Exhumation Of Deep Crustal Rocks Research Articles

Splitting a large pluton by Cretaceous crustal extension: Evolution of the Ziyuan Detachment and crustal thinning of the South China Block

The Cretaceous extensional province of the South China Block (SCB), and the decratonization-induced extension in the North China Block (NCB), were both controlled by the Late Mesozoic subduction of the Izanagi/Paleo-Pacific Plate. Different from the metamorphic core complexes exhuming deep crustal rocks of the NCB, extension of the SCB is expressed by numerous half-graben basins and detachment of upper-middle crustal rocks, but its mode and mechanism remain unclarified. At the westernmost of this extensional province, the Early Paleozoic Yuechengling-Miao'ershan Massif, composed of the ductilely deformed Yuechengling pluton and undeformed Miao'ershan pluton, records Late Mesozoic detachment and exhumation. Magnetic fabrics of the western Yuechengling pluton are consistent with structural fabrics, while the NE-SW trending magnetic lineation and NE-SW striking magnetic foliation of the Miao'ershan pluton and the undeformed Yuechengling pluton reflect a pre-existing magma flow structure. Integrating our structural observation, anisotropy of magnetic susceptibility (AMS) results with gravity modeling, we reveal the deep geometry of the extensional dome and restore the original structure before the Late Mesozoic Ziyuan detachment. The mode of extension argues for a single batholith split into two separate massifs, thinning the crust of the central SCB. The shallow-dipping Ziyuan detachment fault (10°-30°) may account for the large horizontal extension but low exhumation of mid-crustal rocks in the SCB, in contrast to the large exhumation of deep crustal rocks in the NCB.

Exhumation of the crustal-scale Gaoligong strike-slip shear belt in SE Asia

The lateral extrusion of blocks is a well-known geological process during continent–continent collision and is always expressed by either brittle strike-slip faults or ductile shear zones. However, the vertical motion along such fault systems remains poorly constrained. The Gaoligong shear zone formed the western boundary of the Indochina block during India–Eurasia collision, resulting in the exhumation of deep crustal rocks, including a large volume of syn-tectonic granites. Combined zircon U–Pb dating and 40Ar/39Ar thermochronology show that both the unfoliated and foliated granitic intrusions were emplaced during the Early Cretaceous (125–112 Ma), post-magmatic melting occurred from the Early Oligocene (c. 35 Ma) and subsequent cooling took place during the Mid-Miocene (c. 13 Ma). The average emplacement depth of Early Cretaceous samples showed that at least 15 km of the hanging wall of the Gaoligong shear zone must have been removed by vertical motion during shearing. Syn-shearing exhumation underlines the role of the initiation of lateral motion of the shear zone by magma-assisted rheological weakening and exhumation at high ambient temperatures within the shear zone. A new model links magmatic channel flow underneath the Tibetan Plateau with magma intrusions and the high geothermal gradients along the shear belts such as the Gaoligong shear zone.Supplementary material: Methods, electron probe microanalysis test results and zircon laser ablation inductively coupled plasma mass spectrometry U–Pb results are available at https://doi.org/10.6084/m9.figshare.c.5598365

Devonian–Early Carboniferous thrust tectonics in the Old Red Sandstone Molasse Basin, North‐East Greenland

AbstractIn East Greenland, sedimentation during the Middle Devonian‐earliest Carboniferous interval was characterized by the Old Red Sandstone Molasse Basin deposits. The continental succession recorded folding and intrabasinal unconformities associated with the activation of extensional detachments and core complex formation which post‐dated high‐P granulite facies metamorphism at c. 403 Ma. The extensional collapse responsible for the initiation of the Devonian basin and associated bimodal magmatism must be understood in the context of continued continental convergence during the Early Carboniferous, required by ultrahigh‐pressure metamorphism in North‐East Greenland at c. 355 Ma. Along a small nunatak within the Wordie Gletscher in Payer Land, NE Greenland (lat. 74°N), Precambrian gneisses recording Devonian granulite‐facies metamorphism are in tectonic contact with the molasse deposits along an ESE‐ward dipping thrust fault: the Scotstounhill Thrust. This paper will discuss the role of Devonian contractional structures and contemporaneous extensional faulting and present a new tectonic model that accounts for the exhumation of deep crustal rocks by these coupled mechanisms.

Anatomy of an extensional shear zone leading to the exhumation of the middle crust within the Canigou dome (Eastern Pyrenees, Axial Zone)

To investigate the mechanism of exhumation of deep crustal rocks in hot orogens, we focus on the southwestern part of the Variscan Canigou gneiss dome (Pyrenees) where the transition between the middle and upper crust can be observed. On the basis of new structural data, geological mapping, cross sections, microstructural analyses, and petrologic observations, we propose that flat-lying foliation within the gneiss dome and steep cleavage developed above it formed coevally. We note the presence of regional-scale shear zones at the boundary between the middle crust (Núria-Canigou Unit) and upper crust (Puigmal unit). At this boundary, the E-W South Canigou Shear Zone is an extensional shear zone showing top-to-the southwest kinematics. Deformation, which is localized within the andalusite-biotite transition zone, occurred under retrogressive conditions, from high-grade ductile to brittle conditions during the main deformation stage (D2). The tectonic contact was folded during a later stage (D3). We propose that the shape of the Canigou dome resulted from the activation of extensional shear zones at the top of the orthogneiss, leading to partial exhumation of high-grade rocks. The Canigou gneiss dome can be considered an extensional dome that formed during late Variscan (ca. 310-290Ma) transpression.

Early Miocene rapid exhumation in southern Tibet: Insights from P–T–t–D–magmatism path of Yardoi dome

Reconstructing the evolution of Gneiss domes within orogenic belts poses challenges because domes can form in a variety of geodynamic settings and by multiple doming mechanisms. For the North Himalayan gneiss domes (NHGD), it is debated whether they formed during shortening, extension or collapse of the plateau, and what is the spatial and temporal relationship of magmatism, metamorphism and deformation. This study investigates the Yardoi dome in southern Tibet using field mapping, petrography, phase equilibria modelling and new monazite ages. The resulting P–T–time–deformation–magmatism path for the first time reveals the spatial and temporal relationship of metamorphism, deformation and magmatism in the Yardoi dome: a) the dome mantle recorded prograde loading to kyanite-grade Barrovian metamorphic conditions of 650 ± 30 °C and 9 ± 1 kbar (M2) in the Early Miocene (18–17 Ma); b) the main top-to-the-north deformation fabric (D2) formed syn- to post-peak-metamorphism; c) the emplacement of leucorgranites related to doming is syn-metamorphism at 19–17 Ma. The link between the detachment shear zone in the Yardoi dome and the South Tibetan detachment system (STDS) is confirmed. By comparing with orogen-scale tectonic processes in the Himalaya, we suggest that north-south extension in a convergent geodynamic setting during Early Miocene accounts for formation of the Yardoi dome. In a wider tectonic context, the Early Miocene rapid exhumation of deep crustal rocks was contemporaneous with the rapid uplift of southern Tibet and the Himalayan orogen.

Oligocene crustal anatexis in the Tethyan Himalaya, southern Tibet

Recent studies in the Xiaru and Malashan gneiss dome of the Tethyan Himalaya, southern Tibet identify that Xiaru and Paiku tourmaline-bearing leucogranite dike formed at ~28–29Ma. Together with ~28Ma Kuday garnet-bearing leucogranites, it is demonstrated that the Himalayan orogenic belt experienced a major episode of crustal melting in the Mid-Oligocene. Geochemical data indicate that three suites of leucogranite are characterized by large variations in the major and trace element compositions as well as Sr–Nd isotope systematics, which could be explained by combined fractional crystallization and relative contributions of micas and accessory phases dissolved into a crustal melt during decompressional melting of metapelitic rocks. Documentation of Oligocene partial melting of crustal rocks could indicate that the exhumation of deep crustal rocks in the Himalayan orogenic belt could have started as early as Oligocene.

Middle–Late Mesozoic sedimentary provenances of the Luxi and Jiaolai areas: Implications for tectonic evolution of the North China Block

Provenances of sedimentary rocks may provide important constraints on the tectonic evolution of the North China Block (NCB). Previous studies have demonstrated that the northern NCB (NNCB) and the Xing-Meng orogenic belt (XMOB) supplied massive detritus southward into the hinterland of the NCB during the Jurassic. In order to study the evolution of sedimentary provenance during the Middle–Late Mesozoic, U–Pb geochronology and Hf isotopic geochemistry of detrital zircon grains and chemical compositions of detrital garnets from sandstones in the Luxi and Jiaolai areas, eastern NCB, were analyzed in combination with published data on the Jurassic sandstones. The Late Paleozoic–Mesozoic (367–139Ma) zircons in the lowermost Cretaceous Mengyin Formation samples from the Luxi area show εHf(t) values of −15.3 to −3.2 and +1.3 to +10.0, which are very similar to the results of analyses of the Jurassic formations. Further, the increased amount of Mesozoic zircons and granulite-derived garnets in the Mengyin Formation samples, compared to those in the Jurassic samples, indicates there was more detritus supply from the NNCB than from the XMOB. In the overlying Qingshan Formation samples, zircon grains do not exhibit Paleozoic ages, but most of them have Early Cretaceous ages and negative εHf(t) values, which are similar to the zircon grains extracted from the widespread Early Cretaceous igneous rocks in the NCB. This suggests that the provenance might have changed to a locally derived source. In contrast, the zircon population of the Early Cretaceous sandstones from the Jiaolai basin is dominated by grains of mid-Neoproterozoic age (700–900Ma) which signifies contribution from the Sulu orogen. Moreover, the detrital garnet assemblages of sandstones in the Luxi area are not consistent with those from representative metamorphic rocks in the Sulu orogen. The above results seem to confirm that the Mesozoic sedimentary provenance of the Luxi area had no evident connection with the Sulu orogen. Instead, the differential uplifting and exhumation of deep crustal rocks in the NNCB and the XMOB, associated with closure of the Mongol–Okhotsk Ocean, might exert a dominant control on sedimentary provenances in the wide hinterland of the NCB from the Jurassic period to ∼136Ma (the upper boundary age of the Mengyin Formation).

Consequences of open-system melting in tectonics

Partial melting and melt drainage from deep suprasolidus crust in orogens has important consequences for tectonics. Melt extraction along prograde segments of clockwise P – T paths reduces fertility and increases the density and strength of residual crust, which has implications for further melt production during decompression. Using calculated P – T phase diagrams, implications of stepwise melt loss along clockwise P – T paths for pelite and greywacke are assessed, and density of the progressively more residual source and the potential role of buoyancy in the exhumation of deep crustal rocks are evaluated. Two model P – T paths are considered: isobaric heating at 1.2 GPa followed by decompression to 0.4 GPa at 750, 820 and 890 °C, and prograde heating from the fluid-present solidus at 1.2 GPa to 860 °C at 1.8 GPa followed by isothermal decompression to 0.4 GPa. Both closed-system (undrained) and conditionally open-system (drained by intermittent melt loss) conditions are assessed. If melt is drained along clockwise P – T paths in suprasolidus crust then lower quantities of melt will be generated during decompression than sometimes inferred in tectonic models. Instead, the role of melt transfer through suprasolidus crust and melt accumulation at shallow levels in the anatectic zone should be considered rather than simply invoking the generation of large volumes of melt in decompressing crust.

Early carboniferous wrenching, exhumation of high-grade metamorphic rocks and basin instability in SW Iberia: Constraints derived from structural geology and U–Pb and 40Ar–39Ar geochronology

New U–Pb and 40Ar–39Ar geochronology and structural data from high- to medium grade metamorphic shear zones of the Ossa-Morena Zone, and structural data from Early Carboniferous basins (Ossa-Morena Zone and South-Portuguese Zone), place additional constraints on the Variscan tectonics in SW Iberia. A zircon U–Pb age of 465±14Ma (Middle Ordovician) measured on migmatite from the Coimbra–Cordoba shear zone is interpreted as the age of protolith crystallization. This age determination revises the information contained in the geological map of Portugal, in which these rocks were considered to be Proterozoic in age.This paper describes the evolution of Variscan wrench tectonics related to the development of shear zones, exhumation of deep crustal rocks and emplacement of magma in the Ossa-Morena Zone basement. In the Coimbra–Cordoba shear zone (transpressional), migmatites were rapidly exhumed from a depth of 42.5km to 16.6km over a period of ca. 10Ma in the Viséan (ca. 340–330Ma), indicating oblique slip exhumation rates of 8.5 to 10.6mm/yr (Campo Maior migmatites) and 3.2mm/yr (Ouguela gneisses) respectively. In the Évora Massif, the gneisses of the Boa Fé shear zone (transtensional) were exhumed from 18.5 to 7.4km depth in the period ca. 344–334Ma (Viséan), with exhumation oblique slip rates of 2.8 to 4.2mm/yr. At the same time, the Early Carboniferous basins of SW Iberia were filled by turbidites and olistoliths, composed mostly of Devonian rocks. The presence of olistoliths indicates significant tectonic instability during sedimentation with large-scale mass movement, probably in the form of gravity slides. Deformation and metamorphism dated at 356±12Ma, 321±13Ma and 322±29Ma respectively suggests that Variscan wrench movements were active in SW Iberia during the Early Carboniferous for a period of at least 35Ma.

Neotectonic study of the Western Crete and implications for seismic hazard assessment

A detailed study has been realized in the framework of a large-scale seismotectonic survey in Western Crete (Southern Greece), for the creation of a revised neotectonic map in a scale of 1:50.000, including the recognition and mapping of the main neotectonic faults and the evaluation of their seismic potential. For this reason, the faults under investigation were distinguished as active, possible active and inactive. Kinematic data and striations were used to estimate the corresponding stress field geometry. Two distinctive stress phases were recognized, operating after the Middle Miocene extensional exhumation of deep crustal rocks. The first N-S extension phase (D1) took place during Mid-Upper Miocene to Lower Pliocene, forming large normal faults, trending mainly E-W, that bound the large Neogene basins. The second phase (D2) took place during late Pliocene-Quaternary times, forming medium-to-large normal faults that trend mainly N-S, related to an E-W extension. In the E-W trending D1 faults, a younger strike-slip striation usually occurs, compatible with the later D2 kinematics. Smaller, mainly NE-SW trending faults, with significant lateral displacement, indicate a kinematic compatibility to the more recent D2 phase. Some of these faults act as transfer zones between the larger N-S trending D2 faults. Considering the fault length and the using several geological criteria for their seismic risk evaluation, we recognized 13 large major fault zones in the study area, six of which were considered as active, while three as possible active faults. Results obtained from the analysis of fault plane solution information verify both the determined active (D2 phase) stress field results, as well as the local kinematic behavior of the neotectonic faulting. Moreover, a detailed seismic hazard analysis, involving both probabilistic and deterministic approaches, shows a significant spatial variation of the various hazard measures, with the seismic hazard of the westernmost part of study area being controlled by the neighboring higher seismicity neotectonic faults

Rapid exhumation of deep crust in an obliquely convergent orogen: The Kaoko Belt of the Damara Orogen

The exhumation of deep crustal rocks and juxtaposition of structural‐metamorphic domains from different depths in a transpressional orogen may occur during the prograde evolution of the orogen by vertical extrusion or during the retrograde evolution of the orogen via extension. Metamorphic petrology, kinematics, and thermochronology of strike‐slip shear zones in the Kaoko Belt of the Damara Orogen are used to evaluate extrusion and extension processes in this transpressional orogen. Mineral assemblages and deformation mechanisms record shearing at pressures of 4–6 kbar and temperatures of ∼550°C for the Three Palms mylonite zone, 600–650°C for the Purros mylonite zone, and ∼630–700°C for the Village and Khumib mylonite zones. The Three Palms mylonite zone, which separates the accreted Coastal Terrane from the former passive margin of the Congo Craton, exhibits progressive deformation during decreasing temperatures through lower greenschist facies and into the brittle field, with consistent oblique normal shear indicators. Lower‐temperature fabrics and brittle features also overprint the Village mylonite zone. The207Pb‐206Pb (titanite) and40Ar/39Ar (hornblende, muscovite, and biotite) data indicate rapid cooling, at rates of 30–100°C/Ma, of all high‐grade structural domains in the core of the Kaoko Belt between circa 535 and 525 Ma, which is about 20–30 Ma younger then peak metamorphism. The40Ar/39Ar data from synkinematic muscovite fish in the retrograde shear zone assemblages indicate that the Khumib, Village, and Three Palms mylonite zones were actively deforming at temperatures below 350°C at circa 530–524 Ma. These data indicate that the high‐grade metamorphic rocks of the Kaoko were rapidly exhumed and juxtaposed after the main transpressive deformation by oblique extension. Reactivation of the shear zones and tectonic exhumation of high‐grade structural domains from beneath the accreted Coastal Terrane was caused by collision in the Damara Belt and terminal accretion of Gondwana.

Tectonothermal analysis of high-temperature mylonitization in the Coimbra–Córdoba shear zone (SW Iberian Massif, Ouguela tectonic unit, Portugal): Evidence of intra-continental transcurrent transport during the amalgamation of Pangea

Mylonites in pelitic and quartzofeldsphatic gneisses from the Ouguela tectonic unit (Coimbra–Córdoba shear zone, SW Iberian Massif) have been studied as an example of high-temperature ductile deformation associated with transcurrent tectonics. Detailed microstructural and P– T analysis indicates that ductile deformation evolved from a metamorphic peak at approximately 650–750 °C and 7.5–9.5 kbar (quartzofeldsphatic gneisses) and 730–790 °C and 7.5–9.5 kbar (pelitic gneisses) to retrograde conditions at 500–575 °C and 4.5/5.5–6.5/7.5 kbar (quartzofeldsphatic gneisses) and 525–600 °C and 3.5/4.5–5.5/7.5 kbar (pelitic gneisses). Following the metamorphic peak, exhumation was very fast. The P– T trajectory, which does not reach the curve for granite melting, is distinct that of isothermal decompression. Instead, the progressive and contemporaneous decrease in pressure and temperature was a direct response to strong heat dissipation along the contacts between the ascending slice and the adjacent blocks. The horizontal component of exhumation path, calculated for middle and shallower crustal levels, sum to ca. 57 km to 94 km (for the pressure peak). Assuming this offset acted in the Viséan during a time interval of ca. 9 Ma, the estimated exhumation horizontal slip rate is in the order of 6.3 to 10.4 mm/yr, which corresponds to an exhumation oblique-slip exhumation rate of 6.6 to 10.7 mm/yr (for ductile deformation). These values indicate that the transcurrent tectonic displacements accommodated by these mylonitic are similar to those of modern intra-continental shear zones, such as the still active Karakoram Fault (8.3 mm/yr) in the Himalayas. The Coimbra–Córdoba shear zone is therefore a typical intra-continental transcurrent zone with ten-to-one hundred kilometre along-strike mass movement of material that aided the exhumation of deep crustal rocks. Study of this large-scale structure in the SW Iberian Massif is therefore central to models of orogenic deformation during the amalgamation of Pangea.

Transcurrent continental tectonics model for the Ossa-Morena Zone Neoproterozoic?Paleozoic evolution, SW Iberian Massif, Portugal

The Ossa-Morena Zone (SW Iberian Massif) was affected by continuous orogen-parallel transcurrent continental tectonics from the Neoproterozoic to the Carboniferous times, involving transtension (TT) and transpression (TP) processes that co-existed together, occurred separately in neighbouring regions by the means of strain partitioning or even worked diachronically. A first stage of transpression TP1 took place during the Late Neoproterozoic–Lower Cambrian as a result of Cadomian arc-continent collisional processes. Structures generated by transtension TT1 from Cambrian to Lower Devonian were related to strong lithosphere stretching responsible for the development of basins controlled by major detachments, tilting, rifting and important tectono–thermal diachronic processes. Denudation phenomena and inhibition of sedimentation related with thermal uplift (asthenosphere upwelling) and consequent subsidence caused by isostatic equilibrium, involving generalized transgressions, were processes responsible for major unconformities. The Variscan TP2-TT2 episodes that followed diachronically TP1-TT1, by maintaining the orogen-parallel transport direction, were concomitant with syntectonic deposition of continental basins in the OMZ and foreland basins in the SPZ. TT2 local transtension and tectonic exhumation of deep crustal rocks along major shear zones, favoured the opening of tectonic troughs filled up by sediments and volcanism. TP2 shortening have generated fold axes parallel to the orogen-strike and composite dissymmetric flower structures.

Miocene to Holocene exhumation of metamorphic crustal wedges in the NW Himalaya: Evidence for tectonic extrusion coupled to fluvial erosion

The Himalayan crystalline core zone exposed along the Sutlej Valley (India) is composed of two high‐grade metamorphic gneiss sheets that were successively underthrusted and tectonically extruded, as a consequence of the foreland‐directed propagation of crustal deformation in the Indian plate margin. The High Himalayan Crystalline Sequence (HHCS) is composed of amphibolite facies to migmatitic paragneisses, metamorphosed at temperatures up to 750°C at 30 km depth between Eocene and early Miocene. During early Miocene, combined thrusting along the Main Central Thrust (MCT) and extension along the Sangla Detachment induced the rapid exhumation and cooling of the HHCS, whereas exhumation was mainly controlled by erosion since middle Miocene. The Lesser Himalayan Crystalline Sequence (LHCS) is composed of amphibolite facies para‐ and orthogneisses, metamorphosed at temperatures up to 700°C during underthrusting down to 30 km depth beneath the MCT. The LHCS cooled very rapidly since late Miocene, as a consequence of exhumation controlled by thrusting along the Munsiari Thrust and extension in the MCT hanging wall. This renewed phase of tectonic extrusion at the Himalayan front is still active, as indicated by the present‐day regional seismicity, and by hydrothermal circulation linked to elevated near‐surface geothermal gradients in the LHCS. As recently evidenced in the Himalayan syntaxes, active exhumation of deep crustal rocks along the Sutlej Valley is spatially correlated with the high erosional potential of this major trans‐Himalayan river. This correlation supports the emerging view of a positive feedback during continental collision between crustal‐scale tectono‐thermal reworking and efficient erosion along major river systems.

Syncontractional extension and exhumation of deep crustal rocks in the east Greenland Caledonides

Tectonic models for the North Atlantic Caledonides typically show Baltica being subducted below Laurentia prior to late orogenic collapse and exhumation of deep crustal rocks, conveniently explaining the high‐ and ultrahigh‐pressure provinces in the Scandinavian Caledonides. However, these models offer no tectonic explanation for the deformation and overthickening of the overriding Laurentian plate. Here we document major anatectic midcrustal ultraductile top to the (N)W shear zones in the east Greenland Caledonides that may account for this orogenic thickening. U‐Pb dating of monazite and 40Ar/39Ar dating suggests that high‐grade fabrics (∼ 435–424 Ma) and granites (∼427 and 422 Ma) involved in these zones show that these were contemporaneous with syntectonic granites involved in the Fjord Region Detachment Zone (FRDZ), a major top to the east extensional fault. More than 100 km of top to the NW overthrusting has been documented in the foreland of the orogen, yet we show that rocks in the hanging wall of the thrust were thinned by orogen‐parallel extension rather than thickened during emplacement, thereby exhuming the underlying active midcrustal thrust zones. This is documented by Late Silurian (∼419 Ma) to Early Devonian (∼401 Ma) 40Ar/39Ar‐muscovite cooling dates from rocks uplifted along major normal faults and intrusion of mafic dikes (∼ 419 Ma) into extensional faults. Folding and thrusting in the foreland probably continued well beyond this time, and underthrust high‐grade nappes probably were exhumed in the triangular zone between the thrust faults (west) and the structurally higher FRDZ (east) before final NW vergent thrust emplacement over low‐grade sedimentary rocks. Final cooling of the metamorphic core below closure temperatures for Ar diffusion in K‐teldspar occurred in the early Carboniferous. Major folds previously regarded as having formed during the main Caledonian event at ∼425 Ma may in part be up to 100 m.y. younger. Some of these folds also deform sedimentary rocks associated with collapse basins of Middle Devonian to Carboniferous age.

Late orogenic extension in Hellenides

In the Hellenic orogen both typs of late orogenic extension, associated with deep crustal parts exhumation, are recognized during the Tertiare: In the areas of Olympos-Ossa and Pelion Mts in Northern Greece, as well as in the island of Crete in Southern Greece a bivergent late orogenic extension is recognized. Nappes collapse took place immediately above the cold accretionary wedge while compression was active at depth. Heer high pressure assemplages were good preserved. On the contrary, in the Rhodope and Cyclades areas an asymmetric extension dominates. Heer extensional exhumation of deep crustal rocks took place in the high thermal flow back-arc region and high pressure metamorphic rocks were highly overprinted by greenschist to amphibolite facies metamorphism. Partial melting and granitoids intrusions followed the high grade metamorphic reworking of the rocks. Tertiary late orogenic extension in the Hellenides tooke place simultaneously with successive subductions processes and crustal thickening at the front of the extended plate, forming with the associated compression a SW-ward migrated system. Extension started in the Rhodope massif during the Eocene/Oligocene to be reached in the Olympos, Ossa, Pilion and Cyclades areas in the Oligocene/Miocene and final in the Crete island at the more external Hellenides, during the Mid-Miocene. Changes in the rate of convergence between Africa and Eurasia associated with retreating plate boundaries conditions allowed the successive, extensional exhumation of the deep crustal rocks in the Hellenides. Assymetric collapse in the back-arc area was possibly favoured, because the high potential energy of the thickened crust in the active orogenic arc was counteracted by the continuing subduction along the boundaries of the converging segments of Africa and Eurasia. Symmetric collapse of the overthickened crust above the cold accretionary prism was favoured probably, due to an increasing of the upward pressure produced by the unterplating of the lithospheric slap beneath the accretionary wedge.

Transpressive tectonism in the eastern Penokean orogen, Canada: Consequences for Proterozoic crustal kinematics and continental fragmentation

Results of a structural analysis of Archean basement, Huronian cover and various igneous rocks in the Sudbury area, Ontario, and regional tectonometamorphic correlations suggest that structures in the eastern Penokean orogen formed dominantly by dextral transpression. Bulk horizontal shortening started with the ca. 2.4 to 2.2 Ga Blezardian orogeny and was accompanied by peak metamorphism and local granitoid magmatism. During the 1.89 to 1.83 Ga Penokean orogeny, horizontal dextral shear parallel to the curved axis of the eastern Penokean orogen gained in importance, and is most likely related to docking of Proterozoic magmatic arc terranes to the Superior Province margin. This tectonic activity led to distributed, heterogeneous deformation of Archean basement rocks and exhumation of deep-crustal rocks in the Sudbury area and the Kapuskasing structural zone, but may have failed to accrete Paleoproterozoic terranes in the Lake Huron area. The lack of evidence for horizontal extension following Blezardian and Penokean contraction suggests that crustal fragmentation of the Archean supercontinent started at 2.45 Ga. Finally, the Great Lakes tectonic zone and the Murray fault, master dislocations in the Penokean orogen, may have acted as crustal weakness zones which influenced the orientation and development of 1.1 Ga Midcontinent Rift segments.

Petrology of an intrusion‐related high‐grade migmatite: implications for partial melting of metasedimentary rocks and leucosome‐forming processes

Intrusion‐related migmatites comprise a substantial part of the high‐grade part of the southern Damara orogen, Namibia which is dominated by Al‐rich metasedimentary rocks and various granites. Migmatites consist of melanosomes with biotite+sillimanite+garnet+cordierite+hercynite and leucosomes are garnet‐ and cordierite‐bearing. Metamorphic grade throughout the area is in the upper amphibolite to lower granulite facies (5–6 kbar at 730–750 °C). Field evidence, petrographic observations, chemical data and mass balance calculations suggest that intrusion of granitic magmas and concomitant partial melting of metasedimentary units were the main processes for the generation of the migmatites. The intruding melts were significantly modified by magma mixing with in situ partial melts, accumulation of mainly feldspar and contamination with garnet from the wall rocks. However, it is suggested that these melts originally represented disequilibrium melts from a metasedimentary protolith. The occurrence of LILE‐, HFSE‐ and LREE‐enriched and ‐depleted residues within the leucosomes implies that both quartzo‐feldspathic and pelitic rocks were subjected to partial melting. Isotope ratios of the leucosomes are rather constant (143Nd/144Nd (500 Ma): 0.511718–0.511754, ε Nd (500 Ma): −3.54 to −5.11) and Sr (87Sr/86Sr (500 Ma): 0.714119–0.714686), the metasedimentary units have rather constant Nd isotope ratios (143Nd/144Nd (500 Ma): 0.511622–0.511789, ε Nd (500 Ma): −3.70 to −6.93) but variable Sr isotope ratios Sr (87Sr/86Sr (500 Ma): 0.713527–0.722268). The most restitic melanosome MEL 4 has a Sr isotopic composition of 87Sr/86Sr (500 Ma): 0.729380. Oxygen isotopes do not mirror the proposed contamination process, due to the equally high δ18O contents of metasediments and crustal melts. However, the most LILE‐depleted residue MEL 4 shows the lowest δ18O value (<10). Mass balance calculations suggest high degrees of partial melting (20–40%). It is concluded that partial melting was promoted by heat transfer and release of a fluid phase from the intruding granites. High degrees of partial melting can be reached as long as the available H2O, derived from the crystallization of the intruding granites, is efficiently recycled within the rock volume. Due to the limited amounts of in situ melting, it seems likely that such regional migmatite terranes are not the sources for large intrusive granite bodies. The high geothermal gradient inferred from the metamorphic conditions was probably caused by exhumation of deep crustal rocks and contemporaneous intrusion of huge masses of granitoid magmas. The Davetsaub area represents an example of migmatites formed at moderate pressures and high temperatures, and illustrates some of the reactions that may modify leucosome compositions. The area provides constraints on melting processes operating in high‐grade metasedimentary rocks.

Magmatism as an essential driving force for formation of active metamorphic core complexes in eastern Papua New Guinea

The D'Entrecasteaux Islands in eastern Papua New Guinea are composed of a number of active metamorphic core complexes which have been intruded by granodiorite plutons during their formation. The plutons do not appear to have been intruded by diapiric processes as previously suggested. Late, relatively undeformed plutons form flat‐lying bodies which crosscut structural boundaries and are strongly discordant to core complex shear zones. Granodiorite magmatism and the development of the metamorphic core complexes have occurred in a linear zone which coincides with a zone of thick crust and rugged topography. It is proposed that plutonism facilitated deformation in ductile extensional shear zones which resulted in tectonic exhumation of deep crustal rocks and formation of the metamorphic core complexes. The source of the plutons is thought to be related to a linear zone of mantle upwelling beneath the islands related to the propagation of the Woodlark seafloor spreading center into continental crust. It is suggested that a localized heat source of this type, which can provide heat and magmatic material to the crust, is essential for the development of metamorphic core complexes.