Doming Event Research Articles

Deformation Mechanisms During the Syn‐Orogenic Extrusion of the High‐Pressure Phyllites‐Quartzites Unit in the Central and Northern Peloponnese, Greece

AbstractThe High‐Pressure Phyllites‐Quartzites (PQ) unit of the External Hellenides is exposed in tectonic windows in the central and northern Peloponnese (Greece). Understanding the deformation history of this unit is essential to interpreting the Oligo‐Miocene evolution of the External Hellenides belt and its associated exhumation events. This study integrates new field observations and microtectonic analyses with previous studies to offer a comprehensive deformation model of the PQ unit since the Late Oligocene. The first deformation phase (D1), captures the progressive incorporation of the PQ into an orogenic wedge. This phase is largely overprinted and only preserved as relict features. The second phase (D2) displays coeval top‐to‐the‐ENE and top‐to‐the‐WSW localized ductile shear. A transition is observed from top‐to‐the‐ENE non‐coaxial deformation at the upper parts of the nappe to intense isoclinal folding (refolding S1) at the lower structural levels. We associate D2 with the ductile syn‐orogenic exhumation of the PQ within an extrusion wedge, accompanied by greenschist‐facies retrogression. In the third phase (D3), semi‐brittle to brittle extensional fault planes cut through the previous ductile structures. D3 faults exhibit extensional kinematics in all directions on the flanks of exhumation domes. This phase correlates with a late‐orogenic doming event, marking the final exhumation stage of the PQ unit in the upper crust. The exhumation of high‐pressure units results from the interplay between ductile syn‐orogenic extrusion and continuous underplating within the subduction zone. This underplating maintains vertical movements and uplift of the units, initiating a 3D upper‐crustal extensional collapse along low‐angle normal faults.

Extrusion tectonism of Indochina reassessed: constraints from 40Ar/39Ar geochronology from the Day Nui Con Voi metamorphic massif, Vietnam

The extrusion tectonic model for the southeastern margin of the Himalayan orogeny links the crustal shear activity along the Red River Shear Zone (RRSZ) to the opening of the South China Sea (SCS). The Day Nui Con Voi (DNCV) metamorphic massif in northern Vietnam strikes NW-SE, is bounded by the RRSZ to the south and continues along the strike where it meets the SCS. The DNCV is thus a critical area to document thermotectonic history in order to advance our understanding of the tectonic evolution of Indochina extrusion and its relationship to the opening of the SCS. Our new 40Ar/39Ar data combined with microstructural and petrological analyses constrained the timing of the left-lateral shearing of the RRSZ and revealed the thermal evolution of the DNCV metamorphic massif. Three ductile deformation events were observed. D1 formed NNW-SSE striking upright folds under granulite to upper amphibolite facies conditions. D2 was a horizontal to sub-horizontal folding event that occurred at amphibolite facies conditions. D3 was a doming event that formed NW-SE striking up-right folds bounded by left-lateral shearing mylonite belts along the two limbs. The S/C fabrics were defined by muscovite fish, quartz + albite + K-feldspar aggregates, and muscovite folia. The D3 doming event exhumed the DNCV metamorphic massif from amphibolite facies conditions to the lower greenschist facies conditions. The 40Ar/39Ar ages obtained from amphibole (∼26 Ma), phlogopite (∼25 Ma), muscovites (∼24-23 Ma), biotite (∼25-23 Ma), and K-feldspars (∼25-22 Ma) from different structural domains of the DNCV metamorphic massif indicated a rapid exhumation ∼26–22 Ma. We interpreted this as the time period for the D3 event, with the onset of left-lateral shearing occurring around 24 Ma based on ages obtained from syn-kinematic muscovites. This age was much younger than the initiation of sea-floor spreading of the SCS (since 32 Ma) but coincided with the age for the ridge jump event in the SCS. Based on these new data, we proposed that extrusion tectonism cannot be the cause for the initial opening of the SCS. Rather, the extrusion of the Indochina block was temporally correlative with the southward ridge jump event of the already opened SCS.

Late Cenozoic cooling and evolution history of the Kangmar dome in southern Tibet: Insights from inverse thermal modeling

The North Himalayan Gneiss Domes, which are essential parts of the Cenozoic extensional structures in Southern Tibet, record the thermal and tectonic processes that occurred after the India-Asian collision and are thought to be effective structures regulating post-collision intracontinental deformation. However, it is still unclear how these domes are formed and how they contribute to the regulation process. Here, we performed detailed geological mapping, elevation transect sampling, low-temperature thermochronological testing, and 3D modeling on the Kangmar dome, which is located west of the N‒S treading Yadong-Gulu rift, and its core-cover contact fault is suspected to be the northern continuation of the South Tibetan Detachment System (STDS). Our analysis revealed a discrepancy in the deformation histories of the dome’s northern and southern portions. We proposed a model in which the core-cover contact fault of the Kangmar dome was a part of the South Tibetan Detachment System and the doming event that occurred at ∼12.2 Ma was dominated by thrust stacking of the southward mid-crustal channel flow. The rapid cooling following the middle Miocene was possibly influenced by the N‒S Trending Yadong-Gulu rift activity. The present landscape was shaped by the incision of the Nianchu River, which was accompanied by increased glacial activity during the Pleistocene. Our findings enhance the intracontinental deformation patterns following collisions and shed light on the numerous domes in Himalayas and other orogenic belts.

Paleotemperature investigation of the Variscan southern external domain: the case of the Montagne Noire (France)

The Montagne Noire located in the southern part of the French Massif Central represents the northern part of the South-Variscan Foreland. It is subdivided into three parts. The granite-migmatite Axial Zone dome is surrounded by non- or weakly metamorphosed Paleozoic sedimentary series. Both northern and southern flanks of the Montagne Noire dome are deformed by km-scale, south to southeast facing recumbent folds and thrusts sheets. The Raman Spectroscopy of Carbonaceous Material (RSCM) method, carried out in the low-grade metamorphic rocks of the southern flank of the Montagne Noire, yielded temperatures comprised between 400 °C near the dome, and 230 °C in the southern domain. Three Raman geothermometers were used to cover this temperature range. RSCM temperatures comply qualitatively with previous estimates based on illite crystallinity, conodont color alteration, and fluid inclusions carried out in the same area, which document a metamorphic temperature increase towards the dome. The isotherms cut across the different nappe contacts and are oriented parallel to the southern margin of the Axial Zone. This temperature distribution supports the idea that the thermal structure was acquired during the Axial Zone dome emplacement. The thermal structure acquired during the recumbent folds emplacement and burial of the sedimentary series is totally overprinted by the doming event. In addition, in a domain relatively remote from the Axial Zone dome, the RSCM measurements yielded significantly higher temperatures than illite crystallinity. This discrepancy points to a higher sensitivity of RSCM to short-lived thermal events than illite crystallinity, possibly because of more efficient kinetics of the carbonization reaction. On the other hand, high RSCM temperatures analysed far from the Axial Zone, between 300 °C and 360 °C could be explained by the presence of granitic plutons under the foreland basin.

Deciphering the Sardic (Ordovician) and Variscan deformations in the Eastern Pyrenees, SW Europe

Detailed geological mapping of the La Cerdanya area (Canigó unit, Eastern Pyrenees) provides new data characterizing the different structural styles exhibited by Cambrian–Lower Ordovician (Jujols Group) and Upper Ordovician successions. Their unconformable contact, related to the Sardic Phase, ranges from 0° (paraconformity) to 90° (angular discordance). The Jujols Group rocks topped by the unconformity are affected by Sardic foliation-free open folds. The pre-Sardic succession, the Sardic Unconformity and the lower part of the post-Sardic succession (Rabassa Conglomerate and Cava formations) are cut and offset by several Late Ordovician NNE–SSW-trending synsedimentary extensional faults associated with hydrothermal activity, which dramatically affected the thickness of the lower part of the Upper Ordovician succession. We relate (1) the Mid-Ordovician Sardic uplift and erosion, and (2) a Mid- to Late-Ordovician upward propagating extensional fault system bounding the outline of half-grabens, subsequently infilled by alluvial deposits, to a thermal doming event (about 475–450 Ma) that led to the uplift and stretching of the Ordovician lithosphere. Thermal doming may be caused by mafic magma underplating and responsible for the coeval calc-alkaline magmatic activity broadly developed in the Eastern Pyrenees. We discuss the similarities between the Mid-Ordovician Sardic Unconformity and other Early Paleozoic unconformities described in neighbouring areas. Finally, we suggest a geodynamic scenario in which a regional-scale thermal event was related to the opening of the Rheic Ocean that led to the drift of Avalonia from the SW European margin of Gondwana.

The stratigraphic and structural record of the Cretaceous Jianghan Basin, central China: Implications for initial rifting processes and geodynamics

The stratigraphic and structural characteristics of the initial phase of continental rift basins have been widely studied. However, the initial rifting geodynamic processes in many rift basins remain poorly understood because the relevant structures and stratigraphic successions tend to be deeply buried in result of continued rift evolution. Using an extensive database of geological (stratigraphic and structural) and geophysical data we investigate when and how rift initiation occurred in the Jianghan Basin. The correlation of the Lower Cretaceous strata across the basin reveals that they were deposited within a series of localized depressions distributed on the basin margin while the Late Cretaceous tectonic stage was characterized by widespread rifting with a maximum stratal thickness of ∼4500 m. The major faults controlling this Late Cretaceous sediment distribution are radially striking, suggesting a distributed, transtensional stress system or multi-directional extension during the Late Cretaceous. It is a common feature that pre-rift basement strata of the major faults in the hanging wall are older than that in the footwall and become progressively older approaching the fault plane, indicating a reactivation of pre-existing unroofed fault-related folds. Together with the regional geodynamic context for the South China Block, we divide the initial rifting processes into two distinct stages. During the Early Cretaceous, the lithosphere beneath the Jianghan Basin got rapidly thinned under the influence of the large-scale roll-back and dehydration of the subducted Pacific slab. Meanwhile, the upwelling asthenosphere and intruded dykes/magma heated and weakened the lithosphere, leading to thermal doming of most of the Jianghan Basin. However, on the basin margin, which was relatively unaffected by the thermal doming event, a set of localized depression sequences were deposited. Due to the Early Cretaceous lithospheric thinning, the lithosphere was thin enough to rift during the Late Cretaceous. Under the diapirism of the continuously upwelling asthenospheric mantle, the pre-existing thrusts with radial strikes simultaneously underwent extensional reactivation, forming a series of normal faults with multiple orientations. By providing the detailed stratigraphic and structural evidence for active rifting model, this study provides new insights into the processes of early rift initiation.

Genesis of the Zhaxikang epithermal Pb-Zn-Sb deposit in southern Tibet, China: Evidence for a magmatic link

The Zhaxikang Pb-Zn-Sb deposit is one of the most important deposits in the Southern Tibet metallogenic belt. Based on field geology, petrography, melt- and fluid inclusions and C-H-O isotopes, we describe and discuss the mineralization, alteration, and their possible link with magmatic fluids. Our results show that the Zhaxikang deposit shares many geological and geochemical similarities with typical intermediate-sulfidation (IS) epithermal deposits. The Pb-Zn-Sb mineralization is closely related to Fe-Mn carbonate- and silicic alterations, which formed the outer rim around the greisen in the Cuonadong Dome. Orebodies occur mainly as structurally-controlled veins and breccia dikes, with major minerals include sphalerite, galena, pyrite, arsenopyrite, and Fe-Mn carbonates. Main stage ore-forming fluids were of medium temperature (214–292°C), low salinity (2.6–5.3wt.% NaCl eqv.) and CO2-bearing.Melt/fluid inclusions in beryl and quartz from the pegmatite indicate that the primary magmatic fluids were derived from the melt-fluid immiscibility. The magmatic fluids were of low salinity (0.2–7.9wt.% NaCl eqv.), high temperature (298–457°C) and CO2-rich, and contained minor CH4, N2, C2H6, C3H8 and C6H6. The presence of Mn-Fe carbonates and daughter gahnite minerals in the beryl-hosted inclusions indicates high Mn, Fe and Zn contents in the parental magma and related magmatic fluids. This implies a genetic link between magmatic fluids and the Pb-Zn-Sb mineralization, as also supported by Ar-Ar dating and H-O-C isotopic evidence. We suggest that the Zhaxikang is best classified as an IS epithermal deposit, and the ore-forming fluids are likely to be magma-derived. Boiling of the magmatic fluids led to high-salinity fluids and metal enrichment. High regional geothermal gradient caused by the thermal doming event may have facilitated long distance transportation of magmatic fluids, and led to the formation of a wide alteration zone and distal Pb-Zn-Sb mineralization. The temperature drop and meteoric water involvement may have precipitated the Pb-Zn-Sn minerals in the distal fault systems.

The ocean-atmosphere response to wind-induced thermocline changes in the tropical South Western Indian Ocean

In the Indian Ocean basin the sea surface temperatures (SSTs) are most sensitive to changes in the oceanic depth of the thermocline in the region of the Seychelles Dome. Observational studies have suggested that the strong SST variations in this region influence the atmospheric evolution around the basin, while its impact could extend far into the Pacific and the extra-tropics. Here we study the adjustments of the coupled atmosphere-ocean system to a winter shallow doming event using dedicated ensemble simulations with the state-of-the-art EC-Earth climate model. The doming creates an equatorial Kelvin wave and a pair of westward moving Rossby waves, leading to higher SST 1–2 months later in the Western equatorial Indian Ocean. Atmospheric convection is strengthened and the Walker circulation responds with reduced convection over Indonesia and cooling of the SST in that region. The Pacific warm pool convection shifts eastward and an oceanic Kelvin wave is triggered at thermocline depth. The wave leads to an SST warming in the East Equatorial Pacific 5–6 months after the initiation of the Seychelles Dome event. The atmosphere responds to this warming with weak anomalous atmospheric convection. The changes in the upper tropospheric divergence in this sequence of events create large-scale Rossby waves that propagate away from the tropics along the atmospheric waveguides. We suggest to repeat these types of experiments with other models to test the robustness of the results. We also suggest to create the doming event in June so that the East-Pacific warming occurs in November when the atmosphere is most sensitive to SST anomalies and El Nino could possibly be triggered by the doming event under suitable conditions.

Structure and evolution of the Northeastern German Basin and its transition onto the Baltic Shield

The post-Permian sequence stratigraphical and structural evolution of the Northeastern German Basin and its transition onto the Baltic Shield has been studied in the Bay of Mecklenburg (SW Baltic Sea) by means of seismic interpretation. Five major sequences have been identified: Middle Triassic, Upper Triassic, Jurassic, Cretaceous and Cenozoic. Time–isochore maps allowed the identification of several phases of salt pillow growth. The contemporaneity of active salt tectonics and the well studied tectonic evolution of the Northeastern German Basin suggest a causative correlation. The E–W directed extension during the Triassic-Early Jurassic marking the beginning break-up of Pangaea is seen as the trigger process for the first period of salt movement. A fault system outside the limit of the Zechstein evaporates is understood as the consequence of thin-skinned faulting and brittle thick-skinned deformation that accompanied this extension. The observed pronounced erosion of Upper Triassic and Lower Jurassic strata is considered to result from the uplift due to the Mid North Sea Doming event in Middle Jurassic times. The seismic data show an undisturbed Late Cretaceous succession which reflects a period of rising sea level, tectonic quiescence and no salt movement. In contrast to the salt pillows which emerged above Triassic fault systems in the westernmost Baltic and western North German Basin, the Cenozoic salt movement activity is the most pronounced. This period of reactivated salt pillow growth started coevally with the onset of the Alpine orogeny at the Cretaceous/Cenozoic transition when the Africa-Arabian plate collided with Eurasia. Generally, no significant faults were identified in the overburden of the salt floored southern Bay of Mecklenburg where ductile Zechstein salt decouples deep rooted faulting from supra-salt deformation.

Identification of mantle plumes in the Emeishan Large Igneous Province

The plume hypothesis has been recently challenged largely because some fundamental aspects predicted by the modeling of plumes are found to be lacking in some classic hotspot regions. This review paper summarizes recent achievements made in the late Permian Emeishan continental flood basalt province in southwest China. Data from various disciplines are evaluated by comparing observation against prediction of the plume hypothesis. It is shown that 7 out of the 9 most convincing arguments in support of mantle plumes are found in the Emeishan large igneous province (LIP). In particular, sedimentological data show unequivocal evidence for a lithospheric doming event prior to the Emeishan volcanism. This observation, the presence of hightemperature magmas, emplacement of immense volume of magmas over a short time span and the spatial variation in basalt geochemistry are all consistent with predictions of plume modeling, thus providing strong support for the validity of the mantle plume hypothesis.

Comment on: New PGE and Re/Os-isotope data from lower crustal sections of the Vredefort dome and a reinterpretation of its "crust-on-edge" profile: R.J. Hart, I. McDonald, M. Tredoux, M.J. de Wit, R.W. Carlson, M. Andreoli, D.E. Moser and L.D. Ashwal (Vol. 207 (1/2), 173-184)

Ever since Slawson (1976) suggested that the Archaean crystalline basement rocks in the core of the Vredefort dome southwest of Johannesburg might represent “crust-on-edge”, debate has existed about the extent to which the deep crust was exhumed by the ~2.02 Ga, impact-induced doming event. In a suite of papers since 1981, Hart and co-workers (Hart et al. , 1981; 1990a; b; 2004; De Wit et al. , 1992; Tredoux et al. , 1999; Moser et al. , 2001; Flowers et al. , 2003) postulated that doming (and subsequent erosion) exposed a complete section through the crust of the central Kaapvaal craton, and that even rocks of the uppermost mantle are exposed near the centre of the dome. Their argument has been compiled based on: Geophysical evidence has also been used in support of this model - a positive Bouguer gravity anomaly in the central parts of the dome (Maree, 1944; Stepto, 1990) was interpreted by Hart et al. (1990b) as indicative of the existence of dense upper mantle rocks close to surface, …

Palaeoproterozoic dome-forming structures related to granulite-facies metamorphism, Jequié block, Bahia, Brazil: petrogenetic approaches

Dome structures in the São Francisco craton (Northeast Brazil), have been identified in the regions of Brejões and Santa Inês within the granulitic Jequié block. Domes are mainly composed of CH6 charnockites, which intrude various granulites of the basement. The Brejões dome is surrounded by a rim of supracrustal rocks. The enderbite–charnockite suites CH1 and CH2, which are partially included in the basement and were previously dated ca 2.8–2.7 Ga, are similar to modern calc-alkaline suites and could be derived by partial melting of an enriched tholeiite (degree of fractionation ca 24–23%). An older Archaean component is suggested by negative ɛ Nd values at 2.8–2.7 Ga which range between −1 and −4 and Nd model ages with an average ca 3.1 Ga. These magmatic suites are ascribed to the amphibolite-facies based on preserved prograde mineral assemblages and results of geochemical modelling. The CH6 rocks formed in domes are of granitic composition. They mainly derive from partial melting of the 2.7 Ga CH2 basement suite. The hercynite-quartz assemblage in supracrustal rocks surrounding the Brejões dome suggests very high temperature (>1000 °C) for the CH6 magmas which would have induced partial melting in adjacent supracrustal rocks, forming garnet-cordierite-bearing granitic melts. Emplacement of domes is constrained by the age of the granulite facies metamorphism in the Jequié block (ca 2.1 Ga) and the average cooling age of monazite which occurred in the syn-dome charnockite and the surrounding heterogeneous granulites of the basement at 2045 ± 2.5 Ma. A southern dome exhibits a younger monazite age of 2026 ± 1 Ma, which may suggest a broader age span for the doming event.

Evolution of North Himalayan gneiss domes: structural and metamorphic studies in Mabja Dome, southern Tibet

Field, structural, and metamorphic petrology investigations of Mabja gneiss dome, southern Tibet, suggest that contractional, extensional, and diapiric processes contributed to the structural evolution and formation of the domal geometry. The dome is cored by migmatites overlain by sillimanite-zone metasedimentary rocks and orthogneiss; metamorphic grade diminishes upsection and is defined by a series of concentric isograds. Evidence for three major deformational events, two older penetrative contractional and extensional events and a younger doming event, is preserved. Metamorphism, migmitization, and emplacement of a leucocratic dike swarm were syntectonic with the extensional event at mid-crustal levels. Metamorphic temperatures and pressures range from ∼500 °C and ∼150–450 MPa in chloritoid-zone rocks to 705±65 °C and 820±100 MPa in sillimanite-zone rocks. We suggest that adiabatic decompression during extensional collapse contributed to development of migmatites. Diapiric rise of low density migmatites was the driving force, at least in part, for the development of the domal geometry. The structural and metamorphic histories documented in Mabja Dome are similar to Kangmar Dome, suggesting widespread occurrence of these events throughout southern Tibet.

The Eastern prolongation of the Zanskar Shear Zone (Western Himalaya)

This paper aims to describe the possible SE extension of the Zanskar Shear Zone and its relation with other extensional and compressional structures. New structural and metamorphic data were collected in the Baralacha La, Yunam, Lingti region and a new geologic map of the studied area is proposed. The new data reveal that an E-verging syncline (the Kenlung Serai fold), formerly interpreted as a late backfold is, in fact, related to an early northward underthrusting of India below Asia before the main NE movement related to nappes emplacement. This succession of movements is coherent with the displacement of India postulated by several authors from paleomagnetic data. The well defined Zanskar Shear Zone can be followed towards the SE in a zone of low angle normal faults (the Tapachan normal fault zone) located at the front of the North Himalayan nappes. The change in style from a ductile shear zone to a set of low angle normal faults is related to a change of tectonic level and metamorphic grade. The location of these extensional structures at the front of the North Himalayan nappes as well as their tectonometamorphic history is totally similar to what is known farther NW for the Zanskar Shear Zone. High angle normal faults such as the Sarchu faults are late structures related to vertical extrusion accommodated by doming. Late NE-verging backfolds develop mainly in the hanging wall of the high angle normal faults and are therefore associated with the doming event. The new data are incorporated into a kinematic model for the “Crystalline nappe”.

Sequence stratigraphy of the Jurassic of the Danish Central Graben

A sequence stratigraphic framework is established for the Jurassic of the Danish Central Graben based primarily on petrophysical log data, core sedimentology and biostratigraphic data from about 50 wells. Regional seismic lines are used to assist in the correlation of some wells and in the construction of isochore maps. In the Lower Jurassic (Hettangian–Pliensbachian) succession, five sequences have been identified. The Middle Jurassic is subdivided into four sequences that together span the uppermost Aalenian/lowermost Bajocian to the Callovian. In the Upper Jurassic, better well coverage permits greater stratigraphic resolution, and 11 sequences are identified and mapped. On the basis of the sequence stratigraphic correlation and the construction of isochore maps for individual sequences, the Jurassic basin history of the Danish Central Graben can be subdivided into seven discrete phases: (1) Shallow marine and offshore sediments deposited in a prerift basin extending from the North Sea to the Fennoscandian Border Zone (Hettangian–Pliensbachian). (2) Uplift and erosion in association with a Toarcian–Aalenian North Sea doming event. A major hiatus represents this phase in the study area. (3) Terrestrial and marginal marine sedimentation during initial rifting (latest Aalenian/earliest Bajocian – Late Callovian). (4) Early Oxfordian – Early Kimmeridgian transgression during and after a rift pulse. The sedimentary environment changed from coastal plain and marginal marine to fully marine. (5) Regression associated with a cessation or slowing of subsidence during a structural rearrangement that took place in the Late Kimmeridgian during a break in the main rift climax. Shallow to marginal marine sandstones were deposited above an erosion surface of regional extent. (6) Deep-water mudstones deposited in a composite graben with high subsidence rates related to rift pulses (latest Late Kimmeridgian – middle Middle Volgian). (7) Deposition of organic-rich mudstones and turbidite sandstones during the late Middle Volgian – Early Ryazanian. The main basin shallowed, became more symmetrical and experienced a decreasing rate of subsidence, recording the onset of the post-rift stage. A relative sea-level curve is constructed for the Middle–Late Jurassic. It shows close similarity to published eustatic (global) and relative (North Atlantic area) sea-level curves in the latest Bathonian – late Early Kimmeridgian, but differs in the Late Kimmeridgian – Middle Volgian interval, probably due to the high rate of subsidence in the study area.

隠岐島後における末期中新世，隠岐アルカリ火山岩類の地質とマグマ供給系

A huge volume of lavas and pyroclastic rocks were erupted in Oki-Dogo island in the latest Miocene. They compose the Oki Alkaline Volcanic Group, the Hei Trachyte Group, the Tsuzurao Volcanic Group and the Tokage Trachyte Group. We examine the geology, petrography and geochemistry mainly for the Oki Alkaline Group, and present a magma plumbing story in the latest Miocene age. The rocks from the Oki Alkaline Volcanic Group are shoshonite, trachyandesite, trachyte and rhyolite, based on their chemical composition and stratigraphy. The trachytes are divided into Trachyte-1 (Nb 35-50 ppm) and -2 (Nb 49-75 ppm), and the rhyolites are divided into Lower Rhyolites (Moderate-SiO2: 71.4-73.6 wt%), Middle Rhyolites (High-SiO2: 74.7-75.7 wt%), and Upper Rhyolites (Low-SiO2: 70.7-71.6 wt%). Lower Rhyolites, Trachyte-1 were formed at the early volcanic stage. Middle Rhyolites, Trachyte-1 were formed at the middle stage, and Upper Rhyolites, Trachytes-1 and -2, trachyandesite and shoshonite were formed at the later stage. Rhyolitic lava domes with different compositions and feeder dikes of trachyte corresponding to individual volcanic vents occur at a number of places. This may indicate that the eruptions took place from several different vents aligned along a circular fracture. Lower Rhyolites were derived from the lower crust, and Middle Rhyolites were derived from middle crust by partial melting processes. While, Upper Rhyolites, Trachyte-2 and trachyandesite were generated by fractional crystallization from shoshonite magma. The heat source triggered melting of the crusts is inferred to be latent heat of crystallization in the mafic magma. These rhyolite to shoshonite magmas could have ascended through the ring fractures formed by the doming event, and have consolidated as the rocks composing the Oki Alkaline Volcanic Group. After the formation of this group, Cauldron collapse, and it is followed by the eruption of the Hei Trachyte Group and the Tsuzurao Volcanic Group. In the early Pliocene, The tectonic environment of the crust changed from a compressive to an extensional stress field. Then the eruption of the Omine Group and the Tokage Trachyte Group took place.

Evolution of the Kangmar Dome, southern Tibet: Structural, petrologic, and thermochronologic constraints

Structural, thermobarometric, and thermochronologic investigations of the Kangmar Dome, southern Tibet, suggest that both extensional and contractional deformational histories are preserved within the dome. The dome is cored by an orthogneiss which is mantled by staurolite + kyanite zone metasedimentary rocks; metamorphic grade dies out up section and is defined by a series of concentric kyanite‐in, staurolite‐in, garnet‐in, and chloritoid‐in isograds. Three major deformational events, two older penetrative events and a younger doming event, are preserved. The oldest event, D1, resulted in approximately E‐W trending tight to isoclinal folds of bedding with an associated moderately to steeply north dipping axial planar foliation, S1. The second event, D2, resulted in a high strain mylonitic foliation, S2, which defines the domal structure, and an associated approximately N‐S trending stretching and mineral alignment lineation. Shear sense during formation of S2 varied from dominantly top S shear on the south dipping flank of the dome to top N shear on the north dipping flank. The central part of the dome exhibits either opposing shear sense indicators or symmetric fabrics. Microtextural relations indicate that peak metamorphism occurred post‐D1 and pre‐ to early D2 deformation. Quantitative thermobarometry yields peak metamorphic conditions of ∼445°C and 370 MPa in garnet zone rocks, increasing to 625°C and 860 MPa in staurolite + kyanite zone rocks. Pressures and temperatures increase with depth and northward within a single structural horizon across the dome and the apparent gradient in pressure is ∼20% of the expected gradient, suggesting that the rocks were subvertically shortened after the pressure gradient was frozen in. Mica 40Ar/39Ar thermochronology yields 15.24 ± 0.05 to 10.94 ± 0.30 Ma cooling ages that increase with depth and young northward within a single structural horizon across the dome. Diffusion modeling of potassium feldspar 40Ar/39Ar spectra yield rapid cooling rates (∼10–30°C/Myr) between ∼11.5 and 10 Ma and apatite fission track ages range from 7.9 ± 3.0 to 4.1 ± 1.9 Ma, with a mean age of ∼5.5 Ma. Both data sets show symmetric cooling across the dome between ∼11 and 5.5 Ma. The S2 mylonitic foliation, peak metamorphic isobars and isotherms, and mica 40Ar/39Ar isochrons are domed, whereas potassium feldspar 40Ar/39Ar and apatite fission track isochrons are not, suggesting that doming occurred at ∼11 Ma. Our data do not support simple, end‐member metamorphic core complex‐type extension, diapirism, or duplex models for gneiss dome formation. Rather, we suggest that the formation of extensional fabrics occurred within a zone of coaxial strain in the root zone of the Southern Tibetan Detachment System (STDS), implying that normal slip along the STDS and extensional fabrics within the Kangmar Dome were the result of gravitational collapse of overthickened crust. Subsequent doming during the middle Miocene is attributed to thrusting upward and southward over a north dipping ramp above cold Tethyan sediments. Middle Miocene thrust faulting in the Kangmar Dome region is synchronous with continued normal slip along the STDS and thrust motion along the Renbu Zedong thrust fault, suggesting that extension and contraction was occurring simultaneously within southern Tibet.

Low pressure‐high temperature metamorphism in the Vredefort Dome, South Africa: Anticlockwise pressure‐temperature path followed by rapid decompression

AbstractPelitic metasediments of the Witwatersrand Supergroup in the Vredefort Dome contain textural evidence for a two‐stage metamorphic history. The pressure‐temperature (P‐T) path derived for the rocks indicates that they were subjected to a high geothermal gradient (≈40°C km‐1), mid‐amphibolite facies, Mla metamorphic event concomitant with thickening of the overlying upper crust. Peak temperatures of 570–600°C were reached during Mla at depths of 14–16 km. After initial isobaric cooling following Mla, the rocks experienced rapid exhumation during a high strain rate deformation event associated with the formation of the dome. This event led to the development of abundant pseudotachylite and a brittle cleavage in these rocks. These features are overgrown. together with the Mla assemblages, by a low P M1b paragenesis comprising microporphyroblastic cordierite+biotite. Estimated P‐T conditions during Mlb were < 3.5 kbar, ≈500–;530°C.The two‐stage P‐T path is incompatible with existing contact metamorphic and diapiric models that have attempted to link the mid‐amphibolite facies metamorphism with the formation of the Vredefort Dome. Instead, it indicates that the doming event occurred some time after the peak of a regional low P‐high T metamorphic event, during cooling of the terrain. Given an age of 2.02 Ga for the doming event, the Mla event is attributed to a widespread magmatic‐thermal event on the Kaapvaal Craton that accompanied the formation of the Bushveld Complex at 2.05–2.06 Ga. The Mlb event developed in response to the localized exhumation of deep parts of this terrain while the crustal geotherm was still elevated (≈30°C km‐1). The association, elsewhere in the dome, of pseudotachylite with shock metamorphic features generated at pressures in excess of 20–120 kbar indicates that exhumation was accomplished by the impact of a large meteorite into the terrain.

Domal structures and high‐grade metamorphism in the Higher Himalayan Crystalline, Zanskar Region, north‐west Himalaya, India

ABSTRACT In the main Himalayan range in the Ladakh‐Zanskar area, domal structures have been observed at structurally deeper levels in the tectonic unit of the Higher Himalayan Crystalline. Their formation occurred during a second, temperature‐dominated phase (M2) of high‐grade regional metamorphism, characterized by the semipelitic paragenesis of sillimanite‐K‐feldspar and incipient anatexis. The doming event reveals a local system of synmetamorphic uplift superimposed on a regional system of northeast‐southwest trending compression. In the main Himalayan range the development of the dominant S2 foliation is related to deformation during the doming phase, which started early in the M2 event. The deformation propagated continuously north‐east and south‐west with time. In the north‐east, on the northern slopes of the main Himalayan range, this deformation is expressed by extensional shear movements of the upper tectonic levels finally leading to the late‐ to postmetamorphic normal fault system of the Zanskar shear zone. Towards the south‐west, deformation is expressed by compressional movements, e.g. at the Main Central Thrust (MCT) in the Kishtwar window area. The observed compression and extension is inferred to relate to an increased uplift of the domal bulges of the tectonic Kishtwar window and of the whole main Himalayan range.

Thermoelastic bending of the lithosphere: implications for basin subsidence

Summary. Thermoelastic stresses are capable of producing significant lithospheric deflection. A systematic approach to modelling this effect is presented, in which the lithosphere is presumed to behave as a thin, elastic (or viscoelastic) plate on a fluid substrate, and lateral variations in basal heat flow induce both vertical buoyant loads and thermoelastic bending moments. The amount of uplift or subsidence produced by a given heat source depends on a number of factors, including the strength, duration and lateral extent of the thermal anomaly, and the thickness, density, rigidity and viscosity of the plate. The mechanical response of the plate is characterized by two distinct length scales (one for shearing, the other for bending) and the deformation produced depends critically on the scaled width of the heat source. The plate acts as a low pass spatial filter in response to thermal loads, but has a narrow band pass filter response to applied moments. The temporal response to an abrupt change in basal temperature or heat flow also depends on the ratio of the thermal diffusion time for the plate versus the viscoelastic relaxation time of the material. A rapid, localized increase in basal heat flow will initially produce a central depression and a peripheral uplift. Thus, while it is often assumed that the main depositional phase in platform basins is associated with the cooling and contraction of the lithosphere following an earlier thermal doming event, it is shown that, if conditions are right, a basin may also form in direct response to the initial heating event. Either way, subsequent thermal equilibration and viscous stress relaxation will tend to modify the initial deflection profile.