan Orogeny was the major deformation in the buried rocks of the study area, suggesting a shared structural history with the Mount Isa Inlier to the west.

Possible eastward tectonic transport and northward gravitational tectonic collapse in the Arabian–Nubian shield of western Ethiopia

The Neoproterozoic low-grade volcano-sedimentary and medium to high-grade gneissic domains in the Galesa area of western Ethiopia, which represent part of the Arabian–Nubian Shield, are separated by NNE-trending thrusts that are often marked by transposed mafic–ultramafic assemblages. These litho-tectonic domains preserve early Neoproterozoic east–west shortening deformations in the form of east-facing recumbent isoclinal folds and east-verging thrusts with mylonitic fabrics containing down-dip stretching lineations along discrete shear zones within the metavolcano-sedimentary domains. These early structures are re-folded by N-trending, E-verging tight to open upright map-scale folds. The E–W shortening and thrust stacking culminated with the development of the N-trending sinistral strike-slip Baruda shear zone and lithospheric thickening and exhumation, and subsequent development of low-angle normal-slip shearing along the ENE-trending Donbon shear zone interpreted to have developed during orogenic extensional collapse.

Structural Modification of the Komatiite-Associated Harmony Nickel Sulfide Deposit, Leinster, Western Australia

The Harmony nickel sulfide deposit (3.2 Mt of ore) is situated along the eastern margin of the Agnew-Wiluna greenstone belt, Eastern Goldfields, Western Australia. The deposit occurs 2 km north of the larger Rocky’s Reward (15.9 Mt) and Perseverance (50.7 Mt) deposits. The mine stratigraphy trends north-northwest–south-southeast, dips moderately (30°–80°) to the west, is overturned, and metamorphosed to middle-amphibolite facies grades (about 550°C, 3 kbars). Stratigraphic footwall units to the west of the nickel-bearing komatiite include ca. 2720 to 2725 Ma (volcano-) sedimentary rocks and rhyodacite volcanic and volcaniclastic units. Hanging-wall units comprise mainly reworked sedimentary or volcaniclastic rocks interlayered with at least two barren spinifex-textured komatiite units exposed in the mine area. Exhalative pyrite-rich pelitic units and thin amphibolite units are locally present in the footwall and hanging wall. The nickeliferous komatiite is thickest in the center of the deposit and thins to the north, south, and at depth. Although massive sulfides and minor disseminated sulfides are mainly located along the sheared western footwall contact, minor massive sulfides occur throughout the komatiite and country rock in fold hinges, boudin neck areas, brittle-ductile shear zones, piercement cusps, piercement veins, and brittle faults. Footwall and hanging-wall units have a strong composite fabric defined by compositional layering and several subparallel axial planar foliations. East-west shortening resulted in the folding of the composite fabric to form abundant north-northwest–trending, tight, asymmetric, moderately (20°–40°) N- and S-plunging folds. The asymmetric folds thicken the komatiite and orebodies in the center of the pit and control deposit-scale orebody plunge directions. The attenuated north-northwest–trending komatiite fold limbs are boudinaged and concentrate pods of massive sulfides in boudin neck positions. Relaxation in horizontal shortening coincided with open recumbent folding of the steeply dipping limbs of earlier upright asymmetric folds. A resumption of east-west shortening in a brittle stress regime resulted in the remobilization of massive sulfides into east-west–trending brittle faults. The concentration of massive sulfides in brittle-ductile to brittle structures indicates that massive sulfide remobilization spanned a diverse range of deformation and metamorphic conditions. The present geometry and distribution of stratigraphy and orebodies at Harmony reflect a complex interplay of primary magmatic and secondary overprinting processes. Remobilized massive sulfide orebodies are an important exploration target in other deformed and metamorphosed terranes that host primary Ni-Cu-(PGE) deposits. Although footwall units are the preferential host to remobilized massive sulfide shoots, massive sulfides are commonly remobilized within shear zones and faults zones and hosted by hanging-wall units up to 50 m from their primary footwall position. In cases of folding, orebodies tend to be elongate parallel to the plunge direction of fold axes or mineral stretching lineation, and potential exploration targets occur along the trend of known folded ore shoots and more specifically along the plunge line of such shoots.

Fault and vein relationships in a reverse fault system at the Centenary orebody (Darlot gold deposit), Western Australia: Implications for gold mineralisation

The Centenary orebody within the Darlot gold deposit is located in the Yandal greenstone belt in the Yilgarn Craton, Western Australia. At Centenary, moderately (∼45°) west dipping reverse faults and steeply dipping (>70°) faults of variable strike failed during gold mineralisation in response to sub-horizontal east-west shortening and sub-vertical extension. Gently dipping veins are temporally, genetically and spatially related subsidiary structures to west dipping reverse and steeply dipping faults. Line analyses of subsidiary vein distributions in 23 drill cores around Centenary suggest that the gold-related subsidiary veins are localised within a 200–300 m wide tabular linking damage zone between three west dipping faults (Thompson, Lords and Walters). The damage zone is a laterally stepping relay zone between the Thompson and Lords–Walters faults and has a pull-apart geometry. Anomalous vein-related extensional strain (>0.005), vein density (>0.20) and power-law vein thickness population characteristics ( D t 0.58–1.84) distinguish this zone from the surrounding rock. Within the linking damage zone the highest number and volume of veins are observed at the tip of the Walters fault. At the fault tip, the exponent of the power-law distribution of vein thickness is highest ( D t 1.84) indicating that vein-related extensional strain is distributed on a high number of relatively small thickness veins. At approximately 300 m distance from the fault tip the densities of veins and the measured exponents of power-law vein thickness distributions are lower (<0.80 and D t <1.3, respectively). However, bulk vertical extensional strain remains high (>0.005), indicating that subsidiary vein material is concentrated on a greater number of anomalously thick veins. These systematic variations suggest that the fault tip imparted a strong control on vein localisation. Strain localisation within the linking damage zone is complex with coefficients of variation of vein spacing greater than one implying vein clustering. Gently dipping veins occur as wing crack arrays to the reverse faults and also in arrays comprising curviplanar, intersecting networks of subsidiary veins. The linking damage zone corresponds closely with the Centenary gold resource indicating that it has been an important locus for the focussed flux of gold-bearing hydrothermal fluids. However, within the damage zone, individual veins and vein arrays on the tens of metre scale do not always correlate with high gold grade indicating additional complexity within the system.

40Ar/39Ar thermochronology constraints on the timing of Proterozoic basement exhumation and fault ancestry, southern Sangre de Cristo Range, New Mexico

Proterozoic ancestry is increasingly being proposed for both Ancestral Rocky Mountain (Pennsylvanian-Permian) and Laramide-age (Late Cretaceous–early Tertiary) faults in the southwestern United States. We test this hypothesis by using 40 Ar/ 39 Ar thermochronology on Proterozoic basement rocks in the southern Sangre de Cristo Range, New Mexico. Hornblende and mica yield ages consistent with basement rocks cooling below ∼500 °C after ca. 1.4 Ga regional metamorphism, followed by protracted basement residence at temperatures between 300 and 400 °C until the Neoproterozoic. K-feldspar age spectra are dominated by Neoproterozoic apparent ages and constrain basement exhumation from mid-crustal depths (∼10 km) to within a few kilometers of the surface during the Neoproterozoic. Samples were also collected across the north-striking, steeply west-dipping Montezuma fault, which bounds the eastern Rocky Mountain range front and has known Ancestral Rocky Mountain and Laramide-age movement. Contrasting 40 Ar/ 39 Ar K-feldspar–derived thermal histories from opposing sides of the fault require two episodes of Neoproterozoic fault movement. Overall, rocks to the west record multiple diffusion domain (MDD) thermal histories, which indicate that cooling was caused by 5–7 km of basement exhumation between 1000 and 800 Ma, whereas rocks to the east were exhumed later, between 750 and 600 Ma. Assuming that the present-day fault geometry was established in the Proterozoic, west-side-up throw late in the Grenville orogeny (ca. 1000 Ma) accommodated east-west shortening. West-side-down extensional movement at ca. 750 Ma is inferred to be associated with the incipient rifting of the supercontinent Rodinia.

Recent crustal deformation of İzmir, Western Anatolia and surrounding regions as deduced from repeated GPS measurements and strain field

To investigate contemporary neotectonic deformation in İzmir, Western Anatolia and in its neighborhood, a relatively dense Global Positioning System (GPS) monitoring network was established in 2001. Combination of three spatially dense GPS campaigns in 2001, 2003 and 2004 with temporally dense campaigns between 1992 and 2004 resulted in a combined velocity field representing active deformation rate in the region. We computed horizontal and vertical velocity fields with respect to Earth-centered, Earth-fixed ITRF2000, to Eurasia and to Anatolia as well. The rates of principal and shear strains along with rigid-body rotation rates were derived from velocity field. Results show east–west shortening between Karaburun Peninsula and northern part of İzmir Bay together with the extension of İzmir Bay in accordance with general extension regime of Western Anatolia and Eastern Agea. East–west shortening and north–south extension of Karaburun Peninsula are closely related to right-lateral faulting and a clockwise rotation. There exists a block in the middle of the peninsula with a differential motion at a rate of 3–5 ± 1 mm/year and 5–6 ± 1 mm/year to the east and south, respectively. As is in Western Anatolia, north–south extension is dominant in almost all parts of the region despite the fact that they exhibit significantly higher rates in the middle of the peninsula. Extensional rates along Tuzla Fault lying nearly perpendicular to İzmir Bay and in its west are maximum in the region with an extension rate of 300–500 ± 80–100 nanostrain/year and confirm its active state. Extensional rates in other parts of the region are at level of 50–150 nanostrain/year as expected in the other parts of Western Anatolia.

Europe from the Variscan to the Alpine cycles

Abstract The time span between the Variscan and Alpine cycles is not devoid of any major tectonic activity, and corresponds to the Cimmerian cycle. Between the Early Permian and Late Triassic, the Eocimmerian cycle was marked by the closure of Palaeotethys and opening of Neotethys and of an array of south Eurasian back-arc basins. This was followed by the break-up of Pangaea and the Early Jurassic opening of the central Atlantic and Alpine Tethys. However, in the area of the Eocimmerian collision, the geodynamic evolution is relatively uninfluenced by this event, and a new cycle of Cimmerian deformation affected the Hellenides, Dinarides, Balkans and Pontides in Jurassic-Early Cretaceous times. The anti-clockwise rotation of Africa during the Late Cretaceous heralded the onset of Alpine orogenic processes, characterized first by major east-west shortening, and opening and closure of younger oceanic basins of back-arc type.

Extensional collapse in the Neoproterozoic Araçuaí orogen, eastern Brazil: a setting for reactivation of asymmetric crenulation cleavage

The Araçuaí orogen of eastern Brazil is one of many Brasiliano/Pan African orogens formed during the Neoproterozoic assembly of Gondwana. Its western edge, bordering the São Francisco craton, is the Serra do Espinhaço fold-thrust belt, in which top-up-to-the-west (reverse-sense) faults, west-verging folds (F 1), and east-dipping spaced to phyllitic cleavage (S 1) developed. We have found that the kinematics of deformation changes markedly at the hinterland margin of this fold-thrust belt. Here, beneath a plateau known as the Chapada Acauã, metadiamictite and fine-grained pelitic schist comprise an east-dipping belt that contains an assemblage of structures indicative of top-down-to-the-east (normal-sense) shear. This assemblage includes a cascade of F 2 folds that refold F 1 folds and verge down the dip of the belt's enveloping surfaces, vertical tension gashes, and top-down-to-the-east rotated clasts. Based on the presence of these structures, we propose that the plateau exposes a regional-scale normal-sense shear zone, here called the Chapada Acauã shear zone (CASZ). Because F 2 folds refold F 1 folds, normal-sense shear in the CASZ occurred subsequent to initial west-verging thrusting. Considering this timing of motion in the CASZ, we suggest that the zone accommodated displacement of the internal zone of the Araçuaí orogen down, relative to its foreland fold-thrust belt, and thus played a role in extensional collapse of the orogen. The CASZ trends parallel to preserved thrusts to the west, and thus may represent an inverted thrust fault. Notably, throughout the CASZ, S 1 schistosity has been overprinted by a pervasive, west-dipping asymmetric crenulation cleavage (S 2). The sigmoid shape of S 1 surfaces in S 2 microlithons require that slip on each S 2 surface was top-down-to-the-west. S 2 cleavage is axial-planar to the down-dip verging F 2 folds. Based on its geometry, we suggest that S 2 cleavage initiated either as an antithetic extensional crenulation cleavage during reverse-sense shear, or as a near vertical asymmetric crenulation cleavage formed during east–west shortening of a preexisting east-dipping schistosity. Subsequent normal-sense shear in the CASZ reactivated this cleavage, causing clockwise rotation of S 2 domains (as viewed looking along-strike to the north), in a manner similar to that of rotational ‘bookshelf faults’. Such movement could have accommodated concomitant vertical flattening of the CASZ during extensional collapse.

Missing Idaho arc: Transpressional modification of the 87Sr/86Sr transition on the western edge of the Idaho batholith

The North American Cordillera is characterized by subduction-related Mesozoic batholiths with voluminous middle Cretaceous plutonism. In the Cordillera, the Idaho batholith is anomalous, containing only a narrow (<10 km) band of middle Cretaceous plutons on its extreme western margin. These plutons are spatially coincident with the Late Cretaceous, crustal-scale, dextral transpressional western Idaho shear zone. Finite strain analysis indicates that there is significant east-west shortening in the western Idaho shear zone. A reconstruction using isotopic variations as strain markers, and comparing them to the undeformed middle Cretaceous Sierra Nevada batholith, results in a middle Cretaceous magmatic arc in western Idaho similar in width (∼85– 100 km) to other sections of the Cordilleran magmatic arc. The tectonic implications include: (1) a full magmatic arc existed in Idaho in the middle Cretaceous prior to Late Cretaceous deformation; (2) the currently sharp 87Sr/86Sr transition in western Idaho represents structural modification of the batholith after emplacement (i.e., the steep isotopic gradient results from deformation); and (3) as a result of Late Cretaceous shortening in the western Idaho shear zone, the main part of the Idaho batholith (i.e., post–ca. 90 Ma) intruded farther eastward into thick North American lithosphere. Consequently, the main part of the Idaho batholith has a stronger continental signature than the other coastal batholiths.

Kinematic nature and origin of regional-scale ductile shear zones in the central Yilgarn Craton, Western Australia

Regional-scale, northwest- and northeast-trending ductile shear zones, 1- to 6-km wide and over 100-km long, are the most conspicuous structures in the central Yilgarn Craton, Western Australia. They are commonly developed along granite–greenstone contacts, but are centred in strongly foliated granite and granitic gneiss. A wide range of kinematic indicators, including well-developed S–C fabrics, C′-type shear band cleavage, and abundant asymmetric porphyroclasts, consistently indicate sinistral movement on northwest-trending shear zones and dextral movement on northeast-trending shear zones. Major planar fabrics within the ductile shear zones include (1) a northwest- or northeast-trending foliation parallel to the shear zones, (2) a northerly trending gneissic banding, and (3) a northerly trending foliation. The northwest- and northeast-trending foliations with associated asymmetric structural elements and a shallowly plunging mineral lineation were developed during a progressive, inhomogeneous east–west shortening event (D 3) characterized by low-temperature, noncoaxial deformation. Formation of the northerly trending gneissic banding and associated symmetric structural elements is attributed mainly to east–west coaxial shortening (D 2) in a high-temperature state during, or shortly after, granite intrusion. The northerly trending foliation within the ductile shear zones commonly forms the S-fabric of S–C fabrics that was either developed during D 3, same time as the C-fabric, or was reactivated from S 2 foliations. Therefore, major planar fabrics in the regional-scale ductile shear zones of the central Yilgarn Craton can be ascribed to two tectonic events (D 2 and D 3), and deformation within the shear zones includes both coaxial and noncoaxial components.