Dextral Transpression Research Articles

Mineral fabrics unravel syn-magmatic deformation and constrain Permian and Jurassic tectonic regimes in arc plutons from the southern Colombian Andes

ABSTRACT Mineral fabric analyses of Permian La Plata and Jurassic Paez arc plutons provide information about the interplay of tectonic deformation and magma emplacement in the southern Colombian Andes. We combined field mapping, microstructural data, AMS fabrics, rock magnetic properties, vorticity analysis, and thermobarometry to study the internal fabrics of the plutons. Our results indicate that the studied plutons have well-developed NE magmatic and mylonitic foliations, with the latter being distinctive for the Permian pluton. These foliations are consistent with the N65°E/63°W Permian and N43°E/82°W Jurassic AMS magnetic fabrics. Magnetic lineations record subhorizontal (Permian) and vertical (Jurassic) magmas upwards in the NE–SW trajectories. The orientations of all the plutonic fabrics indicate structurally controlled magma emplacement consistent with compressional tectonics and bulk NW–SE crustal shortening. Microstructural features recorded in the plutons reveal magmatic to sub-magmatic and solid-state deformation developed at high- to low-temperatures during their syn-tectonic emplacement history. Thermobarometric calculations indicate similar shallow emplacement conditions (~10 km depth) in the La Plata and Paez plutons. The application of vorticity analysis shows that the magnetic fabrics and syn-emplacement of the Permian pluton developed by simple-shear dominated dextral transpression, whereas the AMS fabrics for the Jurassic syn-tectonic intrusion may have been associated with pure-shear contractional deformation.

Oroclinal arcs of the Variscan Belt: a consequence of transpression during the consolidation of Pangaea

Arcuate traces of large structures characterize many mountain chains. The Variscan Belt is not an exception and traces one of the tightest oroclines on Earth, the Ibero-Armorican Arc, as well as more open oroclines in the Eastern Moroccan Meseta, Central Iberia, the French Massif Central and the Bohemian Massif. All the Variscan arcs are considered to be true or secondary oroclines and are late orogenic features, but their timing and mechanisms of deformation differ. Models explaining their origin have been proposed for some individual arcs and are generally controversial. This paper aims to interprete the ensemble of Variscan arcs, highlighting their age relative to previous orogenic features as well as to those associated with arc development. Several mechanisms operated to form the arcs, the most important being ductile transcurrent shearing and rigid–plastic indentation, with shortening parallel to the orogen having a secondary role. These mechanisms acted at different time intervals, their participation or relative importance differs for each arc and they gave rise to distinct associated structures. The development of the arcs is viewed as related to late Variscan dextral transpression provoked by the displacement of Laurussia to the east relative to Gondwana during the Serpukhovian to early Permian.

Kinematics of the Clarke River Shear Zone (northeastern Australia) and implications for the tectonic evolution of the Tasmanides

The Clarke River Shear Zone marks the boundary between the Thomson and Mossman orogens in the northeastern Tasmanides. To better understand the kinematic history of this east-northeast-trending shear zone, and its role during the tectonic history of eastern Gondwana, we investigated rock deformation along a segment of the Clarke River Shear Zone. Based on field observations, interpretation of aeromagnetic data and microstructural analysis, we show that the Clarke River Shear Zone experienced at least three phases of reactivation. Gently plunging stretching lineation and kinematic indicators, such as steeply plunging asymmetric folds and asymmetric porphyroclasts, correspond to an early phase of sinistral strike-slip ductile shearing. Subsequent general dextral shear deformation is suggested based on the presence of S-tectonites, symmetric porphyroclasts and kinematic indicators with dominant dextral and subordinate sinistral shear senses. This phase, which occurred under amphibolite-facies metamorphic conditions, was contemporaneous with the emplacement of the Craigie Tonalite, for which we obtained a new zircon U–Pb crystallisation age of 410.3 ± 1 Ma. A later reactivation phase along the Clarke River Shear Zone, under brittle conditions, is indicated by the presence of cataclasite zones and chlorite–epidote alteration. Based on our findings, and regional information on the tectono-magmatic evolution of northeastern Queensland, we suggest that the Clarke River Shear Zone was possibly a sinistral transform fault during the Silurian. During the Devonian, it is possible that deformation along the Clarke River Shear Zone was related to the transition between the non-retreating subduction zone in the north, and the highly mobile (retreating) plate boundary farther south. However, the dextral kinematic component associated with this deformation is not fully explained. Brittle deformation, likely associated with dextral transpression during the Carboniferous, marked the last stage of activity along the Clarke River Shear Zone.

Conjugate shear model for emplacement of mid-crustal plutons during dextral transpression, southern Nova Scotia, Canada: tectonic and petrological consequences

The South Mountain Batholith (SMB) is a syn-tectonic composite batholith emplaced in the upper crust within the Meguma terrane between 380 and 370 Ma during the later stages of the Neoacadian orogeny. Coeval plutons in southern Nova Scotia are surrounded by mid-crustal (∼4 kbar) andalusite–staurolite aureoles and are discordant to northeast-trending, regional Neoacadian folds. Detailed field studies, combined with published results, indicate emplacement within a dextral transpressional regime during the transition from distributed (D1) to focused heterogeneous strain (D2), which provided vertical conduits that facilitated magma ascent. The Port Mouton Pluton intruded along P-orientated crustal-scale fractures as a series of subvertical granitic sheets, which were progressively rotated and folded with ongoing dextral shearing. By contrast, the Barrington Passage Pluton intruded between crustal-scale, antithetic (sinistral) P-shear fractures and spread laterally between them as pulsed increments to form a layered, subhorizontal, and sill-like complex resembling the laccolithic structure of the SMB. The SMB was emplaced below the Meguma Supergroup, with magma derived from underthrust Avalon terrane and Silurian Rockville Notch Group. The lack of mantle components in the SMB suggests transpressional orogenesis facilitated conductive crustal heating without significant mantle addition, consistent with low p-wave velocities of Meguma lower crust. 400–355 Ma zircons, recorded either as inherited grains in granites or in felsic granulite xenoliths, imply the Neoacadian thermal anomaly extended for 45 Ma, but magmatism represented only ∼40% of that perturbation.

Convergence and transpression along the paleo-Pacific margin of Australia during the Paleozoic: insights from the Pinnak Sandstone at Mallacoota, northeast Victoria

Underpinning almost every Paleozoic paleogeographic reconstruction and tectonic model of the Lachlan Orogen is the interpretation of the structural geology related to crustal collision and amalgamation. Bounding the eastern margin of the Lachlan Orogen is a north-trending corridor of Cambrian–Ordovician turbidites that in northeast Victoria are recognised as the Pinnak Sandstone or previously known as the Mallacoota Beds. This sequence contains an accretionary prism related early S* foliation, syn-sedimentary dykes, slump structures, early faults or décollements and early quartz veins. These indicate anisotropic compaction by fluid extraction in a shallow marine turbidite sequence that underwent localised deformation (D1), which can be attributed to the Benambran Orogeny (ca 450 Ma). There was a change in the rheological properties during the ensuing late Silurian D2 Bindian Orogeny (ca 420 Ma). This event produced north–south-trending upright, east-verging folds (F2) with a marked partitioning of strain between the different sedimentary units. Folds plunge south at ∼30°, are tight to isoclinal, with local collapsed hinges and limb thrusts. This D2 deformation along the paleo-Pacific margin involved dextral transpression and a southerly propagation of the deformation front. Superimposed on these structures is an east–west paleostress pattern and two episodes of faulting (D3 and D4). In the first, brittle strike-slip faults were accompanied by ductile folding related to east–west shortening during the Middle–Late Devonian Tabberabberan Orogeny (ca 385–370 Ma). These were accompanied by the emplacement of the Sandpatch Point Granite. The second episode (D4) of strike-slip faulting reactivated many of the earlier faults during further east–west shortening and was related to the Carboniferous Kanimblan Orogeny (ca 354–343 Ma). This is best seen close to the granite contact where the strong crystallised granite reinforced the contact metamorphosed greywacke sequence, inhibiting ductile deformation so that faults deformed in a brittle fashion.

Transtension or transpression? Tectono-metamorphic constraints on the formation of the Monte Grighini dome (Sardinia, Italy) and implications for the Southern European Variscan belt

This work presents an integrated structural, kinematic, and petrochronological study of the Monte Grighini dome within the Variscan hinterland–foreland transition zone of Sardinia (Italy). The area is characterised by dextral transpressive deformation partitioned into low- and high-strain zones (Monte Grighini shear zone, MGSZ). Geothermobarometry of one sample of sillimanite-bearing mylonitic metapelite indicates that the Monte Grighini shear zone developed under high-temperature (~ 625 °C) and low-pressure (~ 0.4–0.6 GPa) conditions. In situ U–(Th)–Pb monazite geochronology reveals that the deformation in the shear zone initiated at ca. 315 Ma. Although previous studies have interpreted the Monte Grighini shear zone to have formed in a transtensional regime, our structural and kinematic results integrated with constraints on the relative timing of deformation indicate that it shows similarities with other dextral ductile transpressive shear zones in the Southern European Variscan belt (i.e., the East Variscan Shear Zone, EVSZ). However, dextral transpression in the Monte Grighini shear zone started later than in other portions of the EVSZ within the framework of the Southern European Variscan Belt due to the progressive migration and rejuvenation of deformation from the core to the external sectors of the belt.Graphical abstract

A Complex Interplay Between Pluton Emplacement, Tectonic Deformation, and Plate Kinematics in the Cretaceous Sierra Nevada Magmatic Arc, California

AbstractThe relation of plate kinematics to the structural record of arc plutons and their host rocks is complex and still not fully understood. We address this issue through a combination of field mapping, structural analysis, anisotropy of magnetic susceptibility analysis, and fabric modeling in the Late Cretaceous Tuolumne Intrusive Complex, Sierra Nevada, California. A pattern of anti‐clockwise rotation from ∼NNW–SSE to WNW–ESE steep foliations and change in fabric ellipsoid shape from oblate to prolate was revealed in successively emplaced Kuna Crest (∼95–92 Ma), Half Dome (∼92–89 Ma), and Cathedral Peak (∼89–84 Ma) granodiorites. The numerical model indicates that the Kuna Crest was emplaced in a transpressional setting with an angle of convergence α = 60–40°, whereas the Half Dome and Cathedral Peak required simultaneous vertical constriction overprinted by transpression with α = 35–15°. This transition, which occurred at ∼92 Ma, is accompanied by a shallowing of the lineation plunge observed also in other ∼88–84 Ma central Sierra Nevada plutons. Provided that the Cretaceous Sierra Nevada arc was constructed during overall dextral transpression, these transitions reflect significant changes in kinematics, where ∼107–92 Ma plutons were emplaced during pure shear‐dominated transpression, which was followed by a transition to wrench‐dominated transpression recorded in ∼92–84 Ma plutons. Such a transition in kinematics is explained as a result of progressively increasing obliquity of the relative convergence of the Farallon plate subducting beneath the North American continental margin, in agreement with most paleogeographic reconstructions.

Jura-Cretaceous synorogenic magmatism and relations to supercontinental rifting in the northwestern U.S. Cordillera

AbstractChanges in lithospheric composition and strength associated with rifting of the Rodinian supercontinent (ca. 780–485 Ma) served to localize Mesozoic tectonic activity in the McCall region of west-central Idaho, USA. Although discrimination diagrams support slab failure melting, ca. 150–121 Ma calcic magmatism in the Hazard Creek complex and chemically similar intrusive rocks of northeastern Oregon, USA (e.g., Cornucopia stock), is best explained by progressive loading of oceanic crust. Westward-directed thrusting across the Laurentian continental margin and adjacent island-arc terranes (Olds Ferry/Wallowa; Blue Mountains province) led to partial melting of metabasaltic source rocks at pressures and depths sufficient for garnet stability (>10 kbar, ~35 km). Early pluton emplacement during right-oblique arc–continent collision overlapped in time with subduction zone reorganization, which involved a change in dip direction (westward to eastward) under the accreting Blue Mountains block. Ensuing magmatic activity predated and was concurrent with ca. 116 Ma thrust reactivation of the Wallowa/Olds Ferry boundary, which temporally overlapped with contractional deformation along the terrane–continent accretionary boundary. Later synorogenic magmatism was focused along the accretionary boundary, where rising anatectic melts were caught between the eastward-impinging/northward-migrating (obliquely colliding) terrane block and continental margin backstop (Rodinian rift architecture). Middle Cretaceous dextral transpression viewed in this manner precludes the need for an outboard “hit-and-run” collider (Insular superterrane) and thus, a post-accretion shear zone in western Idaho.

Subsurface structure of the Tarfaya-Laayoune-Dakhla basin (West African Craton Margin, Morocco) revealed from the analysis of aeromagnetic data

The Tarfaya-Laayoune-Dakhla basin is the largest coastal basin in the Moroccan Atlantic margin. It is filled by a thick Meso-Cenozoic sedimentary cover that overlies unconformably the NW margin of the West African Craton and its Proterozoic to Paleozoic peripheral orogenic belt. In this study, we aim to decipher the structure of hidden part of these terranes and discuss the tectonic evolution of the basin. To achieve these objectives, aeromagnetic data were processed by the application of edge enhancement and depth estimation operators. The combination of their results with geological information and boreholes data allowed the construction of a detailed structural map of the subsurface of the study area. The resulted map delineates three distinct tectonic domains: the Northeastern Domain, the Western Domain and the Southern Domain. These domains exhibit geological evidences of four major tectonic periods: the Eburnean, Pan-African and Variscan orogenies, and the Triassic rifting. The Eburnean dextral transpression event formed a shear-zone with a large magmatic body in the Western Domain. This domain was not affected by the subsequent Pan-African orogeny and the following Ediacaran intracontinental rifting phase. Meanwhile, this event led to the formation of an N- to NNE-trending large half-graben structure in the Northeastern Domain, and an Ediacaran bimodal magmatic series in the Southern Domain. The Paleozoic Variscan orogeny has reactivated the N- to NNE-trending normal faults, allowing thrusting in these domains. Eventually, the Central Atlantic opening event reworked pre-existing reverse faults resulting increases in depth in the Northeastern and Western Domains, meanwhile, the extension was reduced in the Southern Domain.

Petrology, age, and tectonic setting of the Park Spur pluton, Aspy terrane, central Cape Breton Highlands, Nova Scotia, Canada

The Park Spur pluton was emplaced in the Aspy terrane of the central Cape Breton Highlands at 374.2 ± 2.9 Ma (U–Pb zircon). It consists mainly of muscovite-biotite monzogranite with small areas of garnet-bearing muscovite monzogranite and abundant pegmatite and aplite dykes. On its northern margin the pluton intruded metamorphic rocks of the Ordovician–Silurian Cape North Group and associated orthogneiss and on the south it intruded low-grade metamorphic rocks of the Silurian Calumruadh Brook Formation. Deformation along the southern margin of the Park Spur pluton is consistent with emplacement during dextral transpression between the Bras d’Or and Aspy terranes. The Late Devonian age combined with S-type petrological characteristics show that the Park Spur pluton and nearby Canal pluton are related to the ca. 375 Ma Black Brook Granitic Suite, all interpreted to have been emplaced during later stages of the Acadian orogeny in Ganderia as a result of delamination.

Redefinition of the Petersburg batholith and implications for crustal inheritance in the Dinwiddie terrane, Virginia, USA

Field relations as well as geochemical and petrologic studies of meta-igneous rocks assigned to the Pennsylvanian–Permian Petersburg batholith identify at least two distinct rock types: foliated metagranitoid gneiss and massive to porphyritic granite. Foliated metagranitoid gneiss of mostly granodioritic composition is geochemically distinct from associated massive and porphyritic granitic rocks. These gneissic rocks yield radiometric ages from ca. 425 Ma to ca. 403 Ma and document that many of the rocks assigned to the late Paleozoic Petersburg batholith are 100 m.y. older than the youngest portions of the composite batholith and are part of an earlier infrastructural terrane. Two samples of massive equigranular granite southwest of Petersburg, Virginia, yield ages of ca. 321 Ma and ca. 317 Ma, which are 15–20 m.y. older than ca. 300 Ma ages for porphyritic granite, massive granite, and monzodiorite near Richmond, Virginia. Geologic mapping shows that the Early Pennsylvanian granite southwest of Petersburg is separated from Late Pennsylvanian to early Permian granite near Richmond by a map-scale septum of Silurian–Devonian foliated metagranitoid gneiss, referred to herein as the informal Pocoshock Creek gneiss. Laser ablation–inductively coupled plasma–mass spectrometry data from one sample of a quartz-muscovite felsic schist xenolith show a peak age mode of ca. 529 Ma that we interpret to be the maximum depositional age. Inherited zircons from foliated metagranitoid gneiss and massive equigranular granite range from ca. 631 Ma to ca. 376 Ma, but many are Cambrian. Neoproterozoic–Cambrian quartz-muscovite felsic schist and amphibolite, Silurian–Devonian Pocoshock Creek gneiss, and Pennsylvanian–Permian granite comprise a fault-bounded terrane referred to herein as the Dinwiddie terrane. Ages of inherited cores in zircon from igneous rocks and limited detrital zircon geochronology suggest the terrane is of peri-Gondwanan affinity. U/Pb ages of healed fractures in zircon grains from foliated metagranitoid gneiss indicate low-grade deformation of the gneiss at ca. 378–376 Ma, while ca. 320–280 Ma rims on many grains record intrusion of late Paleozoic granite. The temperature-time-deformation history of the Dinwiddie terrane is distinct from the adjacent Goochland and Roanoke Rapids terranes. Orogen-scale dextral transpression likely translated the Dinwiddie terrane southward during the Alleghanian orogeny, at which time they were intruded by Pennsylvanian to Permian granite.

Oblique lateral extrusion during dextral transpression along the Beardmore-Geraldton belt, Canada

The Beardmore-Geraldton belt (BGB) is a boundary zone between the Wabigoon and Quetico subprovinces of the Archean Superior Province, Canada. It consists of interleaved metavolcanic and metasedimentary panels that were imbricated during an early D1 thrusting event and regionally folded during a D2 event. D1 and D2 steepened stratigraphy and produced contact-parallel shear zones and a regional bedding-parallel cleavage, which controlled the development of a D3 dextral transpression zone. During D3, the panels deformed by pure shear and were overprinted by a second regional cleavage. Bedding and the early cleavage acted as subvertical planes of anisotropy that were reactivated during D3 dextral shearing. Z-shaped drag folds and pre-existing D1 and D2 folds are parallel to a shallowly-plunging (20°–30°) stretching lineation, which together with the fold axes rotated towards the extensional flow apophysis during D3 shearing. As the lineation and fold axes are oblique to the sub-horizontal shear direction expressed by the intersection between synthetic shear bands and the early cleavage, the reactivated shear zones are transpression zones with shallowly-plunging lateral extrusion and triclinic symmetry. Thus, the shear zones are transpressive within a regional tectonic transpression zone that extends across the BGB and over 500 km along the Quetico-Wabigoon boundary.

Palinspastic restoration of Variscan oroclines – implications for dextral transpression and terrane affinities

Abstract The Paleozoic Variscan orogen in Europe has a markedly circuitous trace for which several different origins have been postulated, including deformation around promontories on the colliding continental margins, extrusion within the collision zone, folding of a ribbon continent and collision with a substantial dextral component. Adopting the latter assumption, unfolding of the large, steeply plunging folds (‘oroclines’) of Iberia and the Moroccan Meseta requires more than 4000 km of dextral lateral translation of Laurussia with respect to Gondwana. Constraints on the age of the folding require that this lateral translation occurred in mid-late Carboniferous time. Significant dextral translation of Laurussia with respect to Gondwana late in the Variscan collision is supported by palaeomagnetic data for the two supercontinents, although the exact timing of this relative motion is not well constrained due to large uncertainties in the palaeomagnetic pole positions. Our palinspastic reconstruction of the major Variscan folds of Iberia places the rocks of the South Portuguese Zone of western Iberia adjacent to the Rhenish Massif; for instance, as part of Avalonia. By the same token, the Sehoul Block in the Moroccan Meseta probably originated at the western end of Cadomia, although nothing in our analysis precludes it also being derived from Avalonia.

Structural Restoration of Cretaceous Inversion Events in the Bjørnøyrenna Fault Complex, Western Barents Shelf

Inversion structures including folds, reverse faults are observed along the Bjørnøyrenna Fault Complex in the western Barents Sea, although the fault complex is extensional in origin and developed in mid-Jurassic to Early Cretaceous. Subsidence along the fault complex was interrupted in Early Cretaceous (Valanginian to early Barremian) because of syn-rift localized tectonic inversion, itself related to the uplift of the Loppa High. The Early Cretaceous inversion caused dextral transpression along the boundary faults adjacent to the Loppa High. The second phase of inversion is interpreted to be Late Cretaceous (mid-Cenomanian) in age, coeval to the deposition of the Kolmule Formation in the Bjørnøyrenna Fault Complex. The later phase of compression is of regional significance and related to NW‒SE directed far field stresses in Late Cretaceous which caused head-on inversion in the study area. The aim of present study is to identify and decipher eventual Cretaceous inversion structures in the Bjørnøyrenna Fault Complex by means of structural restoration. To these aims, 2D MOVETM, structural modeling and analysis software by Midland Valley Exploration Ltd (Glasgow, Scotland), is used and three key seismic lines crossing the central and northern segments of the Bjørnøyrenna Fault Complex are restored. Key profiles 1 and 2 reveal null point positions at the base of the Cretaceous (Hekkingen Formation). Null point positions show progressive compressional inversion of syn-rift Early Cretaceous deposits (Knurr Formation). Below and above null points the geometries of the restored faults show normal and reverse faulting respectively. The results of the restored key profiles 1 and 2 confirm reverse faulting at the Lower Cretaceous triggered by inversion of the study area. The restored sections also show positive inversion features associated with folding of the hanging wall of the base of the Upper Cretaceous (Kolmule Formation). The reconstruction of the amount of eroded material on the footwall block also suggests reverse faulting of the base of the Upper Cretaceous. In key profile 3 the footwall block is eroded up to the base of the Upper Cretaceous (Kolmule Formation) due to the uplift of the Loppa High. The corresponding restored section shows a compressional anticline associated with both Early and Late Cretaceous inversion events.

Combined X-Ray Computed Tomography and X-Ray Fluorescence Drill Core Scanning for 3-D Rock and Ore Characterization: Implications for the Lovisa Stratiform Zn-Pb Deposit and Its Structural Setting, Bergslagen, Sweden

Abstract We present the results of a pilot study that integrates automated drill core scanning technology based on simultaneous X-ray computed tomography (XCT) and X-ray fluorescence (XRF) analyses to provide high-spatial-resolution (<0.2 mm) information on 3-D rock textures and structures, chemical composition, and density. Testing of its applicability for mineral exploration and research was performed by scanning and analyzing 1,500 m of drill core from the Paleoproterozoic Lovisa stratiform Zn-Pb sulfide deposit, which is part of a larger mineral system also including Cu-Co and Fe-(rare earth element) mineralization, hosted by the highly strained West Bergslagen boundary zone in south-central Sweden. The obtained scanning data complements data derived from structural field mapping, drill core logs, and chemical analysis as well as from multiscale 3-D geologic modeling at Lovisa. Data integration reveals macro- and mesoscopic folding of S0/S1 by asymmetric steeply SE-plunging F2 folds and N-striking vertical F3 folds. Stretching lineations, measured directly from the scanning imagery, trend parallel to F2 fold hinges and modeled ore shoots at the nearby Håkansboda Cu-Co and Stråssa and Blanka Fe deposits. The textural character of the Lovisa ore zones is revealed in 3-D by XCT-XRF scanning and highlight remobilization of Zn and Pb from primary layering into ductile and brittle structures. The downhole bulk geochemical trends seen in scanning and traditional assay data are generally comparable but with systematic variations for some elements due to currently unresolved XRF spectral overlaps (e.g., Co and Fe). The 3-D deformation pattern at Lovisa is explained by D2 sinistral transpression along the West Bergslagen boundary zone in response to regional north-south crustal shortening at ca. 1.84–1.81 Ga. Local refolding was caused by D3 regional east-west crustal shortening resulting in dextral transpression along the West Bergslagen boundary zone, presumably at ca. 1.80–1.76 Ga. Based on polyphase ore textures and modeled ore shoots aligned to F2 fold hinges, we postulate that D2 and D3 transpressive deformation exerted both a strong control on ore remobilization and the resulting orebody geometries at Lovisa and neighboring deposits within the West Bergslagen boundary zone. We conclude that the combined XCT-XRF drill core scanning technique provides a valuable tool for 3-D ore and rock characterization, generating continuous downhole data sets, with the potential for increasing precision and efficiency in mineral exploration and mining.

Deformation and uplift at the transition from oceanic to continental subduction, Sumba Island, Indonesia

• The Australian margin boundary collision produces forearc dextral transpression . • Interplate transfer of shear stress and forearc block escape induce trench migration. • Northeastward folding has been propagating across Sumba Island since the Pliocene . • One-third of Sumba Island was inundated by flexure and gravitational collapse . The transition along the strike of the Sunda subduction zone, from oceanic subduction in the west to subduction of continental Australian lithosphere in the east is envisioned as one of the canonical examples of the structural changes that take place within an overriding plate when a continental lithosphere wedge enters a subduction zone. Yet, the along-strike offset of the trench toward the Australian margin represents a structural response opposite to the predictions of numerical models. To understand this paradox, we analyse the morphotectonic evolution of Sumba island located at the transition from oceanic to continental Indo-Australian lithosphere subduction. Drainage evolution allows us to constrain the topographic evolution of the island since the Pliocene. Flights of uplifted coral reef terraces document Quaternary deformation. Focal mechanisms of shallow crustal earthquakes constrain the current stress field. Together, these data reveal that the island is affected by dextral en-échelon folding. Offshore, west of the island, reverse and strike-slip focal mechanisms evidence an active dextral transpressional zone. The emergence of the island and dextral shearing of the accretionary prism were triggered by subduction of the western lateral boundary of the Australian continental margin. We contend that the Plio-Quaternary tectonic evolution of the region, with transpression and migration of the trench toward the Australian margin is primarily dictated by shear stress transfer from the lower plate to the overriding plate, favored by strong interplate coupling, and by southwestward escape of the Savu-Sumba block following the impingement of the Australian continental margin against Timor island.

Flow of the partially molten crust in the Variscan foreland revealed by U–Th–Pb dating of metamorphism, magmatism and deformation (Agly Massif, Eastern Pyrenees)

In this contribution, we investigate the spatial and temporal evolution of mid-crustal flow in the Agly Massif (North Pyrenean Zone) that represents the southern foreland of the Variscan orogenic plateau. In the Agly Massif, the middle crust is represented by an Ediacarian–Devonian series of metasedimentary rocks that recorded high-grade metamorphism synchronously with crustal thinning (D2) and dextral wrenching (D3) during Carboniferous. D2 crustal thinning formed a penetrative S2 flat-lying foliation and localized C2 extensional shear zones with a top-to-the-North kinematics. D2 planar fabrics are deformed by a D3 dextral transpression localized into a two-kilometer wide high-strain zone. We performed LA-ICPMS U–Th–Pb dating on zircon and monazite from small magmatic bodies and from metamorphic rocks showing strain features relative to D2 and/or D3. Our results, compiled with published data, argue that the middle crust of the Agly Massif reached high-temperature and suprasolidus conditions at ca. 325–320 Ma and was partially molten until ca. 300 Ma. They also indicate that the D2 thinning and top-to-the north shearing was active from ca. 325–290 Ma. D2 extension and D3 transpression were synchronous from ca. 308–290 Ma. Making a comparison with the Pyrenean Axial Zone, the Montagne Noire and the French Central Massif, we propose a two-step tectonic model for the mid-crustal flow with a horizontal flow towards South in both the orogenic plateau and the southern forelands between ca. 325 and 310 Ma and locally reoriented into an E–W longitudinal flow between 310 and 300 Ma in high-strain dextral strike-slip shearing domains.

Strain localisation and transcurrent reactivation in the granulite facies Kalinjala Shear Zone at Port Neill, South Australia

The central portion of the Kalinjala Shear Zone, in the Port Neill region, South Australia, is recognised as a northeast-trending anomalously magnetic corridor coinciding with a sub-vertical transpressional dextral shear zone. This shear zone is a primary lithospheric structure, separating the nucleus of the Archean Gawler Craton from a suite of Paleoproterozoic granites (ca 1850 Ma) and earlier paragneiss. At Port Neill, there is an apparent 17 km dextral offset of a pre-existing Bouguer gravity anomaly by the Kalinjala Shear Zone. This is attributed to Kimban-aged (ca 1740–1690 Ma) deformation that is accompanied by peak granulite facies metamorphic conditions reaching pressures of ∼1.0 GPa and temperatures of ∼800 °C. This is consistent with the presence of leucosomes, within the sheared fabric, in the axial surface of mylonitic folds and in mafic gneisses. Strain has been localised within and adjacent to mafic dykes, predominantly hosted in the granitic rocks, with the highest strained areas having the greatest evidence of leucosome extraction. The local orientation of stretching lineations in this shear zone do not necessarily correlate with the transport direction but may reflect along- and across-strike variations in finite shear strain into alternating zones dominated by either constrictional or a superimposed flattening strain. Overprinting the dominant dextral movement are sinistral shears, with horizontal strike-slip movements, which post-date the peak metamorphic assemblages and are generally confined to sites containing late K-feldspar-rich pegmatites. The major movement recorded in the Kalinjala Shear Zone was both horizontal and vertical, occurring in response to regional exhumation of the Cornian-aged (ca 1850 Ma) sequences in the east, during the Kimban Orogeny, and was basically controlled by the nature of lithospheric plate convergence during accretion of a continental backarc onto the Gawler Craton. KEY POINTS Dextral transpression with late-sinistral along-strike extension and east-up exhumation is associated with 17 km of horizontal offset along the central section of the Kalinjala Shear Zone. Deformation is dominated by three major Kimban-aged (ca 1730–1690 Ma) events, which overprint older ensialic backarc metasedimentary rocks, orthogneiss and granites of the Donington Suite. Granulite facies metamorphic conditions control the nature of deformation and strain distribution within the shear zone. The Kimban events record the collision of the Gawler Craton nucleus with the southern margin of the North Australian Craton.

IN-SIMS zircon U-Pb geochronology from the Snowbird tectonic zone Large igneous Province (STZ LIP), western Churchill Province, Canada

IN-SIMS (in-situ secondary ionization mass spectrometry) micro-zircon geochronology resolves the age of the Chipman and Kazan mafic dike swarms exposed along the central segment of the Snowbird Tectonic Zone (STZ), the geophysically defined feature separating the Rae and Hearne subprovinces of the western Churchill Province. Long-considered syn-kinematic with respect to Paleoproterozoic transpressional deformation, the primitive composition of the STZ Large Igneous Province (LIP) and granulite grade metamorphic overprint has made establishing crystallization ages difficult. In-situ U-Pb isotopic analysis of micro-zircon yielded scattered and discordant results, except for four zircon crystals, which yielded a weighted mean of 2113 Ma +/- 22 (n = 8 analyses; MSWD: 0.46). This age is similar to other mafic complexes in the western Churchill Province including the Griffin Gabbro sills interpreted to have formed during opening of the Manikewan Ocean. Nd isotopic analyses of Kazan dikes paired with geochemical data suggest that the Chipman and Kazan dikes were asthenospherically-derived but contaminated by a metasomatized lithospheric root. At ca. 2.1 Ga, the STZ was the site of intracratonic extension within a coherent Rae-Hearne crustal block (Snowbird LIP Phase I). Existing data suggests that subsequent tectonism led to focused dextral transpression along the trace of the dike swarm and attests to a long-lived interplay of preexisting weaknesses and deformation.

Strain pattern and vorticity analysis in the transpressional Kamandan area within the Sanandaj-Sirjan Metamorphic Belt, Iran

New structural data on radiolarite and quartz mylonite rocks from the well-exposed Kamandan area are presented with the aim to shed light on the quantitative finite strain and vorticity analyses and strain partitioning along the Zagros transpression zone. Duplex structures, imbricate fans, thrust sheets and shear zones are well developed in the Kamandan region, located in the Sanandaj-Sirjan Metamorphic Belt, western Iran. Kinematic shear sense indicators associated with Kamandan shear zone show top-to-the-NE sense of ductile shearing deformation. Integrated quantitative finite strain and kinematic vorticity analyses of deformed rocks indicate deformation pattern and geometry. The strain parameter measurements vis. strain ratios and strain ellipsoid shape (k) show that radiolarite and quartz mylonite rocks have deformed in the constrictional to approximately plane-strain conditions. The quartz mylonite rocks highlight general-shear (sub-simple shear) deformation (Wm = 0.68–0.81) associated with the spatial increase in simple-shear component (47%–60%) from NE to SW. Our results of structural, strain, and vorticity analyses confirm a dextral transpression with strain partitioning into the pure-shear and simple-shear components in the Kamandan shear zone. Estimated Wm-values display the θ angle between the transpressional boundary and the maximum instantaneous strain axis (ISA) is the range from 63 to 69°. Accordingly, the oblique convergence angle (α) between the flow apophyses was estimated at about 35–48° that causes a horizontal shortening of ∼32–42% perpendicular to the boundaries of the Kmandan shear zone.