Basement-cover Relationships Research Articles

Crustal structure and basement-cover relationship in the Dangerous Grounds, offshore North-West Borneo, from 3D joint CSEM and MT imaging

Accurate mapping of crustal thickness variations and the boundary relationships between sedimentary cover rocks and the crystalline basement is very important for heat-flow prediction and petroleum system modeling of a basin. Using legacy industry 3D data sets, we investigated the potential of 3D joint inversion of marine controlled-source electromagnetic (CSEM) and magnetotelluric (MT) data incorporating resistivity anisotropy to map these parameters across subbasins in the Dangerous Grounds in the southwestern rifted margin of the South China Sea, where limited previous seismic and potential field basement interpretations are available for comparison. We have reconstructed 3D horizontal and vertical resistivity models from the seabed down to [Formula: see text] depth for a [Formula: see text] area. The resistivity-versus-depth profile extracted from our 3D joint inversion models satisfactorily matched the resistivity and lithologic well logs at a wildcat exploration well location chosen for model validation. We found that the maximum resistivity gradients in the computed first derivative of the 3D resistivity volumes predict a depth to basement that matches the acoustic basement. The models predict the presence of 2 to approximately 5 km thick electrically conductive ([Formula: see text]) sedimentary cover atop an electrically resistive ([Formula: see text]) crystalline crust that is underlain by an electrically conductive ([Formula: see text]) upper mantle at depths that vary laterally from approximately 25 to 30 km below sea level in our study area. Our resistivity variation with depth is found to be remarkably consistent with the density distribution at Moho depth from recent independent 3D gravity/gradiometry inversion studies in this region. We suggest that 3D joint inversion of CSEM-MT, seismic, and potential field data is the way forward for understanding the deep structure of such rifted margins.

Basement–cover relationships and deformation in the Northern Paraguai Belt, central Brazil: implications for the Neoproterozoic–early Paleozoic history of western Gondwana

The Northern Paraguai Belt, at the SE border of the Amazonian Craton, central Brazil, has been interpreted as a Brasiliano–Pan-African ( c. 650–600 Ma) belt with a foreland basin, recording collisional polyphase tectonism and greenschist-facies metamorphism extending from the late Precambrian to the Cambrian–Ordovician. New structural investigations indicate that the older metasedimentary rocks of the Cuiabá Group represent a Tonian–Cryogenian basement assemblage deformed in two contemporaneous fault-bounded structural sub-domains of wrench-dominated (rake <10°) and contraction-dominated (rake ∼30–40°) sinistral transpression, with tectonic vergence towards the SE. The younger late Cryogenian to early Cambrian sedimentary rocks lying to the NW of the Cuiabá Group are non-metamorphic and display only pervasive brittle transtension characterized by normal-oblique faults, fractures and forced drag folds with no consistent vergence pattern. Our analyses suggest that an unconformity separates the metasedimentary Cuiabá Group basement of the Northern Paraguai Belt from the unmetamorphosed sedimentary cover. It is proposed that the latter units were deposited during a post-glacial marine transgression (after c. 635 Ma) in a post-collisional basin. The Paraguai Belt basement and its post-collisional sedimentary cover share a number of significant geological similarities with sequences in the Bassarides Belt and Taoudéni Basin in the SW portion of the West African Craton.

2. 45 Ga break-up of the Archaean continent in Northern Fennoscandia: Rifting dynamics and the role of inherited structures within the Archaean basement

Constraints from restoration of minor rift-related faults preserved from subsequent Palaeoproterozoic overprint along the southern margin of the Peräpohja Belt, northern Fennoscandia indicate local NE-SW extension directions prevailed at the time of 2.44 Ga rifting of the Archaean continent. This palaeostress field is attributed to pull-apart setting of the Peräpohja Belt at a left overstepping zone of major sinistral N-S trending deformation zones. Consequently, the variably overprinted NW-SE trending structures of the Peräpohja Belt, including the NW-SE central graben were originally generated as normal faults. The proposed setting is compatible with a conducted reconnaissance sandbox analogue model where the orientation of the major normal faults was controlled by the ambient stress field within the step-over zone between strike-slip faults. By contrast, the faults bounding E-W to ENE-WSW trending basement horsts are the result of reactivation of older basement structures underlying the pull-apart basin. Based on the results, we infer that the final break-up of the Archaean continent in northern Fennoscandia utilized the 2.44 Ga rift-related NW-SE trending crustal anisotropy (parallel with dyke swarms), with pre-existing Archaean N-S trending structures reactivated as second-order shear zones in-between. The voluminous syn-rift 2.5–2.4 Ga and later pulses of mafic magmatism (2.32–1.98 Ga) are considered indicative of active-type rifting which onset was controlled by a mantle plume centred within a supercraton formed by the Superior, Hearne and Karelian (-Kola) cratons. The presented model supports the continuity of the Archaean craton beyond the Caledonian Orogen towards north-west and explains its segmentation, suggesting a model for the localization of the Palaeoproterozoic supracrustal belts of Northern Fennoscandia. Moreover, this paper provides an approach to understand the basement-cover relationships and the structural signatures within these relatively shallow supracrustal belts which are highly prospective for mineral deposits.

Lithologies, structure and basement-cover relationships in the schist belt of the Dom Feliciano Belt in Uruguay

ABSTRACT: This work is the result of a multiyear effort to use field geology to describe lithologies, to establish contact relationships and to create a sketch of the tectonic evolution of the Meso- to Neoproterozoic metasedimentary successions within the Schist Belt of the Dom Feliciano Belt. This low-grade metamorphic cover rests on the high-grade metamorphic basement of the La China and Las Tetas complexes. This basement is Archean-Paleoproterozoic in age. The Schist Belt is overlapped unconformably by the Barriga Negra formation. The Lavalleja complex and the Barriga Negra formation both deformed together during the D2 deformation event (~ 570 -540 Ma), but the Barriga Negra only partially recorded the D2 transpressive event, whereas the Lavalleja complex was affected by both the D1 tangential event and the D2 event. Event D1 would have developed a fold nappe with vergence to the south. This hypothesis is supported by different structures: (i) recumbent and upright folds oriented E-W, (ii) subhorizontal mylonitic foliation in marbles (calc-schists), (iii) stretching lineations plunging towards the SW in metaconglomerates of the Las Tetas Complex, and (iv) a reworking of the subhorizontal foliation parallel to the Sarandí del Yí strike-slip shear zone.

Integration of palaeomagnetic data, basement-cover relationships and theoretical calculations to characterize the obliquity of the Altomira–Loranca structures (central Spain)

Abstract The main objective of this work is to characterize the structures belonging to the Altomira Range and Loranca Basin (SW Iberian Chain, Central Spain) in terms of understanding their present-day orientation, highly oblique with respect to the NW–SE orientation of adjacent structures of the Iberian Chain. The Altomira and Loranca fold and thrust belts present a slightly curved geometry with a general north–south orientation at their central sector, and structures oriented NNE–SSW and north–south to NNW–SSE at their northern and southern sectors, respectively. Palaeomagnetic data from Middle Eocene–Lower Miocene rocks (including clays, fine sandstones and limestones) reveal the absence of vertical-axis rotations in the central sector of the studied area, where structures are oriented north–south, and up to 17° of clockwise and 21° of anticlockwise vertical-axis rotations in the northern and southern sectors, respectively. These data are supported by calculations of the theoretical vertical-axis rotations from shortening estimates and basement-cover structural relationships. This approach highlights the importance of integrating different datasets to characterize the obliquity of fold and thrust belts. Supplementary material: Complete information about the paleomagnetic sites and directions is available at http://www.geolsoc.org.uk/SUP18841

Positive inversion tectonics in foreland fold-and-thrust belts: A reappraisal of the Umbria–Marche Northern Apennines (Central Italy) by integrating geological and geophysical data

Unraveling the tectonic style in the outer zones of fold-and-thrust belts is generally puzzling because the basement–cover relationships are often hidden in the subsurface as in the outer Northern Apennines of Italy. This study aims to reconstruct the deep setting of the Northern Apennine foreland thrust belt by integrating surface structural–geological and subsurface seismic reflection profile and well data, corroborated by a gravity–magnetic modeling. A remarkable mountain ridge, the Umbria–Marche Apennine Ridge (UMAR), which corresponds to a prominent area of structural and topographic elevation, characterizes the morphology of the Northern Apennines. This mountain ridge is constituted by Meso-Cenozoic carbonates involved in Neogene compressive structures and is surrounded by wide exposures of foredeep deposits. The basement–cover relationships are poorly constrained and both thin- and thick-skinned tectonic styles have been applied. The interpretation of subsurface data allowed recognizing a thick pre-Jurassic sedimentary sequence filling a late Paleozoic(?)–Triassic symmetric fault-bounded extensional basin, lying directly below the UMAR. This deep-rooted basin underwent positive inversion during the Neogene compression and thrust–fold development. The reconstructed thick-skinned inversion tectonic model is consistent with both the modest amount of shortening and the remarkable structural elevation of the UMAR.The outcomes of this study reveal that prominent mountain ridges occurring in foreland thrust belts are most likely related to the deep-rooted basement-involved positive inversion of pre-existing extensional basins.

An Alpine-style Ordovician collision complex in the Sierra de Pie de Palo, Argentina: Record of subduction of Cuyania beneath the Famatina arc

The Caucete Group and structurally overlying Pie de Palo Complex in western Argentina are characterised by two generations of west-verging folds and thrust-related shear zones, which formed under amphibolite facies conditions. The Caucete Group is separated from the Pie de Palo Complex by the Pirquitas thrust. These structures are interpreted to have formed as a result of a progressive deformation, generated during Middle Ordovician, underthrusting of the Laurentian-derived Cuyania microcontinent beneath the active Famatina margin. Geometrical relationships are most simply explained if the Pie de Palo Complex was basement to the Caucete Group prior to Ordovician orogenesis. We propose that this basement-cover relationship was established during Cambrian rifting of the Cuyania microcontinent from Laurentia. The Pirquitas fault may have been initiated during this extension prior to its long-lived remobilization as a thrust. We cannot rule out the possibility that the Pie de Palo Complex was exotic with respect to the Caucete Group, but for this to be possible we have to introduce an extra generation of structures, for which no evidence is preserved. The deformation was characterised by early strain localization followed by a more homogeneously distributed non-coaxial flow during F 2. Thermal softening probably dominated over fabric softening during this stage.

Detrital zircon, detrital titanite and igneous clast U–Pb geochronology and basement–cover relationships of the Colonsay Group, SW Scotland: Laurentian provenance and correlation with the Neoproterozoic Dalradian Supergroup

A multidisciplinary provenance study, including in situ U–Pb dating of detrital titanite, was undertaken on the enigmatic low-grade metasediments of the Colonsay Group, SW Scotland to determine their source and assess previous correlations with major Neoproterozoic sedimentary sequences within the Scottish Caledonides. Sm–Nd model ages ( t DM) of the Colonsay Group range from 1664 to 2140 Ma, suggesting a Palaeoproterozoic source. Laser Ablation ICPMS U–Pb detrital zircon ages from six Colonsay Group formations are consistent with sources within Laurentia with predominant input from a ca. 1780 Ma source (Rhinns Complex) and some Grenvillian ( ca. 1.3–0.95 Ga), Pinwarian ( ca. 1.51–1.45 Ga), Labradorian ( ca. 1.71–1.62 Ga) and Ketilidian ( ca. 1.9–1.75 Ga) detritus. Archaean (> ca. 2.5 Ga) input is minimal. Clasts from the basal Octofad Sandstone Formation indicate input from ca. 1795 Ma and ca. 1400 Ma sources (U–Pb SIMS zircon ages). U–Pb (SIMS) analyses of detrital titanite record Grenville metamorphic events in the source terranes and suggest a slightly younger maximum depositional age of ca. 942 Ma compared with the youngest detrital zircon age of ca. 1025 Ma. The data support the interpretation that the Octofad Sandstone Formation is part of the Colonsay Group and that the Colonsay Group rests unconformably on the Rhinns Complex on Islay. Deposition of the Colonsay Group is, therefore, tied to the margins of Laurentia, possibly in the foreland to the Grenville Belt, as the Rhinns Complex does not record any Grenville deformation. The data are consistent with deposition of the Colonsay Group in a foreland basin related to the Rigolet phase ( ca. 1000–980 Ma) of the Grenville Orogeny. On the basis of detrital zircon U–Pb ages the Colonsay Group best correlates with the lowermost Dalradian Grampian Group of Scotland. This correlation has significant implications for Dalradian basin dynamics, timing of Dalradian deposition and timing of Dalradian deformation. In addition, the unconformable contact between the Colonsay Group and the Rhinns Complex on Islay would make it one of the few places where the basement to the Dalradian can be observed.

Basement–cover relationships of the Kalak Nappe Complex, Arctic Norwegian Caledonides and constraints on Neoproterozoic terrane assembly in the North Atlantic region

The Kalak Nappe Complex (KNC) has been regarded as Baltica passive margin metasediments telescoped eastwards onto the Baltic (Fennoscandian) Shield during the Caledonian Orogeny. Recent studies have questioned this interpretation, instead pointing to a Neoproterozoic exotic origin. In an effort to resolve this controversy we present a Sm–Nd and U–Th–Pb study of gnessic units, traditionally considered as the depositional basement, along with cover rock sediments and intrusives. Late Palaeoproterozoic gneisses now beneath the KNC were deposited after 1948 ± 33 Ma, before intrusion of the Tjukkfjellet Granite at 1796 ± 3 Ma, and were affected by later melting events at 1765 ± 9 and 1727 ± 9 Ma. These gneisses are interpreted as part of the Baltic Shield and underlie the KNC across a tectonic contact. An unconformity between psammites of the KNC and other paragneisses previously considered as its Precambrian basement is reinterpreted as a modified sedimentary contact between Neoproterozoic metasediments. These metasediments have statistically very similar detrital zircon populations with grains as young as 1034 ± 22, 1025 ± 32 and 1014 ± 14 Ma. The results indicate that the KNC sediments were deposited during the Neoproterozoic in basins along the Laurentian margin of eastern Rodinia and were not connected to Baltica via a depositional basement. Dating of the 851 ± 5 Ma Eidvågvatnet and 853 ± 4 Ma Nordneset granites shows that intrusive material associated with the Porsanger Orogeny (c. 850 Ma) affected a considerable region of the upper KNC terrane. Later Neoproterozoic events at 711 ± 6, 687 ± 12 and 617 ± 6 Ma are also recognised the latest of which may be an expression of rifting. Since early Neoproterozoic magmatism (c. 840–690 Ma) is unknown in Baltica, these results support an exotic origin for the KNC terranes.

Polycyclic evolution of the Late Neoproterozoic basement in the Hermitage Flexure region (southwest Newfoundland Appalachians): New evidence from the Cinq-Cerf gneiss

The Cinq-Cerf gneiss represents one of the oldest tectonic elements yet documented within the “Avalonian” basement complex of the Hermitage Flexure region of the Newfoundland Appalachians. This gneiss is a composite unit that includes both supra-crustal and granitic gneisses, the latter having an igneous crystallization age of 675 + 12/−11 Ma. The timing of the gneiss-forming event (D 1 polyphase deformation) is constrained by the protolith age of the orthogneiss and the 584 + 7/−6 Ma crystallization age of granodiorite intrusive into gneiss. The emplacement of the granodiorite is coeval with the previously dated 585 Ma eruption of felsic tuff within nearby low-grade stratified elements of the pre-Silurian basement, north of the Grand Bruit Fault Zone (GBFZ), adding support to models of a Late Neoproterozoic basement-cover relationship between the gneiss and the low-grade volcano-sedimentary sequence. Younger Neoproterozoic magmatism in the gneissic basement is recorded by emplacement of 557 + 14/−5 Ma gabbro and the intrusion of 547 + 2/−7 Ma diorite dykes. The latter have a recrystallized tectonic fabric that hosts 448 + 9/−3 Ma titanite. Peak Appalachian-cycle overprinting of the Cinq-Cerf gneiss occurred in mid- to late Silurian time, and commenced with high-temperature deformation (D 2) of the gneiss, associated with synkinematic intrusion of Western Head granite at 431.5 ± 1 Ma. Mafic dyke swarms that intrude the gneiss and the granite contain 420 ± 3 Ma titanite. This age reflects new growth during Silurian retrogression and mylonitization of the mafic dykes. The D 3 mylonitization is widespread and linked to thrusting of the Cinq-Cerf gneiss along the GBFZ during the 420–418 Ma closure of the northerly adjacent Silurian volcano-sedimentary cover basin. New data bearing on the Late Neoproterozoic record of the basement block of the western Hermitage Flexure further demonstrates similarities in magmatic, stratigraphic and metallogenic record documented in Avalonian rocks of east Newfoundland's Connaigre, Burin and Avalon peninsulas, pointing to a Neoproterozoic linkage with the rocks in these areas. The early to mid-Paleozoic history of the Hermitage Flexure basement indicates that, during that time, the basement block evolved as a terrane separate from the Avalon Zone (sensu stricto).

Relative sea level change along the Slave craton coastline: Characteristics of Archean continental rifting

The >2.8 Ga siliciclastic-dominated, craton-cover succession with subordinate mafic volcanic rocks at Beniah Lake in the Slave craton has been informally referred to as the Bell Lake group. The Beniah Lake succession, up to 1 km thick, is composed of 7–120 m thick coarsening-upward (CU) sequences arranged with the following lithofacies architecture: (1a) argillite–sandstone, (1b) mafic volcanic lithofacies, (2) sandstone–argillite, (3a) quartz arenite and (3b) conglomerate lithofacies. A pan-Slave unconformity, indicating an autochthonous 2.8 to 4.013 Ga basement–cover relationship is present and locally well exposed. The lithofacies and their arrangement are consistent with a tide-influenced coastal setting, whereby the change from argillite–sandstone to quartz arenite lithofacies represents an initial estuary system, defining transgression with a maximum flooding surface, evolving into a fan-delta, defining progradation with a maximum regressive surface. The mafic volcaniclastic lithofacies and associated pillowed flow units, suggesting drowning of the siliciclastic succession, coincide with a change from rift to drift tectonics, as crustal attenuation facilitated magma ascent. The Slave-wide unconformities and lithofacies contacts permit a sequence stratigraphic approach, with the major Slave basement–quartz arenite unconformity suggestive of a 1st order sea level change coinciding with the proposed (super) continent break-up or rifting. The consistent stacking of CU-sequences with capping subaqueous volcaniclastic-volcanic units are considered the expression of 2nd order sea level cycles probably caused by changes of spreading rates at oceanic ridges. The CU-sequences and their thicknesses constitute 3rd order sea level changes and reflect the immediate response to crustal thinning. Time constraints for all these orders remain elusive but the hierarchy is consistent with sequence stratigraphic events. The observed relative sea level change is attributed primarily to tectono-eustasy where mantle dynamics caused rifting, basin subsidence and mafic volcanism. The latter occurred as the crust was sufficiently attenuated.

Archaean tectonics: a review, with illustrations from the Slave craton

Abstract The tectonic evolution of Archaean granite-greenstone terranes remains controversial. Here this subject is reviewed and illustrated with new data from the Slave craton. These data show that a thick, c. 2.7Ga, pillow basalt sequences extruded across extended sialic basement of the Slave craton at a scale comparable with that of modern large igneous provinces. The pillow basalts do not represent obducted oceanic allochthons. Basement-cover relationships argue for autochthonous to parautochthonous development of the basaltic greenstone belts of the west-central Slave craton, an interpretation that is further supported by geochemical and geochronological data. Similar data exist for several other cratons and granite-greenstone terrains, including the Abitibi greenstone belt of the Superior craton, where stratigraphic and subtle zircon inheritance data are equally incompatible with accretion of oceanic allochthons. Many classical granite-greenstone terrains, including most well-documented komatiite occurrences, thus appear to have formed in extensional environments within or on the margins of older continental crust. Closest modern analogues for such basalt-komatiite-rhyolite-dominated greenstone successions are rifts, marginal basins and volcanic rifted margins. Indeed, these environments have high preservation potential compared with fully oceanic settings. Collapse and structural telescoping of these highly extended volcano-sedimentary basins would allow for the complex structural development seen in granite-greenstone terrains while maintaining broadly autochthonous to parautochthonous tectonostratigraphic relationships. Seismic reflection profiles cannot discriminate between these telescoped autochthonous to parautochthonous settings and truly allochthonous accretionary complexes. Only carefully constructed structural-stratigraphic cross-sections, allowing some degree of palinspastic reconstruction, and underpinned by sufficient U-Pb zircon dating, can address the degree of allochthoneity of greenstone packages. Furthermore, seismic reflection profiles are essentially blind for the steep structures produced by multiple phases of upright folding and buoyant rise of mid- to lower-crustal, composite, granitoid and gneiss domes. Such structures are ubiquitous in granite-greenstone terrains and, indeed, most of these terrains appear to have experienced at least one phase of convective overturn to re-establish a stable density configuration, irrespective of the complexities of the pre-doming structural history. Buoyant rise of mid- to lower-crustal granitoid and gneiss domes can explain the typical size and spacing characteristics of such domes in granite-greenstone terranes, and the coeval deposition of late-kinematic, ‘Timiskaming-type’ conglomerate-sandstone successions in flanking basins. The extensional and subsequent contractional evolution of granite-greenstone terrains may have occurred in the overall context of a plate tectonic regime (e.g. volcanic rifted margins, back-arc basins) but highly extended, intraplate, rift-like settings seem equally plausible. Explaining the evolution of the latter in terms of Wilson cycles is misguided. Periods of intense rifting and flood volcanism (e.g. 2.73–2.70 Ga) may have been related to increased mantle plume activity or perhaps catastrophic mantle overturn events. Although there is evidence for plate-like lateral movement in late Archaean time (e.g. lateral heterogeneity of cratons, arc-like volcanism, cratonscale deformation patterns, strike-slip faults, etc.), the details of how these plate-like crustal blocks interacted and how they responded to rifting and collision appear to have differed significantly from those in Phanerozoic time. The most productive approach for Archaean research is probably to more fully understand and quantify these differences rather than the common emphasis on the superficial similarities with modern plate tectonics.

Obliquely convergent tectonics and granite emplacement in the Trans-Saharan belt of Eastern Nigeria: a synthesis

The Eastern Nigeria terrane belongs to the 3000 km-long Trans-Saharan belt which was formed in the Neoproterozoic, between 750 and 500 Ma by continental collision between the converging West African craton, Congo craton and East Saharan block. The study area consists mostly of gneisses and migmatites that underwent granulite facies metamorphism (>800 °C, 800 MPa). In contrast with Western Nigeria and Cameroon, no basement-cover relationship has been identified which is in agreement with a monocyclic metamorphic history. Around 640 Ma, the area underwent a nappe tectonics with eastward displacement, emplacement of Bt–Ms granites and granulite facies peak. Later on, around 615 Ma, nappes were towards the north. Numerous Hbl–Bt granite plutons were emplaced around 585 Ma during north–south strike-slip regional deformation. The anisotropy of magnetic susceptibility fabric helps in defining strain localization along narrow north–south to NNE–SSW vertical shear-zones during the exhumation history. Exhumation of the terrane, assessed using U–Pb, Ar–Ar and Rb–Sr methods, was presumably slow with uplift rates around 0.2–0.5 mm/year. Preliminary estimates of contemporaneous horizontal movements suggest that they were one order of magnitude larger.

In-situ U–Pb zircon ages for Early Ordovician magmatism in the eastern Pyrenees, France: the Canigou orthogneisses

U–Pb age determinations with an ion probe on zircons within orthogneisses from the Canigou Massif, eastern Pyrenees, give an emplacement age for their granitic protolith of 475±10 Ma (Arenigian). This invalidates the previous interpretation of basement-cover relationships between the orthogneisses and the overlying Lower Cambrian Canaveilles metasedimentary series, and corroborates the intrusive character of the orthogneisses, which is further supported by field evidence. The data also define two groups of inherited zircons (Paleo/Meso-Proterozoic and Cadomian) in addition to partial lead loss during the Hercynian Late Carboniferous high temperature–low pressure metamorphic event. These results are in agreement with ages reported from other orthogneissic bodies across the entire Variscan chain of western Europe, and indicate the existence of a widespread Cambro-Ordovician magmatic event.

Age of Palaeozoic granites and metamorphism in the Tuvino-Mongolian Massif of the Central Asian Mobile Belt: loss of a Precambrian microcontinent

The Tuvino-Mongolian Massif (TMM) was previously interpreted as a Precambrian block within the Central Asian Mobile Belt. According to this idea, it consists of tectonic slices composed of metamorphic rocks of pre-Mesoproterozoic basement that experienced two episodes of regional metamorphism, and Mesoproterozoic ‘cover rocks’ that were reworked together with the basement during high-grade metamorphism. Zircon U–Pb dating of granitoids from all metamorphic complexes demonstrates that the earliest metamorphic event occurred at 536±6 Ma, significantly later than the deposition of the cover rocks. Regional upper amphibolite-facies metamorphism, which affected all metasedimentary units of the TMM, occurred in the short time interval of 497±4 to 489±3 Ma. We propose that there is no simple basement-cover relationship in the Tuvino-Mongolian Massif. Instead, the massif consists of tectonic domains, composed of heterogeneous metasedimentary successions with distinct pre-metamorphic and pre-tectonic histories that were juxtaposed by thrusting prior to 497 Ma and then metamorphosed under upper amphibolite facies conditions. We suggest that this thrusting event was associated with early Palaeozoic collisional processes related to amalgamation of the Central Asian Mobile Belt. Our new model for the evolution of the TMM implies derivation of Neoproterozoic terrigeneous metasediments from ∼800–900 Ma granitoids formed in an Andean-type active continental margin setting.

Rift basin architecture and stratigraphical implications for basement-cover relationships in the Neoproterozoic Grampian Group of the Scottish Caledonides

A revised and expanded lithostratigraphy is presented for the lower part of the Dalradian Supergroup in the Central Highlands. Stratigraphical correlations combined with lateral facies variations and geophysical studies are used to define a framework of NE-SW-trending marine basins that formed during a major phase of Neoproterozoic rifting. These basins extended rapidly and infilled with up to 5.5 km of turbiditic deposits, later thermal subsidence is indicated by the regional development of shallow marine shelf environments. The margins to these basins are characterized by lateral facies and thickness changes, stratigraphical omission and onlap relationships of both Grampian and Appin group strata onto a basement of predominantly gneissose strata. Whilst there is clear evidence for a stratigraphical and sedimentological break at the base of the Grampian Group there is presently insufficient structural or metamorphic evidence to prove an orogenic unconformity. Geochronological data confirm the presence of Precambrian events in the basement but have yet to do so in the cover.

Basement–cover relationships and orogenic evolution in the Central East Greenland Caledonides

Recent structural, petrological and geochronological data from the Fjord Region of central East Greenland requires revision of tectonic models for this segment of the Caledonian-Appalachian orogen. The data document the existence of a regionally extensive, orogen parallel, east-dipping extensional detachment zone (Fjord Region Detachment Zone) separating high grade, infracrustal para- and orthogneisses and migmatites in the footwall or Central Metamorphic Complex (basement) from overlying Neoproterozoic to Middle Ordovician metasedimentary rocks in the hanging wall. A tectonically disturbed depositional contact exists between the basement gneisses and overlying amphibolite grade Neoproterozoic metasedimentary rocks along the western margin (towards the foreland) of the Central Metamorphic Complex. Top-to-the-east movement on the Fjord Region Detachment Zone and associated extensional faults in the hanging wall post-dates orogen-perpendicular contractional structures in the hinterland. U-Pb and 40Ar-39Ar ages from the detachment zone and underlying infracrustal gneisses indicate that extensional movement on the detachment zone started as early as in the Late Silurian and continued into the Middle Devonian. We have found no evidence in the upper crustal level in the hinterland of an Early Devonian contractional event as indicated in some previous models. Sedimentological as well as isotopic data from the foreland suggest that extension in the hinterland was contemporaneous with folding and thrusting in the foreland. To explain these relationships we envision a scenario where upper crustal extension in the hinterland triggered upwelling of a low viscosity (“fluid”) lower crust in the hinterland, represented by the eastern part of the Central Metamorphic Complex. The hinterland welt exerted enough subhorizontal stress to cause east-west shortening in the foreland. Diapiric upwelling of the inferred “fluid” lower fluid crust was facilitated by invasion of large volumes of granitic melt dated to c. 425 Ma.

Late Proterozoic rift faults and basement—cover relationships within the Ben More thrust sheet, NW Scotland

Recently proposed models for the tectonic evolution of Northern Scotland, involving lithospheric extension during the late Precambrian, are supported by new discoveries and interpretations of early normal faults within the Moine Thrust Belt. Such structures control the preser­vation of Torridonian sediments beneath the sub-Cambrian unconform­ity. The Ben More thrust ramp from basement into the Cambrian cover may have been controlled by the early faults. Major synclines within the Ben More thrust sheet appear as tightenedhalf-grabens and thus are composite structures. This may require a re-examination of the field relationships between these folds and the apparently syn-tectonic alkaline intrusions that have been used to date thrust activity. The Ben More thrust sheet of the Moine Thrust Belt contains the most easterly outcrops of Torridonian Supergroup (late Proterozoic) in Scotland (Fig. 1). These sediments and their Lewisian basement are unconformably overlain by the Cambrian quartzites of the Eriboll Sandstone Group. The stratigraphic relationships of these units, with the two un­conformities (Cambrian on Torridonian, Torridonian on Lewisian) discordant so that the Cambrian sediments overstep from Toridonian onto Lewisian basement, have been known for over a century. However, the nature of the overstep and its implications for the Precambrian arrangement of Torridonian and Lewisian rocks, is less established. Soper & Barber (1979) inferred the presence of major upright folds with wavelength in excess of 30 km so that Torridonian sediments lie preserved in synclines while the crests of the anticlines are eroded beneath the sub-Cambrian unconformity. This model for Precambrian basement–cover relationships has

Single-zircon Pb evaporation geochronology constrains basement-cover relationships in the lower Hecla Hoek Complex of northern Ny Friesland, Svalbard

The Hecla Hoek Complex, a ∼ 18-km-thick succession in the Svalbard Caledonian Eastern Terrane, has been interpreted to be a more or less continuous volcano-sedimentary sequence. Amphibolite-facies rocks in the lower parts appear to grade conformably up into almost unmetamorphosed Neoproterozoic and Early Paleozoic strata in the middle and upper parts of the succession. However, recent work in the Ny Friesland transpressional orogen has indicated that the lower part of this Hecla Hoek Complex is composed of at least three thrust sheets. Each thrust sheet is made up of Paleoproterozoic ( ∼ 1750 Ma) granites, locally intruded into older metasedimentary rocks, and overlain by younger sedimentary cover, mainly composed of psammites and quartzites, locally with conglomerates. This isotopic study, focuses on basement-cover relationships in one of the thrust sheets in northern Ny Friesland, where a granitic unit (the Instrumentberget Granitic Gneiss) is overlain by conglomerates and quartzites (Polhem Formation). Ages obtained using Pb evaporation and conventional UPb techniques on both the granitic gneisses and granitic boulders in overlying conglomerates, yield similar ages of ∼ 1740 Ma, suggesting that the latter are derived from the former. Additional Pb evaporation analyses on detrital zircons from two quartzite samples higher up in the Polhem stratigraphy have given information about other source ages. One of these quartzitec samples, from northern Ny Friesland, is dominated by detrital zircon ages of 1870–2040 Ma; however, this sample also confirms the existence of a 1740-Ma source. Another quartzite within the same formation, collected 70 km farther south in Ny Friesland and from a higher stratigraphic level, has provided detrital zircon ages of 1320–2710 Ma; the youngest zircon age in this sample constrains the maximum age for the deposition of the Polhem Formation. These sediments may be as young as Early Paleozoic, but are thought to be Mesoproterozoic, because of the intercalation of abundant amphibolites, which are notable for their absence in the eastern Ny Friesland Neoproterozoic and younger succession. The younger age (< 1740 Ma) of some of the zircons in the Polhem sediments strongly support the recent mapping and structural interpretation that the lower part of the Hecla Hoek Complex is a tectonostratigraphic succession composed of several thrust sheets.

Déformation hercynienne et relation socle/couverture dans le domaine du Bas-Draˆa (Anti-Atlas occidental, Maroc)

Hercynian deformations in the western anti-Atlas area are controlled by the reactivation of faults in the Precambrain basement. The Bas-Draˆa anticline zone forms the southernmost Precambian basement massif covered by Infracambrian and Palaeozoic sedimentary formations. Relative motions of basement blocks induce in the cover an heterogeneous ductile deformation associated with a SE vergence. Structural data and finite strain measurements suggest the basement-cover relationships and late Cabboniferous age of the regional tectonics. The uplift of the Bas-Draˆa basement associated with the southeast thrusting induced a punching motion in the cover. A dextral shear component locally involved the rotation of structures or en echelon folding. The deformation is very intense near the basement boundaries, but decreases away from the boundary, as well as upwards within the sedimentary cover. To the southeast the deformation is very weak in the Carboniferous on the Saharan domain. The Bas-Draˆa area represents the last mobile block of Hercynian Belt foreland near the stable West African Craton. Vertical m movements are dominant versus wrench shifting.