Eastward Thrusting Research Articles

Termination of the Ganderian Cambrian–Ordovician Miramichi terrane in east-central Maine, northern Appalachian orogen, USA

The Ganderian Cambrian–Ordovician Miramichi terrane narrows in east-central Maine and terminates at the junction of faults that separate it from the mostly Silurian Central Maine/Aroostook–Matapedia basin (CMAM) to the northwest and Fredericton trough to the southeast. The terrane was emergent after Middle Ordovician recumbent folding and shed sediment to both adjacent depocenters. Its boundary faults are the youngest deformation events and play important roles in its termination, but do not by themselves explain it. The presence of distinctive CMAM strata southeast of the northwest boundary fault indicates that the first step in developing the current relationships was an episode of hitherto unrecognized late Silurian eastward thrusting. In the northern (Danforth) segment of the terrane, intermediate facies CMAM strata were thrust onto their Miramichi source rocks. The thrust sheet was deformed by Acadian upright folds, then dissected by dip-slip offset along boundary and internal faults prior to intrusion of the 409 ± 2 Ma Skiff Lake pluton. Subsequent erosion isolated a remnant of the thrust sheet as the Dill Hill klippe, its allochthonous CMAM strata isolated among Miramichi rocks. The southern (Greenfield) segment experienced similar events, but current relationships are different and timing of the late-stage faults is not well constrained. Allochthonous CMAM strata may have overridden the Miramichi terrane completely, so that a remnant of distinctive CMAM strata is now exposed east of the Miramichi terrane in fault contact with rocks of the Fredericton trough. The entire Greenfield segment is interpreted as a fault block exposed within the thrust sheet.

Diachronous collision in the Seve Nappe Complex: Evidence from Lu–Hf geochronology of eclogites (Norrbotten, North Sweden)

AbstractThe collision of Baltica and Laurentia during the Caledonian Orogeny happened atc. 400–420 Ma. However, subduction and collision processes also took place before this main collisional phase and the tectonic history of these is still not fully resolved. The Seve Nappe Complex in Sweden has recorded these earlier phases. The Seve Nappe Complex in Norrbotten (North Swedish Caledonides) comprises four superimposed nappes emplaced by eastward thrusting (from base to top according to the conventional structural interpretation): Lower Seve Nappe, Vaimok, Sarek, and Tsäkkok Lenses. Eclogites occur in the Vaimok and Tsäkkok Lenses. The Vaimok Lens represents rocks of the Baltican continental margin intruded by Neoproterozoic dolerite dikes which were later eclogitized and boudinaged. By contrast, eclogites of the Tsäkkok Lens are former oceanic basalts associated with calcschists, possibly representing the ocean–continent transition between Baltica and Iapetus. Previous age determinations for eclogitization yielded various ages betweenc. 500 and 480 Ma, in contrast to younger (460–450 Ma) ages of ultra high‐Pmetamorphism in the Seve Nappe Complex further south in Jämtland. Eclogites from the Vaimok (one sample) and Tsäkkok (three samples) lenses were dated using Lu–Hf garnet geochronology. Garnet from all samples shows prograde zoning of major element and Lu contents and yielded well‐defined isochrons of the following ages: 480.4 ± 1.2 Ma (Vaimok); 487.7 ± 4.6 Ma, 486.2 ± 3.2, 484.6 ± 4.6 Ma (Tsäkkok). The ages from Tsäkkok are interpreted to date the burial of the Iapetus–Baltica ocean–continent transition in a west‐dipping subduction zone aroundc. 485 Ma, and the age from the structurally deeper Vaimok Nappe the following subduction of the continental margin. Previously reported ages of 500 Ma and older are not supported by this study. The age difference between eclogites in the Seve Nappe Complex in Jämtland (c. 460–450 Ma) and Norrbotten (c. 488–480 Ma) may reflect the collision of an island arc with an irregularly shaped passive continental margin of Baltica or alternatively the collision of a straight margin with a microcontinent (Sarek Lens) accreted to the upper plate.

Regional subsurface mapping and 3D flexural modeling of the obliquely converging Putumayo foreland basin, southern Colombia

The Putumayo foreland basin (PFB) is an underexplored hydrocarbon-bearing basin located in southernmost Colombia. The PFB forms a 250 km long segment of the 7000 km long corridor of Late Cretaceous-Cenozoic foreland basins produced by eastward thrusting of the Andean mountain chain over Precambrian rocks of the South American craton. We have used approximately 4000 km of 2D seismic data tied to 28 exploratory wells to describe the basin-wide structure and stratigraphy. Based on seismic interpretation and comparison with published works from the southward continuation of the PFB into Peru and Ecuador, three main across strike, structural zones include (1) the 20 km wide, western structural zone closest to the Andean mountain front characterized by inversion of older, Jurassic half-grabens during the Late Miocene, (2) the 45 km wide, central structural zone characterized by moderately inverted Jurassic half-grabens, and (3) the 120 km wide, eastern structural zone (ESZ) characterized by the 40 km wide, north–south-trending Caquetá arch. The five mainly clastic tectonosequences of the PFB include (1) Lower Cretaceous pre-foreland basin deposits, (2) upper Cretaceous-Paleocene foreland basin deposits, (3) Eocene foreland basin deposits related to the early uplift of the eastern Cordillera, (4) Oligocene-Miocene underfilled, foreland basin deposits, and (5) Plio-Pleistocene overfilled, foreland basin deposits. We used 3D flexural modeling to identify the elastic thickness ([Formula: see text]) of the lithosphere below the PFB, and to model the location of the sedimentary-related and tectonically related forebulges of Cretaceous to Oligocene age. Flexural analysis finds two pulses of rapid, foreland-related subsidence first during the Late Cretaceous-early Paleocene and later during the Oligocene-Miocene. Despite the present-day oblique thrusting of the mountain front, flexure of the PFB basement has produced a tectonic forebulge now located in ESZ and controls a basement high that forms the eastern updip limit for most hydrocarbons found in the PFB.

Structural, metamorphic and geochronological insights on the Variscan evolution of the Alpine basement in the Belledonne Massif (France)

A structural and petrochronological study was carried out in the southern part of the Belledonne crystalline massif. A first tectonometamorphic event, Dx, corresponds to the eastward thrusting of the Chamrousse ophiolitic complex characterized by a low-temperature–moderate-pressure metamorphism reaching 0.535 ± 0.045 GPa and 427.5 ± 17.5 °C. A subsequent D1 deformation is defined by a penetrative S1 foliation that mostly dips toward the west and displays an E–W- to NE–SW-trending mineral and stretching lineation L1. D1 is associated with a top-to-the east shearing and is responsible for the crustal thickening accommodated by the eastward nappe stacking and the emplacement of the Chamrousse ophiolitic complex upon the Rioupéroux-Livet unit. This event is characterized by an amphibolite facies metamorphism (0.58 GPa ± 0.06; 608 ± 14 °C) that attains partial melting at the base of the nappe pile (0.78 ± 0.07 GPa; 680.5 ± 11.5 °C). LA-ICP-MS U-Pb dating of monazite grains from the mica schists of the Rioupéroux-Livet unit constrain the age of D1 to 337 ± 7 Ma. The D2 tectono-metamorphic event is characterized by NE–SW trending, upright to NE-verging synfolial folding. Folding associated with D2 is pervasively developed in all lithotectonic units with the development of a steeply-dipping S2 foliation. In particular, D2 involves the uppermost weakly metamorphosed Taillefer unit. LA-ICP-MS U-Pb dating performed on detrital zircon grains shows that the Taillefer conglomerates was deposited during the Visean. A zircon SIMS U-Pb age of 352 ± 1 Ma from a plagioglase-rich leucocratic sill of the Rioupéroux-Livet unit is interpreted as the age of magmatic emplacement. Our results suggest that the D2 event took place between 330 Ma and 310 Ma. We propose a new interpretation of the tectonometamorphic evolution of the southern part of the Belledonne massif, focusing on the Middle Carboniferous stages of the Variscan orogeny.

Kinematics and significance of a poly-deformed crustal-scale shear zone in central to south-eastern Madagascar: the Itremo–Ikalamavony thrust

Across the crystalline basement of Madagascar, late Archaean rocks of the Antananarivo Block are tectonically overlain by Proterozoic, predominantly metasedimentary units of the Ikalamavony and Itremo Groups of the Southwest Madagascar Block. The generally west-dipping tectonic contact can be traced for more than 750 km from NW to SE and is referred to here as the Itremo–Ikalamavony thrust. The basal units of the SW Madagascar Block comprise metasedimentary quartzites with the potential to preserve a multistage deformation history in their microstructures. Previous studies suggest contrasting structural evolutions for this contact, including eastward thrusting, top-to-the-west directed extension and right-lateral strike-slip deformation during the late Neoproterozoic/Ediacaran. In this study, we integrate remote sensing analyses, structural and petrological fieldwork, as well as microstructural investigations of predominantly quartz mylonites from the central southern segment of the contact between Ankaramena and Maropaika. In this area, two major phases of ductile deformation under high-grade metamorphic conditions occurred in latest Neoproterozoic/early Phanerozoic times. A first (Andreaba) phase produces a penetrative foliation, which is parallel to the contact between the two blocks and contemporaneous with widespread magmatism. A second (Ihosy) phase of deformation folds Andreaba-related structures. The investigated (micro-)structures indicate that (a) juxtaposition of both blocks possibly already occurred prior to the Andreaba phase, (b) (re-)activation with top-to-the-east thrusting took place during the latest stages of the Andreaba phase, (c) the Ihosy phase resulted in regional-scale open folding of the tectonic contact and (d) reactivation of parts of the contact took place at distinctively lower temperatures post-dating the major ductile deformations.

Neotectonics of the Eastern Margin of the Tibetan Plateau: New Geological Evidence for the Change from Early Pleistocene Transpression to Late Pleistocene‐Holocene Strike‐slip Faulting

AbstractWe present in this paper some new evidence for the change during the Quaternary in kinematics of faults cutting the eastern margin of the Tibetan Plateau. It shows that significant shortening deformation occurred during the Early Pleistocene, evidenced by eastward thrusting of Mesozoic carbonates on the Pliocene lacustrine deposits along the Minjiang upstream fault zone and by development of the transpressional ridges of basement rocks along the Anninghe river valley. The Middle Pleistocene seems to be a relaxant stage with local development of the intra‐mountain basins particularly prominent along the Minjiang Upstream and along the southern segment of the Anninghe River Valley. This relaxation may have been duo to a local collapse of the thickened crust attained during the late Neogene to early Pleistocene across this marginal zone. Fault kinematics has been changed since the late Pleistocene, and was predominated by reverse sinistral strike‐slip along the Minshan Uplift, reverse dextral strike‐slip on the Longmenshan fault zone and pure sinistral strike‐slip on the Anninghe fault This change in fault kinematics during the Quaternary allows a better understanding of the mechanism by which the marginal ranges of the plateau has been built through episodic activities.

The Late Neoproterozoic/Early Palaeozoic evolution of the West Congo Belt of NW Angola: geochronological (U‐Pb and Ar‐Ar) and petrostructural constraints

Terra Nova, 24, 238–247, 2012AbstractU‐Pb zircon and Ar‐Ar amphibole and biotite ages from rocks of the West Congo Belt of Angola indicate that this area underwent two main deformation events of amphibolite facies grade at c. 540 and 490 Ma, which were followed by tectonically assisted exhumation during eastward thrusting of the hinterland domain onto the foreland domain. High‐grade conditions in the West Congo Belt are 20–40 Ma younger than in the Ribeira–Araçuai Belt of Brazil, its South American counterpart, or than in the westernmost Kaoko Belt of Namibia, its African correlative. In the present state of knowledge, a more appropriate counterpart to the southern part of the West Congo Belt may be restricted to the Cabo Frio Terrane in the eastern Ribeira Belt, which yields a broadly similar evolution.

CONTRASTING SEQUENCES OF METAPELITIC MINERAL-ASSEMBLAGES IN THE AUREOLE OF THE TILTED NELSON BATHOLITH, BRITISH COLUMBIA: IMPLICATIONS FOR PHASE EQUILIBRIA AND PRESSURE DETERMINATION IN ANDALUSITE SILLIMANITE-TYPE SETTINGS

The Nelson Batholith is a ca. 1,800 km 2 Jurassic intrusive body in southeastern British Columbia surrounded by a contact aureole, 0.7–1.8 km wide, developed in graphitic argillaceous rocks that show only minor variations in bulk composition. Contrasting prograde sequences of mineral assemblages are developed around the aureole in a regular pattern, reflecting different pressures of contact metamorphism. The following assemblages are seen going from lower to higher pressure (all assemblages contain muscovite + biotite + quartz ± Mn-rich garnet): (1) cordierite-only assemblages, (2) mix of cordierite-only and cordierite + andalusite assemblages, locally with cordierite + K-feldspar and andalusite + K-feldspar assemblages at higher grade, (3) andalusite-only assemblages, with sillimanite + andalusite assemblages and locally sillimanite + K-feldspar assemblages at higher grade, (4) staurolite-only assemblages, (5) staurolite ± andalusite assemblages, with sillimanite-bearing and locally sillimanite + K-feldspar assemblages at higher grade. The higher-pressure sequences with staurolite ± andalusite are restricted to the aureole surrounding the east half of the batholith, whereas the lower-pressure cordierite ± andalusite are restricted to the aureole surrounding the west half of the batholith and its northern and southern tips. The sequences of mineral assemblages correspond closely to the facies series of Pattison & Tracy (1991) and are interpreted to represent a series of approximately isobaric metamorphic field-gradients below the Al2SiO5 triple point, providing an excellent opportunity to evaluate thermodynamically calculated low-pressure phase equilibria in the metapelitic system. The total difference in pressure represented by the contrasting assemblages is about 1.0 kbar, showing that they are a sensitive measure of small differences in pressure within the stability field of andalusite. Thermobarometry results from the aureole are moderately consistent with the mineral assemblage constraints, but carry pressure uncertainties larger than the total range of pressure represented by the aureole's assemblages. Pressures of the intrusive rocks derived from hornblende barometry are scattered, and many do not agree with the pressure constraints from the aureole. The mineral-assemblage constraints indicate down-to-the-west post-metamorphic tilting of the batholith and aureole of about 10°, interpreted to be due to a combination of eastward thrusting of the Nelson Batholith over crustal-scale ramps during Cretaceous–Paleocene shortening and Eocene east-side-down normal motion on the Slocan Lake – Champion Lakes fault system that forms the western boundary of the batholith.

Finite strain pattern in Andriamena unit (north-central Madagascar): evidence for late Neoproterozoic–Cambrian thrusting during continental convergence

This paper deals with the late Neoproterozoic–Cambrian tectonic evolution of a part of north-central Madagascar, which is characterized by the occurrence of a mafic-ultramafic sequence (the Andriamena unit) overlying a gneissic-granitic basement. The finite strain pattern has been determined by carrying out a SPOT satellite image analysis, structural mapping of specific areas and kinematic analyses of shear zones. Structural investigations reveal the presence of two superposed finite strain patterns, D1 and D2. The D1 event is related to the emplacement of the Andriamena unit on the top of the gneissic-granitic basement. The western contact between these units is a major mylonitic zone characterized by a non-coaxial strain regime consistent with a top-to-east displacement. We suggest that the Andriamena unit originated as a lower crustal fragment of a middle Neoproterozoic continental magmatic arc related to the closure of the Mozambique Ocean. This fragment was thrusted onto the gneissic-granitic basement after 630 Ma, i.e. the age of emplacement of characteristic stratoid granites found only in the lower unit. The D2 event is related to east-west horizontal shortening mainly accommodated by F2 upright folds. In-situ electron microprobe dating of monazites from the Andriamena unit constrains the age of the D1 and D2 events between 530 and 500 Ma under amphibolite to granulite-facies conditions (5–7 kbar, 650–700 °C). The eastward thrust emplacement of the Andriamena unit (D1) followed by the horizontal shortening (D2) are ascribed to the same Cambrian tectonic regime (i.e. east-west convergence). Such D1–D2 bulk strain pattern has been recognized throughout Madagascar and at various structural levels of the crust: in the lower crust in Southern Madagascar and in the uppermost crustal level in the SQC unit (central Madagascar). The D1–D2 event is interpreted to result from the continental convergence of the Australia–Antarctica block and the Madagascar, India, Sri Lanka block during the final amalgamation of Gondwana.

Pan-African deformations in the basement of the Negele area, southern Ethiopia

Polydeformed and metamorphosed Neoproterozoic rocks of the East African Orogen in the Negele area constituted three lithostructurally distinct and thrust-bounded terranes. These are, from west to east, the Kenticha, Alghe and Bulbul terranes. The Kenticha and Bulbul terranes are metavolcano-sedimentary and ultramafic sequences, representing parts of the Arabian–Nubian Shield (ANS), which are welded to the central Alghe gneissic terrane of the Mozambique Belt affinity along N–S-trending sheared thrust contacts. Structural data suggest that the Negele basement had evolved through three phases of deformation. During D1 (folding) deformation, north–south upright and inclined folds with north-trending axes were developed. East and west-verging thrusts, right-lateral shearing along the north-oriented Kenticha and Bulbul thrust contacts and related structural elements were developed during D2 (thrusting) deformation. The pervasive D1 event is interpreted to have occurred at 620–610 Ma and the D2 event ended prior to 554 Ma. Right-lateral strike-slips along thrust contacts are interpreted to have been initiated during late D2. During D3, left-lateral strike-slip along the Wadera Shear Zone and respective strike-slip movements along conjugate set of shear zones were developed in the Alghe terrane, and are interpreted to have occurred later than 557 Ma. The structural data suggest that eastward thrusting of the Kenticha and westward tectonic transport of the Bulbul sequences over the Alghe gneissic terrane of the Mozambique Belt, during D2, were accompanied by right-lateral strike-slip displacements along thrust contacts. Right-lateral strike-slip movements along the Kenticha thrust contact, further suggest northward movement of the Kenticha sequence during the Pan-African orogeny in the Neoproterozoic. Left-lateral strike-slip along the orogen-parallel NNE–SSW Wadera Shear Zone and strike-slip movements along a conjugate set of shear zones completed final terrane amalgamation between the Arabian–Nubian Shield and the Mozambique Belt in Neoproterozoic southern Ethiopia.

Ordovician magmatism, deformation, and exhumation in the Caledonides of central Norway: An orphan of the Taconic orogeny?

Research Article| October 01, 2002 Ordovician magmatism, deformation, and exhumation in the Caledonides of central Norway: An orphan of the Taconic orogeny? Aaron S. Yoshinobu; Aaron S. Yoshinobu 1Department of Geosciences, Texas Tech University, Lubbock, Texas 79409, USA Search for other works by this author on: GSW Google Scholar Calvin G. Barnes; Calvin G. Barnes 1Department of Geosciences, Texas Tech University, Lubbock, Texas 79409, USA Search for other works by this author on: GSW Google Scholar Øystein Nordgulen; Øystein Nordgulen 2Norwegian Geological Survey, Leiv Eirikssons 39, 7491 Trondheim, Norway Search for other works by this author on: GSW Google Scholar Tore Prestvik; Tore Prestvik 3Department of Geology and Mineral Resources Engineering, Norwegian University of Science and Technology, N-7491 Trondheim, Norway Search for other works by this author on: GSW Google Scholar Mark Fanning; Mark Fanning 4Research School of Earth Sciences, Australian National University, Canberra ACT-0200, Australia Search for other works by this author on: GSW Google Scholar R.B. Pedersen R.B. Pedersen 5Geological Institute, University of Bergen, Allegaten 41, N-5007 Bergen, Norway Search for other works by this author on: GSW Google Scholar Geology (2002) 30 (10): 883–886. https://doi.org/10.1130/0091-7613(2002)030<0883:OMDAEI>2.0.CO;2 Article history received: 23 Mar 2002 rev-recd: 08 Jun 2002 accepted: 19 Jun 2002 first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Aaron S. Yoshinobu, Calvin G. Barnes, Øystein Nordgulen, Tore Prestvik, Mark Fanning, R.B. Pedersen; Ordovician magmatism, deformation, and exhumation in the Caledonides of central Norway: An orphan of the Taconic orogeny?. Geology 2002;; 30 (10): 883–886. doi: https://doi.org/10.1130/0091-7613(2002)030<0883:OMDAEI>2.0.CO;2 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 SocietyGeology Search Advanced Search Abstract Magmatism, contractional deformation, and extension associated with the exhumation of high-pressure rocks in the Scandinavian Caledonides are commonly attributed to the Silurian-Devonian Scandian orogeny, in which eastward thrusting of allochthonous terranes over Baltica was followed by extensional collapse and exhumation. New fieldwork and U-Pb geochronology coupled with recent pressure-temperature estimates within the highest thrust sequence of the Caledonian orogen indicate that an earlier phase of west-directed contractional deformation was punctuated by migmatite-producing events and voluminous magmatism ca. 477–466 Ma and ca. 447 Ma, followed by exhumation in the Late Ordovician. Al-in-hornblende and GASP thermobarometry indicate that emplacement of a suite of 448–445 Ma plutons caused partial migmatization at pressures of 700–800 MPa. Subsequent isothermal exhumation to pressures of 400 MPa occurred while the host rocks were still partially molten. Rates of exhumation may have ranged from 2 to 11 mm·yr−1 or greater. These data provide evidence for a previously unrecognized phase of exhumation in the Caledonides and for aerially extensive west-vergent deformation. Deformation and magmatism associated with these events may be related to Taconic-age orogenesis near Laurentia, where the highest nappe sequences of the Scandinavian Caledonides probably resided during early Paleozoic time. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Lithospheric modeling of the Ordovician foreland basin in the Puna of northwestern Argentina: on the influence of arc loading on foreland basin formation

In a qualitative sedimentological model of the Ordovician basin in the Puna of northwestern Argentina, the temporal and spatial distribution of predominantly marine volcanoclastic facies was interpreted to indicate a basin evolution from extensionally related subsidence to that driven by crustal down flexure. Sedimentological data were taken to reflect a Middle Ordovician eastward thrusting of an extinct Early Ordovician arc over part of its back-arc basin in the course of the collision of the allochthonous Arequipa Massif terrane with the South American margin. To test the original qualitative sedimentological interpretation of the Puna basin, an infinite beam elastic plate loading model was applied using the constraints of the field data. Contrary to the original interpretation, results show that the Puna basin can be modeled as resulting entirely from the effects of lithospheric loading caused by the progressive contruction of a static load, i.e., a volcanic arc. Two loading events, one in the Early Ordovician, and one in the Llanvirnian, with tectonic load thicknesses of 6000 and 2000 m, respectively, best explain the observed basin geometry. The onlap patterns on the distal margin on the peripheral bulge are reproduced well using a flexural rigidity of 10 22 N m representing an equivalent elastic thickness of 14 km of the loaded plate. Higher values for the flexural rigidity are considered unlikely, as they result in an enhanced peripheral bulge which uplifts marine near-shore deposits on the distal margin to unrealistic subaereal positions. The model results fit a foreland basin development where crustal thickening was caused by the construction of a static volcanic arc on a previously passive margin. Modeling the movement of thrust loads over parts of the basin according to traditional models of foreland basin formation in this case leads to model results which are inconsistent with field data, even when moderate thrust rates are considered.

Seismic reflection profiling in the Boulder batholith, Montana

Seismic reflection profiling combined with gravity data allows more exact determination of the geometry of the controversial Boulder batholith of Montana, reveals laminated structure of the lower crust beneath the batholith, and identifies the Moho at a depth of 38 km. The batholith has inward-dipping contacts, the dip being about 50° on the west side, on the basis of seismic data; and the depth to the batholith floor is constrained between 12 and 18 km, indicating a great volume for the batholith. The Boulder batholith was emplaced between 80 and 70 Ma during an eastward thrusting in the fold-and-thrust belt. A presumed basal decollement of the thrust system might coincide with the batholith floor and may correspond to the top of the lower-crustal layering at a depth of 18 km.

Structure and Kinematics of a Bent Fold-and-Thrust Belt: The Oil-Bearing Outer Eastern Carpathians (Romania): ABSTRACT

The east-vergent fold-and-thrust belt of the Eastern Carpathians changes strike by 70{degrees} in the south. It formed from early Cretaceous to Recent times and contains the oil-productive province of Ploiesti. To understand orocline formation, a detailed kinematic analysis of folding-thrusting by means of mesoscale fault-slip analysis was carried out. Most stations revealed mainly subhorizontal contraction axes which fan around the bending zone from 110{degrees} in the N to 160{degrees} at the southwestern end of the Eastern Carpathians. Since this fanning pattern is consistent for rocks ranging from Lower Cretaceous to at least Miocene, its formation is interpreted as syn- or post-Miocene in age. The strain pattern probably resulted from the following processes: (a) Cretaceous to Oligocene thrusting and folding. Contractional axes representing this process trend normal to fold axes and were calculated from the oldest fault sets in outcrops exhibiting polyphase brittle deformation. (b) Miocene to Recent eastward thrusting and folding coeval with dextral wrenching in the southern part of the N-S-trending fold-and-thrust belt. The second process is characterized by curved strain trajectories and caused rotation of older structures. As Miocene and post-Miocene structures are strongly influenced by wrench tectonics new tectonic models are needed to guide exploration in themore » oil province of Ploiesti.« less

The Caledonian Heilhornet Pluton, north-central Norway: geological setting, radiometric age and implications for the Scandinavian Caledonides

The Heilhornet Pluton is a medium-sized ( c . 50 km 2 ), sub-circular, discordant intrusion which penetrates the thrust zone separating the Helgeland Nappe Complex from paragneisses in the Vestranden region, north-central Norway. The intrusion consists of biotite ± hornblende granite and ranges in composition from 61.5 to 72.5 wt.% Si0 2 The initial Sr-ratio ( c . 0.7070) shows that crustal sources have been involved in the petrogenesis of the granite. A U-Pb zircon age of 444 ± 11 Ma for the pluton indicates that juxtaposition of the nappe complex with metasedimentary rocks in Vestranden took place prior to or during the late Ordovician. A consequence of this is that the general correlation of the metasedimentary rocks in Vestranden with the Seve Nappes must be revised. Final eastward thrusting of the complex, together with a yet unknown part of the Vestranden supracrustal sequence, probably took place during the middle Silurian to early Devonian, Scandian, orogenic event.

Thomas Simpson (1808-1840)

Thomas Simpson was born in the north of Scotland and graduated from the University of Aberdeen with more than competence. He was enrolled in the service of the Hudson's Bay Company, and in the New World he exemplified the popular conception of the clever academic launched into society. He expressed the utmost contempt for his colleagues and in a letter assured his brother Alexander his talents would secure him speedy advancement. This arrogance made him most unpopular in the service and caused George Simpson, in doubt of his kinsman's fitness to command, to appoint Chief Factor Peter Warren to lead the expedition he was planning to extend the northern coastal survey earlier initiated by John Franklin and John Richardson. Although the leadership escaped him, Thomas Simpson was made responsible for the actual survey work, an arrangement worked admirably. ... Simpson proved his zeal and adaptability on this 1837 assignment. When the boats were blocked by ice, he took to the shore to do the work on foot; on seeing the ice had receded, he borrowed an umiak to finish the work. No one applauded his success more than himself: Mine alone is the victory, he wrote. Dease is an unworthy, indolent, illiterate soul. In the next season - 1838 - when an eastward thrust from Point Turnagain was halted by ice, Simpson landed and added 160 km to the map on foot. The summer of 1839 proved more friendly. They sailed through Simpson Strait, which divided King William Island from the continent, passed the estuary of Back's Fish River - to become grimly memorable 15 years later - and reached Boothia Isthmus at the mouth of the Castor and Polux River. The ruthless Simpson still did not spare the boat crews; overruling the kindly Dease, he slowed the return voyage to map parts of th south shores of King William and Victoria islands, forcing the crews to ascend the Mackenzie River in sub-zero weather with ice masses already floating downstream. ... On the American prairie, Simpson met a violent death has never been fully explained. ... Asked one evening whether they should encamp, he answered moodily that was just as the others chose. They were pitching the tent when Simpson shot two of them with his double-barrelled gun. He came forward declaring he had done no wrong, they had plotted to murder him. The two who remained mounted their horses and galloped off to join a wagon train was not far behind. The next day one of them returned with four men who testified they found two bodies lying in the open, while Simpson was stretched in his tent with the top of his head blown off. ....

Biostratigraphic Units and Tectonism in Mid-Cretaceous Foreland of Wyoming, Colorado, and Adjoining Areas: ABSTRACT

Chronolithologic units and unconformities in mid-Cretaceous formations of the central Rocky Mountains region indicate widespread marine transgressions and regressions as well as recurrent deformation of the foreland in the Western Interior during Cenomanian, Turonian, and Coniacian times (88-96 Ma). The stratigraphic record of the widely recognized Cenomanian and early Turonian transgression, middle Turonian End_Page 856------------------------------ regression, and late Turonian and Coniacian transgression was modified in several areas by episodes of slight uplift and attendant erosion. The most evident tectonism was in western Montana during the middle to late Cenomanian (93-94 Ma), in western Wyoming and adjoining areas during the early Turonian to earliest middle Turonian (90-91 Ma), in north-central Colorado, eastern Wyoming, and northwestern Wyoming in the early late Turonian (89.8 Ma), and in northeastern Colorado, Wyoming, and southwestern Montana in the late late Turonian (89.3 Ma). Crestlines of most of the swells trend generally either northwest or northeast. The tectonism of the mid-Cretaceous foreland corresponds in age to displacements of thrusts in the Sevier orogenic belt of southwestern Wyoming and southeastern Id ho. Furthermore, much of the foreland deformation probably reflects episodes of eastward thrusting in the thrust belt. End_of_Article - Last_Page 857------------

Panafrican to Hercynian deformations in the Mauritanides and tectonic significance of gravity anomalies

The Mauritanides-Rokelides chain, which extends along the western margin of the West-African craton, is characterized by roughly northward structural trends and, at least for the northern two thirds, eastward thrusting of large amplitude. The style and timing of deformation change from south to north along the chain. In Sierra Leone, Guinea and southeastern Senegal (the Rokelides), the main event is attributed to the Panafrican orogeny, apparently with only limited thrusting. The belt of gravity highs associated with that part of the orogen is interpreted as a west dipping suture zone. In northeastern Senegal and central Mauritania, this Panafrican event is masked by Caledonian and Hercynian events, which resulted in strong folding and far-travelled thrusting. The positive gravity anomaly, closely parallel to the belt, records the trace of the collision of two continental blocks (a Taconian event?). In northern Mauritania and south Morocco, the age of emplacement of large nappes is Hercynian and their geodynamic interpretation requires that a trench or ocean has closed farther west, presumably beneath the eastern U.S. Atlantic Coastal Plain.

Evolution of the lithosphere in Pakistan

The geological setting of Pakistan in the framework of the modern concept of plate tectonics is unique in the sense that, within an area of about 800,000 km 2, critical tectonic junctions of different interacting plates and microplates are present in an environment where field exposures are excellent. Here we discuss the dynamics of these various plate boundaries. Two types of active plate boundaries are conspicuous: (1) convergent boundaries characterized by continent-continent collision, obduction, and thrusting in the northern region of the Himalaya and by oceanic crust subduction with a volcanic arc and a wide accretionary wedge in the southern region of Chagai and Makran; (2) a transform boundary, the Chaman transform zone, characterized by very large strike-slip and lesser thrusting. The Chaman transform zone connects the Makran convergence zone, where oceanic lithosphere is being subducted beneath the Lut and Afghan microplates, with the Himalayan convergence zone, where the Indo-Pakistan lithosphere is underthrusting Eurasia. The Chaman zone is at present an intracontinental plate boundary with oblique motion, characterized by north-south strike-slip faults and eastward thrusting and folding in the Kirthar-Sulaiman mountain belt. This mountain belt, the northwestern margin of the Indo-Pak subcontinent, was an Atlantic-type margin from the late Paleozoic until the Cretaceous. In the Cretaceous, the continental margin became a plate boundary; a thrust belt was formed in the Paleocene, and fragments of the oceanic crust were obducted, either as thrust sheets (Muslimbagh) or as an ophiolitic melange (Bela and Waziristan).

Alpine model for the internal metamorphic zones of the North American Cordillera

The tectonic significance of the internal metamorphic zones of the North American Cordillera remains elusive. We propose a general interpretation in which the tectonic features (pervasive unidirectional transverse lineation, recumbent folding, eastward thrusting) of the Shuswap Complex (Canada) are related to a large-scale intracontinental alpine-type eastward shearing. This shearing results from the Jurassic collision between the allochthonous Stikine block and the North American craton, after a westward Triassic subduction. We extend this model to the United States Cordillera, despite some differences due to the late occurrence of a major extension.