pre-Permian Basement Research Articles

The Val Biandino Intrusive Suite (central Southern Alps, N Italy): new geochronological and geochemical data on the Early Permian magmatic activity in the Southalpine Domain

The Early Permian in the present-day Europe area was a time when a major tectonic shift occurred, leading from the tectonic collapse of the Variscan orogeny to the crustal extension and thinning that characterized the Early Permian times. Crustal extension was associated with extensive magmatism at different crustal levels: from gabbro in the lower crust or at the mantle/crust transition to subaerial high-SiO2 volcanic activity. In the whole Southalpine Domain, the Early Permian intrusive bodies occur from the west (e.g. Ivrea-Verbano Complex and “Graniti dei Laghi”) to the east (Ifinger, Brixen and Cima d’Asta intrusive complexes). Among these, in the central Southern Alps (comprised between the Giudicarie Belt and the Lake Como), minor intrusive complexes also occur. The Val Biandino Intrusive Suite consists of two magmatic units: the Val Biandino Quartz-Diorite (VBQD) and the Valle di San Biagio Granite (VSBG). The first of them consists of gabbro-diorite to granodiorite bodies associated with leucocratic cordierite-bearing granitic dikes that intruded the pre-Permian basement. To the west, a W-dipping normal fault of Permian age represents the boundary between this unit and the Valle di San Biagio porphyric granite. All rock varieties of the Val Biandino Intrusive Suite display a high-K calc-alkaline affinity with metaluminous to peraluminous character. Field crosscutting relationships point to a late generation of the cordierite granites of the Val Biandino Quartz-Diorite unit with respect to the more mafic types. SHRIMP U–Pb zircon dating provided an age of 285.2 ± 1.9 Ma for a cordierite granite of the Val Biandino Quartz-Diorite unit and 283.2 ± 1.9 Ma for the porphyric Valle di San Biagio Granite. Geochemical data suggest that gabbro-diorite, quartz-diorite, granodiorite and leucogranite are not co-magmatic. The existing gaps in term of SiO2 wt% and the higher HREE contents in mafic and intermediate rocks with respect to granite coupled with similar LREE in all rocks support this hypothesis. The high Rb/Sr ratios (> 1) in leucogranite, together with the occurrence of white mica and cordierite point to a significant contribution of crustal partial melting in their genesis. The Val Biandino Intrusive Suite was likely formed through the interaction of magma genesis at the mantle/crust transition and partial melting of the heterogeneous pre-Permian basement of the Southalpine Domain. This scenario explains the strong heterogeneity displayed by the relatively small intrusive complex and the strong crustal signature exhibited by all the magmatic types of the Val Biandino Intrusive Suite.

Reinterpretation of geophysical surveys of pre-Permian basement in SW Poland: structural evolution and its implications for prospecting Cu and other ores

Reinterpretation of geophysical surveys of pre-Permian basement in SW Poland: structural evolution and its implications for prospecting Cu and other ores

Coupled U–Pb and Lu–Hf zircon data of Late Devonian magmatism from Bajo de La Leona (Deseado Massif, Argentina): Tracking the southernmost exposure of the Devonian magmatic arc of Patagonia

In Bajo de La Leona, located in the eastern Deseado Massif, pre-Permian basement rocks of the Río Deseado Complex crop out. They consist of high- to medium-grade metamorphic rocks intruded by two Paleozoic granitoids, namely the Mina de La Leona Granite and the Laguna Larga Granite. The latter is the main goal of this work. It is a calc-alkaline to high-K calc-alkaline, ferroan, calcic to alkali-calcic, and mainly peraluminous monzoleucogranite which plots within the I&S-type and volcanic-arc fields of the tectonic discrimination diagrams. Its trace elements composition (e.g., Sr/Y, LaN/YbN, Sm/Yb, Eu/Eu*, Dy/Dy*, Dy/Yb) suggests plagioclase ± clinopyroxene fractionation in a shallow source at low pressures. Assimilation could have played a role in magma differentiation, thus the magmatic development of the Laguna Larga Granite is likely a consequence of assimilation-fractional crystallization (AFC) processes. Coupled LA-ICP-MS U–Pb and Lu–Hf zircon data for the studied granite, as well as Sm–Nd whole-rock data, are here presented for the first time. A Famennian U–Pb zircon crystallization age of 361.9 ± 1.7 Ma was obtained, together with εHf(t) zircon values from -15 to +6 with TDM-Hf model ages between 2.0 and 0.9 Ga. A thin crust, as suggested by low values of thickness proxies such as Sr/Y (1.06–1.83) and LaN/YbN (3.97–4.96), would allow emplacement of juvenile mantle-derived material, giving rise to the suprachondritic εHf(t) values. A mantle-derived source is also suggested by a suprachondritic εNd(t) whole-rock value of +2.34 (TDM-Nd = 0.93 Ga). On the other hand, the subchondritic εHf(t) values could be related to wall rock assimilation. In a regional context, the obtained results for the Laguna Larga Granite allow to correlate it with coeval rocks from northern Patagonia (e.g., San Martín de los Andes batholith, Paleozoic Central Patagonian igneous-metamorphic belt). Therefore, the Devonian magmatic arc might have extended from the North Patagonian Andes and the western North Patagonian Massif to the eastern Deseado Massif along a NNW-SSE-striking belt, confirming previous proposals.

U–Pb ages of magmatic and detrital zircon of the Döhlen Basin: geological history of a Permian strike-slip basin in the Elbe Zone (Germany)

The post-orogenic evolution of Variscan Central Europe is characterized by the formation of numerous basins. The early Permian Dohlen Basin is located in the Elbe Zone (Germany) and is bordered by metamorphic rocks of the Erzgebirge and numerous Variscan magmatic complexes. The NW–SE-oriented basin is evidence for a major rearrangement of stress fields during the post-Variscan reactivation of fault zones in Central Europe. Eleven samples of magmatic rocks and sediments have been analyzed with respect to their U–Th–Pb isotope ratios and geochemical composition. Of three magmatic samples (two tuffs, one trachyandesite), we analyzed 170 zircon grains. The Unkersdorf Tuff of the Unkersdorf Formation gave an age of 294 ± 3 Ma (Upper Asselian to Sakmarian), whereas a trachyandesite of the same formation was dated at 293 ± 5 Ma (Lower Artinskian to Lower Asselian). The Wachtelberg Ignimbrite (Upper Bannewitz Formation) showed an age of 286 ± 4 Ma (Artinskian to Lower Kungurian). As the first study, we also analyzed 984 detrital zircon grains of nine Late Paleozoic Central European sandstone and conglomerate samples of the Niederhaslich Formation and the Bannewitz Formation with respect to their U–Pb age composition. All sediments but two yielded two distinct age groups between 295 and 340 Ma and 530–750 Ma, as well as a minor amount of Precambrian zircon ages. Geochemical data points to an active margin setting with developing strike-slip basins. The data suggests a c. 10 Ma lasting basin formation during the second culmination of volcano-tectonic activity with basic to intermediate melts. The second youngest formation (Niederhaslich Formation) consists predominantly of pre-Permian basement material, which implies only minor volcanic activity and erosion from adjacent basement blocks. On the contrary, the uppermost and youngest Bannewitz Formation features strong evidence for volcanic activity in the neighboring area of the basin. The present study strongly suggests a rapid basin development and further shows how the evolution of the Dohlen Basin is proof for several post-Variscan tectonic reactivation phases in Sakmarian and Lower Kungurian of Central Europe. Finally, our results exemplarily show how basin evolution may be characterized by radiometric data of detrital zircon grains.

GEOLOGY OF THE ZECHSTEIN BASEMENT OF THE LEGNICA-GŁOGÓW COPPER DISTRICT (LGOM) AND ITS SURROUNDINGS: A CRITICAL OVERVIEW

On the background of a critical review of earlier studies and concepts concerning the structure of the pre-Permian basement of LGOM, the authors’s own, modified version of its tectonic model is presented, accompanied by that explaining the sedimentation and areal extent of the Lower Permian sedimentary cover. Integrated sedimentological, structural and tectonic analysis was applied in conjunction with geophysical analysis based on gravimetric and magnetic data. As a result of the analysis, the pre-Permian basement of the LGOM area is interpreted to be part of the northern Variscan internides and, at the same time, to constitutea northerly extension of the Fore-Sudetic block (Sieroszowice block) . Within the LGOM area, in its southern part, there are middle Odra fault zone, comprising the southern middle Odra fault (Lubin trough) and northern middle Odra fault, separated with the Sieroszowice block. In the middle part of the LGOM area located is the Głogów trough, cut by the Silesia-Lubusz fault in its northern segment and by the southern part of the vast middle Odra deep fracture zone.

Tectonics of the Musandam Peninsula and northern Oman Mountains: From ophiolite obduction to continental collision

ABSTRACT The tectonics of the Musandam Peninsula in northern Oman shows a transition between the Late Cretaceous ophiolite emplacement related tectonics recorded along the Oman Mountains and Dibba Zone to the SE and the Late Cenozoic continent-continent collision tectonics along the Zagros Mountains in Iran to the northwest. Three stages in the continental collision process have been recognized. Stage one involves the emplacement of the Semail Ophiolite from NE to SW onto the Mid-Permian–Mesozoic passive continental margin of Arabia. The Semail Ophiolite shows a lower ocean ridge axis suite of gabbros, tonalites, trondhjemites and lavas (Geotimes V1 unit) dated by U-Pb zircon between 96.4–95.4 Ma overlain by a post-ridge suite including island-arc related volcanics including boninites formed between 95.4–94.7 Ma (Lasail, V2 unit). The ophiolite obduction process began at 96 Ma with subduction of Triassic–Jurassic oceanic crust to depths of > 40 km to form the amphibolite/granulite facies metamorphic sole along an ENE-dipping subduction zone. U-Pb ages of partial melts in the sole amphibolites (95.6– 94.5 Ma) overlap precisely in age with the ophiolite crustal sequence, implying that subduction was occurring at the same time as the ophiolite was forming. The ophiolite, together with the underlying Haybi and Hawasina thrust sheets, were thrust southwest on top of the Permian–Mesozoic shelf carbonate sequence during the Late Cenomanian–Campanian. Subduction ended as unsubductable cherts and limestones (Oman Exotics) jammed at depths of 25–30 km. The Bani Hamid quartzites and calc-silicates associated with amphibolites derived from alkali basalt show high-temperature granulite facies mineral assemblages and represent lower crust material exhumed by late-stage out-of-sequence thrusting. Ophiolite obduction ended at ca. 70 Ma (Maastrichtian) with deposition of shallow-marine limestones transgressing all underlying thrust sheets. Stable shallow-marine conditions followed for at least 30 million years (from 65–35 Ma) along the WSW and ENE flanks of the mountain belt. Stage two occurred during the Late Oligocene–Early Miocene when a second phase of compression occurred in Musandam as the Arabian Plate began to collide with the Iran-western Makran continental margin. The Middle Permian to Cenomanian shelf carbonates, up to 4 km thick, together with pre-Permian basement rocks were thrust westwards along the Hagab Thrust for a minimum of 15 km. Early Miocene out-of-sequence thrusts cut through the shelf carbonates and overlying Pabdeh foreland basin in the subsurface offshore Ras al Khaimah and Musandam. This phase of crustal compression followed deposition of the Eocene Dammam and Oligocene Asmari formations in the United Arab Emirates (UAE), but ended by the mid-Miocene as thrust tip lines are all truncated along a regional unconformity at the base of the Upper Miocene Mishan Formation. The Oligocene–Early Miocene culmination of Musandam and late Cenozoic folding along the UAE foreland marks the initiation of the collision of Arabia with Central Iran in the Strait of Hormuz region. Stage three involved collision of Arabia and the Central Iran Plate during the Pliocene, with ca. 50 km of NE-SW shortening across the Zagros Fold Belt. Related deformation in the Musandam Peninsula is largely limited to north and eastward tilting of the peninsula to create a deeply indented coastline of drowned valleys (rias).

Application of the SAR interferometric methods to identify the mobility of the area above the salt diapir in Inowrocław and the regional salt structures in central Poland

Remote sensing interpretation presents the linear structural elements correspond to the tectonic-structural pattern and suggest a block-type character of the pre-Permian basement. They may also indicate the existence of active movements of salt bodies and point at tectonic zones being active during almost recent past. The research on terrain mobility in the Inowroc· aw salt dome area and regional salt structures was conducted with the use of SAR interferometry techniques: PSI and InSAR. All archival scenes (both descending and ascending) available from sat- ellites ERS-1/2 and ENVISAT were used in order to determine the nature of displacement during the last 15 years. To identify the mobility of the pilot study, locally and regionally, the results from SAR interferometry techniques were analysed together with all types of collected data: tectonics, geophysical, geological, geodetic, and structural data. That gave indications about the uplift and subsidence in the salt dome area, as well as information about the types and causes (natural and induced) of mass movements.

Deciphering the geologic memory of a Permian conglomerate of the Southern Alps by pebble P–T estimates

The volcano–clastic sequence of Trompia Valley, which caps the Tre Valli Bresciane Variscan basement (TVB), comprises the Dosso dei Galli Conglomerate (DGC), the oldest deposit containing up to metre-sized metamorphic pebbles. This Lower Permian formation of the Trompia Basin was fed by the erosion products of the Variscan chain. We used microstructural and mineral chemical data on metamorphic pebbles of the DGC to infer a quantitative tectono-thermal evolution of the eroded pre-Permian basement and to compare them with those of TVB and the surrounding Southalpine basement units (tectono-metamorphic units = TMUs). Metapelitic and metaintrusive pebbles record a polyphase metamorphism with two metamorphic re-equilibrations: the first under epidote amphibolite facies (M1, \(T_{{\text{max}}} {\text{--}}P_{T_{\max } }\)) and the second under greenschist facies (M2) conditions. Rock types and metamorphic data largely match those of TVB basement unit. The structural and metamorphic records in the pebbles are pre-Permian, and the conglomerate matrix is non-metamorphic. The DGC deposition age (283 ± 1–280.5 ± 2 Ma) constrains the minimal exhumation age of its basement source. The lack of staurolite bearing assemblages in metamorphic pebbles suggests that the DGC basement source was already exhumed to shallow structural levels (greenschist facies conditions) before the thermal equilibration consequent upon continental crust thickening induced by the Variscan collision.

Phanerozoic evolution of the Inner Mongolia–Daxinganling orogenic belt in North China: constraints from geochronology of ophiolites and associated formations

Abstract The Inner Mongolia–Daxinganling Orogenic Belt (IMDOB), located between the North China and South Mongolia Blocks, consists of several ENE–WSW- to NE–SW-trending zones including dismembered ophiolite blocks, metamorphic rocks and granitoids. Although numerous studies have been carried out on this belt, its tectonic evolution has been a subject of controversy, chiefly because of the lack of reliable geochronological data. Based on a synthesis of newly published geochronological data and our unpublished data for the IMDOB, we define two oceanic basins: Ondor Sum and Hegenshan. The former, probably the main one, was initiated during the Ordovician (>467 Ma) period, whereas the latter, representing a back-arc basin, opened on a pre-Permian basement at, or earlier than, Early Permian times ( c . 295 Ma). These two oceanic basins were separated by a magmatic arc (Sunid–Baolidao), and were probably closed simultaneously when the final orogenesis of the IMDOB occurred during the Triassic period (240–220 Ma). Importantly, the Triassic timing of the final orogenesis of the IMDOB due north of the North China Craton is essentially coeval with that of the Qinling–Dabie–Su–Lu orogenic belt on the southern margin of the North China Craton. It is inferred that this two-sided subduction–collision scenario in the Triassic may have contributed to the Mesozoic lithospheric thinning event of the North China Craton, although the details are unclear.

Inversion-related features along the southeastern margin of the North German Basin (Elbe Fault System)

A seismostratigraphic approach constrained by well data is used for the interpretation of the deformation style along the central Elbe Fault System (EFS) within the sedimentary succession. Structural analysis allows to qualify, to quantify, and to date tectonic events. The stratigraphic interpretation is complicated by the mobilized Upper Permian Zechstein salt and by erosional events. A first-order quantification of the inversion-related uplift is estimated from vertical fault offsets that reach up to 4 km. The main uplift occurred during the Maastrichtian/Paleocene. Amounts of erosion inferred from comparing the strata thickness on top of the Flechtingen High with the surrounding basinal areas range from 3 to 4 km. The data indicate a changing deformation style: Thick-skinned deformation with southwest-dipping thrusts that vertically offset the pre-Permian basement is observed along the Flechtingen High in the central part of the EFS. Thin-skinned deformation occurs in the North German Basin where salt detaches the post-Permian cover from the barely faulted basement. It is concluded that during the Late Cretaceous/Early Tertiary inversion, the EFS responded to regional compression with uplift and formation of an internal high, the Flechtingen High. A stress-sensitive crustal weak zone beneath the EFS could be the reason for the repeated strain localization in the area.

Chronological study of the pre-Permian basement rocks of southern Patagonia

Small outcrops of the basement of the Deseado Massif, consisting of weathered, altered granitoids and their metasedimentary host rocks, were dated by the U–Pb zircon method using thermal ionization mass spectrometry and ion microprobe (SHRIMP). The provenance ages of detrital zircon in the metasediments are typical of material available in the adjacent regions of the Gondwana margin, with prominent components at 1000–1100 and 580±6 Ma, the latter probably approximating the age of sedimentation (latest Neoproterozoic). SHRIMP data for pre-Jurassic granodiorite from a borehole in the Magallanes basin in Tierra del Fuego give a Cambrian age of 521±4 Ma, which confirms published conventional U–Pb dating from the same borehole. SHRIMP ages of 476±4 and 472±5 Ma for granitic cobbles in a Permian conglomerate (La Golondrina Formation) are evidence of nearby Ordovician (Famatinian) intrusive activity. In situ granitoids indicate late Silurian to early Carboniferous ages (422±2, 395±3, and 346±4 Ma). All these events are recognised in the evolution of adjacent South America and the Antarctic Peninsula suggesting that these parts of the Gondwana margin were contiguous throughout their Cambrian to Jurassic history.

Subduction zone polarity in the Oman Mountains: implications for ophiolite emplacement

Abstract Two end-member models have been proposed to account for the structure and metamorphism of rocks beneath the Semail ophiolite in the Oman mountains. Model A involves a single, continuous NE-directed subduction away from the continental margin during the late Cretaceous. The ophiolite and underlying thrust sheets of distal to proximal oceanic sediments were emplaced a minimum of 250 km SW onto the continental margin. Subduction of Triassic-Jurassic oceanic basalts to c. 10 kbar ( c. 39 km depth) led to the accretion of amphibolite-facies rocks to the base of the ophiolite. Thrusting propagated towards the continental margin and ended with subduction of the thinned continental crust to c. 20 kbar ( c. 78 km depth), choking the subduction zone. Buoyancy forces caused the rapid exhumation of eclogites, blueschists and carpholite-grade HP rocks along the NE margin of the continental plate. During the later phase of foreland-propagating thin-skinned thrusting in the SW, NE-facing backfolding and backthrusting occurred in the hinterland, with the final exhumation of the HP rocks. Model B follows recent suggestions that a nascent SW-dipping subduction zone, dipping beneath the continental margin, existed between 130 and 95 Ma, prior to formation and emplacement of the ophiolite. A major NE-facing fold-nappe structure in the pre-Permian basement rocks of Saih Hatat is interpreted as reflecting subduction beneath the margin. Two high-pressure metamorphic events have been suggested, the first predating ophiolite emplacement, the second caused by ophiolite loading. This model is untenable, being based on a misinterpretation of the NE-facing structures in northern Saih Hatat, and on some dubious older 40 Ar/ 39 Ar cooling ages from the eclogite-facies rocks of As Sifah. We conclude that all structures in northern Oman and all the reliable geochronology point to a single emplacement-obduction event lasting from Cenomanian-Turonian time ( c. 95 Ma) when amphibolites were accreted along the metamorphic sole of the ophiolite, to Campanian time, when the continental margin was subducted to the NE producing blueschists and eclogites, to the final thin-skinned emplacement of all thrust sheets, which ended before the Late Maastrichtian, at c. 68 Ma.

L'unité de la Boghen (Nouvelle-Calédonie, Pacifique sud-ouest) : un complexe d'accrétion jurassique. Données radiochronologiques préliminaires UPb sur les zircons détritiques

Abstract Radiochronological dating of detrital zircon extracted from the Boghen terrane metasediments allows a Jurassic age to be assigned. This terrane was formerly considered as the “pre-Permian basement” of New Caledonia. Its sedimentological features, its Late Jurassic high-pressure metamorphism (ca 150 Ma) and its association with the arc-related volcano-sedimentary complex of the Central Chain Terrane indicate that the Boghen terrane was an accretionary complex formed during the Jurassic period along the East-Gondwana active margin. The age spectrum of detrital zircons is consistent with a derivation from the Permian–Mesozoic Southeast-Gondwana arc system and the Antarctic continent. To cite this article: D. Cluzel, S. Meffre, C. R. Geoscience 334 (2002) 867–874.

Late Permian Foraminiferal Biofacies Belts in Turkey: Palaeogeographic and Tectonic Implications

Abstract Upper Permian marine carbonates are distinguished in two contrasting biofacies belts in Turkey. The Southern Biofacies Belt, represented by low-energy inner platform deposits of the Tauride Belt and the Arabian Platform, is rich in algae and smaller foraminifera but poor in fusulines. The Kubergandian and Murgabian stages are missing, although the rest of the Upper Permian consists of monotonous, shallow-marine carbonate deposits. The extremely tectonised and fragmented Northern Biofacies Belt includes the Upper Permian of the Karakaya Orogen and outer platform or platform margin deposits of the Tauride Belt. These deposits are rich in parachomata-bearing fusulines comprising Cancellina, Verbeekina, Afghanella, Sumatrina, Neoschwagerina and Yabeina . The reconstructed biostratigraphic scheme indicates that all Upper Permian stages (Kubergandian-Dorashamian) are present. The lateral continuity of the two biofacies belts is detected by the presence of tongues of he Northern Biofacies Belt pinching out in the Southern Biofacies Belt. Upper Permian blocks in the Karakaya Orogen display similar palaeontologic and biofacies characteristics, with the outer platform or platform margin deposits of the Taurides constituting the northernmost extension of the carbonate platform. This platform was probably facing a basin or a trough to the north. The lack of any transgressive Upper Permian deposits resting unconformably on the pre-Permian basement of the Sakarya Continent strongly suggests that such a basin was located between the Late Permian carbonate platform in the south and the basement rocks of the future Sakarya Continent in the north.

The Polish Sudetes: Caledonian or Variscan?

AbstractThe Polish Sudetes on the NE margin of the Bohemian Massif comprise a complex mosaic of pre-Permian basement units, traditionally included in the Variscides. A hypothesis of significant Caledonian orogenesis in this area originated in the 1920s, was subsequently rejected, and then was recently revived in models which invoked Early Palaeozoic to Early-Mid Devonian subduction and continental collision along a proposed extension of the Tornquist suture zone. We reassess the evidence invoked in support of the Caledonian orogeny, such as supposed regional pre-Upper Devonian unconformity, Ordovician bimodal magmatism and radiometric, palaeontological, palaeomagnetic and structural data, and suggest these are either inconclusive or misinterpreted. On the other hand, the Sudetes record Mid?-Late Devonian blueschist metamorphism followed by an Early Carboniferous regional high temperature event, widespread Late Devonian/Early Carboniferous flysch/molasse sedimentation and abundant granite intrusion in the Carboniferous to Early Permian. We discuss the usage of the term ‘Caledonian’ in a plate tectonic context and suggest it should not be used simply to denote Early to Mid-Palaeozoic tectonic activity. The tectonic evolution of the Sudetes was temporally different from, and resulted from convergence of different crustal domains than that of the British-Scandinavian-Pomeranian Caledonides. The Sudetic Palaeozoic sequences most probably developed on Armorican Neoproterozoic crust and in adjacent oceanic(?) domains and, therefore, the Sudetes form part of the Variscan orogenic belt.

Structural Evolution of the Permian-Triassic Cooper Basin, Australia: Relation to Hydrocarbon Trap Styles

The structural and depositional history of the Cooper basin in eastern central Australia has revealed that the basin is a mildly compressional structural depression controlled by northwest-trending and northeast-trending pre-Permian basement features. Pronounced pre-Permian compressions are indicated by northeast-trending major structures, the Gidgealpa-Merrimelia-Innamincka and Murteree-Nappacoongee trends. Detailed chronostratigraphic facies analysis, with closely spaced palynological control, of the Patchawarra Formation revealed that two pronounced phases of uplift occurred during the Sakmarian. The major intrabasin highs were rejuvenated during these tectonic events, as documented by crestal unconformities (middle and upper Patchawarra unconformities). Evidence of each event is dominantly tectonic in character, with similar depositional patterns over these highs related to each event. These events are also recognizable in midflank areas and basin margins with contemporaneous deposition in deeper parts of the basin. Results from this research show potential for future hydrocarbon discoveries within structural, stratigraphic, and structural/stratigraphic traps in the Cooper basin. Various trap styles are closely associated with faults, unconformities, and lateral facies changes. Lowside fault closures, onlap plays, and unconformity traps are expected to be well ©Copyright 1997. The American Association of Petroleum Geologists. All rights reserved.1Manuscript received February 22, 1995; revised manuscript received August 6, 1996; final acceptance November 12, 1996. 2Geological Survey of Western Australia, 100 Plain Street, East Perth, WA, Australia 6004 (E-mail: n.apak@dme.wa.gov.au). 3National Centre for Petroleum Geology and Geophysics (NCPGG), University of Adelaide, Thebarton Campus, University of Adelaide, SA 5005, Australia. 4Santos Ltd., 101 Grenfell Street, Adelaide, SA 7000, Australia. The authors wish to thank the Cooper Basin Consortium Group of Companies for permission to publish this paper. The drafting staffs at Santos Ltd. and Geological Survey of Western Australia (GSWA) are sincerely thanked for their efforts in preparing figures.This paper is dedicated to the memory of Bill Stuart.

Interpretation of magnetic and gravity surveys in the southern Arabian Gulf, the Strait of Hormuz, and the northwesternmost Gulf of Oman: implications of pre-Permian basement tectonics

A 5 nT aeromagnetic chart and a 5 mGal Bouguer gravity map of the area bounded by latitudes 25°30′–27°10′N and longitudes 55°15′–57°00′E in the southern Arabian Gulf, the Strait of Hormuz, and the entrance of the Gulf of Oman are collectively studied. The total magnetic intensity map (covering the coastal strip and the Arabian side of the southern part of Arabian Gulf) reveals three prominent anomalies (of amplitudes ranging from 30 to 150 nT) related to tectonically-controlled igneous bodies emplaced in the deep-seated pre-Permian basement of the Arabian Shield. The Bouguer gravity map indicates three well-defined trends: an effect duplicating a magnetic source in the basement, a negative belt indicating an anomalous Hormuz salt accumulation in the Southern Gulf Salt Basin, and a continental margin gravity effect marking the transition from continental crust to the Gulf of Oman oceanic crust. The magnetic, and the gravity-derived lineaments are shown to correlate with Gemini-interpreted strike-slip basement faults in the Iran's southeastern Zagros fold zone.

The tectonic history of the East Irish Sea Basin with reference to the Morecambe Fields

Regional seismic lines from the East Irish Sea area and a 2D radial seismic survey across the Morecambe Fields have been used to assess fault pattern evolution and the history of tilting in the East Irish Sea Basin. Extension and fault-controlled sedimentation began in Permian times and continued throughout the Permo-Triassic. Later inversion and erosion has removed the post-Triassic cover in the area. The main extension direction during basin evolution was ENE–WSW to NE–SW. Under this extension, NE–SW-trending basin-bounding faults in the north and south were dominated by strike-slip and oblique displacements. Offset of the pre-Permian basement suggests the amount of post-Carboniferous net extension varies from 10–16%. The basin can be divided into two discrete structural domains which controlled the geological evolution of the East Irish Sea Basin. Basin evolution in the northern part of the basin was controlled by easterly dipping faults which induced tilting towards the southwest or west. The southeastern part of the basin is dominated by two faults which dip to the west or southwest, inducing tilting towards the east. The complex structural and tilting history of the Morecambe Fields arises from their position at the boundary of these two domains. Detailed analysis of this area illustrates how the propagation of the Keys Fault (from the north) and the Crosh Vusta Fault (from the south) competed to dominate tilting history. The faults which now bound the Morecambe Fields show only minor influence on Permo-Triassic sedimentation patterns, implying that the main displacement on these faults post-dated the deposition of the Sherwood Sandstone. The development of these boundary faults is thought to be related to the accommodation of arching between the Keys Fault and Tynwald fault system. The differing tilting histories of the North and South Morecambe Fields are indicated by the differing orientations of a previously shared palaeo-hydrocarbon–water contact. This is now preserved as the top platy-illite surface and is interpreted as arising from the development of a tilt-transfer zone during the propagation and linking of the northerly and southerly propagating fault arrays. Analysis of the tilting history of the Morecambe area has allowed the top platy-illite surface to be mapped outside well control.

Use of topographic lineaments to extend known structure and facies patterns in the Mesozoic of Southern England

A detailed structural map of Southern England was compiled from published geological and geophysical data, carefully integrated with, and extended by, an interpretation of topographic lineament patterns. Pre-Permian basement structures controlled the location of Permian to Lower Cretaceous basin margin faults that were subsequently blanketed by post-tectonic Upper Cretaceous Chalk. Late Cretaceous to Tertiary compression reactivated and inverted earlier Mesozoic structures, imprinting them into the Upper Cretaceous cover. Structural controls on sedimentation are illustrated by maps and sections for key Triassic and Jurassic units.

The tectonic evolution of pre-Permian rocks, Central and Southeastern Oman Mountains

Abstract The structural history of pre-Permian basement rocks exposed in the domal uplifts of Jebel Akhdar, Jebel Nakhl and Saih Hatat, central and southeastern Oman mountains, is dominated by Late Palaeozoic and Late Cretaceous deformation and metamorphism. In Jebel Akhdar pre-Permian rocks have been deformed by large NE-SW trending thrusts and folds and intense cleavage during Late Palaeozoic compression. Later reactivation of these structures accompanied by a major flattening deformation caused by loading may be related to thrusting and folding in the overlying Permian to Cenomanian platform carbonates. These structures are ascribed partly to Late Cretaceous deformation. The complex internal structure of Saih Hatat is attributed to overprinting of possible Proterozoroic and Late Palaeozoic tectonic events by Late Cretaceous thrusting, folding and associated blueschist metamorphism. The identification of a major basement-cover duplex involving pre-Permian rocks and platform carbonates in Saih Hatat suggests that the pre-Permian basement and platform carbonates did not remain as autochthonous units during Late Cretaceous deformation. Instead, both basement and cover have been thrust southwards during emplacement of the Tethyan thrust sheets onto the Arabian continental margin. Sedimentological evidence from the Tertiary sequence reinforces the development of the Saih Hatat dome as a Late Cretaceous culmination above a deep seated thrust which may have been rejuvenated during the Tertiary. In Jebels Akhdar and Nakhl there is no evidence of a basement-cover duplex. Nevertheless, reactivation of pre-Permian structures and imbrication of cover rocks (Mesozoic platform carbonates) in Jebel Akhdar has important implications to the timing of its structural growth. An alternative model of under-thrusting of the N Arabian margin beneath a subduction trench is also proposed to explain the formation and subsequent elevation of the Oman blueschists.