Northwest-southeast Compression Research Articles

Networks of geometrically coherent faults accommodate Alpine tectonic inversion offshore southwestern Iberia

Abstract. The structural styles and magnitudes of Alpine tectonic inversion are reviewed for the Atlantic margin of southwestern (SW) Iberia, a region known for its historical earthquakes, tsunamis and associated geohazards. Reprocessed, high-quality 2D seismic data provide new images of tectonic faults, which were mapped to a depth exceeding 10 km for the first time. A total of 26 of these faults comprise syn-rift structures accommodating vertical uplift and horizontal advection (shortening) during Alpine tectonics. At the regional scale, tectonic reactivation has been marked by (a) the exhumation of parts of the present-day continental shelf, (b) local folding and thrusting of strata at the foot of the continental slope, and (c) oversteepening of syn- and post-rift sequences near reactivated faults (e.g. “passive uplift”). This work proves, for the first time, that geometric coherence dominated the growth and linkage of the 26 offshore faults mapped in SW Iberia; therefore, they are prone to reactivate as a kinematically coherent fault network. They form 100–250 km long structures, the longest of which may generate earthquakes with a momentum magnitude (Mw) of 8.0. Tectonic inversion started in the Late Cretaceous, and its magnitude is greater close to where magmatic intrusions are identified. In contrast to previous models, this work postulates that regions in which Late Mesozoic magmatism was more intense comprise thickened, harder crust and form lateral buttresses to northwest–southeast compression. It shows these structural buttresses to have promoted the development of early stage fold-and-thrust belts – typical of convergent margins – in two distinct sectors.

Present-day kinematics of the northwest Moroccan Atlantic Margin from GNSS data: west southwest extrusion at the western end of the High Atlas

The Global Navigation Satellite System (GNSS) has emerged as a practical and effective technique for studying slow and steady geodynamic movements, enabling continuous monitoring and precise quantification of deformation over different timescales. In Morocco, a network of GNSS stations has been established, offering valuable insights into tectonic processes. This paper focuses on investigating the geodynamic motion of the northwest Moroccan Atlantic Margin. By utilizing GNSS data, subsidence rates and horizontal velocity fields were determined for the first time, providing valuable information for oil and gas exploration activities. The study reveals an active uplift rate of 1 mm/year and a westward horizontal motion of 2.04 mm/year in the Essaouira segment. The paper presents a case study of the Essaouira–Agadir basin (EAB) onshore segment and investigates the anomalous displacement observed in this region compared to other coastal GNSS stations. Possible explanations for the observed movements include local processes such as salt tectonics and regional northwest–southeast compression related to Africa–Eurasia convergence. We suggest that the anomalous movement detected in this work is due to the regional northwest–southeast compression related to Africa–Eurasia convergence imparting an extrusion of the EAB to the west. This research contributes to a better understanding of the geodynamics in the northwest Moroccan Atlantic margin, thereby providing valuable insights for ongoing efforts in oil and gas exploration. Furthermore, it indicates the continued activity of the Agadir fault, which would exhibit a sinistral wrench movement, thus posing a threat to the city of Agadir and its inhabitants.

Multiphase activation of the boundary fault system of the eastern Dampier subbasin, Northwest Shelf of Australia

Investigating the timing, structural style, and development of multiphase extensional fault systems is essential for understanding rift basin evolution and for assessment of structural trap integrity. Borehole-controlled interpretation and analysis of two-dimensional and three-dimensional seismic data sets from the eastern Dampier subbasin indicate that a northeast-trending basement weakness zone was subjected to west-northwest–east-southeast oblique extension in the latest Triassic–late Middle Jurassic, resulting in systematic segmentation of the Rosemary fault system (RFS). Temporal stress change during Cretaceous north-south extension produced complex fault systems along the RFS, including (1) east-west–trending isolated faults with maximum displacement close to their center; (2) east-west–trending abutting faults, which initially nucleated as isolated faults, later abutted against the main structure, showing large displacement accruement on the composite fault; and (3) northeast-southwest–trending splay faults characterized by systematic left-stepping segmentation, with maximum displacement occurring at the point where the splay faults deviate from the main structure. In the Miocene, the RFS was locally reactivated by northwest-southeast compression in the northeastern part of the fault system, developing a compressive fault-propagation fold at the upper tip of the inverted extensional fault. This study suggests that the style of basin boundary fault reactivation depends largely on preexisting structures and temporal stress changes. The fault reactivation style is also a significant factor in influencing basin architecture, sediment distribution, fault linkage processes, and petroleum basin prospectivity.

Flower structures in sandstones of the Paleozoic Inkisi Group (Brazzaville, Republic of Congo): evidence for two major strike-slip fault systems and geodynamic implications

AbstractFew studies have reported field descriptions of flower structures associated with strike-slip faults. This study describes and illustrates flower structures near Brazzaville (Republic of Congo) and explains their implication for the tectonic history of the Paleozoic Inkisi Group. Field observations show that the Inkisi Group is affected by two major strike-slip fault systems. The oldest system is dominated by north-northwest–south-southeast striking sinistral strike-slip faults and minor east–west striking dextral strike-slip faults. The youngest system consists of dominant northeast–southwest striking dextral strike-slip faults and minor northwest–southeast striking sinistral strike-slip faults. Flower structures within these major strike slip faults show four types of arrangements that likely depend on fault growth, propagation and damage zones: (i) flower structures associated with wall damage zones; (ii) flower structures associated with linking damage zones; (iii) flower structures associated with tip damage zones; and (iv) “hourglass” flower structures.Paleostress analysis reveals that both major fault systems originated from two differently oriented pure strike-slip regime stress stages. The first stage, which engendered the first major fault system, developed under northwest–southeast compression (i.e, σ1 = 322°). This phase probably coincided with north–south collision in the southern part of Gondwana in the Permo-Triassic and the Late Cretaceous compression times. The second stress stage, creating the second major fault system, developed under east–west (i.e, σ1 = 078°) compression. This phase is correlated with compression from the east–west opening of the Atlantic Ocean in the Miocene times.

A New Uniform Moment Tensor Catalog for Yunnan, China, from January 2000 through December 2014

AbstractBecause of the collision of the Indian and Eurasian tectonic plates, the Yunnan Province of southwestern China has some of the highest levels of seismic hazard in the world. In such a region, a catalog of moment tensors is important for estimating seismic hazard and helping understand the regional seismotectonics. Here, we present a new uniform catalog of moment tensor solutions for the Yunnan region. Using a grid-search technique to invert seismic waveforms recorded by the permanent regional network in Yunnan and the 2 yr ChinArray deployment, we present 1833 moment tensor solutions for small-to-moderate earthquakes that occurred between January 2000 and December 2014. Moment magnitudes in the new catalog vary from Mw 2.2 to 6.1, and the catalog is complete above Mw∼3.5–3.6. The moment tensors are constrained to be purely double-couple and show a variety of faulting mechanisms. Normal faulting events are mainly concentrated in northwest Yunnan, while farther south along the Sagaing fault the earthquakes are mostly thrust and strike slip. The remaining area includes all three styles of faulting but mostly strike slip. We invert the moment tensors for the regional stress field and find a strong correlation between spatially varying maximum horizontal stress and Global Positioning System observations of horizontal ground velocity. The stress field reveals clockwise rotation around the eastern Himalayan syntaxis, with northwest–southeast compression to the east of the Red River fault changing to northeast–southwest compression west of the fault. Almost 88% of the centroid depths are shallower than 16 km, consistent with a weak and ductile lower crust.

Cooling and exhumation along the curved Albany-Fraser orogen, Western Australia

The Albany-Fraser orogen of Western Australia exhibits a distinct 45° primary (preorogenic) curvature. Consequently, northwest-southeast compression during Mesoproterozoic orogeny was orthogonal to orogenic strike in the east of the orogen, but was oblique in the west. This produced different structural settings in the east and west of the orogen, with a greater component of dextral transpression in the west. We report new 40 Ar/ 39 Ar thermochronology from the east Albany-Fraser orogen, and compare these results to the cooling history of the west to examine how cooling varies between the differently striking domains of a curved orogen. The 40 Ar/ 39 Ar analyses of hornblende, muscovite, and biotite grains encompass a range of metaigneous and metasedimentary lithologies from two lithotectonic domains. The eastern Biranup Zone yields five hornblende cooling ages at ca. 1190 Ma and seven muscovite and biotite cooling ages between ca. 1171 and 1158 Ma. Hornblende and biotite in the southwestern Fraser Zone record cooling between ca. 1217 and 1205 Ma, and the central Fraser Zone reached 40 Ar/ 39 Ar biotite closure temperature at ca. 1157 Ma. Slow 8.2–9.5 °C/m.y. cooling in the eastern Biranup Zone commenced 20 m.y. earlier than 22–33 °C/m.y. cooling in the west Albany-Fraser orogen. The differences in cooling rate are a result of the different structural settings in the east and west. However, similar mica 40 Ar/ 39 Ar cooling ages in the east and west record a convergence in cooling history. This suggests that exhumation had become increasingly decoupled from compressional tectonics, instead driven by more passive processes related to isostatic rebound and erosion.

Petrotectonic framework of granitoids and associated granulites at Nagavalli Shear Zone (NSZ), Eastern Ghats Belt: Evidence of a late transpression orogeny

Megacrystic granitoids associated with migmatitic and metasedimentary gneisses occurring around Nagavalli Shear Zone (NSZ) preserve complex metamorphic and deformation history. Thinly laminated discontinuous banding of quartzofeldspathic layer (S1) in the migmatites is the product of first incipient melting during prograde M1–D1 tectonothermal event. Peak M2–D2 event is manifested by the development of S2 gneissic foliation in all rocks, which is axial planar to rootless folds on S1. Porphyroblastic garnet mantled by leucosomal melt fraction in granitoids, suggest that the rock suffered peak granulite facies metamorphism along with host migmatitic gneisses. The subsequent D3 event deforms differently the massive granitoids and the migmatitic granulite gneisses. The D4 deformation acted as transpression with broad northwest–southeast compression that develops strong discontinuous regional-scale anastomosing shear zones transecting the earlier gneissosity (S2) in the granitoids with prominent sinistral shear sense. It deforms the axial plane of regional folds in migmatites and develops superposed non-plane non-cylindrical folds in outcrop to regional scale. Thus we infer megacrystic granitoids were possibly emplaced in pre- to syn-peak metamorphic event within the host granulites. Granitoids and associated migmatitic gneisses of Late Meso- to Neoproterozoic age suffered subsequent petrotectonic events followed by a sinistral transpression acted along NSZ.

En échelon and orthogonal fault ruptures of the 11 April 2012 great intraplate earthquakes

The Indo-Australian plate is undergoing distributed internal deformation caused by the lateral transition along its northern boundary--from an environment of continental collision to an island arc subduction zone. On 11 April 2012, one of the largest strike-slip earthquakes ever recorded (seismic moment magnitude M(w) 8.7) occurred about 100-200 kilometres southwest of the Sumatra subduction zone. Occurrence of great intraplate strike-slip faulting located seaward of a subduction zone is unusual. It results from northwest-southeast compression within the plate caused by the India-Eurasia continental collision to the northwest, together with northeast-southwest extension associated with slab pull stresses as the plate underthrusts Sumatra to the northeast. Here we use seismic wave analyses to reveal that the 11 April 2012 event had an extraordinarily complex four-fault rupture lasting about 160 seconds, and was followed approximately two hours later by a great (M(w) 8.2) aftershock. The mainshock rupture initially expanded bilaterally with large slip (20-30 metres) on a right-lateral strike-slip fault trending west-northwest to east-southeast (WNW-ESE), and then bilateral rupture was triggered on an orthogonal left-lateral strike-slip fault trending north-northeast to south-southwest (NNE-SSW) that crosses the first fault. This was followed by westward rupture on a second WNW-ESE strike-slip fault offset about 150 kilometres towards the southwest from the first fault. Finally, rupture was triggered on another en échelon WNW-ESE fault about 330 kilometres west of the epicentre crossing the Ninetyeast ridge. The great aftershock, with an epicentre located 185 kilometres to the SSW of the mainshock epicentre, ruptured bilaterally on a NNE-SSW fault. The complex faulting limits our resolution of the slip distribution. These great ruptures on a lattice of strike-slip faults that extend through the crust and a further 30-40 kilometres into the upper mantle represent large lithospheric deformation that may eventually lead to a localized boundary between the Indian and Australian plates.

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.

Seismogenesis and state of stress in the UK

In this paper I present a compilation of focal mechanisms for earthquakes with magnitudes greater than 3.0 M L in the British Isles that can be used to help constrain our understanding of seismicity and it's driving forces in the British Isles. The fault plane solutions consist of both previously published mechanisms for significant British earthquakes, and new solutions calculated from regional and local data for more recent and smaller earth- quakes that were previously unpublished. Focal mechanisms for earthquakes in the UK are dominantly strike-slip with northwest-southeast compression and northeast-southwest tension,or reverse, with northwest-southeast com- pression. In many cases there is also an oblique component to the slip. P and T axes from individual solutions are relatively well constrained in azimuth, though less so in dip, with P-axes orientation for most events clustering between north and north-west, indicating sub-horizontal compression. However, some spatial variation in P- and T-axes orientation is also apparent, with near north/northeast compression and east-west extension in north- west Scotland, changing to northwest-southeast compression in England and Wales. I estimate a best-fitting stress tensor, under the assumption of uniform stress using two different inversion methods for both the entire focal mechanism data set and two different subsets of the data. The results from the two different datasets suggest that there is a significant difference in the stress state between northwest Scotland and England and Wales. Calculated σ 1 directions for England and Wales are northwest-southeast, consistent both with existing stress data and expected stresses from first order plate motions. By contrast, the inversion results for northwest Scotland show near east-west extension with possible σ 1 and σ 2 directions lying in a north south band, and that the magnitudes of σ 1 and σ 2 are similar. The relative magnitude of the principal stresses, R, determined for England and Wales suggests that the intermediate stress σ 2 is close to the average value of σ 1 and σ 3 . The clear difference in the stress inversion results between northwest Scotland and England and Wales suggests that the principal stress directions expected from first order plate motions have been modified in Scotland by local stress conditions due to glacio-isostatic adjustment.

Multi-scale integrated structural and aeromagnetic analysis to guide tectonic models: An example from the eastern Musgrave Province, Central Australia

In many polydeformed Precambrian provinces, the understanding of their tectonic history is limited by insufficient exposure, which leads to difficulty in developing links between local and regional architecture, and results in poorly constrained models of their regional-scale architecture. High-resolution aeromagnetic data allows interpretation and modelling at a scale comparable to structural mapping, and by integrating these methods, these links can be developed with high confidence. As an example, we integrate structural mapping, aeromagnetic data interpretation and 3D magnetic inversions in a multi-scale structural analysis of the polydeformed eastern Musgrave Province in central Australia. Deformation during the ca. 1200 Ma Musgravian Orogeny is characterised by an originally northeast-trending structural grain defined by shear zones and tight to isoclinal folds at all scales. 3D magnetic inversion models of syn-to-post Musgravian granites indicate that this structural grain is predominantly west dipping. This architecture suggests that a northeast-oriented fold and thrust belt developed as a result of northwest–southeast compression in central Australia during the Musgravian Orogeny. The next deformation event is characterised by an array of originally east-southeast trending shear zones that truncate and offset the Musgravian structural grain with apparent dextral displacements of up to 1 km. These shear zones are commonly co-located with mafic and granitic dykes, suggesting a dilational origin, and they are interpreted to have initiated under northeast–southwest extension during the ca. 1080 Ma Giles Event. The architecture of these two Mesoproterozoic events was later extensively overprinted by two deformation events during the Pan-African aged (ca. 550 Ma) Petermann Orogeny. The first is characterised by the widespread reactivation of Mesoproterozoic structures, resulting in a network of shear zones of variable orientation, and the second is characterised by the development of crustal-scale transpressional shear zones, motion on which has caused the rotation of lithospheric blocks and the reorientation of prior structural trends by up to 90°. Collectively, the architecture of these two events suggests broadly north–south crustal shortening in central Australia as an intraplate response to Gondwana assembly. This example demonstrates that the integration of structural geology with aeromagnetic interpretation and modelling can be used to develop high-confidence links between local and regional structures. These links permit the derivation of better constrained models of major tectonic events, particularly in regions where outcrop is poor and the structural architecture is complex.

Record of Late Mississippian tectonics in the new Percé Group (Viséan) of eastern Gaspésie, Quebec

Viséan clastic units and structures at the northwest margin of the upper Paleozoic Maritimes Basin provide information on tectonic events that are only poorly recorded in more central parts of the basin. These continental units are time equivalent to marine sediments of the Windsor Group of Nova Scotia. They are herein assigned to the new Percé Group, which includes the La Coulée and Bonaventure formations, as well as a new unit, the Cap d'Espoir Formation. The latter unit unconformably underlies the Bonaventure Formation in a small but thick sub-basin of the Ristigouche Basin in eastern Quebec. It is characterized by a succession of sandstone and mudstone rhythmites that contrast with the coarse alluvial fan deposits of the overlying Bonaventure Formation. The Cap d'Espoir Formation was sourced from a broad area of subdued topography occupied by the Viséan La Coulée Formation and underlying units. Erosional remnants of the La Coulée Formation are unconformably overlain by the Bonaventure Formation in marginal parts of the Ristigouche Basin, whereas these units are separated by the Cap d'Espoir Formation in more central areas of the basin. The La Coulée and Cap d'Espoir formations underwent tilting and erosion during a normal faulting event that preceded deposition of the fault-controlled Bonaventure Formation. This series of events is interpreted to represent different steps in the reactivation of a pre-Carboniferous dextral strike-slip system in response to northwest–southeast compression during the Viséan in Gaspésie.

Current strain regime in the Western Alps from continuous Global Positioning System measurements, 1996–2001

Four to six years of continuous measurements at 10 permanent Global Positioning System sites in the Western Alps show horizontal residual velocities of ,2 mm/yr with respect to stable Europe; uncertainties range from 0.3 to 1.4 mm/yr. These velocities and the associated strain-rate field indicate that the central part of the range is currently dominated by east-west extension, whereas the southern part shows north-south to northwest-southeast compression. The geodetic and seismotectonic data are consistent with a model where strain is essentially controlled by the counterclockwise rotation of the Adriatic microplate with respect to Eurasia. This rotation, together with the arcuate shape of the contact between the Adriatic microplate and the Alps, induces dextral shear kinematic boundary conditions across the Western Alps, with an additional divergence component in their central part and in Switzerland, and a convergence component in their southern part.

Sedimentology of the Pliocene Upper Onzole Formation, an inner-trench slope succession in northwestern Ecuador

The Pliocene Upper Onzole Formation exposed in the vicinity of Punta Gorda, near Esmeraldas, Ecuador, is composed mainly of fine-grained mud turbidites, having regular vertical sequences of sedimentary structures associated with a positive grading, and bioturbation restricted mostly to the tops of beds. The remainder of beds measured consist of volcanic ash, mud pelagite, and glauconitic silt–sand turbidites. Vertical sequential analysis of stratigraphic sections for the most part show no pronounced trends in bed thickness or grain size. Volcanic ashes are crystal–vitric tuffs occurring in four bedding styles: (A) normally-graded ashes with burrowed gradational tops and sharp wavy bases; (B) ashes that form part of a complex microstratigraphy consisting of thinly-bedded mudstone, silt–sand turbidites, tuffaceous mudstone, and ash beds; (C) less conspicuous ash laminae and ash-filled burrows; and (D) a tuffaceous bed with ash and mud swirled together in convolute layers. Ash chemistry suggests an Andean high-K calc-alkaline provenance. Facies relations, paleontologic data and regional geologic setting suggest sediment accumulation on an inner trench slope in a basin situated oceanward of the Pliocene trench–slope break. Faulting is extensive and reflects two deformational episodes, the youngest involving Holocene marine and fluvial terraces. Faults group into an older, northerly trending, listric set, and a younger, west–northwest-trending high-angle set. Fault striations of both sets suggest dominantly dip–slip motion. The faults reflect an episode of latest Pliocene through Quaternary trench-normal extension in the forearc, followed by trench-parallel extension. Both sets of faults crosscut large northeast-trending regional folds. Post-Pliocene changes in stress regime, from generally northwest–southeast compression, to east–west tension, to north–northeast–south–southwest tension probably reflect change in stress vectors that resulted from the ongoing collision of the leading edge of the South America plate with the thick oceanic crust of the Carnegie Ridge.

The Palaeoproterozoic Woodlands Formation of eastern Botswana-northwestern South Africa: lithostratigraphy and relationship with Transvaal Basin inversion structures

The Woodlands Formation (uppermost Pretoria Group) of eastern Botswana overlies thick quartzites of the Sengoma Formation (Magaliesberg Formation) and comprises a lower unit of interbedded mudrocks and fine-grained recrystallised quartzitic sandstones, succeeded by chaotic and very coarse-grained inferred slump deposits. Within the adjacent western region of South Africa, interbedded mudrocks and quartzitic sandstones stratigraphically overlying the Magaliesberg Formation are now assigned to the lower Woodlands Formation. Within the entire region, interference folding produced by northeast-southwest (F 1 and F 3) and northwest-southeast (F 2) compression, and concomitant faulting characterised inversion of the Pretoria Group basin. This deformation is of pre-Bushveld age and affected all units in the Pretoria Group, including the uppermost Silverton, Magaliesberg and Woodlands Formations, and intrusive Marico Hypabyssal Suite (pre-Bushveld) mafic sills. The Nietverdiend lobe of the Bushveld Complex, intrusive into this succession, was not similarly deformed. Movement along the major Mannyelanong Fault in the northwest of the study area post-dated Transvaal Basin inversion, after which the “upper Woodlands” chaotic slump deposits were formed. The latter must thus belong to a younger stratigraphical unit and is possibly analogous to apparently syntectonic sedimentary rocks (Otse Group) in the Otse Basin of eastern Botswana.

The Sudbury dyke swarm and its bearing on the tectonic development of the Grenville Front, Ontario, Canada

In the Southern Province, Ontario, southeast-striking diabase dykes of the 1235 Ma Sudbury continental swarm intrude folded Huronian strata and Paleo- and Mesoproterozoic plutons. The dykes are truncated at the Grenville Front, southeast of which irregularly oriented metadiabase dykes, correlative in age and chemical composition with the Sudbury swarm, cut gneissic fabrics in the Grenville Province. Some irregularities in dyke trend, including marked deflections in strike at and southeast of the Grenville Front mylonite zone, are primary features, resulting from dyke propagation across pre-existing structure and an associated change in paleostress field. However, other irregularities, in particular the markedly sinuous traces of many dykes, analogous to classic buckle fold forms, are attributable to superimposed (Grenvillian) deformation. The deformation history of the dykes is constrained by a distinctive sequence of macroscopic and microscopic structures. Early deformation, related to northwest-southeast compression and accompanying overthrusting, resulted in layer-parallel shortening and buckling of the dykes in a regime with elements of both pure and simple shear, a process accommodated internally by alignment, bending and kinking of plagioclase and other primary minerals. Coeval metamorphism, manifested by reaction coronas around primary olivine and Fe-Ti oxide, outlasted early deformation. A later period of deformation resulted in tightening of the buckle folds, superseded by faulting, in response to further strain accumulation. The equivalent microscopic-scale structures are late fractures and brittle-ductile microfaults which post-date plastic strain. A cause-and-effect relationship, whereby pre-metamorphic microstructures strongly influenced the location and development of syn- to post-metamorphic microstructures, suggests that the two phases of dyke deformation represent more or less a continuum, separated only by a period of decreased strain rate coinciding with the peak of metamorphism. In relation to existing models for the tectonic development of the northwestern Grenville orogen, the progression of dyke structures and their relationship to metamorphism are best explained by three principal stages: (1) at ≧ 1035 Ma, an early stage of penetrative shortening and overthrusting, correlated with buckling of the dykes and their depression to lower crustal levels; (2) an intermediate stage, commencing some time before 1020 Ma, when compressive stresses waned, enabling thermal relaxation, and ductile extension took place in the interior of the orogen, initiating passive uplift of the dykes; (3) at 1010-980 Ma, a stage of renewed thrusting during which the dykes were further deformed as they were uplifted across the ductilebrittle transition, documenting the final advance of the Grenville orogen toward its foreland.

Structure of the intraplate eastern Palmyride Fold Belt, Syria

The Palmyride Fold Belt is an aulacogen within the Arabian plate linked to the Levantine continental margin, which contains >10 km of Phanerozoic sediments. Inversion of this Mesozoic basin began during Late Cretaceous time, although most deformation was clearly post-Oligocene. Recent field mapping shows that almost all the structures have resulted from a phase of northwest-southeast compression followed by major dextral transpression. Early northwest-southeast compression resulted in northeast-southwest-aligned fold axes in the central and southeast Palmyrides, many of which were subsequently rotated by progressive dextral simple shear during Neogene to Recent time. The north-south-aligned sinistral Dead Sea fault is the major wrench fault aligned at 45° to the maximum compressive stress. The conjugate east-west-striking dextral Jhar fault and the northwest-southeast-aligned Bishri fault have accommodated limited dextral shear and clockwise rotation of northeast-southwest-aligned folds (Jebels Mrah, Ash Shaer). In the northern Palmyrides, dextral strike-slip faults aligned along Reidel fractures (R1) cut the flanks of earlier domal structures (Jebel Abu Rajmein). Subordinate antithetic R2 Reidel shears aligned north-northeast-south-southwest are trans-tensional sinistral strike-slip faults showing minor extension that cut through the earlier domal structures (for example, Ras' al Hawa depression in Jebel Abu Rajmein). Northwest-southeast-aligned faults (for example, Jebel Bilas cross-trend faults) show northeast-southwest extension at right angles to the Palmyride shortening direction. This extension parallels the major Euphrates graben trend in eastern Syria and northwestern Iraq and also parallels some aligned Pliocene-Pleistocene volcanic rifts south of Damascus and in the Jebel Druze (Jebel Arab) region of southern Syria and northern Jordan. A regional Palmyride strain ellipse is presented to explain the geometry of all these structures. The Ad Daww depression is a northeast-southwest-aligned transpressional basin containing around 2 km thickness of Tertiary sediments. The basin is bounded to the west by a sinistral shear couple along the Cherrife structural trend in the central Palmyrides, along the north by the dextral Jhar fault, and along the southeast by the compressional southeast Palmyrides. In the southeast Palmyrides, structural style is one of a series of northeast-southwest-aligned narrow anticlines in the Mesozoic platform sediments with upright to north-west-dipping, southeast-facing axial planes. Shortening amounts are very small in the eastern Palmyrides (∼1 km) and taken up almost entirely by folding, but these increase slightly toward the west to probably ∼20 km in the Cherrife region. Tight box folds and slightly asymmetric folds in the southeast Palmyrides have a localized basal detachment along the Triassic gypsum horizon, which in some examples flows into the fold core (Jebel Hayyan). There is no evidence for a major basal detachment underlying the Palmyrides. All folding and faulting can be related to disharmonic folding of the Mesozoic sediments above a Triassic evaporite sequence and subsequent dextral transpression. The lack of overthrusting explains the absence of any foreland basin caused by lithospheric flexuring on the Rutbah platform. The major Palmyride deformation in Miocene-Pliocene times was concomitant with the opening of the Red Sea, with the closing of the Bitlis-Zagros suture zone in eastern Turkey and central Iran, and with the main sinistral strike-slip motion along the Dead Sea fault.

Ramp-flat geometry within the central Kapuskasing uplift? Evidence from potential field modeling results

Potential field data constrained by seismic reflection, seismic refraction, geologic, and physical-properties data have been used to study the deep crustal structure of the central Kapuskasing uplift. The Val Rita block is interpreted as a thrust sheet of granulites detached at mid-crustal depths and uplifted along a ramp producing an arched, double granulite layer. A thin thrust sheet of southward-thickening granulites best describes the Groundhog River block, whereas simple thrusting along a ramp characterizes the Chapleau block. From this interpretation, a sequential evolution of the Kapuskasing uplift is proposed.At about 2450 Ma, northwest–southeast compression (σ1) resulted in variable thrusting along the Ivanhoe Lake fault zone. This was followed by a period of normal faulting, resulting in the juxtaposition of high- and low-pressure rocks across the Lepage and Saganash Lake faults, with possible truncation of the Val Rita block arch. Further rotation of σ1 to an east–west direction resulted in the reactivation of the Lepage and Saganash Lake faults as dextral transcurrent or transpressional faults accentuating the Val Rita block as an en echelon fold, and distorting the Matachewan dykes. Later (about 2140 Ma), rotation of σ1 into a southwest–northeast direction brought transpression to a halt and a period of intermittent dyke and carbonatite intrusion was initiated, which lasted to about 1200 Ma or just prior to the Grenville orogeny.

Post-Keweenawan compressional faults in the eastern Lake Superior region and their tectonic significance

GLIMPCE aeromagnetic data in eastern Lake Superior are characterized by a series of strong easterly- and northeasterly-oriented gradients that relate to mapped post-Keweenawan faults occurring along the eastern shore. The reversed nature of three of the faults is established through field observations and potential field modelling. Middle Keweenawan volcanic rocks at Mamainse Point are in fault contact on their south side with upper Keweenawan sandstone of Bayfield–Jacobsville type. Gravity modelling suggests that the fault is a low angle thrust dipping to the north. Field observations and high-resolution aeromagnetic data show that it extends inland along the southern margin of the Batchawana Greenstone Belt for at least 17 km. To the west, the Mamainse Point fault may extend across eastern Lake Superior to the Keweenaw Peninsula, linking several offsets in the seismic data that are consistent with the same attitude and sense of displacement. Along the south side of Batchawana Bay at Havilland, sandstones of Bayfield–Jacobsville type are isoclinally folded against a package of upthrust older rocks that include drag-folded middle Keweenawan volcanics. At Grindstone Point, north of Cape Gargantua, a reverse fault separating isoclinally-folded upper Keweenawan sandstones from Archean basement may, on aeromagnetic evidence, be an eastward extension of the Michipicoten Island fault.These faults mark a significant change in the style of late compressional tectonism observed within the Midcontinent Rift. All cut Keweenawan rocks across strike. The inference is that broad north–south or northwest–southeast compression, consistent in timing and orientation with the Grenville Orogeny, led to a reversal of movement along the major graben faults in western Lake Superior and was taken up in the eastern region by reverse faults oriented normal to the extensional axis of the rift.

Clockwise Block Rotations in the Perija Mountains, Venezuela

Paleomagnetic analyses of Mesozoic sedimentary rocks (8 sites, 115 samples) from the Perija Mountains, Venezuela, yield well-grouped directions of both polarities. Fold tests imply that the magnetization of the Jurassic La Quinta Formation is pre-mid-Cretaceous in age and that the Lower Cretaceous Cogollo Group acquired its magnetization prior to its mid-Tertiary deformation. These data as well as the results from the Cretaceous Apon and La Luna Formations yield consistent northeast or southwest declinations and inclinations corresponding to the present latitude of the sampling area. The clockwise rotations indicated by these data are interpreted to be the result of rotation of fault-bounded blocks in a left-lateral strike-slip zone. Faults such as the Cogollo, La Ge, and Totumo faults which intersect the Perija-Tigre fault at low angles, had initially a north-northwest strike. The Late Oligocene northwest-southeast compression which initiated the uplift of the Perija Mountains rotated these faults to their present north-northeast orientation imparting the observed left-lateral slip. Our paleomagnetic data measure the corresponding rotation of the fault-bounded blocks. Model calculations suggest that a displacement on these faults of less than ten kilometers would be sufficient to cause the observed approximately 45[degrees] clockwise rotations.