Crustal Slices Research Articles

Exhumation of UHP/LT rocks due to the local reduction of the interplate pressure: Thermo-mechanical physical modelling

Spatial distribution of UHP/LT terrains suggests that their exhumation is essentially a three-dimensional process that occurs only locally in specific sites along mountain belts. On the other hand, the continental subduction resulting in the formation of UHP/LT rocks takes place along the whole belt. The previously performed by the authors 2-D thermo-mechanical laboratory modelling of continental subduction has shown that exhumation of deeply subducted continental crust is possible only when the effective interplate pressure p n is lower than the lithostatic pressure (low compression subduction regime). At the same time, this modelling showed that for the deeply subducted continental crust to be preserved at low temperature at great depth, the continental subduction should be accompanied by the subduction of the fore-arc block or the arc plate. The latter process occurs only when p n is high (high compression regime). To reconcile both processes we suggest that within the background of a generally high compression regime the interplate pressure can be locally reduced in some specific situations which would then allow the local exhumation of UHP/LT material. Using physical modelling technique we investigate one of such situations that occurs when the frontal part of the overriding plate undergoes (subduction induced or not) extension parallel to the plate boundary with activation or formation of a strike–slip transform fault oblique to the plate boundary (to the interplate zone). The displacement along this fault results in a local reduction of the interplate pressure at the intersection of the fault with the interplate zone. This pressure reduction permits the rise of the deeply subducted low-density continental crust and sediments submitted to UHP/LT conditions under buoyancy force. A 10 km-thick slice of crust detaches at ~ 150 km-depth and moves up along the interplate zone with a starting rate of ca. 3 cm/yr. The ascent rate reduces when the unit reaches crustal depth. The deformation of this unit is small when it first detaches and starts rising, but increases when reaching the base of the overriding plate. The exhuming material is sheared and stretched in a narrow passage at ~ 70 km-depth. Therefore in real conditions the deformation is mainly recorded when the unit is submitted to HP and not UHP conditions.

Thermo-tectonic evolution of the North Singhbhum Mobile Belt (eastern India): A view from the western part of the belt

The arcuate North Singhbhum Mobile Belt (NSMB) comprises multiply-deformed greenschist–amphibolite facies phyllites and schists flanking the centrally-located belt of the Dalma meta-igneous suite of rocks. The NSMB is separated from the Archaean Singhbhum Craton in the south, and the suite of granulite–amphibolite facies gneisses and foliated granites of the Chotanagpur Gneissic Complex to the north, by ductile shear zones. Three fabrics (S 1S, S 2S, and S 3S) formed due to D 1S, D 2S and D 3S deformation events and three metamorphic stages (M 1S: pre-S 2S; M 2S: syn-S 2S; and M 3S: syn/post-S 3S) were identified in phyllites and schists to the south of the Dalma meta-igneous in the NSMB. Reverse-sense transport along north-dipping imbricate thrusts (S 3S) emplaced the high-grade garnet–kyanite–staurolite schists centrally located in the orogen over low-grade muscovite–biotite–chlorite schists in the foreland. The M 1S stage (∼1.5 Ga; EPMA monazite date) in the high-grade gneisses characterized by high P/ T prograde metamorphism ( P > 10 kbar) was followed sequentially by post-loading prograde heating ( T max ∼650 °C) and exhumation – cooling at amphibolite facies (M 2S) and greenschist facies (M 3S: ∼1.3 Ga; EPMA monazite date) conditions. In the northern part of the NSMB, mica schists fringing the CGC document a northward increase in strain (sequential cleavage formation) and temperature (greenschist to amphibolite facies). The strain–temperature spatial gradient is correlated with crustal shortening and heating ( T ∼620 °C, P ∼7 kbar) synchronously with reverse-sense transport of schists along southerly-dipping thrust planes. The structural framework and thermal structure across the mobile belt is correlated with (a) collisional thickening of the orogen (>1.5 Ga), followed by convective removal of the lithospheric root resulting in post-burial heating and (b) syn-collision and exhumation of the orogen (∼1.3 Ga). The exhumation was caused by outward-directed extrusion of the crustal slices along imbricate foreland-vergent thrusts dipping towards the centre of the orogen.

Extrusão tectônica e transporte lateral de massa na porção central do cinturão Paraíba do Sul, seção Três Rios - Matias Barbosa (RJ/MG)

This paper deals with the geometry and kinematics of structures, described along a section cross-cutting the northern limb of a NE-trending regional fan-like geometry. The structure occurs on the core of the Ribeira orogen in SE Brazil. Three structural domains were described, from south to north. In the domain I, the mylonitic foliation runs towards NE with sub-vertical to vertical dip. Stretching lineation is horizontal, and assymmetric structures suggest dextral transpressive motion in the core of the divergence (domain I). Tablet-of-chocolate boudinage suggests secondary vertical stretching and shortening orthogonal to the flow plane occurs. Northwards occurs a crustal slice composed mainly by orthogneiss, and minor metassediments and deformed granitoids, called Paraíba do Sul domain – or domain II. This slice tectonically overlies the granulitic rocks of the Juiz de Fora domain (domain III). Foliation in domain II delineates a guirdle on the stereoplot, with slowly ENE- plunging axes. Stretching lineation plunges slowly to NE, parallel to the girdle axes. Tight to isoclinal folds with axis parallel to the stretching lineation occur within the mylonitic foliation. Slowly southward dipping mylonitic foliation (and associated folds) are deformed by open to tight, nearly upright extension-parallel folds with axial surfaces sub-parallel to the regional transpression zones. L-tectonites in deformed granitoids, and mullion structures parallel to the axes of both families of folds also were described. Shear-sense indicators suggest top-to-SWW shearing, sub-parallel and oblique to the orogenic trend, coeval with a NW-SE contractional component. Later structures are ductile-ruptile top-toNW thrusts and sinistral transtrative shear zones. The structural evolution of the area is interpreted in terms of regional dextral unconfined transpression. Our observations suggest that strain path partitioning during oblique extrusion and convergence may explain coeval parallel orogenic motion either in flat lying or steeply dipping shear zones, and WNW-directed dextral thrusts described in the deeper portions of the belt. Tectonic extrusion may also explain the presence of L-tectonites and the absence of vertical stretching lineation in the highly strained zones in the Paraíba do Sul Belt.

Extrusão tectônica e transporte lateral de massa na porção central do cinturão Paraíba do Sul, seção Três Rios - Matias Barbosa (RJ/MG)

This paper deals with the geometry and kinematics of structures, described along a section cross-cutting the northern limb of a NE-trending regional fan-likegeometry. The structure occurs on the core of the Ribeira orogen in SE Brazil. Three structural domains were described, from south to north. In the domain I, the mylonitic foliation runs towards NE with sub-vertical to vertical dip. Stretching lineation is horizontal, and assymmetric structures suggest dextral transpressive motion in the core of the divergence (domain I). Tablet-of-chocolate boudinage suggests secondary vertical stretching and shortening orthogonal to the flow plane occurs. Northwards occurs a crustal slice composed mainly by orthogneiss, and minor metassediments and deformed granitoids, called Paraiba do Sul domain – or domain II. This slice tectonically overlies the granulitic rocks of the Juiz de Fora domain (domain III). Foliation in domainII delineates a guirdle on the stereoplot, with slowly ENE- plunging axes. Stretching lineation plunges slowly to NE, parallel to the girdle axes. Tight to isoclinal folds with axis parallel to the stretching lineation occur within the mylonitic foliation. Slowly southward dipping mylonitic foliation (and associated folds) are deformed by open to tight, nearly upright extension-parallel folds with axial surfaces sub-parallel to the regional transpression zones. L-tectonites in deformed granitoids, and mullion structures parallel to the axes of both families of folds also were described. Shear-sense indicators suggest top-to-SWW shearing, sub-parallel and oblique to the orogenic trend, coeval with a NW-SE contractional component. Later structures are ductile-ruptile top-to- NW thrusts and sinistral transtrative shear zones. The structural evolution of the area is interpreted in terms of regional dextral unconfined transpression. Our observations suggest that strain path partitioning during oblique extrusion and convergence may explain coeval parallel orogenic motion either in flat lying or steeply dipping shear zones, and WNW-directed dextral thrusts described in the deeper portions of the belt. Tectonic extrusion may also explain the presence of L-tectonites and the absence of vertical stretching lineation in the highly strained zones in the Paraiba do Sul Belt.

Shear structures in the Serra do Azeite shear zone, southeastern Brazil: Transtensional deformation during regional transpression in the central Mantiqueira province (Ribeira belt)

The Serra do Azeite shear zone (SASZ) is a northeast-trending regional structure in southeastern Brazil. Kinematic analysis carried out in the SASZ suggests a ductile sinistral transtensional regime in amphibolite facies conditions. Constrictional, flattened, and simple-shear strain domains occur in mylonites with stretching lineation plunging to the east–southeast. Kinematic indicators suggest oblique top–down-to-the-east–southeast and sinistral strike-slip components along variably oriented shear planes. Results of the simple geometrical construction applied to the shear-zone pattern, coupled with field data and kinematic analysis, suggest that sinistral transtensional shearing resulted from east–northeast-directed crustal extension and sinistral strike-slip displacement, accompanied by north–northeast/south–southwest contraction and vertical thinning. The K/Ar and Ar/Ar cooling ages match the proposed interval for crustal extension in the central Mantiqueira province (0.6–0.58/0.57 Ga) based on ages of alkaline granitoids and volcanic rocks. These data indicate types-I and -S granite magmatism, as well as metamorphism and dextral transpressional deformation along the Ribeira belt. Therefore, we interpret the transtensional regime as a result of southwest-directed lateral extrusion and uplift crustal slices (overall oblique extrusion) during an orogenic-scale partitioned transpressional regime. Our results suggest this regime was coeval with a phase of regional stretching subparallel to the Ribeira belt, which would explain the coexistence of extensional and compressional structures during overall plate convergence.

Dabieshan UHP Metamorphic Terrane: Sr-Nd-Pb Isotopic Constraint to Pre-metamorphic Subduction Polarity

The Qinling-Dabie orogen in east-central China was produced by Triassic collision between two Precambrian cratons. The traditional view about the pre-collisional plate movement is that the South China Block (SCB = Yangtze craton) underwent northerly subduction beneath the North China Block (NCB = Sino-Korean craton). In this paper, we use the Sr-Nd-Pb isotopic tracer technique to test the validity of this view. In all published models, ultrahigh-pressure and high-pressure metamorphic rocks and subjacent "basement" gneisses of Dabieshan are considered to be the deeply subducted, then exhumed crustal slices of the South China Block. Cretaceous igneous rocks, including granitic batholiths generated by partial melting of the Yangtze middle to lower crust and mafic rocks derived from the subjacent upper mantle, should carry the isotopic signatures of the SCB lithosphere. However, Sr-Nd-Pb isotopic tracer analyses indicate that the Cretaceous granitoids and mafic-ultramafic rocks of Dabieshan have an NCB isotopic signature; hence the subducted lithosphere has little resemblance to the Yangtze craton. We conclude that an opposite polarity of the Triassic subduction is more likely, namely that the NCB plate was subducted southward beneath the SCB plate. A new tectonic model is presented, and we suggest that the plate boundary is located in the southern margin of the Dabieshan.

Zircon SHRIMP U–Pb dating for gneisses in northern Dabie high T/ P metamorphic zone, central China: Implications for decoupling within subducted continental crust

Zircon SHRIMP U–Pb ages and cathodoluminescence (CL) images reveal that most zircon separated from two tonalitic gneiss samples in the northern Dabie high T/ P metamorphic zone (NDZ) have four different domains: (1) inherited core, with clear oscillatory zoning, low- P mineral inclusions and high Th/U ratios, indicating the Neoproterozoic age of the protolith; (2) inner-mantle, with homogeneous CL intensity, low Th/U ratios of ≤ 0.09 and occasionally ultrahigh pressure metamorphic (UHPM) mineral inclusions such as diamond, garnet and rutile, which defined a weighted mean 206Pb/ 238U age of 218 ± 3 Ma, corresponding to the age of ultrahigh-pressure metamorphism; (3) outer-mantle with lower Th/U ratio and retrograde mineral inclusions, which yields a retrogressive age of 191 ± 5 Ma; and (4) rim, which is black luminescence, with relatively high Th/U ratios of 0.12–0.40 and a weighted mean 206Pb/ 238U age of 126 ± 5 Ma, indicating an overprint of the Early Cretaceous migmatization. Consequently, four discrete and meaningful age groups have been identified. Remarkably, the U–Pb ages of both UHPM zircon and retrograded metamorphic zircon from the NDZ are significantly younger than the U–Pb ages of 238 ± 3 Ma–230 ± 4 Ma for UHPM zircon and U–Th–Pb ages of 218 ± 1.2 Ma–209 ± 3 Ma for rutile and monazite overgrowths (representing the cooling or retrograded metamorphic time corresponding to amphibolite-facies) from the southern Dabie UHPM zone (SDZ), respectively. Combined with reported ages for UHPM rocks from the NDZ, SDZ and Huangzhen low- T eclogite zone (HZ), we found that the metamorphic ages of these three UHPM units in the Dabie orogen gradually decrease from south to north. This age distribution suggests that the three UHPM units represent 3 exhumed crustal slices, which are decoupled from each other and have different subduction and exhumation histories. Firstly, the subducted Huangzhen crustal slice was detached from the underlying subducted continental lithosphere, and then exhumed because of the buoyancy. In the meantime, subduction of underlying continental plate continued. After the initiation of this process, lithologic separations or decoupling within subducted continental crust occurred repeatedly several times to form an imbricate megastructure in the Dabie UHPM belt.

Collision leading to multiple-stage large-scale extrusion in the Qinling orogen: Insights from the Mianlue suture

The geologic framework of the Phanerozoic Qinling–Dabie orogen was built up through two major suturing events of three blocks. From north to south these include the North China craton (including the north Qinling block), the Qinling–Dabie microblock, and the South China craton (including the Bikou block), separated by the Shangdan and Mianlue sutures. The Mianlue suture zone contains evidence for Mesozoic extrusion tectonics in the form of major strike–slip border faults surrounding basement blocks, a Late Paleozoic ophiolite and a ca. 240–200 Ma thrust belt that reformed by 200–150 Ma thrusts during A-type (intracontinental) subduction. The regional map pattern shows that the blocks are surrounded by complexly deformed Devonian to Early Triassic metasandstones and metapelites, forming a regional-scale block-in-matrix mélange fabric. Five distinct tectonic units have been recognized in the belt: (1) basement blocks including two types of Precambrian basement, crystalline and transitional; (2) continental margin slices including Early Paleozoic strata, and Late Paleozoic fluviodeltaic sedimentary rocks, proximal and distal fan clastics, reflecting the development of a north-facing rift margin on the edge of the South China plate; (3) out of sequence oceanic crustal slices including strongly deformed postrift, deep-water sedimentary rocks, sheeted dikes, basalts, and mafic–ultramafic cumulates of a Late Paleozoic ophiolite suite, developing independent of the rift margin in a separate basin; (4) out-of-sequence island-arc slices; (5) accretionary wedge slices. All the tectonic units were deformed during three geometrically distinct deformation episodes (D 1, D 2 and D 3 during 240–200 Ma). Units 2–4 involved southward thrusting and vertical then southward extrusion of about 20 km of horizontal displacement above the autochthonous basement during the D 1 episode. Thrust slices 20 km south of the Mianlue suture are related to this vertical extrusion due to the same rock assemblages, ages and kinematics. The D 2 and D 3 episodes folded all the units in a thick-skinned style about east–west (D 2) and west–northwest (D 3) axes in the Mianlue suture zone. An early foreland propagating sequence of accretion of Late Paleozoic rocks deposited above the Yangtze craton is not involved in D 1 deformation but is temporally equivalent to the D 2 and D 3 deformation in the Mianlue suture. Two stages of strike–slip faulting mainly occurred at the end of D 2 and D 3, respectively. During D 2 deformation, the Bikou block was obliquely indented to the ESE into the Mianlue suture, rather than being thrust over the Mianlue suture from the north as a part of the Qinling–Dabie microblock. During D 3 deformation, however, the Bikou block was bounded by the south boundary fault of the Mianlue suture, and the Yangpingguan fault on the south. These faults are coeval strike–slip faults, but of opposite senses, and accommodated minor southwestward extrusion of the Bikou block into Songpan–Ganze orogen. The other basement blocks north of the Mianlue suture were extruded eastward by about 20 km of lateral displacement, based on the offset of the Wudang dome, during the D 3 episode due to the northeastward indentation of the Hannan complex of the South China craton. Post-D 3 emplacement of granite, cutting across the strike–slip faults such as the Mianlue suture, provides a minimum age of 200 Ma for D 3 deformation. Therefore, based on insights from the evolution of the Mianlue suture, the D 2 and D 3 episodes in the Mianlue suture and its neighbors are not responsible for and associated with the two-stage extrusion of the Dabie UHP-HP terranes from the Foping dome to the present erosional surface (more than 350 km).

Precollisional, multistage exhumation of subducted continental crust: The Sesia Zone, western Alps

The Sesia Zone within the Tertiary arc of the western Alps is a relic of the subducted part of the Adriatic continental margin along the SE border of the Tethyan ocean. The Sesia Zone comprises three basement nappes which individuated during Late Cretaceous (65–80 Ma) subduction to different depths at high‐pressure (HP, blueschist, eclogite facies) conditions (peak pressures of 1.0–1.2, 1.0–1.5, and 1.5–2.0 GPa). The thrusts bounding these nappes developed where the crust was previously thinned during Jurassic rifting. Crustal‐scale shear zones partly overprinted these early thrusts and exhumed coherent slices of crust containing HP rocks. Initial exhumation of the internal part of the accreted margin involved thrusting (D1) and transpressional shearing (D2) along a subvertical, E‐W trending mylonitic shear zone under retrograde blueschist‐ to greenschist‐facies conditions. This exhumation was nearly isothermal to a depth of about 25 km, where the basement nappes were juxtaposed. Subsequent exhumation of these nappes to a common depth of about 15–20 km occurred in the footwall of a greenschist‐facies, top‐SE extensional shear zone (D3) preserved in some of the highest mountain peaks of the Sesia Zone. New Rb‐Sr mineral ages constrain D2 to have occurred at about 60–65 Ma and D3 at about 45–55 Ma. Thus top‐SE extensional exhumation was broadly coeval with Eocene, SE directed subduction of the Liguro‐Piemont oceanic lithosphere beneath the Adriatic margin. Slow cooling and erosional denudation of the Sesia Zone from 45 to 30 Ma occurred in the hanging wall of the Gressoney extensional shear zone (D4), which itself contributed to the exhumation of Eocene HP and ultra‐HP oceanic rocks in its footwall. By 30 Ma, HP rocks of the Sesia Zone were intruded by shallow granitic plutons which were eroded and redeposited within volcanoclastic sediments. Oligo‐Miocene Insubric backfolding and thrusting (D5) only exhumed northeastern parts of the Sesia Zone, where HP metamorphism is absent or was overprinted by Tertiary temperature‐dominated metamorphism. Most exhumation of continental HP rocks in the Sesia Zone therefore preceded Tertiary Alpine collision and coincided with Late Cretaceous to Early Tertiary subduction of the Adriatic and Tethyan lithosphere. The transition from D2 to D3 in the Sesia Zone is interpreted to mark a change from high‐stress, oblique SE directed subduction and accretion of the distal Adriatic continental margin to NW retreating, low‐stress subduction of the Liguro‐Piemont oceanic lithosphere.

A crustal‐scale cross‐section of the south‐western Alps combining geophysical and geological imagery

AbstractA geotransect across the south‐western Alps from the Pelvoux Massif (external French Alps) to the Dora‐Maira massif (internal Italian Alps), through the Monviso ophiolitic complex, was investigated in the framework of the ‘Géo‐France 3D Alpes’ programme. A new interpretative crustal‐scale section across the south‐western Alps is proposed, combining geological and geophysical 2D/3D data. The Apulian mantle (i.e. the Ivrea body) might be divided into two rigid pieces separated by the downward prolongation of the Penninic frontal thrust. These mantle indenters drive the decoupling of the European crust. Beneath the high to ultra‐high pressure metamorphic nappes, the deep structure results from the stacking of crustal slices extracted from the European lithosphere. The proposed structural model provides a basis for discussing the evidence of the crustal‐scale partitioning of the current strain pattern as well as the location of the seismicity.

Cretaceous and Triassic subduction-accretion, high-pressure-low-temperature metamorphism, and continental growth in the Central Pontides, Turkey

Biostratigraphic, isotopic, and petrologic data from the Central Pontides document major southward growth of the Eurasian continental crust by subduction-accretion during the Cretaceous and Triassic Periods. A major part of the accreted material is represented by a crustal slice, 75 km long and up to 11 km thick, consisting of metabasite, metaophiolite, and mica schist that represent underplated Tethyan oceanic crustal and mantle rocks. They were metamorphosed at 490 degrees C and 17 kbar in mid-Cretaceous time (ca. 105 Ma). The syn-subduction exhumation occurred in a thrust sheet bounded by a greenschist facies shear zone with a normal sense of movement at the top and a thrust fault at the base. A flexural Foreland basin developed in front of the south-vergent high-pressure-low-temperature (HP-LT) metamorphic thrust sheet; the biostratigraphy of the foreland basin constrains the exhumation of the HP-LT rocks to the lbronian-Coniacian, similar to 20 m.y. after the HP-LT metamorphism, and similar to 25 m.y. before the terminal Paleocene continental collision. The Cretaceous subduction-accretion complex is tectonically overlain in the north by oceanic crustal rocks accreted to the southern margin of Eurasia during the latest Triassic-earliest Jurassic. The Triassic subduction-accretion complex is made up of metavolcanic rocks of ensimatic arc origin and has undergone a high pressure, greenschist facies metamorphism with growth of sodic amphibole. Most of the Central Pontides consists of accreted Phanerozoic oceanic crustal material, and hence is comparable to regions such as the Klamath Mountains in the northwestern United States or to the Altaids in Central Asia.

Tectonic Evolution of the Dabie-Sulu UHP and HP Metamorphic Belts, East-Central China: Structural Record in UHP Rocks

The complex structural history of the Dabie-Sulu terrane is deduced from various scales of structural features studied in UHP metamorphic units combined with metamorphic and thermal data. Excluding pre-UHP events, the following sequence of distinct tectonometamorphic stages is suggested: (1) The first deformation, D1, produced weak foliation and lineation in massive eclogite in the P-T stability field of coesite/diamond under low differential stress. (2) The D2 event is mainly inferred from a dominant coesite eclogite-facies texture characterized by a stretching mineral lineation, mesoscale sheath-like folds, and a network of ductile shear zones. (3) D3 structures and fabrics developed shortly after the formation of granulite/amphibolite—facies symplectites. These structures are characterized by a regional, steeply dipping foliation and heterogeneous compositional layering, eclogite boudinages of various dimensions, intrafolial folds, and ductile shear zones forming an anastomosing pattern, leading to tectonic juxtaposition or nappe-like structures defining shear zone-bounded crustal slices. The D3 deformation event was associated with decompressional partial melting and intense retrogressive metamorphism. (4) Post-collisional ductile crustal thinning and extension affected the D3 foliation and compositional layering, producing a regional, gently dipping D4 main amphibolite-facies foliation and stretching lineation, dome- and arc-shaped structures, and low-angle and extensional detachments typified by different stretching directions. All of these structures formed prior to intrusion of Late Mesozoic plutons, faulting, basin development, and tectonic unroofing at shallow crustal levels (D5). The first two stages of ductile structures (D1 and D2) were related to subduction and collision between the Sino-Korean and Yangtze cratons at UHP (>27 kbar) and HP metamorphic conditions at 230-250 Ma. In contrast, D3 and D4 stages of Late Triassic to Jurassic (~230-170 Ma) ductile deformation accompanied exhumation of UHP and HP metamorphic rocks to crustal levels, initially driven by compression and later by extension. D4 structures dominate the map pattern of most UHP and HP metamorphic rocks in the orogen. The final D5 deformation largely controls the present-day geomorphology of the Dabie-Sulu region. A modified tectonic evolution model for the UHP and HP metamorphic belts is proposed. It involves continental subduction and collision between the Yangtze and Sino-Korean cratons, and subsequent polyphase exhumation of the UHP and HP metamorphic rocks.

Architecture of Proterozoic shear zones in the Christie Domain, western Gawler Craton, Australia: Geophysical appraisal of a poorly exposed orogenic terrane

Large-scale Mesoproterozoic crustal shear zones are partially exposed from beneath cover sequences in the Christie Domain of the northwestern Gawler Craton, southern Australia. These structures are associated with gravity and magnetic anomalies that allow them to be mapped under cover, and their three-dimensional geometry and kinematics to be evaluated. Gravity and magnetic forward modelling indicate that these shear zones form an imbricate oblique thrust stack with combined top-to-the-southeast, left lateral transport. The longest shear zones in the stack penetrate the crust to at least 15 km depth, and dip to the northwest at 70°; their inferred sense of motion are consistent with kinematic indicators from sparse outcrops. From northwest to southeast, the stack includes crustal slices bound by the Karari Fault Zone, Tallacootra and Blowout Shear Zones, Colona and Coorabie Fault Zones and the Muckanippie Shear Zone. These structures separate discrete tectonometamorphic packages within the previously undifferentiated Christie Gneiss of the Archean Mulgathing Complex, and imply several generations of transport of material from the lower and middle crust. Three-dimensional inversion of gravity data indicates that the Muckanippie Shear Zone, a shallowly rooted second-order splay of the Coorabie Fault Zone has an antithetic dip (to the southeast), and is part of a positive flower structure. Cross-cutting relationships, coupled with recent reconnaissance geochronology, suggest that the western Gawler Craton, part of the Mawson Continent, was under a long-lasting oblique slip deformational regime after ca. 1590 Ma. This tectonic reworking is correlated with episodic pervasive metamorphism and deformation recorded elsewhere in the Gawler Craton between ca. 1550 and 1450 (Coorabie Orogeny), pointing to a complex history of stabilization in Proterozoic Rodinia. The Tallacootra, Colona and Coorabie structures are all cut at a high angle to their strikes by Mesozoic rift faults of the Southern Rift System, and may form piercing points in reconstructions of Gondwana, and earlier Rodinia configurations.

Thrusting and faulting in metamorphic and sedimentary units of Ligurian Alps: an example of integrated field work and geochemical analyses

In a sector placed in the SE part of the Alps–Apennine junction, a kilometre-scale shear zone has been identified as the Grognardo thrust zone (GTZ), which caused the NE-directed thrusting of metaophiolites (Voltri Group) and polymetamorphic continental crust slices (Valosio Unit) of Ligurian Alps onto Oligocene sediments of an episutural basin known as “Tertiary Piemonte Basin”. The structural setting of the GTZ is due to syn- to late-metamorphic deformation, followed by a brittle thrusting that occurred in the Late Aquitanian times and can thus be related to one of the main contractional tectonic events suffered by northern Apennines. The GTZ was then sealed by Lower Burdigalian carbonate platform sediments (Visone Formation). Transtensive faulting followed in post-Burdigalian times along NW–SE regional faults and displaced the previously coupled sedimentary and metamorphic units. The GTZ thus underwent a plastic-to-brittle evolution, during which carbonate-rich fluids largely sustained the deformation. In these stages, a complex vein network originated within both the metamorphic and sedimentary rocks. Field data and stable isotopic analyses (13C and 18O) of bulk rocks and veins show that fluid–rock interaction caused the carbonatisation of the rocks in the late-metamorphic stages and the cataclasis and recementation, by the action of isochemical cold carbonate groundwater during the thrusting events. Carbonate veins largely developed also during the transtensive faulting stages, with composition clearly different from that of the veins associated to thrust faults, as indicated by the strong depletion in 13C of carbonate fillings, suggesting the presence of exotic fluids, characterised by a high content of organic matter.

Shear wave properties and Poisson's ratios of ultrahigh‐pressure metamorphic rocks from the Dabie‐Sulu orogenic belt, China: Implications for crustal composition

Shear wave velocities (Vs), anisotropy, and shear wave splitting have been measured at pressures up to 600–800 MPa for ultrahigh‐pressure (UHP) metamorphic rocks from the Dabie‐Sulu orogenic belt, China, with a focus on three types of eclogites. Type 1 eclogites are coarse‐grained, unaltered samples showing high densities and high Vs values (4.85 ± 0.06 km/s at 600 MPa); type 2 eclogites are fine‐grained, sheared samples with intermediate Vs values (4.53 ± 0.04 km/s at 600 MPa); and type 3 eclogites are overprinted by amphibolite facies metamorphism and display low Vs values (4.33 ± 0.09 km/s at 600 MPa). The compositional layering and retrograde metamorphism can result in significant anisotropy and shear wave splitting in eclogites, suggesting their plausible contribution to seismic anisotropy in the lower crust, upper mantle, and particularly in subducted slabs. Integrating our P and S wave velocity results with reliable data from previous studies, we estimated the pressure and temperature derivatives of Vp, Vs, and Poisson's ratios for common rock types in the UHP metamorphic belt. The geometric means were used as a mixture rule to invert the lithological and chemical compositions of the layered crust from seismic refraction velocities. The inferred crustal composition suggests that the eclogite‐bearing UHP rocks are tectonic slices of crust that have been thrust along a series of shear zones during the continental collision between the north China and Yangtze cratons, over a normal UHP‐free middle lower crust with overall intermediate composition.

Continental subduction and exhumation of high-pressure rocks: insights from thermo-mechanical laboratory modelling

Thermo-mechanical physical modelling of continental subduction is performed to investigate the exhumation of deeply subducted continental crust. The model consists of two lithospheric plates made of new temperature sensitive analogue materials. The lithosphere is underlain by liquid asthenosphere. The continental lithosphere contains three layers: the weak sedimentary layer, the crust made of a stronger material, and of a still stronger lithospheric mantle. The whole model is subjected to a constant vertical thermal gradient, causing the strength reduction with depth in each lithospheric layer. Subduction is driven by both push force and pull force. During subduction, the subducting lithosphere is heating and the strength of its layers reduces. The weakening continental crust reaches maximal depth of about 120 km and cannot subduct deeper because its frontal part starts to flow up. The subducted crust undergoes complex deformation, including indicated upward ductile flow of the most deeply subducted portions and localised failure of the subducted upper crust at about 50-km depth. This failure results in the formation of the first crustal slice which rises up between the plates under the buoyancy force. This process is accompanied by the delamination of the crustal and mantle layers of the subducting lithosphere. The delamination front propagates upwards into the interplate zone resulting in the formation of two other crustal slices that also rise up between the plates. Average equivalent exhumation rate of the crustal material during delamination is about 1 cm/year. The crust-asthenosphere boundary near the interplate zone is uplifted. The subducted mantle layer then breaks off, removing the pull force and thereby stopping the delamination and increasing horizontal compression of the lithosphere. The latter produces shortening of the formed orogen and the growth of relief. The modelling reveals an interesting burial/exhumation evolution of the sedimentary cover. During initial stages of continental subduction the sediments of the continental margin are dragged to the overriding plate base and are partially accreted at the deep part of the interplate zone (at 60–70 km-depth). These sediments remain there until the beginning of delamination during which the pressure between the subducted crust and the overriding plate increases. This results in squeezing the underplated sediments out. Part of them is extruded upwards along the interplate zone to about 30-km depth at an equivalent rate of 5–10 cm/year.

Basement heterogeneity in the Cathaysia crustal block, southeast China

Abstract Isotope signatures and T DM model ages in Hong Kong and neighbouring Guangdong Province have indicated that the basement of the Cathaysia Block is probably an amalgamation of narrow crustal slices, ranging in age from latest Archaean to Mesoproterozoic. Inheritance ages from zircons contained within Mesozoic volcanic and plutonic rocks also show Proterozoic and Archaean components. Regional gravity survey studies display NNE- to NE-trending Bouguer anomalies that are indicative of sharp changes in rock densities at middle and lower crustal levels. The anomalies displayed on the gravity profile from Guangdong to Hong Kong have been modelled as narrow slices of Archaean and Proterozoic crust. A substantial E-W-trending Bouguer anomaly, which largely parallels the trend of the foliation in the Proterozoic schists of the region, is present to the east of Guangzhou. It is proposed that the basement of the Cathaysia Block consists of an amalgamation of NE- to NNE-trending Palaeo- to Mesoproterozoic and Archaean crustal terranes, which in places have retained the pre-amalgamation E-W-trending tectonic fabric. The discontinuities between the basement terranes, and the E-W structures have strongly influenced the geological evolution of the Phanerozoic sequences and igneous complexes in southeast China. These are most obviously manifest in the regional NE-trending fault and shear zones that displace the cover sequences.

Surface deformation and tectonic setting of Taiwan inferred from a GPS velocity field

We have determined the present‐day surface deformation of Taiwan by computing the velocity gradient field and fault slip from 143 GPS velocity vectors. In southern Taiwan the derived strain and rotation rates and fault slips are indicative of lateral extrusion toward the south. In northern Taiwan we infer the onset of gravitational collapse which is induced by the on‐land extension of the Okinawa Trough. In the eastern Central Range the observed inverted NW‐SE extension is consistent with geological observations and high heat flow measurements. This could be the result of exhumation of crustal material. The model further shows a significant decrease in slip rate northward along the Longitudinal Valley fault at 23.7°N. The northern Coastal Range shows high strain rates and two oppositely rotating blocks. By combining the surface deformation model with seismicity data and seismic tomography we are able to propose a coherent model for the present‐day tectonic activity. Both seismicity and tomography show further evidence for active, southward propagating exhumation of a crustal slice in the eastern Central Range. Offshore east Taiwan we deduce strong evidence of a southward propagating crustal tear fault, accommodating most of the Philippine Sea Plate‐Eurasian Plate convergence. The tear is the crustal response to incipient northwestward subduction of the Philippine Sea Plate. Thus the Ryukyu Trench is bending southward becoming almost perpendicular to the convergence direction, while subduction of the Philippine Sea Plate continues. In this setting a sudden rapid southward propagation of the afore mentioned tear is conceivable.

Lower crustal granulite xenoliths from the Pannonian Basin, Hungary, Part 2: Sr–Nd–Pb–Hf and O isotope evidence for formation of continental lower crust by tectonic emplacement of oceanic crust

Mafic granulite xenoliths from the lower crust of the Pannonian Basin are dominated by LREE-depleted bulk-rock compositions. Many of these have MORB-like 143Nd/144Nd but 87Sr/86Sr is elevated relative to most MORBs. Their δ18O values cover a wide range from +3.8 to +9.5‰. A group of LREE-enriched mafic granulites have higher 87Sr/86Sr (0.704–0.708) and lower 143Nd/144Nd (0.5128–0.5124), with higher δ18O values on average (+7.8 to +10.6‰) than the LREE-depleted granulites. The LREE-enriched granulites are, however, isotopically similar to newly discovered metasedimentary granulite xenoliths. A sublinear correlation in eHf–eNd isotope space has a shallower slope than the crust–mantle array, with the metasedimentary rocks forming the low eHf end member; the radiogenic end is restricted to the LREE-depleted granulites and these overlap the field of MORB. Pb isotopes for the LREE-depleted samples are less radiogenic on average than those of the LREE-enriched and metasedimentary xenoliths, and metasedimentary granulites have consistently higher 208Pb/204Pb. The wide range in δ18O over a restricted range in Nd and Sr isotope values, in combination with the predominance of LREE-depleted trace-element compositions, is consistent with an origin as a package of hydrothermally altered oceanic crust. The existence of δ18O values lower than average MORB and/or mantle peridotite requires that at least some of these rocks were hydrothermally altered at high temperature, presumably in the oceanic lower crust. The low 143Nd/144Nd of the LREE-enriched mafic granulites cannot be explained by simple mixing between a LREE-depleted melt and an enriched component, represented by the recovered metasediments. Instead, we interpret these rocks as the metamorphic equivalent of the shallowest levels of the ocean crust where pillow basalts are intimately intercalated with oceanic sediments. A possible model is accretion of oceanic crustal slices during subduction and convergence, followed by high-grade metamorphism during the Alpine orogeny.

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

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