Oblique Continental Collision Research Articles

Topography Response to Horizontal Slab Tearing and Oblique Continental Collision: Insights From 3D Thermomechanical Modeling

AbstractThe horizontal propagation of slab detachment (slab tearing) is known to control lateral migration of the mountain uplift along the collisional belt. However, along‐strike differential collision due to an oblique passive margin geometry can make the topography response more complex. In this study, we employ 3D thermomechanical modeling to distinguish between the lateral migration of the mountain topography driven by slab tearing and oblique continental collision itself. In our models, slab breakoff is triggered by the transition from oceanic to continental subduction, occurring earlier on one side of the passive margin than on the other due to the initial oblique configuration. However, once slab breakoff has begun, it spreads horizontally in the form of tearing at high velocity (∼38–118 cm yr−1), and associated topographic uplift also propagates with the same velocity. In contrast, the along‐strike migration of subsequent continental collision and related topographic uplift propagation is typically much slower (∼2–34 cm yr−1). Similarly, the vertical magnitude of surface uplift caused by slab tearing is higher (up to 10 mm yr−1) than the following collision phase (<4 mm yr−1). The parametric analysis reveals that slab tearing velocity and the associated horizontal propagation of mountain uplift depends on obliquity angle and slab age, whereas the migration of collision‐induced topographic growth is controlled by the convergence velocity and obliquity angle. Finally, we show that presence of microcontinental block detached from the passive margin leads to spatial and temporal transition from horizontal to vertical slab tearing and more intense syn‐collisional mountain building.

What controls structural variations along the Zagros Collision Zone? Insights from geophysical observations and thermo-mechanical modelling

The Zagros Collision Zone is a complex tectonic region formed as a consequence of the collision between Arabia and Eurasia after the subduction of the Neo-Tethys ocean. The NW-SE striking Zagros orogen consists of the following parallel tectonic units (from SW to NE): Zagros Fold and Thrust Belt (ZFTB), Sanandaj–Sirjan Metamorphic Zone (SSZ), and Urumieh–Dokhtar Magmatic Arc (UDMA). In this study, we perform a combined analysis of recent geophysical data, revealing pronounced differences in the crustal and lithospheric structure along the Zagros Mountains. The northwestern sector shows a fairly uniform crustal thickening across the broad symmetric orogen from the ZFTB to the UDMA. In contrast, in the central Zagros, the transition from a relatively narrow zone of high elevations and high-frequency relief in the ZFTB to a smoother surface topography of the SSZ and UDMA occurs with an abrupt increase in Moho depth below the SSZ. The last observation has recently been interpreted as a result of “relamination” process, where the felsic upper crust of the Arabian plate underthrust the mafic crust of the Iranian plate. We present geodynamic numerical models of crustal relamination during continental collision and compute static gravity field of the resulting structures. We show that oblique closure of the Neo-Tethys affects lateral variations in the style and extent of crustal relamination, which control the observed along-strike changes in crustal configuration and orogen altitude. In particular, a narrow and higher orogen (as in the central Zagros) develops in the experiments with a young and wide oceanic plate, whereas an old and narrow subducting plate tends to form a broad and lower topography (as in the northwestern Zagros). This is geometrically consistent with the progressive closure of the Neo-Tethys from NW to SE during the oblique continental collision between Arabia and Eurasia.

The electrical structure of the lithosphere in the southeastern margin of the Amazonian craton: Contribution to West Gondwana tectonics

A high electrical conductivity belt near the southeastern margin of the Amazonian craton, known as Paraguay-Araguaia Conductivity Anomaly (PACA), has been discovered and mapped over a distance of more than 1,000 km by a large array of magnetometer measurements. To precisely determine the location and structure of the conductive anomaly, we conducted a 200-km-long magnetotelluric (MT) profile with typical site spacing of 8 km over the northern part of PACA and crossing the Goiás magmatic arc and Araguaia belt in Central Brazil. Three-dimensional resistivity models derived from this data indicate that the PACA is not a single conductor but comprises at least two contiguous bodies at the upper to middle crust. The enhanced conductivity of the northern part of PACA is likely due to sulfide minerals deposited in an inter-arc or back-arc basin that were deformed and subsequently emplaced tectonically beneath the western Goiás magmatic arc. The PACA is truncated to the west by a prominent sub-vertical resistor, attributed to the Transbrasiliano lineament (TBL), a continental-scale structural feature interpreted here as a mylonitic shear zone developed late in the Neoproterozoic to Cambrian tectonic history involving the collision between Amazonian craton and proto-West Gondwana. We cannot confirm the prevailing view of an east-dip subduction of oceanic lithosphere from the Amazonian plate under the Goiás arc terranes. Instead, our results probably represent the record of an oblique continental collision. Regardless of its origin, the PACA may serve as a proxy for the paleo border of the Amazonian plate. Therefore, additional geophysical mapping, especially in areas covered by thick Phanerozoic sediments, is essential to constrain the tectonic evolution of West Gondwana.

Petrology and crustal evolution of the Tartarugal Grande Granulitic Complex - Northeastern Amazonian Craton

The Tartarugal Grande Complex (TGC) is an association of high-grade metamorphic rocks with Archean and Paleoproterozoic protoliths, located at the border of the Archean Amapá and Rhyacian Lourenço domains, in southeastern Guiana Shield, northeast of the Amazonian Craton. It consists of felsic granulites, and aluminous leucogneisses and rare mafic granulites. Both granulites and gneisses are affected by thrust and transcurrent shear zones along NW-SE trend. Migmatization with quartz-feldspathic neosomes, with or without orthopyroxene, and garnet-rich neosomes are present in felsic granulites and leucogneisses, respectively. The gneisses have high silica contents and are peraluminous due to the presence of biotite, garnet, and cordierite, indicating the derivation from metasedimentary sequences. The felsic granulites have medium to high silica content and geochemical signature of calc-alkaline magmatic arc series. The mafic granulites are small bodies embedded in the felsic granulites and leucogneisses. They display geochemical affinities with tholeiitic series and represent ancient diabase dykes. The metamorphic paragenesis in the granulite facies is represented by Pl-antiperhtite + Qtz + Mc-mesoperthite + Opx + Cpx + Bt ± Hbl (enderbitic granulite), Pl-antiperhtite + Qtz + Mc-mesoperthite + Opx ± Bt (charnoenderbitic granulite); Mc-mesoperthite + Qtz + Pl + Opx + Bt (charnockitic granulite), Pl (An60)+Opx + Cpx ± Hbl ± Grt (mafic granulite), and Qtz + Mc + Pl ± Bt ± Grt ± Crd (leucogneiss). The presence of orthopyroxene, clinopyroxene and hornblende in equilibrium in the granulites allows estimating temperatures at 800° ± 20 °C. In addition, non-extensive anatexis under anhydrous conditions produced charnockitic neosomes in the granulites and garnet-bearing leucogranite neossomes in the paragneisses. The presence of cordierite in leucogneisses indicates low-medium pressure conditions around 6–7 kbar. The partial or total replacement of pyroxenes by hornblende and/or biotite, garnet by biotite, clinopyroxene by titanite, and myrmekite intergrowths, indicate retrometamorphism and cooling during exhumation to higher crustal levels. The U–Pb LA-ICP-MS dating on zircon from a charnoenderbitic granulite yielded an age of 2082 ± 5 Ma for the crystallization of the igneous protolith of the granulites. Metamorphic zircons from an enderbitic granulite yielded a U–Pb LA-ICP-MS age of 2045 ± 14 Ma, which corresponds to the high-grade metamorphic episode that has affected the region at the end of the Rhyacian. The age of the protoliths is not constrained but 207Pb/206Pb dates at 2.57–2.70 Ga indicated inherited crystals and/or xenocrystals from Archean source. In agreement with previous works, we conclude that the TGC consists of calcium-alkaline magmatic rocks and pelitic sedimentary sequences formed during the development of a Rhyacian magmatic arc at the edge of the Archean Amapá Block. This was followed by a thermo-tectonic event at the end of Rhyacian, around 2.05 Ga, under high-grade conditions reaching the granulite facies, with restricted anatexis. Records of the tectonic exhumation of these terranes were defined by partial disequilibrium of some mineral phases in amphibolite facies. This Late-Rhyacian thermo-tectonic event is contemporaneous to the ultra-high temperature metamorphism (2.09–2.03 Ga) of the Bakhuis Granulite Belt in Suriname. The thermal differences between these two high-grade metamorphic terrenes may be related to the different crustal levels where they settled. Late-Rhyacian asthenospheric upwelling by crustal stretching during oblique continental collision seems to be the more plausible scenario as previously proposed, even if a slab break-off scenario cannot be ruled out.

Oblique continental collision and the formation of syn-collisional A-type granites: insights from the Early Jurassic Baoji granite suite in the Qinling orogenic belt, central China

ABSTRACT A-type granites are conventionally interpreted to be formed in late post-collisional or anorogenic settings. However, we firstly identified an Early Jurassic syn-collisional A-type granite suite in the Qinling orogenic belt: the Baoji granite suite. A comprehensive investigation into its mineralogical, petrological, geochronological, and geochemical characteristics can shed light on the genesis of syn-collisional A-type granites and geodynamic evolution of collisional orogeny. The Baoji granite suite contains two types of rocks: (1) A-type granites including biotite syenite, coarse-grained, porphyritic, and fine-grained syenogranites, and alkali-feldspar granite, and (2) magnesian monzodiorite; both types are coeval, with zircon U–Pb ages of ca. 190 Ma. The A-type granites yield A1-type trace element signatures and large variations in trace element contents, with Ba contents of 16.9–762 ppm, Sr of 11.0–211 ppm, Rb of 193–645 ppm, and Eu of 0.08–1.24 ppm. Their radiogenic isotope compositions (87Sr/86Sri = 0.70631–0.70637; εNd = −5.18 to −3.84; εHf = −5.01 to +2.76) are similar, and overlap those of Neoproterozoic OIB-like basic volcanics from the South Qinling belt. These characteristics show that the A-type granites originated from the anatexis of OIB-like lower crust and underwent fractional crystallization in a silicic magma reservoir. The monzodiorite has low SiO2 contents (49.82–53.29 wt.%) and enriched radiogenic isotopic compositions (87Sr/86Sri = 0.70554–0.70638; εNd = −11.0 to −5.28) contains abundant hornblende and biotite (~40%), and exhibits evidence of magma mingling. It also contains two groups of zircon: pristine and unzoned zircon that yields negative Hf isotope compositions (εHf = −9.42 to −3.14), and xenocrystic zircon that yields similar Hf isotope compositions (εHf = −3.54 to +1.85) to the A-type granites. These characteristics demonstrate that the monzodiorite was derived from the differentiation of basaltic partial melts of an enriched mantle source, and mingled with the A-type granitic magma during its ascent and emplacement into a silicic magma reservoir. Along with regional records of the Early Jurassic magmatism, deformation, metamorphism, and sedimentation, the genetic models for the Baoji A-type granites and monzodiorite suggest that the Middle Triassic to Early Jurassic oblique continental collision (ca. 230–190 Ma) produced coupled EW-trending strike-slip and NE-trending extensional faults at crustal levels and Rayleigh–Taylor (gravitational) convective instabilities at mantle depths in the Qinling orogenic belt, which controlled the production of the syn-collisional A-type granite suite. Our work also indicates that the A-type granites can be formed in syn-collisional setting in response to oblique continental collision and the resultant slow foundering of a high-density lithospheric root; therefore, we suggest that the syn-collisional A-type granites are evidence for oblique continental collision.

GEBCO and ETOPO1 gridded datasets for GMT based cartographic Mapping of Hikurangi, Puysegur and Hjort Trenches, New Zealand

The study focused on the comparative analysis of the submarine geomorphology of three oceanic trenches: Hikurangi Trench (HkT), Puysegur Trench (PT) and Hjort Trench (HjT), New Zealand region, Pacific Ocean. HjT is characterized by an oblique subduction zone. Unique regional tectonic setting consist in two subduction zones: northern (Hikurangi margin) and southern (Puysegur margin), connected by oblique continental collision along the Alpine Fault, South Island. This cause variations in the geomorphic structure of the trenches. PT/HjT subduction is highly oblique (dextral) and directed southwards. Hikurangi subduction is directed northwestwards. South Island is caught in between by the “subduction scissor”. Methodology is based on GMT (The Generic Mapping Tools) for mapping, plotting and modelling. Mapping includes visualized geophysical, tectonic and geological settings of the trenches, based on sequential use of GMT modules. Data include GEBCO, ETOPO1, EGM96. Comparative histogram equalization of topographic grids (equalized, normalized, quadratic) was done by module ’grdhisteq’, automated cross-sectioning – by ’grdtrack’. Results shown that HjT has a symmetric shape form with comparative gradients on both western and eastern slopes. HkT has a trough-like flat wide bottom, steeper gradient slope on the North Island flank. PT has an asymmetric V-form with steep gradient on the eastern slopes and gentler western slope corresponding to the relatively gentle slope of a subducting plate and steeper slope of an upper one. HkT has shallower depths < 2,500 m, PT is <-6,000 m. The deepest values > 6,000 m for HjT. The surrounding relief of the HjT presents the most uneven terrain with gentle slope oceanward, and a steep slope on the eastern flank for PT, surrounded by complex submarine relief along the Macquarie Arc. Data distribution for the HkT demonstrates almost equal pattern for the depths from -600 m to ₋2,600 m. PT has a bimodal data distribution with 2 peaks: 1) -4,250 to -4,500 m (18%); 2) -2,250 to -3,000 m, < 7,5%. The second peak corresponds to the Macquarie Arc. Data distribution for HjT is classic bell-shaped with a clear peak at -3,250 to -3,500 m. The asymmetry of the trenches resulted in geomorphic shape of HkT, PT and HjT affected by geologic processes.

Diachronous initiation of post-collisional magmatism in the Arabia-Eurasia collision zone

The continental collision between Arabia and Eurasia which gave rise to the Caucasus-Iran-Anatolia (CIA) volcanic province provides a unique opportunity for understanding collisional zone magmatism. This study reports a comprehensive dataset of ages and geochemical compositions of volcanic rocks formed during the initial phase of post-collisional magmatism in the CIA province. The age data indicate a diachronous onset of volcanism that began ~17 Ma in SE Anatolia, and propagated northward from ~11 to 9 Ma toward NE Anatolia and NW Iran. The rocks are characteristically bimodal, with dominantly basic (SiO2 = 48–52 wt%) and silicic (SiO2 = 58–71 wt%) components that feature significant isotopic variations (εNd = +6 to −5), suggesting two principal magma sources: (1) a juvenile mantle-derived component, and (2) an older continental crust component. We therefore attribute the volcanic initiation to a migrating post-collisional extension regime caused by the successive breakoff of subducted Neo-Tethyan slabs. Subsequent volcanism that began from ~6.5 Ma resulted in a wide spectrum of calc-alkaline and alkaline rocks, with mafic to felsic lithologies in the entire province. From ~2 Ma, volcanism ceased in the western CIA province, and started propagating eastward and southeastward to SE Iran, following the Urumieh-Dokhtar magmatic belt, consistent with the notion of an oblique continental collision between Arabia and Eurasia.

Magmatic response to slab tearing and resultant crustal evolution during scissor-like oblique continental collision: Insights from Triassic mafic and felsic intrusions in the Qinling orogen, China

Characterization of the magmatic response to scissor-like oblique continental collision and of the associated geodynamic mechanisms and crustal evolution is crucial to understanding the formation of collisional orogens. This paper presents geochronological, elemental, and isotopic data of three Triassic mafic–felsic intrusions formed during scissor-like continental collision in the Qinling orogenic belt (QOB), China. The biotite pyroxenite, gabbro, quartz monzonite, and syenite of the Jiuzigou (JZG) mafic–felsic intrusive complex were emplaced at 233–231 Ma and have extremely high Ba and Sr contents, fractionated rare-earth elements, insignificant Eu anomalies, and enriched Sr-Nd isotopes. A negative relationship between εNd(t) values and SiO2 content suggests that the JZG complex was formed by assimilation and fractional crystallization (AFC) processes of enriched-mantle-derived melts. The Pangjiahe (PJH) and Laowangou (LWG) granite porphyries were emplaced at 224 Ma and 215 Ma, respectively, and are characterized by enriched large-ion lithophile elements, depleted high-field-strength elements, and enriched Sr and Nd isotopic compositions (0.70565–0.70648 and − 3.22 to −5.33 respectively), which indicate that these granite porphyries were sourced from mafic lower crust of the South Qinling terrane. The relatively high Mg numbers (45.8–49.4) might indicate mixing of crustal melt and minor mafic magma for the PJH granite porphyries. In contrast, the low Mg numbers (24.8–41.4) and obvious negative Eu anomalies of the LWG granite porphyries suggest an origin by fractional crystallization of crustal melts. The spatial–temporal distribution of the Triassic intrusions emplaced during the continental collision is proposed to have been produced by southeastward slab tearing in the QOB. Slab tearing may be a common geodynamic process during oblique collision generally. The origin and evolution of Triassic mafic and felsic intrusions in the QOB reveal that this oblique-collisional orogenic belt likely underwent a transition from net crustal growth to crustal reworking as collision progressed.

Development of the Ervidel-Roxo and Figueirinha-Albernoa volcanic sequences in the Iberian pyrite Belt, Portugal: Metallogenic and geodynamic implications

The Ervidel-Roxo (ER) and Figueirinha-Albernoa (FA) volcanic sequences in the Iberian Pyrite Belt (IPB) in Portugal form two elongated and independent exposures trending WNW-ESE. The ER sequence is mainly composed of rhyolites with subordinated rhyodacites and rocks of intermediate (“andesitic”) composition, whereas the FA succession comprises mostly rhyodacitic to dacitic rocks. Both sequences experienced metamorphic recrystallization under greenschist facies conditions and heterogeneous deformation during the Variscan orogeny. Temperatures obtained with the chlorite geothermometer show that a late hydrothermal event is locally recorded, possibly reflecting the maintenance of high geothermal gradients in the upper crust due to (moderate to) rapid crustal uplifting after completion of the oblique continental collision at the SW Iberian Variscides.Zircon saturation temperature (TSatZrn) and Ti-in-zircon thermometry were combined to identify volcanic sequences related to magmatic centers able to sustain long-lived high-temperature hydrothermal systems, separating them from sequences developed in centers that support the progression of hydrothermal activity under lower temperature conditions. A significant number of rhyolites from ER yield TSatZrn > 800 °C, while the rhyodacitic to dacitic rocks in the FA are characterized by TSatZrn values in the 630–790 °C range. In both cases, TSatZrn underestimates variably the onset temperature of zircon crystallization as indicated by Ti-in-zircon thermometry (T ≈ 870 °C, Roxo; T ≈ 920 °C, Albernoa and T ≈ 825 °C, Figueirinha). The obtained results also suggest that melts related to rhyolite-dominant sequences have a considerably lower zircon inheritance component in comparison with those linked to series of prevailing (rhyo)dacitic nature.The TSatZrn results obtained for ER and FA volcanic sequences, together with an extensive database compiled from literature and representing non-altered felsic volcanic rocks with SiO2 > 60 wt% and 1 < M = (Na + K + 2Ca)/Al·Si (cation proportions) ≤ 1.9, suggest that: (i) rhyolitic magmas feeding volcanic centers nearby or closely related to volcanic-hosted sulfide ore systems resulted mostly from melts generated at T higher than 800 °C, being under-saturated in zircon or including minor amounts of inherited zircon; and (ii) rhyodacitic to dacitic facies often included in barren or weakly mineralized volcanic sequences were most likely derived from melts formed under lower temperatures (700–775 °C) preserving a large inherited zircon fraction.

Evidence for underplating in the genesis of the Variscan synorogenic Beja Layered Gabbroic Sequence (Portugal) and related mesocratic rocks

The Beja Layered Gabbroic Sequence (LGS) is a mafic, layered synorogenic intrusion that was emplaced at the SW border of the Ossa Morena Zone (OMZ) at ca. 350Ma during the early stages of the Variscan oblique continental collision. The LGS represents the primitive member of the Beja Igneous Complex (BIC), which records part of an important Variscan magmatic event to the north of the SW Iberian suture that led to the formation of several igneous complexes. Although LGS primary magmatic features are well-preserved from post-crystallization tectono-metamorphic events, the magma chamber processes were influenced by the Variscan regional stress field which affected also the development of the magmatic layering and associated foliation.The baric evolution recorded in LGS (from 9 to 4kbar) is compatible with early crustal underplating of juvenile basaltic magma at the Moho. Mild lower crustal contamination resulted in Nb-Ta-Rb-Th depletion and higher REE fractionation compared to MORB. Contamination proceeded mainly by assimilation-fractional-crystallization (AFC) at final level of emplacement.The obtained results suggest that the spatially associated BIC mesocratic rocks are genetically related to LGS, both deriving from an equivalent mantle source. It is proposed that the underplating of basaltic magmas at the lower crust gave rise to a deep crustal hot zone. This allows BIC evolution to be explained as a single, long-lived magmatic event of progressive geochemical and lithological diversification due to the involvement of distinct crustal components. The most important components were incorporated at depth by the reworking of OMZ lower crustal rocks, with the involvement of middle crustal rocks later on, and of upper crustal contamination during the final stages of emplacement.

Internal architecture and Fe–Ti–V oxide ore genesis in a Variscan synorogenic layered mafic intrusion, the Beja Layered Gabbroic Sequence (Portugal)

The Beja Layered Gabbroic Sequence (LGS) intruded the SW border of the Ossa Morena Zone at ca. 350Ma during the early stages of the Variscan oblique continental collision in Iberia. It represents an exceptional case-study for the petrogenesis and metallogenesis of the rare class of synorogenic layered intrusions.LGS oxidised parental magmas (+1.7±0.5 ΔFMQ) evolved to relatively reduced ferro-basaltic magmas (±0.5 ΔFMQ) after extensive differentiation (SB I–SB II–ODV I Series: Fo88-54) and gave rise to ODV I Series oxide-rich ferro-gabbros and massive oxide ores, that represent a potential economic resource for Fe, Ti and V. Vanadium enrichment in ODV I melt peaked at the formation of basal ores and was followed by heterogeneous depletion at prevailing high Fe–Ti contents within other oxide-rich domains of the Series. Geological, geophysical and geochemical evidence suggest that density driven processes were fundamental to oxide accumulation and segregation within ODV I basal domains.The SB I–SB II–ODV I suite was followed by monotonous gabbroic Series (ODV II, III, BRG I, II) developed from repeated magma replenishments with similar initial composition (Fo78). The prevailing oxidising conditions (Fo76-66 ΔFMQ+.5 to+1.0) and the buffering effect of recurrent replenishments constrained the Fe–Ti enrichment within a typical calc-alkaline differentiation trend; decreasing recharge/crystallisation rates locally produced oxide accumulations at ≈+1Δ FMQ. The coupled ƒO2 decreasing with Fe-enrichment throughout LGS is consistent with crystallisation under conditions closed to oxygen exchange. It is inferred that the large amount of oxides that precipitated throughout ODV I Series required transient, episodic openings of the system favoured by the tectonically active character of the magma chamber.Modelling of melt compositions supports that LGS parental magmas were high-alumina, primitive basalts (#Mg0.63), which after some differentiation gave rise to SB II Series (#Mg0.59) preserved as a chilled margin. The SB I parental magmas may account for the monotonous Series compositional spectrum, whereas ODV I Series model melts may be derived as residual magmas from SB II Series. ODV I melts show strong Fe–Ti enrichment and Si depletion, consistent with a Fenner trend, wherein the formation of extreme differentiation products was precluded by the arrival of new magma batches related with the monotonous Series.

An investigation into the fault patterns in the Chadegan region, west Iran: Evidence for dextral brittle transpressional tectonics in the Sanandaj–Sirjan Zone

The NW–SE trending Sanandaj–Sirjan Zone (SSZ) is the internal part of the Zagros continental collision zone, which mainly consists of metamorphic rocks deformed in a dextral transpressional zone. This dextral transpression is attributed to brittle deformation related to late Cenozoic Arabia–Eurasia oblique continental collision. Major NW-trending faults, including the Dalan, Garmdareh, Yasechah, Sheida, and Ben faults, are reverse faults with a dextral strike-slip component. These faults were displaced by NW-trending synthetic and NE-trending antithetic faults. There are also E-trending thrusts and N-trending normal faults developing in directions that are, respectively, almost normal and parallel to the major shortening direction. The NW-trending Ben, Yasechah, and Sheida faults are NE-dipping faults, and the Dalan and Garmdareh faults are SW-dipping faults. These faults indicate the presence of a transpressive flower structure zone that probably led to the exhumation of Jurassic high-grade metamorphic rocks, such as eclogite, in the central part of the study area.

3-D imaging of Marlborough, New Zealand, subducted plate and strike-slip fault systems

SUMMARY We present a 3-D seismic velocity model of the northern South Island, New Zealand, using several combined passive and active-source data sets. This area is the transition from Hikurangi subduction in the North Island to oblique continental collision along the Alpine fault and the Southern Alps, and geodetically observed deformation is consistent with plate motion being primarily taken up on a series of rotating strike-slip faults. We carry out simultaneous inversion for Vp and Vp/Vs and hypocentres using earthquake data recorded by the permanent seismic network and three temporary seismic arrays, also using active-source marine airgun data recorded at onshore stations. We image the subducted Hikurangi slab as a continuous unbroken feature extending from the active subduction zone to the north. Beneath the Marlborough region, the 3-D hypocentres define the subducted slab as a smooth curved feature to over 250-km depth, whereas below the junction of the Awatere and Alpine faults, the seismicity abruptly ends at 100-km depth. The subducting Pacific plate has a thick crust, including greywacke upper crust, which slows subduction. The interaction of the subducted plate with the translating overlying plate forms a 50-km-long zone of apparent crustal thickening with Vp < 7.0 km s−1 observed to 40-km depth. The Australian Plate has a high-velocity block to 65-km depth, aligned with the Separation Point pluton and trending oblique to the subducted slab. This zone of thicker Australian lithosphere may also hinder subduction. The Marlborough faults trend about 25° to the strike of the underlying subducted slab, so that the relation of faults to the slab varies along the fault strikes, and the overlying plate ranges in thickness from 20 to 50 km along the faults. Thus, the fault interaction with the subducted slab varies, from translating material directly above the subducting plate, to bottoming into a thick ductile region. The lower crust has high Vp/Vs under the Marlborough faults (except for a region of Haast schist), which may indicate excess fluid released by the subducted plate.

Transpressive dextral shear in the Italva-Itaperuna section, Northern State of Rio de Janeiro, Brazil

Structural analysis carried out on a segment of the Neoproterozoic Ribeira Belt, southeastern Brazil, show that it represents part of the transpressive dextral orogen related to the Central Mantiqueira Province. NNE-trending and steeply dipping regional mylonitic belts form anastomosed geometry, and describe a map-scale, S-C-like structure that is characterized by their deflection towards NE near the Além Paraíba Lineament. Lithological and structural control related to deformation partition were responsible for the formation of felsic mylonitic granulites with S-type granites lenses developed in ductile shear zones, alternated with less deformed intermediate to basic granulites associated with charnockites. The dextral shear sense indicators are consistent with transpressive deformation in the region and are common especially at the border of the main shear zones. The presence of S-type leucogranite may lead to variations of linear and planar relationships, which result in local extension zones. These elements are consistent with oblique continental collision considering the São Francisco Craton as a stable block.

Tectonics of the Simplon massif and Lepontine gneiss dome: deformation structures due to collision between the underthrusting European plate and the Adriatic indenter

This study presents a review of published geological data, combined with original observations on the tectonics of the Simplon massif and the Lepontine gneiss dome in the Western Alps. New observations concern the geometry of the Oligocene Vanzone back fold, formed under amphibolite facies conditions, and of its root between Domodossola and Locarno, which is cut at an acute angle by the Miocene, epi- to anchizonal, dextral Centovalli strike-slip fault. The structures of the Simplon massif result from collision over 50 Ma between two plate boundaries with a different geometry: the underthrusted European plate and the Adriatic indenter. Detailed mapping and analysis of a complex structural interference pattern, combined with observations on the metamorphic grade of the superimposed structures and radiometric data, allow a kinematic model to be developed for this zone of oblique continental collision. The following main Alpine tectonic phases and structures may be distinguished: 1. NW-directed nappe emplacement, starting in the Early Eocene (~50 Ma); 2. W, SW and S-verging transverse folds; 3. transpressional movements on the dextral Simplon ductile shear zone since ~32 Ma; 4. formation of the Bergell – Vanzone backfolds and of the southern steep belt during the Oligocene, emplacement of the mantle derived 31–29 Ma Bergell and Biella granodiorites and porphyritic andesites as well as intrusions of 29–25 Ma crustal aplites and pegmatites; 5. formation of the dextral discrete Rhone-Simplon line and the Centovalli line during the Miocene, accompanied by the pull-apart development of the Lepontine gneiss dome – Dent Blanche (Valpelline) depression. It is suggested that movements of shortening in fan shaped NW, W and SW directions accompanied the more regular NW- to WNW-directed displacement of the Adriatic indenter during continental collision.

LPHT metamorphism in a late orogenic transpressional setting, Albera Massif, NE Iberia: implications for the geodynamic evolution of the Variscan Pyrenees

AbstractDuring the Late Palaeozoic Variscan Orogeny, Cambro‐Ordovician and/or Neoproterozoic metasedimentary rocks of the Albera Massif (Eastern Pyrenees) were subject to low‐pressure/high‐temperature (LPHT) regional metamorphism, with the development of a sequence of prograde metamorphic zones (chlorite‐muscovite, biotite, andalusite‐cordierite, sillimanite and migmatite). LPHT metamorphism and magmatism occurred in a broadly compressional tectonic regime, which started with a phase of southward thrusting (D1) and ended with a wrench‐dominated dextral transpressional event (D2). D1 occurred under prograde metamorphic conditions. D2 started before the P–T metamorphic climax and continued during and after the metamorphic peak, and was associated with igneous activity. P–T estimates show that rocks from the biotite‐in isograd reached peak‐metamorphic conditions of 2.5 kbar, 400 °C; rocks in the low‐grade part of the andalusite‐cordierite zone reached peak metamorphic conditions of 2.8 kbar, 535 °C; rocks located at the transition between andalusite‐cordierite zone and the sillimanite zone reached peak metamorphic conditions of 3.3 kbar, 625 °C; rocks located at the beginning of the anatectic domain reached peak metamorphic conditions of 3.5 kbar, 655 °C; and rocks located at the bottom of the metamorphic series of the massif reached peak metamorphic conditions of 4.5 kbar, 730 °C. A clockwise P–T trajectory is inferred using a combination of reaction microstructures with appropriate P–T pseudosections. It is proposed that heat from asthenospheric material that rose to shallow mantle levels provided the ultimate heat source for the LPHT metamorphism and extensive lower crustal melting, generating various types of granitoid magmas. This thermal pulse occurred during an episode of transpression, and is interpreted to reflect breakoff of the underlying, downwarped mantle lithosphere during the final stages of oblique continental collision.

Three-dimensional lithospheric deformation and gravity anomalies associated with oblique continental collision in South Island, New Zealand

SUMMARY Isostatic considerations exhibit differences between the northern, central and southern parts of the Pacific‐Australian plate collision in South Island, New Zealand. In the northern part mean elevations are moderate and the gravity low is small; the central part contains the highest elevations, and gravity and elevations correspond to each other relatively well; and in the southern part the gravity low is strongest whereas the mean elevations are moderate again. These differences indicate changes in the character of the isostatic compensation and are explained by increased thickening and widening of the crustal root from north to south, and also by the long wavelength gravity response to a mantle density anomaly that increases towards the south. A simple 3-D gravity model is derived that includes the detailed crustal structures from the South Island GeopHysical Transect (SIGHT) experiment as well as a high-density anomaly in the mantle inferred from teleseismic data. The model indicates that cold and, therefore, dense upper mantle material penetrates the asthenosphere to a greater extent in the south, similar to the behaviour of an apparently highly ductile lower crust. As plate reconstruction suggests more lithospheric shortening in the north, our model corresponds to lithospheric material escaping laterally to the south, almost perpendicular to the compression caused by lithospheric shortening of the mantle. Therefore, in addition to the prevailing mantle shear in New Zealand, there may also be a component of extrusional mantle creep beneath the Southern Alps orogen, which could have caused some of the observed large seismic anisotropy in this region. We may have also found evidence for submerged Eocene‐Miocene oceanic lithosphere beneath the southeastern part of South Island that has been unaccounted for after plate reconstruction.

Anatomy and formation of oblique continental collision: South Falkland basin

The South Falkland basin is a partially filled Cenozoic foreland basin located south of the Falkland Plateau. It was formed by flexure of this southern edge of the South American plate when the load represented by Burdwood Bank collided. This continental fragment belongs to the predominantly oceanic Scotia plate. Flexure probably started in early Cenozoic times and has continued to the present day. The whole region is submarine and so the detailed stratigraphy and structure of the basin has been well imaged by seismic reflection profiling. The clarity of this imagery has made analysis of structures within the collision zone possible. The plate boundary itself is an active oblique thrust fault which has controlled the growth of a frontal fold. There is evidence for older phases of thrusting and folding further south. Within and beneath the sediments which blanket the flexed South American plate, normal faulting occurs on a variety of scales. Episodes of stratigraphic growth associated with the largest of these faults demonstrates that they were active during flexural bending. We have modeled the development of the South Falkland basin using two different approaches, both of which are based upon the simplest elastic model. Inverse modeling of free‐air gravity and bathymetric profiles suggest that the elastic thickness of the loaded crust is 5–20 km. A complementary approach based upon the spectral analysis of free‐air gravity and bathymetry shows that the elastic thickness is 15 ± 5 km. Both techniques indicate that the flexed continental lithosphere is weak, a conclusion supported by the presence of normal faults within the flexed plate. A small increase in elastic thickness from west to east appears consistent with a change in the density and penetration of normal faulting.

Geochemistry of Volcanic Rocks, Albernoa Area, Iberian Pyrite Belt, Portugal

Volcanic rocks from the Albernoa area essentially consist of calcalkaline quartz-feldspar-phyric coherent and hyaloclastic rhyodacites, and alkaline and tholeiitic basaltic rocks. Binary plots show that high-field-strength elements behaved as immobile elements, and allow for the identification of two felsic rock suites. Silica and alkali mobility, however, is reflected by compositional scatter on major-element diagrams: felsic rocks display rhyolitic to apparent andesitic compositions, and the mafic rocks display basaltic to apparent dacitic compositions. Silica and alkali mobility was focused along fracture networks and within the matrices of hyaloclastic breccias. Problematic classification of geotectonic setting for the felsic rocks is a reflection of anomalous high-field-strength element systematics; this probably results from a low temperature of crustal fusion, causing decreased solubility of the refractory phases in which these elements reside. The mafic rocks, however, evidently were generated in an extensional setting without involvement of subduction; the existence of apparent arc signatures was caused by crustal assimilation. This is compatible with volcanism in an attenuated continental lithosphere setting, due to strike-slip tectonics during oblique continental collision.

Lithospheric structure across oblique continental collision in New Zealand from wide‐angle P wave modeling

Oblique convergence between the Australian and Pacific plates in South Island, New Zealand, has resulted in crustal thickening and distributed deformation within both plates. We measure this thickening and image the deformation with seismic wide‐angle data along a 600 km long transect that spans the plate boundary. P wave arrival times from 34,000 rays are used to construct a two‐dimensional model for seismic velocity with depth. Crustal velocities average 6.1 and 6.2 (±0.23) km/s for the Australian and Pacific sides of the boundary, respectively. Upper mantle velocities average 8.1 (±0.25) km/s. Distributed deformation is indicated by the following observations: a reduction in upper crustal and midcrustal velocities by 4% west of the plate boundary, ascribed to flexural bending stresses; a velocity reduction of 8% immediately east of the Alpine fault, linked to high pore fluid pressures; 17 km of crustal thickening to form a 44(±1.4) km deep crustal root that is offset 10–20 km SE of the highest mountains; a velocity reversal below the midcrust in the eastern part of the crustal root, caused either by heating or an artifact from seismic anisotropy; strong thickening of a 7.0(±0.4) km/s fast lower crust within the crustal root; and a low‐velocity zone in the Australian upper mantle due to sampling the slow orientation of upper mantle anisotropy.