Orogenic Channel Research Articles

Tectonics, geochronology, and petrology of the Walker Top Granite, Appalachian Inner Piedmont, North Carolina (USA): Implications for Acadian and Neoacadian orogenesis

Abstract The Walker Top Granite (here formally named) is a peraluminous megacrystic granite that occurs in the Cat Square terrane, Inner Piedmont, part of the southern Appalachian Acadian-Neoacadian deformational and metamorphic core. The granite occurs as disconnected concordant to semi-concordant plutons in migmatitic, sillimanite zone rocks of the Brindle Creek thrust sheet. Locally garnet-bearing, the Walker Top Granite contains blocky alkali feldspar megacrysts 1–10 cm long in a groundmass of muscovite-biotite-quartz-plagioclase-alkali feldspar and accessory to trace zircon, titanite, epidote, sillimanite (xenocrysts), and apatite. It varies from granite to granodiorite and contains several xenoliths of biotite gneiss, amphibolite, quartzite, and in one location encloses charnockite (here formally named Vale Charnockite). New sensitive high-resolution ion microprobe U-Pb zircon magmatic crystallization ages obtained from the plutons of the Walker Top Granite are: 407 ± 1 Ma in the Brushy Mountains; 366 ± 2 Ma in the South Mountains; and 358 ± 5 Ma in the Vale–Cat Square area. An age of 366 ± 3 Ma was obtained from the Vale Charnockite at its type locality. Major-, trace-element, and isotopic chemistry indicates that Walker Top is a high-K, peraluminous granite, plotting as volcanic arc or syn-collisional on tectonic discrimination diagrams and suggests that it represents deep-seated anatectic magma with S- to I-type affinity. The alkali calcic, ferroan Vale Charnockite likely formed by deep crustal melting, and similar geochemical and trace-element compositions suggest a similar tectonic origin as Walker Top Granite. The discontinuous nature of the Walker Top Granite plutons precludes it intruded as a volcanic arc. Instead, the peraluminous nature, common xenoliths of surrounding country rock, and geochemical and isotopic signatures suggest it formed by partial melting of Cat Square and Tugaloo terrane rocks. Following emplacement and crystallization, Walker Top plutons were deformed into elliptical to linear shapes—SW-directed sheath folds—enveloped by partially melted, pelitic and quart-zofeldspathic rocks. Collectively, Walker Top and other plutons helped weaken the crust and facilitate lateral crustal flow in a SW-directed, tectonically driven orogenic channel during the Acadian-Neoacadian event. A comparison with the northern Appalachians recognizes a similar temporal magmatic and deformational history during the Acadian and Neoacadian orogenies, although while the Walker Top Granite intruded the lower plate during eastward subduction beneath the peri-Gondwanan Carolina superterrane, the northern Appalachian plutons intruded the upper plate during subduction of the Avalon superterrane westward beneath Laurentia. We hypothesize that a transform fault, located near the southern end of the New York promontory, accommodated oppositely directed lateral plate motion and different subduction polarity between the Carolina and Avalon superterranes during the Acadian and Neoacadian orogenies.

Insights into the dynamics of an orogenic wedge from lubrication theory: Implications for the Himalayan tectonics

Using lubrication theory, we develop a mechanical model to evaluate the dynamic relation between an orogenic wedge and the overriding plate. The model suggests that the subducting plate motion produces a dynamic pressure in the wedge, which supports the gravity load of the overriding plate lying above it (stable condition). A drop in the dynamic pressure results in a rotational collapse of the overriding plate (unstable condition). We present an analytical solution to provide an estimate of the dynamic pressures in orogenic wedges. The lubrication model is also implemented in computational fluid dynamics (CFD) simulations to perform real scale numerical experiments, considering mainly three variables: 1) plate convergence angle (α), 2) subduction rate (us), and 3) viscosity ratio (R) between the overriding plate and wedge. The overriding plates attain mechanical stability when us exceeds a threshold value (us∗); otherwise, they become unstable as us<us∗. A linear increase of us∗ with α as well as R widens the unstable fields. We demonstrate that the stable to unstable transition initiates a crustal flow channel in the orogenic wedge, driven by the collapse of the overriding plate. Such a collapse is associated with a kinematic transformation from contraction to extension in the overriding plate. Based on geological proxies, this article finally explains the formation of the orogenic channel in the Himalaya as a consequence of the transition from stable to unstable state in overriding Tibet, lying above the Himalayan wedge, when the Indo-Asia collision rate reduced to ~5 cm/yr at ~22 Ma.

Compressional metamorphic core complexes, low-angle normal faults and extensional fabrics in compressional tectonic settings

AbstractMetamorphic core complexes (MCCs) are interpreted as domal structures exposing ductile deformed high-grade metamorphic rocks in the core underlying a ductile-to-brittle high-strain detachment that experienced tens of kilometres of normal sense displacement in response to lithospheric extension. Extension is supposedly the driving force that has governed exhumation. However, numerous core complexes, notably Himalayan, Karakoram and Pamir domes, occur in wholly compressional environments and are not related to lithospheric extension. We suggest that many MCCs previously thought to form during extension are instead related to compressional tectonics. Pressures of kyanite-and sillimanite-grade rocks in the cores of many of these domes are c. 10–14 kbar, approximating to exhumation from depths of c. 35–45 km, too great to be accounted for solely by isostatic uplift. The evolution of high-grade metamorphic rocks is driven by crustal thickening, shortening, regional Barrovian metamorphism, isoclinal folding and ductile shear in a compressional tectonic setting prior to regional extension. Extensional fabrics commonly associated with all these core complexes result from reverse flow along an orogenic channel (channel flow) following peak metamorphism beneath a passive roof stretching fault. In Naxos, low-angle normal faults associated with regional Aegean extension cut earlier formed compressional folds and metamorphic fabrics related to crustal shortening and thickening. The fact that low-angle normal faults exist in both extensional and compressional tectonic settings, and can actively slip at low angles (&lt; 30°), suggests that a re-evaluation of the Andersonian mechanical theory that requires normal faults to form and slip only at high angles (c. 60°) is needed.

Migmatization, Granite Generation and Melt Accumulation in the Himalayan Orogenic Channel, Central and Eastern Bhutan

In Central and Eastern Bhutan Himalaya, the Great Himalayan Sequence (GHS) reveals mesoscopic structures within the migmatite–leucogranite association due to crustal anataxis above the Main Central Thrust (MCT). The first phase of dominant melting generates stromatitic migmatite along the main foliation during high grade of metamorphism, possibly by dehydration melting. Subsequent ductile strike–slip shearing caused in situ melting in dilatational sites to produce structureless, non-foliated patchy leucogranite leucosome as well as in boudin necks and post-tectonic patches. In addition, melt-enhanced deformation caused doming of accumulated melt and subsidiary ductile shear zones on either margins of these domes. Surrounded by biotite-rich melanosome, leucosomes destroy the pre-existing foliation during new anatectic phase, which post-dates earlier stromatitic migmatite. These migmatites are the snapshot of mutual relations between newly-developed migmatite and leucogranite melt, and signify the transportation of Himalayan Orogenic Channel to the extreme south in Central and Eastern Bhutan over the Lesser Himalayan sedimentary belt along the MCT.

Eclogite as a seismic marker in subduction channels: Seismic velocities, anisotropy, and petrofabric of Cabo Ortegal eclogite tectonites (Spain)

Research Article| March 01, 2011 Eclogite as a seismic marker in subduction channels: Seismic velocities, anisotropy, and petrofabric of Cabo Ortegal eclogite tectonites (Spain) B. Ábalos; B. Ábalos † 1Departamento de Geodinámica, Universidad del País Vasco, P.O. Box 644, E-48080 Bilbao, Spain †E-mail: benito.abalos@ehu.es Search for other works by this author on: GSW Google Scholar D.M. Fountain; D.M. Fountain 2National Science Foundation, Earth Sciences Division, Geosciences Directorate, Tectonics Program, Room 785, 4201 Wilson Boulevard, Arlington, Virginia 22230, USA Search for other works by this author on: GSW Google Scholar J.I. Gil Ibarguchi; J.I. Gil Ibarguchi 3Departamento de Mineralogía y Petrología, Universidad del País Vasco, P.O. Box 644, E-48080 Bilbao, Spain Search for other works by this author on: GSW Google Scholar P. Puelles P. Puelles 1Departamento de Geodinámica, Universidad del País Vasco, P.O. Box 644, E-48080 Bilbao, Spain Search for other works by this author on: GSW Google Scholar Author and Article Information B. Ábalos † 1Departamento de Geodinámica, Universidad del País Vasco, P.O. Box 644, E-48080 Bilbao, Spain D.M. Fountain 2National Science Foundation, Earth Sciences Division, Geosciences Directorate, Tectonics Program, Room 785, 4201 Wilson Boulevard, Arlington, Virginia 22230, USA J.I. Gil Ibarguchi 3Departamento de Mineralogía y Petrología, Universidad del País Vasco, P.O. Box 644, E-48080 Bilbao, Spain P. Puelles 1Departamento de Geodinámica, Universidad del País Vasco, P.O. Box 644, E-48080 Bilbao, Spain †E-mail: benito.abalos@ehu.es Publisher: Geological Society of America Received: 18 Dec 2009 Revision Received: 15 Feb 2010 Accepted: 17 Feb 2010 First Online: 08 Mar 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 © 2011 Geological Society of America GSA Bulletin (2011) 123 (3-4): 439–456. https://doi.org/10.1130/B30226.1 Article history Received: 18 Dec 2009 Revision Received: 15 Feb 2010 Accepted: 17 Feb 2010 First Online: 08 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 B. Ábalos, D.M. Fountain, J.I. Gil Ibarguchi, P. Puelles; Eclogite as a seismic marker in subduction channels: Seismic velocities, anisotropy, and petrofabric of Cabo Ortegal eclogite tectonites (Spain). GSA Bulletin 2011;; 123 (3-4): 439–456. doi: https://doi.org/10.1130/B30226.1 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 High-pressure, sheath-like fold-nappes made of normal–mid ocean ridge basalt (N-MORB) eclogites, high-pressure volcanic-arc granulites, arc-root lithospheric mantle peridotites, and shallow crustal volcanosedimentary gneisses were amalgamated at deep realms of an Eohercynian orogenic channel and currently crop out in the Cabo Ortegal Complex. Compressional seismic-wave velocity measurements determined in eclogite at confining pressures of 600 MPa range from 8.4 to 7.3 km/s, whereas anisotropy varies between 0.7% and 3.2%. Shear-wave velocities range from 4.8 to 4.2 km/s, and the respective anisotropies vary between 0.1% and 3.4%. The Poisson's ratios are low (0.25–0.265), independent of the degree of retrogression or deformation. Seismic velocities and anisotropy are related to the fabric of omphacite, and calculated eclogite anisotropy is not symmetrical with respect to the tectonic fabrics. Calculated seismic-reflection coefficients induced by lithological, structural, and rock petrofabric variations of eclogites show that these can be identified as reflectors imprinting a particular, penetrative, and dipping fabric to discrete seismic-profile segments of paleosubduction settings. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

An Andean type Palaeozoic convergence in the Bohemian Massif

The geological inventory of the Variscan Bohemian Massif can be summarized as a result of Early Devonian subduction of the Saxothuringian ocean of unknown size underneath the eastern continental plate represented by the present-day Teplá-Barrandian and Moldanubian domains. During mid-Devonian, the Saxothuringian passive margin sequences and relics of Ordovician oceanic crust have been obducted over the Saxothuringian basement in conjunction with extrusion of the Teplá-Barrandian middle crust along the so-called Teplá suture zone. This event was connected with the development of the magmatic arc further east, together with a fore-arc basin on the Teplá-Barrandian crust. The back-arc region – the future Moldanubian zone – was affected by lithospheric thinning which marginally affected also the eastern Brunia continental crust. The subduction stage was followed by a collisional event caused by the arrival of the Saxothuringian continental crust that was associated with crustal thickening and the development of the orogenic root system in the magmatic arc and back-arc region of the orogen. The thickening was associated with depression of the Moho and the flux of the Saxothuringian felsic crust into the root area. Originally subhorizontal anisotropy in the root zone was subsequently folded by crustal-scale cusp folds in front of the Brunia backstop. During the Visean, the Brunia continent indented the thickened crustal root, resulting in the root's massive shortening causing vertical extrusion of the orogenic lower crust, which changed to a horizontal viscous channel flow of extruded lower crustal material in the mid- to supra-crustal levels. Hot orogenic lower crustal rocks were extruded: (1) in a narrow channel parallel to the former Teplá suture surface; (2) in the central part of the root zone in the form of large scale antiformal structure; and (3) in form of hot fold nappe over the Brunia promontory, where it produced Barrovian metamorphism and subsequent imbrications of its upper part. The extruded deeper parts of the orogenic root reached the surface, which soon thereafter resulted in the sedimentation of lower-crustal rocks pebbles in the thick foreland Culm basin on the stable part of the Brunia continent. Finally, during the Westfalian, the foreland Culm wedge was involved into imbricated nappe stack together with basement and orogenic channel flow nappes.

Paleoproterozoic granitic gneisses of the Dinggye and LhagoiKangri areas from the higher and northern Himalaya, Tibet: Geochronology and implications

Granitic gneisses have been widely found in crystalline rocks in the Dinggye area of the Higher Himalaya (HHM) and the LhagoiKangri area of the North Himalaya (NHM), Tibet. In the HHM, the gneisses intruded in the granulite-amphibolite facies metamorphosed sedimentary rocks, known as Nyalam group. In the NHM, the gneisses intruded in the amphibolite facies metamorphosed ones, known as LhagoiKangri group. These granitic gneisses are peraluminous monzonitic granites in terms of their mineral assemblage, and are considered as being derived from metamorphosed sedimentary rocks by anatexis based on the transitional relationship of the gneisses with their migmatitized wall rocks. Zircons are similar in crystal shape and interior structure from both gneisses. Most of them are euhedral or subhedral elongated prism-shaped transparent crystals, with fine oscillatory zoning, showing the magmatic genesis. Some of them are short prism-shaped and with relict core inherited from magma source and oscillatory zoning mantle crystallized from magma. SHRIMP U-Pb dating of zicons shows that both the granitic gneisses in the HHM and NHM are Paleoproterozoic (1811.6±2.9 Ma and 1811.7±7.2 Ma, respectively). These ages are similar to those (1815 to 2120 Ma) from granitic gneiss which is widely distributed in the Lesser Himalaya (LHM). The ages of inherited zircons (>2493.9±7.0 Ma, 2095.8± 8.8 Ma, 1874±29 Ma) exhibit the possible presence of several thermal events in Paleoproterozoic. All of the ages suggest the same India basement beneath the different units in Himalaya area, and do not support the idea that the HHM and NHM are accretionary terranes in Pan-Africa orogenic event. The fact that the basement in HHM is as old as or even younger than LHM is inconsistent with the presently prevalent orogenic models such as either extrusion of low-viscosity mid-crust or orogenic channel.

The Appalachian Inner Piedmont: an exhumed strike-parallel, tectonically forced orogenic channel

Abstract The Appalachian Inner Piedmont (IP) extends along orogenic strike some 700 km from North Carolina to Alabama. Its physical attributes contrast with those of other Appalachian tectonic elements: gentle dip of dominant foliation; imbricate stack of fold nappes; dominant sillimanite-grade metamorphism and near ubiquitous migmatization; heterogeneous, non-plane deformation; and earlier S-foliations transposed to C-foliations southeast of the mid-Palaeozoic Brevard fault zone forming a 10–20 km wide amphibolite-facies shear zone along the western flank of the IP. The IP contains west- and SW-directed thrust sheets and mineral stretching lineation, sheath folds on all scales, and other indicators that define a curved crustal flow pattern throughout the belt. Field and modern geochronologic data confirm that the IP is not exotic. It contains a Laurentian component (eastern Tugaloo terrane) and an internal terrane (Cat Square) that contains both Laurentian and Gondwanan detrital zircons, separated by the Brindle Creek fault. Cat Square terrane rocks likely accumulated in a Devonian remnant ocean that closed beginning c. 400 Ma. The complex but consistently asymmetric, NW- to west- to SW-directed flow pattern throughout the IP reflects confinement beneath a &gt; 15 km thick overburden produced during subduction of Cat Square and Laurentian components beneath the approaching Carolina superterrane along the Central Piedmont suture. Oblique NE-to-SW transpressive subduction to &gt; 15 km depth initiated partial melting, forcing escape from the collision zone in an along-strike orogenic channel. The IP detached from rocks to the west of the mid-Palaeozoic Brevard fault zone as the collision zone tightened and the IP mass flowed c. 200 km southwestward in the channel. The top of the channel is preserved at the NE end of the IP, and the base (Brevard fault zone) is preserved to the west and SW. As an exhumed orogenic channel, the curved IP flow paths may provide insight for middle to lower crustal deformation and flow in modern orogens.

Chronological constraints on the pre-orogenic history, burial and exhumation of deep-seated rocks along the eastern margin of the Variscan Orogen, Bohemian Massif, Czech Republic

Key lithological units of the high-grade eastern margin of the Bohemian Massif were dated using the U-Pb and Pb-Pb methods on zircons in order to establish a chronological framework for the geodynamic evolution of the Variscan orogenic root. The protolith ages for metagranitoids, orthogneisses and granulites of thickened lower and middle crust reveal the existence of magmatic activity that occurred over a 100 million year time interval from Cambro-Ordovician to early Devonian times, probably related to discontinuous intracontinental rifting of Neoproterozoic crust. Our geochronological data suggest that the eastern part of the orogenic root represents thermally softened and rifted Neoproterozoic crust, preserved farther to the east as the Brunia microcontinent. Zircon ages for felsic granulites, high-grade gneisses of the lower crust and of a syn-convergence granodioritic intrusion in the upper crust indicate that thickening and exhumation of the crust occurred during a narrow time interval between 370 and 340 Ma. Exhumation of the lower crust to mid-crustal levels was a localized process that occurred at ∼340 Ma and was associated with crustal-scale folding in the internal part of the root as well as orogenic channel flow along the eastern collisional margin. Both types of exhumation mechanisms were driven by deep-level wedging (indentation) of the easterly Brunia continent, followed by deposition of heavy minerals and pebbles derived from high-pressure rocks in the adjacent foreland basin. Final orogenic development was characterized by NE-SW dextral transpressive shearing parallel to the Brunia margin as well as dextral trans tension associated with activity along the Elbe lithospheric fault. These processes affected the marginal parts of. the orogenic root and were accompanied by 330 to 325 Ma old syntectonic granitoid intrusions along reactivated lithotectonic boundaries. Rotation of the assembled orogenic belt, accompanied by lithospheric faulting driven by westerly subduction roll-back, may be the most plausible model to explain late deformation of the orogenic root.

The northern Inner Piedmont, southern Appalachians, USA: kinematics of transpression and SW-directed mid-crustal flow

Analysis of new and existing field, structural, petrologic, and geochronologic data in the northern Inner Piedmont (IP), southern Appalachians, provide insight into the processes and kinematics of Neoacadian (Late Devonian–Mississippian) crustal flow. The IP is characterized by 360–345 Ma middle to upper amphibolite facies metamorphism, gentle dip of the dominant S 2 foliation, and curved L 2 mineral lineation pattern. Recognized map-scale sheath folds are cored by Henderson Gneiss or outlined by Devonian granitoids, and record SW-directed axial elongations of 120–486%. Net Neoacadian fault displacement exceeds 400 km, with over 200 km associated with SW-directed, orogen-parallel flow. Map-scale sheath folds and mineral lineation patterns suggest that NW- and W-directed flow rooted to the east was deflected SW by an oblique buttress in the footwall beneath the Neoacadian Brevard fault zone. Kinematic vorticity estimates remain in the realm of sub-simple shear across the IP, and approaches simple shear in the Neoacadian Brevard fault zone. Curved IP flow paths provide a model for middle to lower crustal deformation and flow in ancient to active orogens like the Himalayas, including the possibility that the IP is an exhumed orogenic channel.

Structural assemblage of high‐pressure mantle and crustal rocks in a subduction channel (Cabo Ortegal, NW Spain)

High‐pressure metamorphic fold‐nappes and ductile thrusts occur in the Cabo Ortegal upper allochthons. They involve arc‐root lithospheric mantle peridotites, high‐pressure volcanic‐arc granulites, normal mid‐oceanic ridge basalt eclogites and shallow crustal volcanosedimentary high‐pressure gneiss units. Confluence of units from such disparate tectonic settings occurred at an eo‐Hercynian convergent plate margin. Their internal deformation, juxtaposition, and high‐pressure synmetamorphic reworking resulted from kinematically simple but intense and polyphase ductile deformations. Fold nappes exhibit a remarkable sheath‐like form and contain mineral and stretching lineations defined by high‐pressure mineral assemblages subparallel to fold axes. We argue that these structures outline the internal organization of high‐pressure sheets at deep tectonic realms of an oblique subduction/collision orogenic channel.

Himalayan metamorphic sequence as an orogenic channel: insight from Bhutan

The Bhutan Himalayas differ from the rest of the Himalayas in two major ways: (i) low-grade sedimentary rocks lie above the Greater Himalayan Sequence as klippen (i.e. erosional remains of the South Tibetan Detachment); and (ii) an out-of-sequence thrust, the Kakhtang thrust, lies structurally above the klippen, and it doubles the exposed thickness of the Greater Himalayan Sequence. Our field observations and geochronological data constrain the main kinematic events in the Bhutan Himalayas. Crystallisation ages of leucogranite dykes deformed by the Main Central Thrust and the South Tibetan Detachment indicate that these two structures operated together between 16 and 22 Ma. The out-of-sequence Kakhtang thrust was active at 10–14 Ma and was concurrent with reactivation of the South Tibetan Detachment. Restoration of the Bhutan Himalayas prior to the out-of-sequence thrusting shows that the Greater Himalayan Sequence was the core of a long, low-viscosity crustal channel extending under the Tibetan plateau. We propose that the gravity-driven southward extrusion of the channel material from underneath the Tibetan plateau caused the inverted metamorphic sequence in the Lesser Himalayan Sequence and in the Greater Himalayan Sequence. This process also led to occurrences of present-day surface rocks that were derived from variable distances down dip, but from similar crustal depths. Such an exhumation pattern can explain the similar peak pressures for the Greater Himalayan Sequence along the length of the Himalayas.

On the relationship between kilometer-scale sheath folds, ductile thrusts and minor structures in the basal high-pressure units of the Cabo Ortegal complex (NW Spain)

Large sheath folds in the basal high-pressure nappes of the Cabo Ortegal complex are described and a kinematic interpretation provided. The principal penetrative and map structures relate to regional D2 deformation, which produced foliations (S2) bearing mineral and stretching lineations (L2) and several types of folds (a-type, sheath-like and ‘folded folds’). The latter structures are subparallel to the trend of the orogen. Their attitude suggests that the units involved shared a common tectonic evolution during progressive ductile deformation of an anisotropic medium. Reconstruction of major geological structures was accomplished through projection of map-scale features onto the ductile flow plane and the plane perpendicular to the ductile flow direction. The structures reconstructed illustrate their development in the deeper structural levels of an orogenic channel subjected to high-pressure metamorphism during the early phases of the Hercynian orogeny in NW Iberia. We argue that orogen-normal tectonic displacements (of up to a few hundreds of kilometers) represent the minor components of the transpression with possibly thousands of kilometers along-strike dextral displacement between the intervening plates (during subduction/collision).