Autochthonous Basement Research Articles

Are the Carpathians tectonically active?: Geomechanical study in deep boreholes in the outer Carpathians (Poland)

Present-day tectonic stress state was investigated in three deep boreholes located in the eastern segment of the Polish Outer Carpathians (POC). Significant rotations of the maximum horizontal stress (SH) were observed in these boreholes, located at the hinge of the anticlines in the upper part of the nappes. For the deepest borehole, D-1 (5.5 km depth), 1D geomechanical modelling was performed to determine the stress gradient profiles. An optimal solution of the model, validated by numerous compressional and extensional failures (breakouts and drilling-induced fractures, respectively) of the borehole wall, was obtained for variable elastic horizontal strain. The strain varies stepwise across the Main Thrust Fault (MTF) and linearly within its walls. The dominance of a strike-slip faulting stress regime was determined for the Carpathian nappes, with contributions from thrust faulting above the MTF and normal faulting below the MTF. A critical stress state for reactivation of preferentially oriented pre-existing faults and fractures was inferred for the competent strata. A consistent interpretation of the variations in stress orientation and magnitude, suggests a contemporary refolding of the anticline at a shallower structural level, enhanced by the reactivation of the MTF and a lack of reactivation of the Carpathian Bottom Thrust. Integration of these results with measurements from previous studies in the eastern segment of the POC indicates a different regional orientation of SH in the autochthonous basement (N-S) and in the nappes (NE-SW). These results indicate a thin-skinned compressive reactivation of the upper part of the accretionary wedge, with the lower part of the nappes remaining passive, or locally prone to minor strike-slip or normal faulting. These results contradict the hypothesis of a contemporaneous extensional collapse of the POC.

The relationship of the oil and gas fields of the Forecarpathian region with the regional faults system and deep structure

The oil and gas fields of Forecarpathian oil and gas region are compared with the structure of earth’s crust and regional faults. Most of the oil and gas condensate fields are localized in Borislav-Pokutia nappe over Precarpathian Fault zone. The majority of gas fields are located in under thrusting zone of the Sambir fault, which is a gently dipping detachment connected with the Forecarpathian fault and other faults of autochthonous basement. The regional gravity low of Carpathian Foredeep is also limited from northeast by the Sambir fault and from the southwest by the Uzhok fault. The Forecarpathian fault tends to the central part of gravity low. Transverse Tyachev-Nadvorna fault displaces the gravity low and delimits the oil and gas condensate fields. Velocity model along the wide-angle reflection and refraction PANCAKE profile shows a deep (25 km) sedimentary trough under the Carpathian Orogen. The Forecarpathian fault coincides with a jump of the Moho under the trough. Along the RomUkrSeis profile under Carpathian Orogen there were revealed a sedimentary trough as deep as 15 km and a low-velocity domain under the trough. From northeast, the trough is limited by the Forecarpathian fault, which goes up to the keel structure of the Moho discontinuity. Structure of the trough under Carpathian Orogen reflects development of East European Platform margin. The earliest stage of the formation of Baltica paleocontinent passive margin is seen in the structure of the trough’s lower part. Autochthonous basement under Borislav-Pokutsky and Skiba nappes includes a Riphean Massif limited by the Krakovets, Forecarpathian and Uzhok faults. The oil and gas condensate fields are located above the Riphean Massif.
 The purpose of this article is to analyze the fault system and the structure of the earth’s crust in the Forecarpathian oil and gas region, as well as their relationship with the distribution of oil and gas deposits, taking into account new WARR profiles and other geological and geophysical data.

The stress field in the frontal part of the Eastern Alps (Austria) from borehole image log data

Stress orientations in the frontal part of the Eastern Alps are inferred from borehole breakouts and drilling-induced tensile fractures depicted in image logs from 62 vertical or slightly deviated wellbores with up to 5km depth. Borehole breakouts (2463 individual features with 1673.9m cumulative length) are observed in all the wells except one, whereas drilling induced fractures only occur in four wells.Microresistivity image log data from the Alpine foreland, the external thrust sheets (allochthonous Molasse, Helvetic- and Flysch nappes) and the autochthonous units in the footwall of the Alpine units reveal consistent in-situ stress orientations with approximately N-S-directed maximum horizontal stresses (SHmax). Indications for stress partitioning across thrusts including the Alpine floor thrust are not observed. In the Western Sector of the study area the mean SHmax is oriented 002° with a standard deviation (SD) of 11.1°. In the central and eastern sector, the mean SHmax is oriented 177° (SD 14°) and 173° (SD 14.3°), respectively, indicating a counterclockwise rotation of SHmax of about 10° from west to east. The observed stress indicators and the analysis of deviated wells suggest a strike-slip regime for the western and Central Sector, whereas a thrust regime is probable in the east of the study area. The thrust regime there coincides with the occurrence of Middle Pleistocene conglomerates which were previously described to be displaced by Alpine thrust faults.The orientations of SHmax fit into the general observation that SHmax directions at the Alpine front are oriented about perpendicular to the strike of the mountain belt and rotate along the Alpine arc. Our stress orientations - including those derived from the autochthonous basement - are, however, markedly different from the NW-SE-directed maximum horizontal stresses reported from the Variscan units in the European foreland north of the study area.

Paleoseismic Evidence for Multiple M w ≥6 Earthquakes in the Eastern Tennessee Seismic Zone during the Late Quaternary

Abstract The eastern Tennessee seismic zone (ETSZ) is the second‐most active seismic zone in the eastern United States, but it has not generated an earthquake larger than M w 4.8 in historic time. Earthquakes are sourced deep in autochthonous basement, and there are no known faults originating at this depth that break the surface. As a result, until recently, there has been virtually no fieldwork to identify Quaternary paleoseismic features in the ETSZ. We present new results from paleoseismic investigations of coseismic features that indicate the ETSZ generated M w ≥6 earthquakes during the late Quaternary. Detailed geologic mapping and trenching near Dandridge, Tennessee, record a northeast‐trending zone of seismically generated features. Optically stimulated luminescence ages delimit timing for the formation of paleoseismic features crosscutting Quaternary alluvium and alluvium‐filled fissures, including a thrust fault with ∼1 m displacement. Collectively, this zone of faults and fissures provides significant evidence that the ETSZ has produced at least three large earthquakes during the late Pleistocene and at least one that exceeded M w 6.

Origin and pre-Cenozoic evolution of the south Qiangtang basement, Central Tibet

The Qiangtang terrane in Central Tibet, with its high-pressure rocks, is a key area to unravel the evolution of the Paleo-Tethys. We present a new 1:50,000 map of the Rongma area in the central Qiangtang terrane, of which relatively little is known so far. We show that the Qiangtang metamorphic belt can be separated in a Paleozoic autochthonous basement and an overlying allochthonous thrust stack of subduction mélange that contains high-pressure rocks and Permian sediments. Detrital zircon ages (youngest peak at 591 Ma) and an ~ 470 Ma granite intrusion age constrain the age of the Qiangtang Basement to be between the Late Precambrian to Middle Ordovician. This is in agreement with the observed unconformity between basement and overlying Mid–Late Ordovician strata. This unconformity and the zircon age spectra are comparable to the Himalaya area. The Qiangtang terrane must thus have been part of the Gondwana supercontinent during the Early Paleozoic. The occurrence of the Late Triassic eclogite and glaucophane-bearing schists in the Central Qiangtang terrane indicates the existence of a suture zone between the North and South Qiangtang terranes before the Late Triassic. This suture zone resulted from closure of the Paleo-Tethys between the North and South Qiangtang terranes. Late Triassic syn-collisional granites and rapidly exhumed high-pressure rocks resulted from the closure of this ocean and final amalgamation of the re-united Qiangtang terranes with Eurasia. Collision of the Lhasa and Qiangtang terranes is correlated with north dipping reverse faults in the south Qiangtang terrane, which were probably related to exhumation of the Qiangtang basement and formation of the Qiangtang Culmination.

Evidence for hyperextension along the pre-Caledonian margin of Baltica

We propose that a mantle-peridotite-bearing mélange unit, which has been mapped for more than 400 km in southern Norway, represents vestiges of deep basin(s) formed by hyperextension of the Baltic margin during the inception of the Caledonian Wilson cycle. In the mélange, which occurs below the crystalline nappes of the Middle Allochthon in southern Norway, solitary mantle peridotites, detrital serpentinites, metabasalts, gabbros and deep basin sediments are interlayered and imbricated with allochthonous, coarse-grained siliclastic sediments and slivers of Proterozoic basement rocks. The siliciclastic sediments have detritus derived from Proterozoic rocks, similar to those of the large crystalline nappes and the autochthonous basement of Baltica. The mélange unit shares many similarities with the hyperextended Tethyan margin complexes that have been described from the Alps, the Pyrenees and several segments of the present-day passive ocean margins, and we interpret it to represent an ancient late Proterozoic to Palaeozoic analogue to present-day hyperextended passive margins. Regional maps show that the mélange continues into the central Scandinavian Caledonides and that this basin assemblage may have had a much wider geographical distribution. The new model suggests that a major reinterpretation of the Lower and Middle Allochthons of the Scandinavian Caledonides is required.

In situ monazite (U–Th)–Pb ages from the Southern Brasília Belt, Brazil: constraints on the high‐temperature retrograde evolution of HP granulites

AbstractIn the southern sector of the Southern Brasília Belt, late Neoproterozoic arc–passive margin collision resulted in juxtaposition of an arc‐derived nappe (the Socorro–Guaxupé Nappe) over a stack of passive margin‐derived nappes (the Andrelândia Nappe Complex) that lies on top of autochthonous basement of the São Francisco Craton. (U–Th)–Pb monazite ages are reported from the high‐grade nappes of the Andrelândia Nappe Complex to better constrain the high‐temperature retrograde evolution. For residual HP granulites from the uppermost Três Pontas–Varginha Nappe, (U–Th)–Pb ages of c. 662 and 655 Ma from low yttrium monazite inclusions in the rims of, or associated with garnet are interpreted to date the late‐stage close‐to‐peak prograde evolution, whereas an age of c. 648 Ma from a similar low yttrium monazite inclusion is interpreted to record post‐peak recrystallization with melt via factures in garnet. For the same nappe, ages of 640–631 Ma retrieved from higher yttrium areas or cores in monazite grains that occur both as inclusions in garnet and in the matrix are interpreted to record growth of monazite either by local breakdown of garnet (±older monazite) and mass exchange with a matrix melt reservoir along cracks or growth from residual melt in the matrix as it crystallized during high‐pressure, close‐to‐isobaric cooling close to the solidus, the temperature of which, at a given pressure, varies with bulk composition of the residual granulites. (U–Th)–Pb ages in the range 620–588 Ma from lower yttrium areas in these monazite grains and from matrix‐hosted patchy monazite are interpreted to date exhumation, as recorded by close‐to‐isothermal decompression and subsequent close‐to‐isobaric cooling. Older monazite ages in this group are interpreted to record late‐stage interaction with melt close to the solidus whereas younger monazite ages are interpreted to record recrystallization of monazite by dissolution–reprecipitation owing to ingress of alkali fluid from the Carmo da Cachoeira Nappe beneath as fluid was released by crystallization of in‐source melt at the solidus. In the underlying Carmo da Cachoeira Nappe, higher yttrium areas in monazite and one single domain monazite yield chemical ages of 619–616 Ma, which are interpreted to date growth as in‐source melt crystallized close to the solidus along the high‐pressure, close‐to‐isobaric segment of the retrograde P–T evolution. Younger (U–Th)–Pb ages of 600–595 Ma retrieved from lower yttrium areas and one single domain monazite are interpreted to record recrystallization of monazite by dissolution–reprecipitation owing to release of fluid at the solidus during exhumation of this nappe. Monazite from the Carvalhos Klippe, interpreted to be correlative with the uppermost nappe, yields a wide range of (U–Th)–Pb ages: for two zoned grains, c. 619 and c. 614 Ma from higher yttrium cores, and c. 583 and c. 595 Ma from lower yttrium rims; and, 592–580 Ma from single domain grains in one sample, and ages of c. 593 and c. 563 Ma from monazite in a second sample. Ages younger than 605 Ma are interpreted to date a fluid‐induced response to the early stages of orogenic loading associated with terrane accretion in the Ribeira Belt to the southeast. The results reported here demonstrate that ages retrieved from monazite that grew close to the solidus in residual granulites from a single tectonic unit will vary from sample to sample according to differences in the solidus temperatures. Further, we show that monazite inclusions may yield ages that are younger than the host mineral and confirm the propensity of monazite to record evidence of tectonic events that are not always registered by other high‐temperature mineral chronometers.

Geochemical/isotopic evolution of Pb–Zn deposits in the Central and Eastern Taurides, Turkey

The Central and Eastern Taurides contain numerous carbonate-hosted Pb–Zn deposits, mainly in Devonian and Permian dolomitized reefal–stramatolitic limestones, and in massive Jurassic limestones. We present and compare new fluid inclusion and isotopic data from these ore deposits, and propose for the first time a Mississippi Valley-type (MVT) mode of origin for them. Fluid inclusion studies reveal that the ore fluids were highly saline (13–26% NaCl equiv.), chloride-rich (CaCl2) brines, and have average homogenization temperatures of 112°C, 174.5°C, and 211°C for the Celal Dağ, Delikkaya, and Ayraklı deposits, respectively. Furthermore, the δ34S values of carbonate-hosted Pb–Zn deposits in the Central and Eastern Taurides vary between –5.4‰ and +13.70‰. This indicates a possible source of sulphur from both organic compounds and crustal materials. In contrast, stable sulphur isotope data (average δ34S –0.15‰) for the Çadırkaya deposit, which is related to a late Eocene–Oligocene (?) granodioritic intrusion, indicates a magmatic source. The lead isotope ratios of galena for all investigated deposits are heterogeneous. In particular, with the exception of the Suçatı district, all deposits in the Eastern (Delikkaya, Ayraklı, Denizovası, Çadırkaya) and Central (Katranbaşı, Küçüksu) Taurides have high radiogenic lead isotope values (206Pb/204Pb between 19.058 and 18.622; 207Pb/204Pb between 16.058 and 15.568; and 208Pb/204Pb between 39.869 and 38.748), typical of the upper continental crust and orogenic belts. Fluid inclusion, stable sulphur, and radiogenic lead isotope studies indicate that carbonate-hosted metal deposits in the Eastern (except for the Çadırkaya deposit) and the Central Taurides are similar to MVT Pb–Zn deposits described elsewhere. The primary MVT deposits are associated with the Late Cretaceous–Palaeocene closure of the Tethyan Ocean, and formed during the transition from an extensional to a compressional regime. Palaeogene nappes that typically limit the exposure of ore bodies indicate a pre-Palaeocene age of ore formation. Host rock lithology, ore mineralogy, fluid inclusion, and sulphur + lead isotope data indicate that the metals were most probably leached from a crustal source such as clastic rocks or a crystalline massif, and transported by chloride-rich hydrothermal solutions to the site of deposition. Localization of the ore deposits on autochthonous basement highs indicates long-term basinal fluid migration, characteristic of MVT depositional processes. The primary MVT ores were oxidized in the Miocene, resulting in deposition of Zn-carbonate and Pb-sulphate–carbonate during karstification. The ores underwent multiple cycles of oxidation and, in places, were re-deposited to form clastic deposits. Modified deposits resemble the ‘wall-rock replacement’ and the ‘residual and karst fill’ of non-sulphide zinc deposits and are predominantly composed of smithsonite.

Alpine metamorphic and tectonic evolution of the Inzecca‐Ghisoni area (southern Alpine Corsica, France)

AbstractIn the Inzecca‐Ghisoni area (southern Alpine Corsica), a complex assemblage of vertically juxtaposed tectonic units, affected by Alpine deformations and metamorphism, crops out. Among them, there are some tectonic units (Parautochthonous Units, i. e. parautochtone of previous studies), that represent fragments of the continental Corse basement (Palaeozoic granitoids and associated volcanic and metamorphic pre‐Carboniferous rocks) and of its Mesozoic to Tertiary sedimentary cover, that are tectonically sliced between the allochthonous Ligurian‐Piedmontese Units (Schistes Lustrés) and the autochthonous basement (Variscan Corsica). The reconstructed polyphase deformation and metamorphic evolution of such units and the finding of high‐pressure/low‐temperature mineral assemblages in the continental‐derived tectonic slices, points to the involvement of the south‐eastern border of the European basement of Corsica in the tectonic processes linked to the Alpine subduction. Copyright © 2009 John Wiley & Sons, Ltd.

SHRIMP U-Pb zircon age constraints on the Late Archaean tectonostratigraphic architecture of the Eastern Goldfields Superterrane, Yilgarn Craton, Western Australia

Reassessment of Late Archaean tectonostratigraphic architecture of the Eastern Goldfields Superterrane is based on 35 new SHRIMP U-Pb zircon ages of supracrustal sequences. The superterrane comprises at least five terranes with ∼2810 to ≤2657 Ma volcanic and sedimentary rocks amalgamated by ∼2660 Ma. Each terrane has distinctive stratigraphy, volcanic facies, geochemistry and timing of volcanism. The Kalgoorlie Terrane comprises a >2715 to ∼2692 Ma plume-influenced, but deep-marine, mafic-ultramafic succession (Kambalda Sequence) overlain unconformably by a 2686 ± 5 to ≤2666 ± 6 Ma arc-related, deep-marine felsic volcaniclastic-shale succession of tonalite-trondjemite-diorite (TTD) affinity (Kalgoorlie Sequence). Felsic volcaniclastic rocks of the Agnew Domain and northern Yandal Belt are included in the Kalgoorlie Terrane, based on their similar age and geochemistry. The terrane also contains enclaves of >2740 Ma volcanic sequences that are remnants of autochthonous basement to the Kambalda Sequence. Arc-related bimodal and intermediate volcanic rocks of the Gindalbie Terrane, ranging in age from 2693 ± 4 to 2676 ± 5 Ma, are coeval with but geochemically distinct from the Kalgoorlie Sequence. They overlie sequences similar in age and composition to volcanic rocks of the Kurnalpi Terrane. The Kurnalpi Terrane comprises deep-marine, calc-alkalic mafic and intermediate volcanic rocks (Kurnalpi Sequence) ranging in age from 2715 ± 5 to 2704 ± 4 Ma, overlain conformably by mafic to ultramafic lavas (Minerie Sequence). Although of overlapping age, arc-related rocks in the Kambalda and Kurnalpi-Minerie Sequences are geochemically different. SHRIMP dating has identified older volcanic sequences in the Laverton (≤2808 ± 8 Ma) and Duketon-Burtville (2805 ± 5 Ma) Terranes, with komatiite-hosted Ni-sulphide deposits at Windarra, in the Laverton Sequence, linked to mafic-ultramafic magmatism ∼100 million years older than in the Kalgoorlie Terrane. Older sequences in the Laverton Terrane are autochthonous basement to the Kurnalpi Sequence. Synorogenic siliciclastic sequences occur as erosional remnants either in synclinal structures or adjacent to terrane-scale faults, and overlie a regional-scale unconformity. Detrital zircon ages indicate deposition after 2657 ± 4 Ma, placing important age constraints on the principal shortening event (D 2) and peak metamorphism. Detrital zircon ages also indicate older sources at ∼2730, ∼2800, ∼2950, ∼3000 Ma and, less commonly, >3400 Ma. Many zircon age-populations correspond to known supracrustal and crustal rocks in the Yilgarn Craton, but some have no known source in the Eastern Goldfields Superterrane. Superterrane-wide distribution and evidence for erosion of all crustal and supracrustal rocks ≥2660 Ma in age requires deposition of these sequences after terrane amalgamation, most likely synorogenic to collision between the Eastern Goldfields Superterrane and the Yilgarn Craton.

100 years Jura décollement hypothesis: how it affects Steinmann’s (1892) “Schwarzwaldlinie”

Steinmann, then professor of geology at Freiburg (Germany), more than a 100 years ago wondered about the southern end of the extensional Rhinegraben and proposed that elements of the graben penetrated the contractional Jura. In particular, he recognized the "Schwarzwaldlinie” in the southern prolongation of the eastern border of the southern Rhinegraben, a line-up of topographic as well as structural irregularities. He conjectured that it was caused by normal faults of the Rhinegraben system. Subsequently—100 years ago—Buxtorf (1907) proposed the hypothesis, that the Jura was a thin-skinned nappe sheared off on Triassic evaporites. In the autochthonous basement underneath the wrinkled skin, the ``Schwarzwald line” is difficult to define. It probably consists of a gentle flexure punctuated by faults that approximately coincides with Steinmann’s original projection, although he sought to identify its constituent faults in the badly deformed allochthonous skin. Current data place the thin-skin elements of the Schwarzwald line in a more westerly, allochthonous position where most of them were reactivated into sinistrally transpressional structures.

Age and origin of the Årdal dike complex, SW Norway: False isochrons, incomplete mixing, and the origin of Caledonian granites in basement nappes

Granitic dikes intruding the upper thrust sheets of the Jotun Nappe Complex, southwest Norway, previously assigned a Sveconorwegian age based on Rb‐Sr data, are redated to 427 ± 1 Ma by U‐Pb. The erroneous Rb‐Sr age can be explained by mixing of magmas with different isotopic compositions. Similar geochemical signatures, evidence of Sr mixing, inherited Proterozoic zircons and synkinematic modes of emplacement are also seen in Silurian granites in the Lifjorden Complex as well as in the Lindås Nappe and the Totland unit of the Bergen Arc Complex. We propose that these granites are related and derived from anatexis of Rb‐poor sediments of Baltica affinity overridden by the nappes during Scandian thrusting. This interpretation is compatible with the presence of the granites in nappes of both oceanic and of continental affinities and their absence in the autochthonous basement.

Successive Paleocene and Eocene infillings of polyphase paleokarsts within the Cretaceous limestones of the Empordà thrust sheets (Catalan Pyrenees, Spain) : relationships between tectonics and karstification

Abstract The Mesozoic series of the southern units of the Pyrenean Empordà thrust sheets (Montgrí and Figueres nappes, Catalonia, Spain) were finally emplaced over the autochthonous basement and its Cenozoic cover during Eocene times. However, they have originally been folded by the “Laramian” compressional event (Late Cretaceous/Early Paleocene), while they were still in their root zone more than 50 km to the N-NE. Postdating the Santonian, the emersion of the Cretaceous tectorogen induced karst formation at the expense of Berriasian to Santonian limestone sequences. Karst cavities of this paleokarst 1 (lapiaz and canyons) were subsequently coated with a fine, red or black, Microcodium-bearing, continental silt, and infilled with marine chaotic breccias. Following a new episode of emersion then erosion, the original paleokarst 1 was cross-cut by newly formed cavities of the paleokarst 2, filled with Lutetian-Bartonian marine breccias. Both types of marine breccias (Paleocene then Eocene in age) are now relatively well dated by means of planktonic foraminifera (Globigerinacea) occurring within the argillaceous-sandy matrix, and for the older ones, within the argillaceous-sandy or carbonate, finely laminated, interbedded hemipelagites, that mark the top of marine sequences tens of centimetres thick. The relationships of the “Laramian” and “Pyrenean” compressional tectonic events, occurring from latest Cretaceous to Bartonian, with the development of paleokarsts 1 and 2 are analysed in the perspective of the progressive southwards emplacement of the Montgrí thrust sheet, during Eocene time.

Collision leading to multiple-stage large-scale extrusion in the Qinling orogen: Insights from the Mianlue suture

The geologic framework of the Phanerozoic Qinling–Dabie orogen was built up through two major suturing events of three blocks. From north to south these include the North China craton (including the north Qinling block), the Qinling–Dabie microblock, and the South China craton (including the Bikou block), separated by the Shangdan and Mianlue sutures. The Mianlue suture zone contains evidence for Mesozoic extrusion tectonics in the form of major strike–slip border faults surrounding basement blocks, a Late Paleozoic ophiolite and a ca. 240–200 Ma thrust belt that reformed by 200–150 Ma thrusts during A-type (intracontinental) subduction. The regional map pattern shows that the blocks are surrounded by complexly deformed Devonian to Early Triassic metasandstones and metapelites, forming a regional-scale block-in-matrix mélange fabric. Five distinct tectonic units have been recognized in the belt: (1) basement blocks including two types of Precambrian basement, crystalline and transitional; (2) continental margin slices including Early Paleozoic strata, and Late Paleozoic fluviodeltaic sedimentary rocks, proximal and distal fan clastics, reflecting the development of a north-facing rift margin on the edge of the South China plate; (3) out of sequence oceanic crustal slices including strongly deformed postrift, deep-water sedimentary rocks, sheeted dikes, basalts, and mafic–ultramafic cumulates of a Late Paleozoic ophiolite suite, developing independent of the rift margin in a separate basin; (4) out-of-sequence island-arc slices; (5) accretionary wedge slices. All the tectonic units were deformed during three geometrically distinct deformation episodes (D 1, D 2 and D 3 during 240–200 Ma). Units 2–4 involved southward thrusting and vertical then southward extrusion of about 20 km of horizontal displacement above the autochthonous basement during the D 1 episode. Thrust slices 20 km south of the Mianlue suture are related to this vertical extrusion due to the same rock assemblages, ages and kinematics. The D 2 and D 3 episodes folded all the units in a thick-skinned style about east–west (D 2) and west–northwest (D 3) axes in the Mianlue suture zone. An early foreland propagating sequence of accretion of Late Paleozoic rocks deposited above the Yangtze craton is not involved in D 1 deformation but is temporally equivalent to the D 2 and D 3 deformation in the Mianlue suture. Two stages of strike–slip faulting mainly occurred at the end of D 2 and D 3, respectively. During D 2 deformation, the Bikou block was obliquely indented to the ESE into the Mianlue suture, rather than being thrust over the Mianlue suture from the north as a part of the Qinling–Dabie microblock. During D 3 deformation, however, the Bikou block was bounded by the south boundary fault of the Mianlue suture, and the Yangpingguan fault on the south. These faults are coeval strike–slip faults, but of opposite senses, and accommodated minor southwestward extrusion of the Bikou block into Songpan–Ganze orogen. The other basement blocks north of the Mianlue suture were extruded eastward by about 20 km of lateral displacement, based on the offset of the Wudang dome, during the D 3 episode due to the northeastward indentation of the Hannan complex of the South China craton. Post-D 3 emplacement of granite, cutting across the strike–slip faults such as the Mianlue suture, provides a minimum age of 200 Ma for D 3 deformation. Therefore, based on insights from the evolution of the Mianlue suture, the D 2 and D 3 episodes in the Mianlue suture and its neighbors are not responsible for and associated with the two-stage extrusion of the Dabie UHP-HP terranes from the Foping dome to the present erosional surface (more than 350 km).

Ongoing tectonic reactivation of the Outer Carpathians and its impact on the foreland: Results of borehole breakout measurements in Poland

The direction of present-day maximum horizontal compression (= S Hmax = tectonic stress) was interpreted for 62 wells in Poland, using the method of borehole breakout analysis of 4-arm and 6-arm dipmeter logs. The study area covers complex tectonic junction of the Carpathian orogen and its foreland, which comprises the East European Craton (EEC) divided by the Teisseyre–Tornquist zone (TTZ) from the Palaeozoic platform of western Poland. For this area, frequent deviation of the S Hmax direction from NW–SE characteristic for the Atlantic ridge push has been interpreted in terms of the ALCAPA tectonic push. In the Upper Silesian segment of the Polish Outer Carpathians (POC), NNE–SSW-oriented S Hmax in the accretionary wedge differs significantly from NNW–SSE S Hmax in the autochthonous basement. The above discrepancy points to uncoupled type of the suture in this segment of the POC. In this scenario the ALCAPA push involves the nappes and is compensated in the top of the basement, which is expressed by systematic S Hmax rotations. In the accretionary wedge of the eastern Małopolska segment, S Hmax follows the trend perpendicular to the strike of nappes. It is in general agreement with NNE–SSW-oriented S Hmax in the autochthonous basement that also parallels the ALCAPA push direction. Similarity in stress orientation between these structural levels implies coupled suture zone in this segment of the POC. Further to the north, ALCAPA push is transmitted into the foreland plate where it causes N–S orientation of S Hmax, as determined for sedimentary cover of the EEC. Within the Baltic portion of the EEC, further S Hmax rotation towards the intermediate NNW–SSE position suggests balance between the ALCAPA push and the ridge push components. Within the TTZ, common S Hmax rotations from N–S to NW–SE indicate structurally controlled accommodation of the ALCAPA push. In the Palaeozoic platform of western Poland, Mesozoic complex of the Fore-Sudetic Monocline reveals NNE–SSW-directed S Hmax that differs from NW–SE-oriented S Hmax in the Variscan accretionary wedge. Here, mechanical decoupling along Zechstein evaporates is likely. Presented set of breakout data from Poland shows that recent compressive reactivation of the Carpathians exerts strong impact on the stress field of the foreland plate at a distance of 700 km from the suture. Due to this effect, the Fore-Carpathian stress domain within the North European stress province can be discriminated.

Crustal properties from S-wave and gravity data along a seismic refraction profile in Romania

VRANCEA'99 is a seismic refraction line that was carried out in 1999 to investigate the deep structure and physical properties of the upper lithosphere of the southeastern Carpathians and its foreland. It runs from the city of Bacau to the Danube River, traversing the Vrancea epicentral area of strong intermediate-depth seismicity and the city of Bucharest. Interpretation of P-wave arrivals led to a velocity model that displays a multi-layered crust with velocities increasing with depth. The range of P wave velocities in the sedimentary cover increases from N to S and a structuring of the autochthonous basement of the Moesian Platform is observed. The crystalline crust displays thickness variations, but at the same time the lateral velocity structure along the seismic line remains almost constant. An intra-crustal boundary separates an upper crust from the lower crust. Within the upper mantle a low velocity zone is detected at a depth of about 55-km. The interpretation of observable S-waves resulted in a velocity model that shows the same multi-layered crust, with S-velocities increasing similarly with depth as the P-waves. The corresponding Poisson's ratio is highly variable throughout the crust and ranges from 0.20–0.35 for the sedimentary cover to 0.22–0.25 for the crystalline crust. The interpretation of the V p, V s and Poisson's ratio in petrological terms suggests a large variety of rocks from sand and clay to sandstone, limestone and dolomite within the sedimentary cover. Within the crystalline crust the most probably rock types are granite, granodiorite, granite–gneiss and/or felsic amphibolite–gneiss in the upper part and gneiss and /or amphibolite in the lower part. Based on the 2-D seismic velocity model, a density model is developed. Density values are assigned to each layer in agreement with the P-wave velocity model and with values accepted for the geological units in the area. After several iterations a good fit between the computed and observed Bouguer anomalies was obtained along the seismic line.

The electrical resistivity structure of Archean to Tertiary lithosphere along 3200 km of SNORCLE profiles, northwestern Canada

Magnetotelluric (MT) measurements to image the three-dimensional resistivity structure of the North American continent from an Archean core to a region of Tertiary assembly were recorded at almost 300 sites along 3200 km of profiles on the Lithoprobe Slave – Northern Cordillera Lithospheric Evolution (SNORCLE) transect in northwestern Canada. At the largest scale, the MT results indicate significant lithospheric thickness variation, from 260 km at the southwest margin of the Slave craton to significantly < 100 km at the southwestern end of the SNORCLE transect in the Cordillera. At intermediate scale, the resistivity results allow broad terrane subdivisions to be made. Several anomalously conductive zones along the SNORCLE transect, in rocks ranging in age from Archean to Tertiary, are attributed to the introduction of either water or carbon into the crust and mantle during subduction processes. At the local scale, the MT data image two major faults crossing the study area, the Great Slave Lake shear zone and the Tintina Fault. The resistivity images show that both the Tintina Fault and Great Slave Lake shear zone form crustal-scale features, and that the Tintina Fault has a remarkably uniform resistivity signature over a 400 km strike length in the study area. Arguably the most controversial conclusion reached is that the MT data do not support the western extension of North American autochthonous basement suggested from interpretation of the seismic reflection data.

Integrating seismic reflection and geological data and interpretations across an internal basement massif: The southern Appalachian Pine Mountain window, USA

Research Article| May 01, 2005 Integrating seismic reflection and geological data and interpretations across an internal basement massif: The southern Appalachian Pine Mountain window, USA John H. McBride; John H. McBride 1Department of Geology, Brigham Young University, P.O. Box 24606, Provo, Utah 84602-4606, USA Search for other works by this author on: GSW Google Scholar Robert D. Hatcher, Jr.; Robert D. Hatcher, Jr. 2Department of Earth and Planetary Sciences, University of Tennessee, 306 Earth and Planetary Sciences Building, Knoxville, Tennessee 37996-1410, USA Search for other works by this author on: GSW Google Scholar William J. Stephenson; William J. Stephenson 3U. S. Geological Survey, M.S. 966, P.O. Box 25046, Denver, Colorado 80225, USA Search for other works by this author on: GSW Google Scholar Robert J. Hooper Robert J. Hooper 4ConocoPhillips, P.O. Box 2197, Houston, Texas 77252-2197, USA Search for other works by this author on: GSW Google Scholar Author and Article Information John H. McBride 1Department of Geology, Brigham Young University, P.O. Box 24606, Provo, Utah 84602-4606, USA Robert D. Hatcher, Jr. 2Department of Earth and Planetary Sciences, University of Tennessee, 306 Earth and Planetary Sciences Building, Knoxville, Tennessee 37996-1410, USA William J. Stephenson 3U. S. Geological Survey, M.S. 966, P.O. Box 25046, Denver, Colorado 80225, USA Robert J. Hooper 4ConocoPhillips, P.O. Box 2197, Houston, Texas 77252-2197, USA Publisher: Geological Society of America Received: 24 Dec 2002 Revision Received: 03 Sep 2003 Accepted: 13 May 2004 First Online: 02 Mar 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (2005) 117 (5-6): 669–686. https://doi.org/10.1130/B25313.1 Article history Received: 24 Dec 2002 Revision Received: 03 Sep 2003 Accepted: 13 May 2004 First Online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation John H. McBride, Robert D. Hatcher, William J. Stephenson, Robert J. Hooper; Integrating seismic reflection and geological data and interpretations across an internal basement massif: The southern Appalachian Pine Mountain window, USA. GSA Bulletin 2005;; 117 (5-6): 669–686. doi: https://doi.org/10.1130/B25313.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract The southern Appalachian Pine Mountain window exposes 1.1 Ga Grenvillian basement and its metasedimentary Paleozoic(?) cover through the allochthonous Inner Piedmont. The issue of whether the crustal block inside the window was either transported above the master Appalachian (late Alleghanian) décollement or is an autochthonous block that was overridden by the décollement has been debated for some time. New detrital zircon geochronologic data from the cover rocks inside the window suggest this crustal block was derived from Gondwana but docked with Laurentia before the Alleghanian event. Reprocessed deep seismic reflection data from west-central Georgia (pr- and poststack noise reduction, amplitude variation analysis, and prestack depth migration) indicate that a significant band of subhorizontal reflections occurs almost continuously beneath the window collinear with the originally recognized décollement reflections north of the window. A marked variation in the décollement image, from strong and coherent north of the window to more diffuse directly beneath the window, is likely a partial consequence of the different geology between the Inner Piedmont and the window. The more diffuse image beneath the window may also result from imaging problems related to changes in topography and fold of cover (i.e., signal-to-noise ratio). Two alternative tectonic models for the Pine Mountain window can partially account for the observed variation in the décollement reflectivity. (1) The Pine Mountain block could be truncated below by a relatively smooth continuation of the décollement. The window would thus expose an allochthonous basement duplex or hors-block thrust upward from the south along the Late Proterozoic rifted continental margin. (2) The window represents localized exhumation of autochthonous basement and cover along a zone of distributed intrabasement shearing directly beneath the window. Either model is viable if only reflector geometry is considered; model (1) is favored if both geometry and kinematics of Blue Ridge–Piedmont thrust sheet emplacement are incorporated. In either model, the southern margin of the window merges to the west with the Iapetan early Alleghanian Central Piedmont suture, which juxtaposes North American–affinity Piedmont rocks to the north and exotic Panafrican rocks of the Carolina (Avalon) terrane to the south. Immediately south of the window, this suture dips southward and merges in the lower crust with the late Alleghanian suture joining the Appalachians with Gondwana. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Cambrian sediments and Proterozoic granites in the Dividalen-Torneträsk area, northern Scandinavia: Palaeomagnetism and U-Pb geochronology

New palaeomagnetic data from the autochthonous Early Cambrian Dividal Group (northern Scandinavia) confirm earlier findings, and a refined palaeomagnetic pole of 58.4[ddot]N and 122.5[ddot]E, places Baltica at intermediate southerly latitudes at ∼535 Ma. Palaeomagnetic data from the immediately underlying granitic basement (pole: 9.8[ddot]N, 226.7[ddot]E) differ markedly from the Dividal sediments. The result of this fieldtest increases the palaeomagnetic reliability of the Dividal Group results. We dated the granitic basement in the eastern part of the Torneträsk area (corresponding to our palaeomagnetic site) to 1786±4 Ma (U-Pb zircon and titanite), whilst deformed allochthonous granites west of lake Torneträsk are slightly older (1800±4 Ma; U-Pb zircon). These ages are compatible with autochthonous basement ages to the east of the study area, but also with ages from basement windows to the west. Preliminary palaeomagnetic data from the 1786±4 Ma granitic basement are clearly pre-Phanerozoic in origin, and comparable and concordant zircon and titanite ages may hint that the magnetisation could be primary. However, the palaeomagnetic pole does not match contemporaneous poles from Baltica, which suggest a tectonic explanation (no palaeohorizontal control), a problem of secular variation (only one site) or a younger but pre-Phanerozoic remagnetisation event, which did not affect the U-Pb system in zircon and titanite.

From Rifted Margins to Foreland Basins: Investigating Provenance and Sediment Dispersal Across Desert Arabia (Oman, U.A.E.)

Arid Arabia, where chemical weathering and anthropic modifications are negligible, is a huge natural laboratory for the study of sediment provenance. Mafic and ultramafic detritus from ophiolite belts is found along the Oman Mountains and on Masirah Island (obduction-orogen provenance). Dominant cellular serpentinite grains from residual mantle harzburgites of the Sama'il ophiolite contrast with gabbroic, diabase, and basaltic grains from igneous crustal rocks of the Masirah ophiolite. Detritus from autochthonous basement and cover rocks is found along the Arabian Sea margin (rift-shoulder provenance). Transition from carbonate-dominated lithic sands (undissected stage) to hornblende-rich plagioclase arkoses (dissected stage) is best documented at the northern edge of the Gulf of Aden. Along the older, strike-slip Owen-basin margin, only locally are feldspars, hornblende, and garnet derived from small outcrops of Pan-African basement. Modern eolian sands from Yemen to the Arabian (Persian) Gulf display marked enrichment in quartz, low plagioclase/feldspar ratios, and depleted dense-mineral assemblages with rounded epidotes, hornblende, garnet, and ultrastables, pointing to extensive recycling also fostered by Quaternary changes. The Rub'-Al-Khali and Wahibah sand seas have distinct quartz/feldspar ratios, reflecting largely separate transport pathways. Dune fields, fed chiefly from rift highlands in the west and south, mix with orogenic detritus in the northeast. Carbonate-dominated sands in coastal UAE are mostly derived from the Zagros Mountains, blown by northerly Shamal winds across the Gulf during Pleistocene lowstands to reach the backbulge depozone and the foreland beyond. Contributions from the Oman Mountains reach only 5-10% of bulk sediment along their southern apron, and mostly indirectly through deflation of Plio-Pleistocene fans. The Sama'il ophiolite supplies enstatite and diopside to the eastern UAE and Wahibah sands; the Masirah ophiolite supplies amphiboles and clinopyroxenes locally along the Batain coast. The composition of desert sediments faithfully mirrors tectonic history of source terranes, deformation style during orogeny, and unroofing level during extension. Bulk petrography coupled with dense-mineral analysis thus represents a powerful tool to investigate not only provenance and sediment transport but also crustal dynamics and geologic evolution at continental scale.