Basement Culmination Research Articles

Supradetachment basins in necking domains of rifted margins: Insights from the Norwegian Sea

AbstractSupradetachment basins at passive rifted margins are a key witness of major‐continental extension, and they may preserve a record from which the amount and rates of extension and metamorphic core complex exhumation may be reconstructed. These basins have mainly been recognised in back‐arc and orogenic collapse settings, with few examples from rifted margins. Using 2D and 3D seismic reflection, wellbore, and gravity anomaly data, we here characterise the three‐dimensional structural and tectonosedimentary evolution of a spoon‐shaped supradetachment basin that was formed in the necking domain of a rifted margin, at the southern limit of the Møre and Vøring segments of the Norwegian rifted margin. The basin, with an areal extent of ca. 2400 km2, and a landward‐rotated syn‐tectonic succession up to ca. 30 km thick (true stratigraphic thickness), is separated from footwall continental margin core complex basement culminations by major large‐offset (>30 km) normal fault complexes characterised by a cross‐sectional geometry whereby an upper, steeper part of the fault gives way to a low‐angle detachment fault at depth. These fault complexes are associated with a tectonic thinning of the continental crust to ca. 11 km, compared with a crustal thickness of ca. 27 km in the proximal domain. The basin is filled by a succession of pre‐, syn‐ and post‐tectonic deposits, that accumulated over time as the basin evolved over a series of rift‐ and detachment faulting events. The 30 km thick syn‐tectonic succession reflects deposition during two separate rifting events, which are disconnected by deposits reflecting a relative short period of tectonic quiescence. The results are discussed in light of examples of supradetachment basins on other rifted margins globally, as well as in the context of the evolution of the Norwegian margin overall.

The proximal domain of the Mid-Norwegian rifted margin: The Trøndelag Platform revisited

Located along the coast of Norway, the Trøndelag Platform area corresponds to the proximal domain of the Mid Norwegian Vøring rifted margin. We investigated its architecture and structural evolution based on interpretation of a dense network of high-resolution seismic reflection profiles, supplemented by comparison with local onshore analogues.The entire area is floored by ductile crustal shearing and regional-scale core complexes that form a corrugated surface with basement culminations that are flanked and sometimes cut by detachment faults. The detachments are segmented, suggesting long-lived and multiphase tectonic activity. The observed geometries are directly comparable to the ones mapped onshore in and around Devonian ‘Old Red Sandstone’ basins in western and south-western Norway, and in the adjacent North Sea. The combination of intensively sheared basement, core complexes, flanking and cutting detachment faults is interpreted to represent the offshore record of the transition from orogenic collapse to the first phases of rifting. The observations demand a revision of our conceptual approach of margins' proximal domains geometries for rifted margins that develop on collapsed orogens.

Early exhumation of the Frontal Cordillera (Southern Central Andes) and implications for Andean mountain-building at ~33.5\xb0S

The Andes are the modern active example of a Cordilleran-type orogen, with mountain-building and crustal thickening within the upper plate of a subduction zone. Despite numerous studies of this emblematic mountain range, several primary traits of this orogeny remain unresolved or poorly documented. The onset of uplift and deformation of the Frontal Cordillera basement culmination of the Southern Central Andes is such an example, even though this structural unit appears as a first-order topographic and geological feature. To solve for this, new (U-Th)/He ages on apatite and zircon from granitoids of the Frontal Cordillera at ~33.5°S are provided here. These data, interpreted as an age-elevation thermochronological profile, imply continuous exhumation initiating well before ~12–14 Ma, and at most by ~22 Ma when considering the youngest zircon grain from the lowermost sample. The age of exhumation onset is then refined to ~20 Ma by combining these results with data on sedimentary provenance from the nearby basins. Such continuous exhumation since ~20 Ma needs to have been sustained by tectonic uplift on an underlying crustal-scale thrust ramp. Such early exhumation and associated uplift of the Frontal Cordillera invalidate the classically proposed east-vergent models of the Andes at this latitude. Additionally, they provide further support to recent views on Andean mountain-building proposing that the Andes at ~33.5°S grew firstly over west-vergent basement structures.

Tectonic and structural setting of the northeastern central Gulf of Suez area using aeromagnetic data

Cumulative qualitative and quantitative analysis of the filtered regional and residual magnetic components of the northeastern central area of the Gulf of Suez, as well as images of the second vertical derivatives of the reduced to the northern magnetic pole map of the total magnetic intensity field images, supplemented with the available geologic information, enabled the precise delineation of the detailed structural configuration of the basement complex, which consequently illustrated the structural deformational pattern of the overlying sedimentary succession. The basement tectonic map reflects a series of N–S to NNW–SSE oriented belts of high and low basement structures. These structures are interrupted by a set of NE–SW crossing diagonal faults having varying throws and creating promising blocks for exploration.An often remarkable correlation between the reduced to the magnetic pole map and the basement relief map is noted, in particular the outline of various oil fields. A larger number of the tilted fault blocks and basement culminations have been outlined and numerous interesting exploration prospects are indicated, which appear to warrant further follow up investigation.

Structural complexities in a foreland thrust belt inherited from the shelf-slope transition: Insights from the Alishan area of Taiwan

The Alishan area of Taiwan spans the transition from the platform with full thickness of the Eurasian continental margin in the north to the thinning crust of its slope in the south. This part of the foreland thrust and fold belt includes important along-strike changes in structure, stratigraphy, and seismic velocities. In this paper we present the results of new geological mapping from which we build geological cross sections both across and along the regional structural trend. Fault contour, stratigraphic cutoff, and branch line maps provide 3-D consistency between the cross sections. Minimum shortening is estimated to be ~15 km with displacement overall to the northwest. A P wave velocity model helps constrain the structure at depth by providing insight into the possible rock units that are present there. P wave velocities of ≥ 5.2 km/s point toward the presence of basement rocks in the shallow subsurface throughout much of the southeastern part of the area, forming a basement culmination. The changes in strike of thrusts and fold axial traces, the changing elevation of thrusts and stratigraphic contacts, and the growing importance of Middle Miocene sediments that take place from north to south are interpreted to be associated with a roughly northeast striking lateral structure coincident with the northern flank of this basement culmination. These transverse structures appear to be associated with the inversion of Eocene- and Miocene-age extensional faults along what was the shelf-slope transition in the Early Oligocene, uplifting the margin sediments and their higher P wave velocity basement during Pliocene-Pleistocene thrusting.

Gondwanan Eoarchean–Neoproterozoic ancient crustal material in Iran and Turkey: zircon U–Pb–Hf isotopic evidence

The Sanandaj–Sirjan Zone is a basement culmination northeast of the Neo-Tethys suture in Iran. In this zone near Azna, granite has a magmatic zircon U–Pb age of 568 ± 11 Ma, with 900–800, ca. 2400, and ca. 3600 Ma inherited cores. The ca. 3600 Ma inherited zircon is the oldest crustal component yet detected in Iran. Near Chadegan, orthogneiss has a magmatic zircon U–Pb age of 637 ± 15 Ma, and carries ca. 1000 and 2000 Ma inherited zircons. Inherited 900–1000 Ma zircons have juvenile initial εHfvalues of ca. +8 to +9, whereas the younger 630 and 568 Ma magmatic zircons show lower initial εHfvalues; however, the 3600 Ma core has initial εHf= 0.0. A Neoproterozoic rim on the inherited 3600 Ma core has the most extreme initial εHfvalue of −18. The Hf isotopic data indicates generation of the magmatic protoliths from a mixture of juvenile Neoproterozoic and Archean sources. Previous studies showed that in Turkey the Central Anatolian Crystalline Complex is underlain by Neo-Eoarchean rocks, the Menderes Massif contains Neoproterozoic granitoids, and that central Iran’s basement and the northern Sanandaj–Sirjan Zone contain Neoproterozoic granitic rocks. This basement terrane is from Gondwana, and was transferred across Paleo-Tethys to dock against Eurasia’s southern margin. Occurrence in Iran and Turkey of Eoarchean crust raises the possibility of sinistral migration of this terrane in the closure of Tethys because the nearest known early Archean crust occurs in northeast India.

The Heggmovatn supracrustals, North Norway—A late Mesoproterozoic to early Neoproterozoic (1050–930 Ma) terrane of Laurentian origin in the Scandinavian Caledonides

Rocks of the “Heggmovatn dome”, North Central Scandinavian Caledonides have traditionally been interpreted as a fold nappe involving Baltican basement rocks. They are also considered to be comparable in age to the orthogneisses in the Glomfjord dome to the south, the Rishaugfjell dome to the east and the Tysfjord culmination to the north, dated to ca. 1.7–1.8Ga. However, while these basement culminations are generally dominated by orthogneisses, the Heggmo area is composed of alternating metapsammites and metapelites intruded by both pre-and post-tectonic granitoids. U–Pb ID-TIMS ages on magmatic zircons from the syn/post-tectonic granitoids indicate a mid-Silurian emplacement age (ca. 430Ma) whereas zircons from a migmatitic biotite-rich pre-tectonic intrusive indicate a Grenvillian/Sveconorwegian crystallization age (ca. 925Ma). U–Pb LA-ICPMS ages on clastic zircons from the Heggmo metapsammites, coupled with ages from cross-cutting granitoid intrusives constrain the period of deposition of the sedimentary sequence to be between ca. 1050Ma and ca. 930Ma.There are strong similarities in lithologies, depositional age, age of clastic zircon populations and magmatic activity, to Laurentian derived supracrustal sequences, particularly, the Krummedal Sequence in Northeast Greenland. Based on these similarities and on its high tectonic position in the Caledonian nappe pile, the Heggmo rocks are reinterpreted to represent an exotic terrane of Laurentian affinity, probably derived from present day NE Greenland, and thrust onto Baltica as part of the Uppermost Allochthon during the Scandian phase (E. Devonian) of the Caledonian orogeny.

Intra-continental Dabashan orocline, southwestern Qinling, Central China

Using field investigation, structural analysis, and 40Ar–39Ar and fission-track geochronology, we synthesize the structural architecture, stress fields, and tectonic evolution of the Dabashan orocline, southwestern Qinling. Three paleostress fields govern deformation: The first (Late Triassic to Early Jurassic, T3–J1) reflects ∼N–S to NNE–SSW shortening in the foreland of the Qinling–Dabie orogen and is the result of the collision of the North China and Yangtze blocks; the principal structures in the Dabashan belt are ∼E-trending folds. The second event (Late Jurassic to Early Cretaceous, J3–K1) shows NE–SW shortening; a fold-thrust belt formed with ∼150km shortening above a detachment layer at 10–12km depth. During this event, the Dabashan foreland acquired its convex-to-southwest oroclinal shape. The third event (Late Cretaceous to Paleogene, K2–E) reflects ENE–WSW or NE–SW extension coeval with the formation of local basins. The Dabashan orocline formed due to pinning of the Qinling–Dabie thrust front at two basement culminations within the subducting Yangtze craton; foreland progradation was made possible by the activation of a Lower–Middle Triassic gypsum-salt detachment of the Sichuan-basin sedimentary pile. During expansion, movements were radially along the orocline. This study gives an example of intra-continental orogeny when terminal deformation is governed by foreland heterogeneities.

Variscan deformation at the northern border of the West African Craton, eastern Anti-Atlas, Morocco: compression of a mosaic of tilted blocks

Abstract North of the Saharan cratonic domain, the Anti-Atlas mountains correspond to the foreland, external fold belt of the Variscan orogen which extends in the Meseta block to the north, and Mauritanides to the southwest. The Anti-Atlas was uplifted during the Mesozoic-Cenozoic, and display several basement culminations (“boutonnières”) amidst the folded Palaeozoic cover. Recent studies in western Anti-Atlas emphasized the basement implication in the shortening process (thick skinned structure). Hereafter we investigate the cover-basement relations in eastern Anti-Atlas south of the Ougnat culmination, based on mapping at scale 1:50,000. The Palaeozoic sequence is much thinner than in the west, and the décollement levels are less important. Flexural slip folds are concentrated along the faults (en échelon folds) and within some rhombic domains crushed between major faults (e.g. Angal-Gherghiz Lozenge), whereas other areas are monoclinal. The main shortening direction deduced from the fold axes trend is directed ~N045°E as in the Ougarta range further to SE. At a regional scale, this shortening direction interferes with a N-S trending one. A sketch map of the top of the basement makes visible a mosaic of S- to SE-ward tilted blocks. The faults between these blocks are inherited from paleofaults which formed during extensional events during the Cambrian, late Ordovician, and (mainly) Middle-Late Devonian. The paleofault array is indicative of a proximal passive margin setting at the northern border of the metacratonic domain. The fault inversion and their dominant strike-slip throw occurred during a late Variscan (Stephanian-Permian) compression event, postdating the NNW-SSE collision of the Meseta block.

Geochemistry and geochronology of the Neoproterozoic Pan-African Transcaucasian Massif (Republic of Georgia) and implications for island arc evolution of the late Precambrian Arabian–Nubian Shield

The Transcaucasian Massif (TCM) in the Republic of Georgia includes Neoproterozoic–Early Cambrian ophiolites and magmatic arc assemblages that are reminiscent of the coeval island arc terranes in the Arabian–Nubian Shield (ANS) and provides essential evidence for Pan-African crustal evolution in Western Gondwana. The metabasite–plagiogneiss–migmatite association in the Oldest Basement Unit (OBU) of TCM represents a Neoproterozoic oceanic lithosphere intruded by gabbro–diorite–quartz diorite plutons of the Gray Granite Basement Complex (GGBC) that constitute the plutonic foundation of an island arc terrane. The Tectonic Mélange Zone (TMZ) within the Middle-Late Carboniferous Microcline Granite Basement Complex includes thrust sheets composed of various lithologies derived from this arc-ophiolite assemblage. The serpentinized peridotites in the OBU and the TMZ have geochemical features and primary spinel composition (0.35) typical of mid-ocean ridge (MOR)-type, cpx-bearing spinel harzburgites. The metabasic rocks from these two tectonic units are characterized by low-K, moderate-to high-Ti, olivine-hypersthene-normative, tholeiitic basalts representing N-MORB to transitional to E-MORB series. The analyzed peridotites and volcanic rocks display a typical melt-residua genetic relationship of MOR-type oceanic lithosphere. The whole-rock Sm–Nd isotopic data from these metabasic rocks define a regression line corresponding to a maximum age limit of 804 ± 100 Ma and εNd int = 7.37 ± 0.55. Mafic to intermediate plutonic rocks of GGBC show tholeiitic to calc-alkaline evolutionary trends with LILE and LREE enrichment patterns, Y and HREE depletion, and moderately negative anomalies of Ta, Nb, and Ti, characteristic of suprasubduction zone originated magmas. U–Pb zircon dates, Rb–Sr whole-rock isochron, and Sm–Nd mineral isochron ages of these plutonic rocks range between ∼ 750 Ma and 540 Ma, constraining the timing of island arc construction as the Neoproterozoic–Early Cambrian. The Nd and Sr isotopic ratios and the model and emplacement ages of massive quartz diorites in GGBC suggest that pre-Pan African continental crust was involved in the evolution of the island arc terrane. This in turn indicates that the ANS may not be made entirely of juvenile continental crust of Neoproterozoic age. Following its separation from ANS in the Early Paleozoic, TCM underwent a period of extensive crustal growth during 330–280 Ma through the emplacement of microcline granite plutons as part of a magmatic arc system above a Paleo-Tethyan subduction zone dipping beneath the southern margin of Eurasia. TCM and other peri-Gondwanan terranes exposed in a series of basement culminations within the Alpine orogenic belt provide essential information on the Pan-African history of Gondwana and the rift-drift stages of the tectonic evolution of Paleo-Tethys as a back-arc basin between Gondwana and Eurasia.

Crustal section based on CROP seismic data across the North Tyrrhenian–Northern Apennines–Adriatic Sea

Using deep seismic reflection data from the Italian lithospheric exploration project CROP in the Central Mediterranean region, a 400-km-long section, composed of three different profiles crossing the Northern Tyrrhenian Sea (CROP M-12A profile), the Northern Apennines (CROP-03) and the Adriatic Sea (CROP M-16) is reconstructed and discussed. New data allow us to outline a seismically consistent tectono-stratigraphic setting for the crust and upper mantle of the Northern Apennines thrust–belt system and its Adriatic foreland. Time–space analysis of the deformation of the investigated chain and identification of existing macrostratigraphic crustal intervals and tectonic units allow a reasonably controlled interpretation of the geodynamic evolution and of the main orogenic stages. Careful seismic reprocessing and application of advanced techniques to key zones of the explored area (such as the Tuscan Archipelago) were determinants in obtaining fundamental information for understanding of the complex lithospheric structures and their evolution. Profile interpretation supports that the Northern Apennine chain is dominated by a compressive thrust system. Crustal extension, assumed by some authors as the dominating tectonic process for the whole Tuscan Apennine area, represents a subordinate geodynamic event of the last stage (Tyrrhenian). In the Early Cretaceous–Late Jurassic, the paleogeographic framework consisted of the Europe and Adria plates separated by the Alpine Tethys Ocean. During the Late Cretaceous–Early Eocene, Adria–Europe convergence (eo-Alpine stage) and subduction beneath the Adria plate closed the Alpine Tethys Sea, with the Tethyan slab being clearly seismically imaged. The first Apenninic geodynamic stage occurred in the Late Oligocene–Early Miocene with the opening of the Balearic Basin, which generated a first “lithospheric root” of the Apenninic chain in the Tuscan Archipelago area. This root is represented by Adria-verging thrust faults that progressively flatten eastward. Upper parts of the west-verging eo-Alpine thrust blocks were truncated by the east-verging thrust faults of the Balearic stage. A deeper seismic reflector, attributed to the top of the asthenosphere, forms a mantle high below the Elba Island. From the Late Miocene to Present, the Corsica basin and western hinterland area were affected by extensional tectonics related to the Tyrrhenian opening, whereas compressional tectonics continued in the eastern hinterland and mostly on the eastward migrating foreland, with development of a second “lithospheric root” constituted by high-angle thrust faults. These faults give rise to a huge basement culmination below the main Apennines watershed. Impressive E-directed gravity-sliding of sedimentary blocks over their sloping basement occur, generating the Umbria–Marche shallow seismicity. Crustal shortening of the Apennines system amounts to 170 km, 14 km of which are due to the eo-Alpine stage, 71 km to the Balearic and 85 km to the Tyrrhenian one. In the frame of Africa–Europe convergence, the Tyrrhenian–Apennines tectonodynamics were mainly conditioned by the Mesozoic paleogeography.

The Central Slave Basement Complex, Part I: its structural topology and autochthonous cover

New field and geochronological data are used to define the distribution of Mesoarchean basement rocks in the south-central Slave Province. This distribution reflects a single contiguous basement terrane that we propose to call the Central Slave Basement Complex. It shows a structural topology that is internally consistent and compatible with known regional folding and faulting events. A sample of a proposed basement gneiss below the Courageous Lake greenstone belt, central Slave Province, has been dated by U-Pb methods and yields an age of 3325 ± 8 Ma, consistent with the new basement distribution. This sample also contains 2723 ± 3 Ma metamorphic zircon and ca. 2680 Ma titanite. The Central Slave Basement Complex is overlain by a thin, discontinuous, but distinctive cover sequence that includes minor volcanic rocks, clastic sedimentary rocks, and banded iron formation. All previously known and some new occurrences of this distinctive cover sequence occur in the immediate stratigraphic hanging wall of the Central Slave Basement Complex, locally overlying a preserved in situ unconformity. We propose to call this post-2.93 Ga cover sequence the Central Slave Cover Group. It is perhaps best typified by detrital chromite-bearing, fuchsitic quartzites. Formal formation names are proposed for the spatially separate occurrences of the Central Slave Cover Group. Detrital zircon ages are presented for one of the formations of the Central Slave Cover Group, the Patterson Lake Formation, which occurs on the western flank of a local basement culmination known as the Sleepy Dragon Complex. The detrital zircon data provide evidence for two discrete basement sources dated at ca. 2943 Ma and ca. 3147-3160 Ma. These detrital ages reinforce the depositional link between the Central Slave Cover Group and underlying crystalline rocks of the Central Slave Basement Complex.

Laser 40Ar/39Ar thermochronology of Archean rocks in Yellowknife Domain, southwestern Slave Province: insights into the cooling history of an Archean granite-greenstone terrane

Laser 40Ar/39Ar ages for hornblende, muscovite, and biotite from Archean plutonic rocks of Yellowknife Domain, southwestern Slave Province, complement U-Pb ages from minerals with higher closure temperatures (zircon, monazite, titanite). Together, the data indicate that the plutonic suites of different ages (Defeat, Prosperous, Morose) coring three major thermal culminations in the domain have distinct Archean cooling histories until below approximately 250°C (biotite closure). The cooling trajectories for successive suites appear to document a decrease in cooling rate over the course of orogeny, possibly reflecting the build-up of heat within the orogen, coupled with a transition to lower rates of exhumation and (or) erosion. Greater structural relief over the Sleepy Dragon basement culmination may be responsible for slower cooling of the Morose Granite situated in its core. At or below the closure temperature of biotite the cooling curves converge with one another, documenting a transition from "localized" to "regional" cooling. The fact that cooling was locality specific until at least the biotite closure temperature suggests that, just as intrusion and related peak metamorphism were diachronous across the region, so was cooling to approximately 250°C. Uniform regional isotherms thus were not established until >200 Ma after the youngest Archean plutonism. The variable disturbance of biotite Ar spectra, but generally not muscovite or hornblende, indicates that the Paleoproterozoic thermal disturbance of Archean rocks in Yellowknife Domain did not exceed 200-300°C.

Laser <sup>40</sup>Ar/<sup>39</sup>Ar thermochronology of Archean rocks in Yellowknife Domain, southwestern Slave Province: insights into the cooling history of an Archean granite-greenstone terrane

Laser 40Ar/39Ar ages for hornblende, muscovite, and biotite from Archean plutonic rocks of Yellowknife Domain, southwestern Slave Province, complement U-Pb ages from minerals with higher closure temperatures (zircon, monazite, titanite). Together, the data indicate that the plutonic suites of different ages (Defeat, Prosperous, Morose) coring three major thermal culminations in the domain have distinct Archean cooling histories until below approximately 250°C (biotite closure). The cooling trajectories for successive suites appear to document a decrease in cooling rate over the course of orogeny, possibly reflecting the build-up of heat within the orogen, coupled with a transition to lower rates of exhumation and (or) erosion. Greater structural relief over the Sleepy Dragon basement culmination may be responsible for slower cooling of the Morose Granite situated in its core. At or below the closure temperature of biotite the cooling curves converge with one another, documenting a transition from "localized" to "regional" cooling. The fact that cooling was locality specific until at least the biotite closure temperature suggests that, just as intrusion and related peak metamorphism were diachronous across the region, so was cooling to approximately 250°C. Uniform regional isotherms thus were not established until >200 Ma after the youngest Archean plutonism. The variable disturbance of biotite Ar spectra, but generally not muscovite or hornblende, indicates that the Paleoproterozoic thermal disturbance of Archean rocks in Yellowknife Domain did not exceed 200-300°C.

Structural development of the Neogene Radicondoli–Volterra and adjoining hinterland basins in Western Tuscany (Northern Apennines, Italy)

This paper presents a geological–structural study of some Neogene hinterland basins of the Northern Apennines, located on the Tyrrhenian side of the chain. These basins developed on the already delineated thrust-fold belt from middle–late Tortonian times. Their evolution has been commonly referred to an extensional tectonic regime, related to the opening of the Tyrrhenian Sea. New data have allowed us to hypothesize a different tectonic evolution for the chain, where compressive tectonics plays a major role both in the external and in the hinterland area. In this frame, the hinterland area located west of a major outcropping crustal thrust (Mid-Tuscany Metamorphic Ridge) has been the target of a geological–structural investigation. The field mapping and structural analysis has been focused on the syntectonic sediments of the Radicondoli–Volterra basin as well as on adjoining minor basins. These basins commonly display a synclinal structure and are generally located in between basement culminations, probably corresponding to thrust anticlines. Sediments of the hinterland basins have been affected by compressive deformation and regional unconformities separate stratigraphic units due to the activity of basement thrusts. In the study area, normal faulting either accommodates the thrusting processes or post-dates compressive deformation. A chronology of faulting and a six-stage evolution of this area are presented, providing further insights for the Neogene tectonic evolution of the Northern Apennines. Copyright © 1998 John Wiley & Sons, Ltd.

Structural development of the Neogene Radicondoli–Volterra and adjoining hinterland basins in Western Tuscany (Northern Apennines, Italy)

This paper presents a geological–structural study of some Neogene hinterland basins of the Northern Apennines, located on the Tyrrhenian side of the chain. These basins developed on the already delineated thrust-fold belt from middle–late Tortonian times. Their evolution has been commonly referred to an extensional tectonic regime, related to the opening of the Tyrrhenian Sea. New data have allowed us to hypothesize a different tectonic evolution for the chain, where compressive tectonics plays a major role both in the external and in the hinterland area. In this frame, the hinterland area located west of a major outcropping crustal thrust (Mid-Tuscany Metamorphic Ridge) has been the target of a geological–structural investigation. The field mapping and structural analysis has been focused on the syntectonic sediments of the Radicondoli–Volterra basin as well as on adjoining minor basins. These basins commonly display a synclinal structure and are generally located in between basement culminations, probably corresponding to thrust anticlines. Sediments of the hinterland basins have been affected by compressive deformation and regional unconformities separate stratigraphic units due to the activity of basement thrusts. In the study area, normal faulting either accommodates the thrusting processes or post-dates compressive deformation. A chronology of faulting and a six-stage evolution of this area are presented, providing further insights for the Neogene tectonic evolution of the Northern Apennines. Copyright © 1998 John Wiley & Sons, Ltd.

Basement–cover relationships in the Shaftesbury area of the Wessex Basin, southern England

AbstractIn the northwestern part of the Wessex Basin of southern England, interpretation of seismic reflection and recently released borehole data provides detailed information of the major unconformity at the base of the sedimentary-fill. Seismic mapping of the top of Variscan basement reveals a concealed, faulted, dome-like Palaeozoic basement culmination or structural ‘high’ lying within the larger Cranborne–Fordingbridge High, southwest of Shaftesbury. This structure profoundly affected the deposition of the overlying Permo-Triassic sequence. The culmination, composed of Carboniferous Limestone, lies to the south of northwards-throwing, basement-controlled, syn-sedimentary faults which form the southern margin of the Mere Basin. In this area, the main phase of faulting occurred during Permo-Triassic times with little or no faulting during the Jurassic. Triassic strata onlap the basement culmination which was overstepped and finally submerged in late Triassic times. The structure has much in common with the basement ‘highs’ at Bruton and in the Mendips and provides further evidence for the existence of such features close to the main lines of Mesozoic faulting within the Wessex Basin. It is considered that differential subsidence related to syn-depositional normal faulting was the dominant factor controlling development of the highs. The simple infilling of a pre-existing post-orogenic topography played a minor role.

Deformation‐related Paleozoic radiogenic lead and strontium additions in Proterozoic marbles from the Rombak‐Sjangeli basement culmination, Scandinavian Caledonides

The Sjangeli supracrustal belt occurs at the eastern margin of the Rombak‐Sjangeli basement culmination that involves Proterozoic basement, which became incorporated into the Caledonian orogen as old basement faults were reactivated. The lead isotopic composition of early Proterozoic marbles from the Sjangeli supracrustal belt fall in the 206Pb/204Pb‐207Pb/204Pb diagram on a linear array that is interpreted as a mixing line. Using the age of granite intrusion and Proterozoic metamor‐phism as source age for the lead (t1 = 1.8 Ga), the corresponding mixing age is t2= 0.4 Ga, which suggests that the radiogenic lead was introduced during the Caledonian deformation of the marbles. The isotopic compositions of lead and strontium, which also is highly variable and radiogenic, are positively correlated, indicating that radiogenic lead and strontium were introduced together. Combining lead and strontium isotope data from the marbles and Paleozoic galena‐calcite veins demonstrates that the lead and strontium were d...

A basement culmination in the Scandinavian Caledonides formed by antiformal stacking (Bångonåive, northern Sweden)

AbstractThe Bångonåive basement culmination, a doubly plunging antiform trending SW-NE in its southern part and SSW-NNE in the north, is part of a major Caledonian antiform in north-central Scandinavia. Crystalline, Proterozoic basement rocks (mainly syenite) are unconformably overlain by a sedimentary cover including tillites at the base, passing up into arkoses, quartzites and shales, capped by black phyllites. This sequence is correlated with the Varangian to Cambrian succession of the Baltoscandian platform farther east. Detailed mapping revealed a succession of five basement-cover horses, which represent the accessible part of an antiformal stack exposed beneath the Middle and Upper Allochthons and taken here as the Lower Allochthon structural level.Caledonian deformation varies in intensity from penetrative near thrusts and in pelitic rocks to very weak in the more internal parts of the horses. A penetrative foliation is associated with the growth of white mica and rare biotite. This early fabric is overprinted by a mylonitic foliation related to localized shear zones, which separate the structural units within the Lower Allochthon. Stretching and mineral lineations trend WNW-ESE and related shear-sense criteria indicate transport (top) towards the ESE. Structural units (horses) are thrust into an antiformal stack and folded around the lowermost horse exposed, which is itself folded into an anticlinal lift-off fold. Towards the northeast, the antiformal stack is overprinted by a pop-up and an out-of-sequence thrust. The latter breached the roof of the Lower Allochthon and transported part of it over the Middle and Upper Allochthons. Further folds are associated with lateral and oblique ramps in the Lower Allochthon. These structures relate very well with the complex fold pattern previously observed in the higher structural units and thrust tectonics provides a straightforward genetic explanation for these folds. Therefore, earlier genetic models of the Bångonåive basement culmination as a simple imbrication of basement into higher units, as a buckling structure or as a gravitational dome structure are rejected here. The structural information, supported by gravimetric data, is consistent with an essentially flat regional detachment surface (2° dip) extending from the present external Caledonian margin to the base of the Bångonåive antiformal stack.

The origin of metamorphic core complexes and detachment faults formed during Tertiary continental extension in the northern Colorado River region, U.S.A.

Metamorphic core complexes form as the result of major continental extension, when the middle and lower continental crust is dragged out from beneath the fracturing, extending upper crust. Movement zones capable of producing such effects evolve in space as well as with time. Deforming rocks in the footwall are uplifted through a progression of different metamorphic and deformational environments, producing a characteristic sequence of (overprinted) meso- and microstructures. The movement zone is folded as the result of the bowing upwards of the lower crust to form a broad basement culmination, as the result of isostatic rebound due to tectonic denudation, but most likely also as the result of local isostatic adjustments due to granite intrusion in the middle crust. A succession of splays branch off from the master detachment fault at depth, excising substantial portions of the lower portions of the upper plate as successive detachment faults eat upwards through it. At the same time, detachment faults incise into progressively deeper levels of the lower plate, although the amount of incision is limited, because the locus of movement remains at approximately the same level in the lower plate. The detachment faults presently observed in the metamorphic core complexes are relatively young features, formed late in the geological evolution of these bodies, and are only the last in a succession of low-angle normal faults that sliced through the upper crust at the upward terminations of major, shallow-dipping, ductile shear zones in the extending Cordilleran orogen. Excisement of listric fault bottoms can explain some of the enigmatic domino-like fault blocks, and other structural relations observed in these terranes. Evidence in support of this model is illustrated from detachment terranes in the northern Colorado River region of southern Nevada, southeastern California and western Arizona, where multiple generations of detachment faults have produced remarkable excisement and incisement geometries.