Thermal Uplift Research Articles

Exhumation of UHP Terranes: A Case Study from the Dabie Mountains, Eastern China

Two processes are suggested to explain how UHP rocks are exhumed from mantle depths. One is removal of the overburden either by erosion or by extension, whereas the other involves the uplifting of the UHP rocks through the overburden. Application of either of these mechanisms to the Dabie Mountains, however, is fraught with difficulty. When combined with previously published data, new studies on metamorphic P-T paths, regional structures, and deep upper-mantle architecture revealed by seismic tomography lend support to a multi-stage exhumation process that operated in the Dabie Mountains. The first stage (230 to 200 Ma) is characterized by ductile deformation, produced during eclogite-facies recrystallization under a geothermal gradient as low as 10°C/km, implying a synsubduction exhumation. Some of the UHP rocks evidently were exhumed to a depth of ˜60 km, as indicated by petrological study of the Shuanghe eclogite. The second stage (200 to 170 Ma) attended ductile deformation and amphibolite-facies retrograde metamorphism. Subduction of the Yangtze block was halted by slab breakoff at a depth of ∼200 km. The resultant geothermal gradient recovered to ∼20″C/km. Slab breakoff permitted buoyancy-driven ascent of the UHP low-density melange to shallow crustal levels in a diapir structure. When the UHP portion of the mountain root rose, the shallow portion was heated to a temperature higher than that of the peak metamorphic pressure. The third stage (170 to 120 Ma) is characterized by extension and thermal uplift, as well as erosion. Sedimentary basins and volcanic rocks developed on both sides of the Dabie Mountains. Gab-bro-pyroxenite intruded the hanging wall of the UHP terrane, and granite, as well as migmatite, developed in that stage. Exhumation mechanisms might include corner flow for the first stage, buoyancy-driven squeezing-up for the second stage, and crustal extension, as well as erosion, for the third. Rupture and loss of the subducted lithospheric plate generated the gravity instability that resulted in exhumation of the subducted UHP section.

New Data Regarding Hotly Debated Topics Concerning UHP Metamorphism of the Dabie-Sulu Belt, East-Central China

Critical but controversial problems in the study of UHP metamorphic rocks from the Dabie-Sulu region include: (1) the possible existence of ophiolitic mélange; (2) the “in situ” versus “foreign” origin of UHP eclogites and their enclosing gneisses; (3) the possible presence and role of fluids during ultrahigh-pressure (UHP) recrystallization; (4) the timing of collision between the Yangtze and Sino-Korean continental blocks; (5) the polarity of syncollisional subduction; and (6) a single-versus multistage exhumation scenario for the UHP rocks. These questions are discussed in light of new geological, geochemical, and isotopic constraints. Our conclusions for the Dabie-Sulu belt are as follows: (1) Mafic-ultramafic blocks are of two distinct origins: one group samples lithosphere of the suprasubductionzone mantle wedge, whereas the second group represents postcollisional magmatic intrusions. Neither lithologic group represents true oceanic crust. (2) Quartzofeldspathic gneisses enveloping the eclogites are of two types— metasedimentary “in situ” and igneous “foreign.” The paragneisses contain UHP garnets + white micas, and are uniformly older (235 ± 5 Ma) than the orthogneisses (210 ± 5 Ma), which are devoid of UHP mineralogic indicators. (3) Fluids were active under UHP conditions and allowed the formation of UHP hydrous phases such as phengite and zoisite. However, the aqueous fluids may have been restricted to certain channels/pathways during exhumation. External fluids were absent until ascent of the UHP rocks to middle-crustal levels. (4) The Yangtze and Sino-Korean continental blocks collided during 230 to 240 Ma, when supracontinental rocks experienced UHP metamorphism. The HP metamorphic event dated as >400 Ma might record a subduction of oceanic crust during the Paleozoic. (5) An ancient mantle wedge is revealed by geochemical characteristics of Mesozoic magmatic rocks developed on the southern margin of the Sino-Korean craton, the hanging wall of the UHP-rock-bearing unit. Seismic tomography images reveal that the Yangtze block extends beneath the Dabie-Sulu orogenic belt. This indicates that both oceanic and continental crust had a northward subduction polarity. (6) Taking petrologic and geochronological data into account, we prefer a multistage exhumation model. The UHP rocks were exhumed rapidly during the first stage (230 to 200 Ma), perhaps reflecting a corner-flow mechanism. Then, buoyancy and mantle upwelling brought the UHP rocks up to middle-crustal levels during the second stage (200 to 170 Ma). Extension and thermal uplift, as well as erosion, eventually exposed the UHP rocks to the surface in the third stage (170 to 120 Ma).

How continents break up: Insights from Papua New Guinea

The Woodlark Basin in the western Pacific forms a continuous system of active continental rifting evolving to well‐developed seafloor spreading. Thin sediment cover in the basin and a dominantly nonvolcanic rift phase permit basement fabric and structures to be imaged by swath mapping and seismic reflection data in the continental and oceanic parts of the basin. Magnetic isochrons indicate a single Euler pole of opening for most of the basin history and allow us to infer the opening kinematics along the rifted margins. In agreement with rigid plate tectonic models, continental rifting initiated geologically synchronously (at ∼6 Ma) along the length of the protomargins within a deforming plate boundary zone. Strain localization and seafloor spreading, however, developed in a time transgressive fashion from east to west within this zone of deformation. Spreading centers formed within the rheologically weaker protocontinental margins surrounded by stronger oceanic lithosphere in the Solomon and Coral Seas. The transition to spreading occurred after a rather uniform degree of continental extension: 200±40 km. Both early and late stage rifting involved high‐ and low‐angle normal faults. We identify distinct styles in the transition from rifting to spreading which we refer to as nucleation, propagation, and stalling. These breakup styles impart varyingly concordant to discordant relationships between the adjacent oceanic and continental rift structures. Continental transform margins which are or were juxtaposed against the ends of spreading centers show no evidence for thermal uplift or igneous underplating. The initial spreading segments achieved much of their length at nucleation (within rift basins separated along strike by accommodation zones), with subsequent lengthening by spreading propagation into rifting continental crust. This early propagation, and the subsequent development of transform faults between initially nontransform spreading segment offsets, produced rift and spreading segmentation boundaries that are not simply correlated. The spreading centers nucleated approximately orthogonal in strike to the opening direction but, as the protomargins were oblique to this direction, nucleation jumps occurred in order to maintain the new spreading centers within the protomargins. Thus stepwise spreading nucleation in order to remain within a rheologically weak zone, rather than rupturing of the lithosphere by stress concentration at the tip of a propagating ridge axis, is the dominant form of the rifting‐to‐spreading transition in the Woodlark Basin.

The prospectivity of the Vøring and Møre basins on the Norwegian Sea continental margin

The focus of exploration activity on the Norwegian Sea Continental Shelf has shifted westwards from the shallow water areas over the Trøndelag Platform and Halten Terrace to the deep water areas over the Cretaceous Vøring and Møre basins. The basins, especially the Vøring Basin, have been involved in a complex tectonic development since the main extensional phase in the late Middle Jurassic to Early Cretaceous, including Cenomanian–Campanian and Maastrichtian–Eocene extension and thermal uplift and intermittent phases of compression/transpression. While the Jurassic is the prospective interval in the Trøndelag Platform and Halten Terrace, the potential in the Vøring and Møre basins is seen mainly in the Cretaceous and Paleocene – the Jurassic is too deeply buried. The two first deep water exploration wells in these basins, 6707/10-1 and 6305/5-1, have already proven two effective plays, with excellent reservoir sands in the Upper Cretaceous and lower Paleocene charged with thermogenic gas in structural traps in rotated fault blocks and compressional domes. Palaeogeographical reconstructions indicate that these reservoir rocks may be widespread within the basin areas. No good oil-prone source rock at a suitable depth of burial is so far proven, but the palaeogeographical reconstructions linked to surrounding well data and regional analyses indicate a chance for the existence of Upper Cretaceous oil source intervals. The thermal history of the basins is as complex as their tectonic history, and is further complicated by the possible effects of the Paleocene–Eocene magmatic activity, making any basin modelling results at the present stage unreliable.

The origin and genesis of major Jurassic unconformities within the triple junction area of the North Sea, UK

Three Jurassic unconformities of intra-Aallenian, intra-Callovian and intra-middle Oxfordian age are consistently identified within the literature for the North Sea region but despite their common recognition these unconformities remain poorly understood and they are essentially spatially and temporally unconstrained. The recent development of a sequence stratigraphic framework for the Jurassic of the North Sea allows for a thorough investigation of the spatial and temporal characteristics of the intra-Callovian and intra-Oxfordian unconformities and these data have important implications for Middle and Late Jurassic rifting within the North Sea region. The intra-Aalenian unconformity is believed to have been generated as a result of an increase in geothermal gradient within the area and subsequent thermal uplift that preceded rifting. The stratigraphic gap associated with this unconformity increases towards the proposed location of a Middle Jurassic, triple junction centred dome. In contrast the stratigraphic break associated with the intra-Callovian and the intra-middle Oxfordian unconformities is found to increase onto what are now intra-rift highs and rift margin areas. These unconformities occur at the same time as distinct changes in the geometry of the fill of the triple junction and also occur at the same time as seismic scale extension. They do not appear to correlate to any supposed global fall in relative sea-level. Stratigraphic data indicate that three phases of extension occurred during the Callovian, middle and late Oxfordian and lastly during the Kimmeridgian and Volgian. Although the intra-Callovian and intra-middle Oxfordian unconformities can be confused with the effects of the Middle Jurassic North Sea dome unconformity, they are in fact the result of extension during the initial Callovian and Middle/late Oxfordian rift phases. The recognition of these regional unconformities suggests that the Middle and Late Jurassic rifting process was punctuated rather than being continuous.

Carbon-isotope composition of Lower Cretaceous fossil wood: Ocean-atmosphere chemistry and relation to sea-level change

The carbon-isotope composition of fossil wood fragments, collected through a biostratigraphically well-constructed Aptian (Lower Cretaceous) shallow-marine siliciclastic succession on the Isle of Wight, southern Britain, shows distinct variations with time. The results indicate that the stratigraphic signature of {delta}{sup 13}C{sub wood} through the Aptian was influenced primarily by fluctuations in the isotopic composition of CO{sub 2} in the global ocean-atmosphere system, as registered in marine carbonates elsewhere, and was not governed by local paleoenvironmental and/or paleoecological factors. Negative and positive excursions in {delta}{sup 13}C{sub wood} through the lower Aptian occur in phase with inferred transgressions and regressions, respectively -- a pattern that contrasts with that observed in many previous studies for different time intervals. The relationship between {delta}{sup 13}C variations and relative sea-level change is tentatively interpreted as a response to various climatic and eustatic factors, relating to rapid sea-floor spreading, thermal uplift of ocean floor, emplacement of plateaus, volcanic CO{sub 2} emissions, weathering, and sedimentary rate.

The importance of fluid pressures and migration to the hydrocarbon prospectivity of the Faeroe–Shetland White Zone

The hydrocarbon prospectivity of the Faeroe–Shetland White Zone, located in the area between the Shetland and Faeroe Islands, was assessed in a regional study that integrated seismic and well interpretations with detailed source-rock geochemistry and predictive basin modelling. The Faeroe Basin formed during a Barremian rifting event followed by subsidence during the Late Cretaceous. The Paleocene began with a period of thermal uplift of basement highs and rapid sedimentation which infilled the submarine topography formed during the Cretaceous, and produced marked overpressuring in the basin. Gradual subsidence continued through the Tertiary except for a significant mid Tertiary inversion event that formed several interesting structures in the basin. New thermal models of basins and a new pressure mechanism for inducing hydrofractures that allow vertical hydrocarbon migration from Jurassic source rocks through Cretaceous mudrocks to Tertiary reservoirs, which we call the ‘whoopee cushion effect’, provide the key controls on the hydrocarbon charge mechanism, timing and petroleum composition. The other crucial elements, source, reservoir, and traps which are present at several stratigraphic levels in the White Zone, are summarized in this paper. The interplay of overpressure, hydrocarbon generation and migration during a complex basin evolution makes the White Zone a highly prospective frontier petroleum province.

Tectonostratigraphical development of the Upper Karooforeland basin: Orogenic unloading versus thermally-induced Gondwana rifting

The origins of the Karoo Basin have never been adequately explained, but twomain models have been suggested: (1) a purely foreland basin (retro-arc) model; and (2) a polyphase successor basin, beginning with extension during Upper Table Mountain Group (Silurian) times to a foreland basin in the Early Karoo (Permian) and extension in the Upper Karoo (Upper Triassic-Jurassic). Subsidence and forward modelling on the known stratigraphy of the Karoo Basin do not fit a simple foreland basin model. This is particularly true for the Upper Karoo where the stratigraphy, stacking patterns and age of the succession suggest that the foreland basin model is not viable, and that continental extension played a major role in late basin development. Evidence for extension, and when this occurred, is provided by: (1) sequence stratigraphical identification of a major Middle-Late Carnian low stand sequence boundary unconformity, related to uplift of a newly emergent source area in the southeast; and (2) the presence of volcanic detritus in Upper Karoo sediments, chemically similar to the Karoo basalts, suggesting a possible chemogenetic link between them and, by implication, with early volcanism and incipient Gondwana rifting, now well-preserved in the rift sequences of East Antarctica. This early volcanism, which began not later than the Carnian (∼230 Ma), and provenance uplift, may be related to a thermal anomaly linked to mantle plume-induced updoming, crustal extension and faulting approximately 40 Ma prior to maximum uplift preceeding Karoo Basalt eruption at 183 Ma. Thermal uplift occurred some 200 km off the present southeast coast of South Africa, just south of the Karoo Hotspot, but away from the direct influence of any known hotspot or hotspot track. This uplift, aligned parallel to the southeast coast of South Africa, at a high angle to the Cape Fold Belt, was located close to the site of later rifting and separation along the Agulhas Falkland Fracture Zone.

Deformation and Metamorphism of the Marble Mountain and Pony Camp Areas, Western Triassic and Paleozoic Belt, Central Klamath Mountains, Northwestern California

The Western Triassic and Paleozoic belt (WTrPz) is a regionally extensive, composite terrane correlative with Cache Creek-affinity rocks, a major crust-forming lithotectonic entity of the North American Cordillera. New structural, stratigraphic, and petrologic data suggest that a large tract of greenschist to amphibolite-grade metavolcanic and metasedimentary rock, previously considered to consist of several separate oceanic terranes, is, instead, a single fault-bounded, volcanic island arc, the Sawyers Bar terrane. It represents a mid-Jurassic, relatively intact, recrystallized nappe complex 5 to 10 km thick, extending over 100 km along strike in the central Klamaths. Protoliths of the complex are interpreted to be Lower Triassic (?) to mid-Jurassic supracrustal, volcanic arc-related units deposited, deformed, and metamorphosed within a suprasubduction zone adjacent to the continental margin. Metamorphism increases monotonically with depth in the nappe, ranging from prehnite-pumpellyite to lower greenschist-grade in the Pony Camp area on the south, through greenschist-grade in the medial Sawyers Bar area, to low-pressure amphibolite-grade metamorphism in the Marble Mountains on the north. The Pony Camp area generally lacks penetrative deformation. In the Marble Mountains, peak metamorphism largely postdates intense deformation; nevertheless, folding of fabrics and brittle deformation are common. The complex is bounded by low-angle, W-vergent, crustal-scale, mid-Jurassic thrusts. The Soap Creek Ridge fault juxtaposes Stuart Fork blueschists over the Sawyers Bar complex. The lower thrust is not definitely established, but must be situated beneath tectonic levels postulated by earlier workers. It may coincide with the previously unrecognized brittle-plastic Isinglass shear zone in the Marble Mountains, and a poorly exposed, unnamed low-angle fault in the Virgin Buttes region west of Pony Camp. In this area, mapping indicates that the Twin Sisters fault is a relatively minor high-angle break within the WTrPz, rather than being a crustal-scale terrane suture. Synmagmatic, brittle extensional faults are common, as are syn and postmetamorphic, regionally extensive, high-angle faults that internally imbricate the WTrPz; the latter are marked by sheared serpentinite. Folds within the Sawyers Bar nappe complex are NE to NW-trending and W-vergent. Structural evidence suggests that W-vergent thrusting, E-W contraction, regional Siskiyou metamorphism, penetrative deformation, and crustal thickening occurred at ∼170 to 165 Ma, and preceded voluminous 167 to 162 Ma calc-alkaline plutonism. In the study areas, waning stages of Siskiyou deformation were characterized by thermal relaxation, uplift, extension, crustal thinning, and E-directed tectonic transport. Nevadan age contraction (≈155 to 150 Ma), prevalent to the west at lower structural levels of the WTrPz, is not recognized in the Sawyers Bar nappe; however, regionally developed open folding of Siskiyou metamorphic fabrics and rare superposed folding and axial-plane cleavage development in the Marble Mountains may reflect a Nevadan event. Brittle deformation that clearly post-dates Siskiyou folding is younger than 150 Ma, but is older than ∼130 Ma, the age of the oldest marine strata that overlie the Klamath province regionally. Kinematic evidence from the eastern Marble Mountains suggests sinistral transtension of possibly latest Jurassic-Early Cretaceous age. Late-stage brittle deformation is permissibly Cenozoic; the Sawyers Bar thrust sheet was tilted a maximum of 30° to the south along the flanks of the Condrey Mountain dome during Cenozoic uplift. The Sawyers Bar nappe complex is similar to other composite terranes in Phanerozoic convergent suture zones throughout the world. Like the Klamath Mountains, these areas also may represent different exposure levels within a single fault-bounded entity rather than an amalgam of disparate terranes.

Analysis of erosion events and palaeogeothermal gradients in the North Alpine Foreland Basin of Switzerland

Abstract For basin modelling purposes, uplift and erosion analysis is as important as subsidence analysis. Within the North Alpine Foreland Basin of Switzerland (NAFB) three major interregional unconformities separate the sedimentary succession: the late Palaeozoic, the base Tertiary and the base Quaternary unconformity. The amount of missing section depends on the palaeogeographic basin position and on different inversion mechanisms (thermal uplift, intraplate stress, tectonic uplift or isostatic rebound movements). Several techniques exist to determine the magnitude and timing of maximum palaeotemperatures and palaeogradients, facilitating direct estimation of missing section. In our study coalification and fission track data are analysed and compared with recently published models concerning erosion in the NAFB. The results indicate that each inversion episode is accompanied by significant erosion (up to several km). The following range of removed section values were reconstructed: (i) late Palaeozoic unconformity: 1000–1200 m; (ii) base Tertiary unconformity: 800–1800 m; (iii) base Quaternary unconformity: 1500–3000 m in the SW, up to 700 m in the NE and 4100–4400 m in parts close to the Alpine front. The reconstruction techniques provide only rough estimates as each approach is based on certain assumptions and calibration uncertainties. Also, the obtained values are higher than those expected from simple evaluation of the preserved stratigraphic record. Calculated palaeogeothermal gradients are very high (80–90 °C km −1 ) during the late Palaeozoic, slightly higher than normal during the late Cretaceous (30–50 °C km −1 ) and lower than normal (15–30 °C km −1 ) during the Cenozoic.

Tectonic Evolution of the Himalayan Collision Belt

AbstractThis paper discusses the tectonic divisions of the Himalayan collision belt and deals with the tectonic evolution of the collision belt in the context of crustal accretion in the front of the collision belt, deep diapirism and thermal–uplift extension and deep material flowing of the lithosphere — backflowing. Finally it proposes a model of the tectonic evolution progressive intracontinental deformation model — of the Himalayan belt.

Isotopic constraints on the influence of the Icelandic plume

Thermally buoyant mantle, in the form of a plume, rises beneath Iceland creating a major topographic anomaly along the Mid-Atlantic Ridge and in the surrounding ocean basin. However, the influence of the Iceland plume on the composition of lavas erupted on adjacent ridges remains a contentious issue. Trace element systematics and radiogenic isotope ratios of Sr, Nd and Pb suggest that the plume influences a region 1200 km in length. In contrast, the 3He anomaly associated with Iceland closely corresponds to the 2400 km ridge section affected by thermal uplift. We present evidence that the Sr, Nd and Pb isotope signature of the Iceland plume is in fact as widespread as its thermal and 3He anomalies. Results imply that much of the source of North Atlantic ridge basalts has been contaminated by lateral outflow of asthenosphere from the Icelandic plume. Consequently, estimates of the average composition of mid-ocean ridge basalt (MORB) sources are likely to be biased by including data from plume-contaminated regions. True MORB values, and perhaps upper mantle geochemistry, can be constrained only by considering data untainted by plume asthenosphere.

Sheared continent–ocean margins: an overview

Continent–ocean fracture zones are the fossil transform offsets located along passive rifted continental margins. Kinematic models identify at least two principal stages in their evolution. During the first stage as rifting proceeds, continent–continent shearing dominates a narrow region in which the transform fault will eventually rupture. High-standing continental marginal ridges 50–100 km wide and bounding deep sedimentary basins, are derived in such settings. In stage two as sea-floor spreading proceeds, the younger oceanic block slides along the active transform, heating the older continental block, and possibly induces thermal uplift and accompanying denudation. Magnetic injection into the continental block at depth may also induce an isostatic uplift. After ridge–transform intersection time, mechanical coupling between the continental and oceanic blocks may influence the stratigraphy and structure of these margins.

Structural evolution of the Isle of Man and central part of the Irish Sea

Abstract Gravity, aeromagnetic, seismic, borehole and outcrop data from the Isle of Man area have been used to study the structural framework and tectonic evolution of the central part of the Irish Sea since the Ordovician. The area has been affected by two major compressional events (Caledonian and Variscan), three rifting events (early Carboniferous, early Permian and ?late Jurassic) and a period of thermal uplift (early Tertiary). A large part of the Isle of Man consists of early Ordovician turbidites (‘basement’) interpreted to have been deposited in a back arc or fore arc basin that was inverted during the Caledonian orogeny. Major NE-SW and subsidiary NW-SE strike-slip faults were initiated at this time, some of which were later reactivated to form important basin controlling faults in the Irish Sea. Post-Caledonian basin subsidence was initiated in the early Carboniferous with a period of NW-SE directed extension followed by thermal sag which ceased at the end of the Carboniferous as a result of Variscan compression. E-W extension in the early Permian led to the development of new extensional faults which have influenced the thickness of the Collyhurst Sandstone in the offshore. Post-rift thermal sag continued throughout the late Permian and the Triassic. A late Jurassic period of extension has been proposed in order to explain post-Triassic offset across major normal faults east of the Isle of Man although direct evidence of syn-rift or post-rift strata has been removed by erosion during uplift in the early Tertiary. This uplift was due to thermal doming of the crust, an event which was associated with the intrusion of a large number of dolerite dykes. The structural evolution of the Isle of Man has also been influenced by a major NW-SE trending basement fracture known as the ‘Central Valley Lineament’ which extends into the offshore to the SE and NW of the island. The lineament appears to be important in defining areas of different hydrocarbon prospectivity in the Irish Sea.

Subduction, mega-shear systems and Late Palaeozoic basin development in the African segment of Gondwana

Basins within the African sector of Gondwana contain a Late Palaeozoic to Early Mesozoic Gondwana sequence unconformably overlying Precambrian basement in the interior and mid-Palaeozoic strata along the palaeo-Pacific margin. Small sea-board Pacific basins form an exception in having a Carboniferous to Early Permian fill overlying Devonian metasediments and intrusives. The Late Palaeozoic geographic and tectonic changes in the region followed four well-defined consecutive events which can also be traced outside the study area. During the Late Devonian to Early Carboniferous period (up to 330 Ma) accretion of microplates along the Patagonian margin of Gondwana resulted in the evolution of the Pacific basins. Thermal uplift of the Gondwana crust and extensive erosion causing a break in the stratigraphic record characterised the period between 300 and 330 Ma. At the end of this period the Gondwana Ice Sheet was well established over the uplands. The period 260–300 Ma evidenced the release of the Gondwana heat and thermal subsidence caused widespread basin formation. Late Carboniferous transpressive strike-slip basins (e.g. Sierra Australes/Colorado, Karoo-Falklands, Ellsworth-Central Transantarctic Mountains) in which thick glacial deposits accumulated, formed inboard of the palaeo-Pacific margin. In the continental interior the formation of Zambesi-type rift and extensional strike-slip basins were controlled by large mega-shear systems, whereas rare intracratonic thermal subsidence basins formed locally. In the Late Permian the tectonic regime changed to compressional largely due to northwest-directed subduction along the palaeo-Pacific margin. The orogenic cycle between 240 and 260 Ma resulted in the formation of the Gondwana fold belt and overall north-south crustal shortening with strike-slip motions and regional uplift within the interior. The Gondwana fold belt developed along a probable weak crustal zone wedged in between the cratons and an overthickened marginal crustal belt subject to dextral transpressive motions. Associated with the orogenic cycle was the formation of mega-shear systems one of which (Falklands-East Africa-Tethys shear) split the supercontinent in the Permo-Triassic into a West and an East Gondwana. By a slight clockwise rotation of East Gondwana a supradetachment basin formed along the Tethyan margin and northward displacement of Madagascar, West Falkland and the Gondwana fold belt occurred relative to a southward motion of Africa.

Permo-Carboniferous Glaciogenic Al Khlata Formation, Oman: A New Hypothesis for Origin of its Glaciation

ABSTRACT Al Khlata Formation is an oil-bearing reservoir in Oman. Its origin during the Late Palaeozoic Gondwana glaciation in the southern part of the Arabian Peninsula is demonstrated by the glaciogenic deposits of the formation in Oman. Outcrops of the Al Khlata Formation occur in a belt parallel to the Huqf fold axis, the best outcrops being found in the two wadis Al Khlata North and South. In the southern wadi, glacial deposits rest directly on dolomites of the Precambrian Khufai Formation showing northeast-southwest trending glacial striations. Earlier workers proposed that the direction of ice flow was from the southwest to northeast. This is not accepted in this paper, where evidence is presented to support an ice-flow direction from northeast to southwest. Later fluvial flow towards the northeast possibly resulted from the collapse of the continental margin towards the new Proto-Arabian Sea and its subsequent subsidence. We propose that the Early Permian glaciation in Oman resulted from crustal uplift just prior to the calving of a microcontinent along the northeast Arabia margin of Gondwana and the creation of Neo-Tethys. It is suggested that the triple-junction area where the Neo-Tethys and the Proto-Arabian Sea were later to meet, was a site of sufficient thermal uplift to become a center of mountain glaciation.

Isostatic compensation of Ishtar Terra, Venus

We have used spherical harmonic representations of the Venus topography and geopotential, obtained from Magellan data, to evaluate isostatic support in several areas within the Ishtar Terra highlands, including the Lakshmi plateau, its surrounding mountain belts, namely Akna and Freyja, and Maxwell Montes, and the Fortuna Tessera province. We find that topography in Ishtar is largely isostatically compensated (>80%). Regional geoid‐topography variations in the subregions can be explained by a combination of Airy (crustal thickening) and thermal (lithospheric thinning) mechanisms, provided Venus has a thick reference thermal lithosphere (∼300–400 km). With the exception of eastern Fortuna, low elevation areas (h < 3–4 km above the mean planetary radius, MPR) with large geoid‐topography ratios (GTR) seem to be associated, to various degrees, with thermal isostasy, whereas the higher areas (h > 4 km above MPR) with small GTRs are almost certainly Airy compensated via thickened crust. Relatively large (>60 km) total Airy crustal thicknesses obtained in the western Ishtar mountain belts, together with a probable basalt‐eclogite phase change, suggest a possible silicic composition for these structures, provided they are older than ∼25–50 Ma. Lakshmi Planum seems essentially thermally supported, with the thermal lithosphere thinned to ∼100 km. We suggest, as one possibility, that the lithospheric thinning process under Lakshmi is delamination of a dense eclogite lower lithosphere layer into the mantle. The decrease in GTR observed in Ishtar between Lakshmi to the west (GTR ∼ 20 m/km), Maxwell and west Fortuna (GTR ∼ 8 m/km), and eastern Fortuna (GTR ∼ 4 m/km) may correspond to a decay in thermal compensation attributed to lithospheric delamination, which would be fairly recent (∼100 Ma) in Lakshmi, partially decayed in west Fortuna, and absent in east Fortuna, where a mostly Airy‐supported topography is essentially relaxed with no thermal uplift. Alternatively, if surficial concentrations in radiogenic elements were prevalent throughout the crust, partial melting of a thickened crust could account for the thermal uplift in Lakshmi and west Fortuna. The zero‐elevation basaltic crustal thickness H ∼ 24 km obtained for the east Fortuna Tessera region may be representative of the ambient crustal thickness in the Venus lowlands. Our findings support multicomponent models for tectonic and volcanic activity in Ishtar. The thick ambient crust and thermal lithosphere implied by this study agree with observational constraints such as support of extreme elevations, large topographic slopes, unrelaxed craters, and the thick elastic lithosphere suggested by flexure studies. If the ambient thermal lithosphere on Venus were to be relatively thin (∼100–200 km), with a cold mantle and radiogenic elements concentrated in the crust, then thermal evolution on Venus may be in quasi‐steady state, with the geodynamic evolution in monotonic decline. However, if the ambient thermal lithosphere is very thick (∼300–400 km), as suggested by our thermal model fits, then it is consistent with the predictions of strongly unsteady state thermal evolution models and an interior which is currently heating up. This would support the view that catastrophic resurfacing on Venus might be episodic.

Quantitative modelling of basin and rheological evolution of the Iberian Basin (Central Spain): implications for lithospheric dynamics of intraplate extension and inversion

Subsidence analysis of Mesozoic sediments in the central part of the Iberian Basin in Central Spain demonstrates that basin evolution was related to three stages of lithospheric stretching, each followed by thermal subsidence. The rifting history was followed by Tertiary inversion. The three extensional phases correlate with rifting stages identified in other parts of the Iberian Basin, on the Betic, Lusitanian and Cantabrian margins of Iberia, and with regional extensional tectonics. The first phase (245-157 Ma), marked by rapid tectonic subsidence in Triassic-Early Jurassic times, yields a lithospheric stretching factor of about 1.10. The second, Oxfordian to earliest Albian (157-112 Ma) in age, is characterised by uplift followed by moderate subsidence. The amount of uplift and subsidence during these phases varies significantly within the basin. The uplift implies subcrustal lithospheric stretching factors ranging from 1.06 to 1.28 assuming a thermal uplift mechanism. The subsequent subsidence corresponds to lithospheric stretching factors in the range from 1.00 to 1.06. The third main phase (95-85 Ma), seen as a Cenomanian-early Senonian acceleration of tectonic subsidence, is modelled with lithospheric stretching factors of about 1.02. The subsidence models are used to reconstruct the paleo-rheological evolution of the area during the Mesozoic, and indicate progressive strengthening of the lithosphere from west to east in the studied area during the Late Jurassic and Cretaceous. For the second phase of rifting and onset of Tertiary basin inversion, strength distributions are not in agreement with localised basin deformation. As an explanation, it is shown that late Variscan faults may have contributed considerably to lithospheric weakening and localisation of extension and inversion.

Heat flow and regional uplift at the north-eastern border of the Ebro basin,NE Spain

Summary The north-eastern border of the Ebro foreland basin is characterized by relatively high elevation. the presence of Neogene to Present volcanism, and a high thermal gradient. These facts suggest that the area is affected not only by the topographic load of the Pyrenees but also by the opening of the Valencia trough. To examine this hypothesis, we first present the results of a surface heat-flow survey carried out in the Osona depression. After selecting those wells which are essentially affected by conductive heat transport and applying topographic and palaeoclimatic corrections, we obtain a corrected surface heat-flow density of 88 ± 16m Wm−2. Assuming this value as representative of the regional conductive heat flow, we relate the resulting lithospheric thermal structure to the topographic elevation assuming local isostatic equilibrium and considering a plausible range of crustal thickness and crustal density. The results obtained together with geothermometric and geobarometric data from xenoliths sampled in the nearby Olot volcanic zone indicate a regionally thinned lithosphere about 60–70 km thick. This thinning is related to the Neogene opening of the Valencia trough whose effects extend beyond the rift shoulder (Catalan Coastal Ranges) affecting a broad area of the Ebro basin. Two different uplift mechanisms are proposed as having taken place in the study area: a mechanical uplift related to Eocene compression, followed by a thermal uplift phase associated with asynchronous differential stretching produced during Neogene extension. The combination of these two mechanisms accounts for the observed vertical movements and the migration of the Middle Miocene and Plio-Quaternary volcanic activity.

Mid-Miocene propagation of the Yellowstone mantle plume head beneath the Columbia River basalt source region

Columbia River flood-basalt volcanism is the product of mid-Miocene distortion of the Yellowstone mantle plume head against the Precambrian margin of North America. The overall migration pattern of Columbia River basalt vents reflects northward propagation of the plume head beneath the west-southwest–migrating North American plate. Thermal uplift mimics the vent migration. Over the 1.5 m.y. of Grande Ronde basalt extrusion, the projected crest of the plume head migrated 105 km (7 cm/yr), from the Chief Joseph dike swarm east-oortheast into the Clearwater embayment, where uplift occurred over the same period at a rate of 0.67 mm/yr. Chemical variations reflect progressive melting of the plume and mantle lithosphere, followed by the mixing of these components with an enriched source from the overriding craton. Rapid thickening of the plume head against the cratonic margin led to decompressional melting and magma rise beneath a thin oceanic lithosphere of accreted terranes.