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The ca. 1 Ga Grenville orogeny was a protracted mountain-building event that culminated in the collision of Laurentia and Amazonia and the formation of the Rodinia supercontinent. While the expression of Grenville orogenesis in present-day crustal structure has been extensively investigated in eastern Canada, evidence for contemporaneous crustal deformation is less well established beneath the eastern United States. Furthermore, the interpretation of a geophysical lineament through the U.S. midcontinent, typically inferred to be the Grenville deformation front, has recently been called into question; an alternative hypothesis is that this feature actually corresponds to an eastern arm of the Midcontinent Rift. Here we present P-to-S receiver functions computed for stations of the Mid-Atlantic Geophysical Integrative Collaboration (MAGIC) experiment, a dense array of broadband seismometers across the central Appalachians and midcontinent. We see evidence for a crustal negative velocity gradient that dips gently (dip angle <10°) to the southeast and extends east from a location near the putative Grenville front, terminating near the Appalachian Mountains. While we cannot date this feature, its location and characteristics are consistent with a shallowly dipping, seismically anisotropic intracrustal shear zone associated with collisional deformation, perhaps during Grenville orogenesis. The similarity between this feature and similar mid-crustal detachments in other orogens, both ancient (Appalachians) and modern (Himalayas), suggests that this style of crustal deformation has been common in continental collisional orogens. [ABSTRACT FROM AUTHOR]

The tectonic origin of the Pennsylvanian-Permian Ancestral Rocky Mountain system is debated. Most models invoke stress from collision along the Ouachita-Marathon belt as the primary driver for Ancestral Rocky Mountain (ARM) (western USA) tectonism, but no existing model has convincingly explained the timing, northeastsouthwest contractional kinematics, and amagmatic nature of this system. We propose that stress generated by transpressional deformation along the southwestern (Sonora) margin of Laurentia combined with that from collision along the Ouachita-Marathon and Nevada margins drove uplift and basin development of the ARM. This model explains ARM timing, uplift kinematics, and lack of magmatism, and is supported by a new generation of late Paleozoic global plate reconstructions. [ABSTRACT FROM AUTHOR]

We present a regional model of plate geometry and kinematics of Southeast Asia since the Late Cretaceous, embedded in a global plate model. The model involves subduction polarity reversals and sheds new light on the origin of the subduction polarity reversal currently observed in Taiwan. We show that this subduction zone reversal is inherited from subduction of the proto-South China Sea plate and owes its current location to triple junction migration and slab rollback. This analysis sheds new light on the plate tectonic context of the Taiwan orogeny and questions the hypothesis that northern Taiwan can be considered an older, more mature equivalent of southern Taiwan. [ABSTRACT FROM AUTHOR]

We provide model evidence for a previously unexplored positive feedback between tectonic strain and fluvial erosion by considering rock erodibility as a function of shear damage. Plastic shear strain permanently damages the upper crust within planar shear zones, providing a greater ease for detachment and transport by fluvial processes. The subsequent rapid erosion of exposed shear zones reforms the topographic stress field in a way that encourages continued accommodation of strain, a positive feedback response that becomes more prominent with greater shear damage. Based on model results and natural examples, rock strength heterogeneity plays a major role in the evolution of drainage network patterns and the capability for valley-scale strain localization in active orogens. [ABSTRACT FROM AUTHOR]

Accretionary and collisional orogenesis in the Tarim and North China cratons during Paleozoic time can be correlated with events associated with the assembly and subsequent incipient dispersal of Gondwana. Zircon U-Pb and Hf isotopic data from the northern margins of the two cratons and neighbors have revealed comparable ∊Hf(t)-time patterns. Zircons with magmatic ages of 500-400 Ma display a large spread and decreasing ∊Hf(t) values with time, whereas 400-310 Ma zircons have dominantly positive ∊Hf(t) values and an overall increasing trend. The marked shift of the zircon Hf array at ca. 400 Ma was most likely related to a major tectonic switch from advancing to retreating accretionary orogenesis, corresponding to the development of regional extension. The commencement of subduction at 500 Ma and establishment of an advancing accretionary orogen along the preexisting passive margin was synchronous with early Paleozoic continental collisional events along the southern margins of the two cratons. The temporal agreement of these events, and their accordance with collision and/or accretion events during Gondwana assembly, suggest that the Tarim and North China cratons likely collided with the northern Australia margin of East Gondwana at ca. 500 Ma. They subsequently dispersed from Gondwana in the Early Devonian, coinciding with switching accretionary tectonics along the northern margins of the two cratons that were possibly induced by the slab rollback of the subducting paleo- Asian Ocean plate. [ABSTRACT FROM AUTHOR]

The sedimentary cover of the North American craton preserved little evidence of the Paleozoic tectonic events that shaped the Appalachian orogen on its eastern side. A notable exception is the NNWtrending Hudson Bay central high, which corresponds to a normalfault array extending for a minimum length of 500 km. A working hypothesis is proposed in which stresses applied to the continental margin during the Silurian-earliest Devonian Salinian orogeny were transmitted over a distance of >1400 km in the continental interior, where they induced the normal-fault reactivation of older structural discontinuities. The shutdown of tectonic activity along the Hudson Bay central high during the latest Early Devonian to earliest Middle Devonian is interpreted as resulting from a change in the direction of plate convergence during the Acadian orogeny. [ABSTRACT FROM AUTHOR]

The analysis of new and published Hf and Nd isotopic data of late Cenozoic Andean arc igneous rocks from central Chile, coupled with our improved knowledge of orogenic processes in the region, reveals a tight link between major magmatic isotopic shifts and different Andean basement domains and timing of the main uplifting event. Oligocene--Miocene magmas from the Western Principal Cordillera show a nearly constant and juvenile composition (εHfI: +5 to +10; ε NdI: +2 to +7), while those from the Eastern Principal Cordillera, formed since early late Miocene, are variably more enriched (ε HfI: -4 to +4; ε NdI: 0 to +3). Post-4.8 Ma magmas from both belts share an enriched signature (ε NdI: -2 to +2) reflecting source contamination from east to west, contrary to the eastward subduction direction, in a process that occurred toward the end of the main Andean uplifting event. This results from the deep western basement underthrusting the orogen, and thus accounts for the westward propagation of the eastern enriched isotopic signatures approximately coeval with thickening and uplifting events. The observed patterns highlight the strong control exerted by the continental lithosphere on the composition of arc magmas over deep controls from the subduction-modified asthenospheric mantle. Moreover, they dynamically represent both (1) the hybridization affecting magmas ascending from the mantle in a heterogeneous continental lithosphere, and (2) the evolution of such lithosphere resulting from the thermal weakening and mass transfer processes occurring underneath cordilleran arcs during mountain building. [ABSTRACT FROM AUTHOR]

The Eocene deep-marine siliciclastic fill of the Ainsa basin, Spanish Pyrenees, gives unprecedented temporal resolution of the causes and timing of coarse elastic sediment supply to a deep-marine basin. Early Eocene tectonic subsidence linked to Pyrenean orogenesis created the Ainsa basin, with water depths of ∼400-800 m above a foundered shallow-marine mixed carbonate and clastic shelf. The ∼25 sandbodies or channelized submarine fans in the basin were controlled by the ∼400 k.y. Milankovitch frequency, with modes at ∼100 k.y. and ∼41 k.y. (possibly stacked ∼23 k.y.) influencing bottom-water conditions in the basin, causing periodic stratification in the water column across a submarine sill at the western basin margin (early Boltaña anticline). Intrabasinal tectonics defined and controlled the position of eight sandy systems and their constituent fans, in a process of seesaw tectonics, by (1) westward lateral offset stacking of sandy fans due to growth of the eastern side of the basin, represented today by the Mediano anticline, and (2) eastward (orogenward) back stepping of the depositional axis of each sandy system, due to phases of relative uplift of the Boltafla anticline. During basin infill, uplift of the Boltaña anticline led to increasing basin narrowing and depositional confinement. Unlike the earlier depositional systems, the youngest deep-marine system was fed from a more southern sediment source between the growth anticlines, as was the overlying deltaic system. All the older deep-marine sandy systems were fed from southeast point sources, from canyons and erosional lower-slope channels eroding the growing Mediano anticline. The depositional style outlined in this paper might be common to other active margins where siliciclastic basins evolve between active thrusts. [ABSTRACT FROM AUTHOR]

Intraplate orogeny remains an enigmatic process in terms of plate tectonics. Regional continental stress fields agree well with stress orientations predicted by the main forces driving and resisting plate motion. However, regional continental stress fields fail to explain major localized compressive plate deformation in continental interiors far away from plate boundaries. Global or plate-wide intraplate stress models typically assume a laterally homogeneous plate rheology and are confined to modeling the present, without addressing the antiquity of the current stress field. An understanding of intraplate stress and deformation requires the linking of spatial variations in continental rheology with time-dependent plate geometry and driving forces. Here we demonstrate how the complex interplay between juxtaposed weak and strong geological provinces and changes in far-field plate boundary forces has caused intraplate orogenesis and tectonic reactivation in southeastern Australia during the Tertiary. Our findings are contrary to the opinion that continental interiors are insensitive to compression at plate margins, and help to explain the mechanisms causing intraplate orogenesis. [ABSTRACT FROM AUTHOR]

The recent dramatic increase in seismic anisotropy and surface global positioning system (GPS) data for central Asia permits a comprehensive examination of the mantle's role in mountain building. A joint analysis of 178 shear-wave-splitting and 2000 GPS observations using a new technique reveals that the crust and lithospheric mantle deform coherently, arguing for crust-mantle mechanical coupling during deformation. The observed spatial variations in anisotropy reflect the large-scale pattern of lithospheric deformation, as well as a change in deformational style from simple shear on the Tibetan Plateau transitioning to pure shear in surrounding regions. [ABSTRACT FROM AUTHOR]

We refine conventional continental-scale geodynamic models by including conversion of mechanical work done by deformation into heat. The intensity of the shear heating is extracted from the Brace- Goetze strength envelope without any additional model parameters or assumptions. Incorporation of this, certainly present, heating rate into a model may result in up to tenfold stress reduction, which is exceeding the effects of variation of common parameters within their uncertainty limits. Shear heating with lithospheric thickening at high integrated strength solves the puzzle of the heretofore-missing heat source recorded by metamorphism, magmatism, and heat flow in mountain building. However, the mechanism is self-limiting as the rising temperature reduces stress and thus the rate of heat production. Thus this is a self-regulating mechanism maintaining a moderate integrated lithospheric strength consistent with results of model-independent forcebalanced calculations and with surface heat flow measurements. [ABSTRACT FROM AUTHOR]

A compilation of geochemical data from >50 active volcanoes where seismically defined Moho depth measurements are available, representing most volcanic arcs around the world, indicates a relation exists between crustal thickness (Moho depth) and the trace element composition of basalts. The best correlation exists for maximum light/heavy rare earth element (REE) ratios, and we use maximum Ce/Y (R2 = 0.90) to show the exponential function with increasing depth, to 50 km. Application to New Zealand, part of the eastern Gondwanan margin through much of the Phanerozoic, shows that higher Ce/Y ratios in arc basalts correspond to two periods of recognized crustal thickening, plus an additional event at ca. 240 Ma, which links New Zealand to the Gondwanide Orogeny. Conversely, low Ce/Y ratios in basalts correlate with periods of recognized extension in eastern Gondwana. Very high Ce/Y ratios in ca. 120 Ma rocks imply an unusually thickened (∼50 km) arc crust, consistent with independent geological estimates of crustal thickness at that time. The correlation shows that crustal thickness variations during orogenesis can be conservatively estimated to within ±3 km. [ABSTRACT FROM AUTHOR]

The extensional Pannonian Basin was formed in a few million years during Miocene time synchronously with contraction in the surrounding Alpine and Carpathian orogens. This system is characteristic of a class of extensional basins that form in the midst of active orogenic (mountainforming) belts. The mechanism that causes this type of geological event is enigmatic but usually has been associated with subduction. We examine a new hypothesis for intra-orogenic extensional basin formation in which gravitational spreading of previously thickened crust triggers gravitational instability of the mantle lithosphere. A basin is formed by lithospheric extension as shortening and lithospheric downwelling occur in the surrounding mountain belts. This mechanism provides a mechanically self-consistent explanation for all of the main structural features of the Pannonian-Carpathian system and presents a plausible alternative to the popular view that subduction and slab rollback have driven the development of this basin. [ABSTRACT FROM AUTHOR]

The ∼2800-km-long Snowbird tectonic zone is one of the most controversial tectonic features of the Canadian shield. Metamorphic and in situ geochronologic data reported here reveal that a 1.9 Ga medium- to high-pressure belt extends along most of this tectonic zone. In contrast to recent interpretations, a collisional origin is indicated by the length of this metamorphic belt, tectonic thickening documented in parts of it, geological contrasts across it, and subduction-type microdiamonds within it. This collisional event marks a pre—1.865 Ga phase of the Hudsonian orogeny involving microcontinent accretion that was fundamental to the growth of Laurentia. [ABSTRACT FROM AUTHOR]

Both Taconian (ca. 450 Ma) and Acadian (ca. 380 Ma) ages of high-grade deformation and metamorphism have been advocated for the Western Blue Ridge, southern Appalachians, United States. To resolve this debate, metamorphic monazite from the Great Smoky Mountains was dated via in situ microsampling plus isotope dilution-thermal ionization mass spectrometry (ID-TIMS). U-Pb ages are 450 ± 5 Ma (±2σ error), and mean 207Pb/206Pb ages are 453 ± 10 Ma. These data indicate that peak metamorphism and high-grade deformation occurred during the Taconian orogeny, although they do allow for a low-grade (subamphibolite-facies) Acadian overprint. Unresolved issues remain regarding the comparability of U-Pb TIMS ages with those obtained via other methods. [ABSTRACT FROM AUTHOR]

Understanding uplift and exhumation in orogenesis requires integrating the effects of both climatic and tectonic forcing. Punctuated thrust activity is widely documented from field studies in a range of mountain belts. The potential for climatic forcing of thrust activity has been a subject of recent debate. Here we use a modeling approach to analyze the behavior of individual thrust units in the context of asymmetric doubly vergent thrust wedges. The model predicts that rates of surface uplift, frontal accretion, and exhumation should be punctuated on a time scale linked to thrust sheet geometry and convergence rates. This time scale ranges from 0.1 to 5 m.y. for various settings, and should be calculated before external forcings such as climate are invoked. [ABSTRACT FROM AUTHOR]

Seven localities distributed along a 180-km-long outcrop within the UK sector of the Laurentian margin were sampled, and all showed two phases of authigenic K-feldspar. The first phase was precipitated from a cool fluid of moderate salinity, and the second phase was precipitated from a hotter, more saline fluid.Ar-Ar ages for the two episodes of authigenesis are ca. 470 Ma and ca. 432 Ma. The data can be interpreted to represent two phases of regional-scale fluid migration related to Ordovician arc accretion and Silurian continent-continent convergence, corresponding to the Grampian and Scandian episodes of the Caledonian orogeny, respectively. [ABSTRACT FROM AUTHOR]

The 40Ar/39Ar hornblende crystallization ages from the Las Pirquitas thrust of the Sierra de Pie de Palo, Argentina, indicate that imbrication of the proto-Andean margin had initiated by Ca. 515 Ma. The ages do not permit the Pie de Palo block to represent basement of the adjacent Argentine Precordillera terrane, but rather require it to be a separate fragment not juxtaposed with the Precordillera prior to the Ordovician. The Pie de Palo block was likely autochthonous to the Famatina arc of the proto-Andean margin in the Early Cambrian. When combined with existing regional ages of magmatism, metamorphism, and deformation, the ages suggest that east-dipping subduction beneath the Famatina arc initiated by the Early to Middle Cambrian. The newly established convergent margin resulted from a westward shift of the plate boundary at the end of the Pampean orogeny in the eastern Sierras Pampeanas. The Famatina arc persisted as an Andean-type active margin until the collision and accretion of the Precordillera terrane in the Middle Ordovician. [ABSTRACT FROM AUTHOR]

Early ENE-striking joints (present coordinates) within both Pennsylvanian coal and Devonian black shale of the Central and Southern Appalachians reflect an approximately rectilinear stress field with a dimension >1500 km. This Appalachian-wide stress field (AWSF) dates from the time of joint propagation, when both the coal and shale were buried to the oil window during the 10-15 m.y. period straddling the Pennsylvanian-Permian boundary. The AWSF was generated during the final assembly of Pangea as a consequence of plate-boundary tractions arising from late-stage oblique convergence, where maximum horizontal stress, SH, of the AWSF was parallel to the direction of closure between Gondwana and Laurentia. After closure, the AWSF persisted during dextral slip of peri-Gondwanan microcontinents, when SH appears to have crosscut plate-scale transcurrent faults at ∼30°. Following >10 m.y. of dextral slip during tightening of Gondwana against Laurentia, the AWSF was disrupted by local stress fields associated with thrusting on master basement decollements to produce the local orocline-shaped Alleghanian map pattern seen today. [ABSTRACT FROM AUTHOR]

Thermal-mechanical numerical experiments are used to consider if the influence of surface erosion during continental plate collision and orogenesis extends beneath the crust into the mantle lithosphere. The models demonstrate that the modification of crustal mass flux by surface denudation can change the evolution of the deforming crust and consequently alter the behavior of the crust-mantle interface. The mantle lithosphere responds to these variable dynamics: with active surface erosion, stable subduction-like plate consumption is maintained; in the absence of erosion, subduction is inhibited by accumulating crust, causing the convergent plates to steepen dip, detach, and reverse consumption polarity. With strong subducting plates, the removal of surface processes can trigger a transition from stable subduction to delamination and/or retreat of the convergent plate. The influence of surface erosion is less important for mantle lithosphere undergoing Rayleigh-Taylor-type dripping and is not significant at ocean plate collision, where there is relatively little buoyant crust to clog the subduction zone. The results of the experiments demonstrate that in certain tectonic regimes the influence of climate-controlled surface processes may reach much deeper into the lithosphere than has previously been appreciated. [ABSTRACT FROM AUTHOR]

The southern margin of the Pilbara Craton in northwestern Australia underwent regional heating, folding, and thrusting as well as extensive fluid flow during collision in the Paleoproterozoic. However, the precise tinting of this event is uncertain. Previous geochronologic studies of Archean basement rocks have yielded a wide range of younger radiometric dates (from ca. 2.4 Ga to ca. 2.0 Ga), interpreted to record multiple hydro- thermal alteration events. In situ U-Pb geochronology of authigenic monazite and xenotime in very low grade Archean metasedimentary rocks from across the craton suggests that the Pilbara was affected by at least two cryptic thermotectonic events, the first between ca. 2430 Ma and ca. 2400 Ma, and the second between ca. 2215 Ma and ca. 2145 Ma. The older event is restricted to three localities in the west, and its cause is unknown. The younger event affected most of the craton (>100,000 km²), spanning a 70 m.y. period from ca. 2215 Ma nearest the collisional margin in the south, to ca. 2145 Ma toward the craton interior in the north. The widespread geographic and stratigraphic distribution of ca 2.2 Ga phosphates suggests that fluid flow was pervasive. This event was probably driven by the northward-advancing Ophthalmian fold-and-thrust belt that developed during protracted collision. The associated low-grade metamorphic front migrated over ∼350 km at an average rate of ∼5.0 mm/yr, placing important constraints on the rate and duration of deformation and metamorphism in orogenic settings. [ABSTRACT FROM AUTHOR]

The Andes, the world's second highest orogenic belt, were generated by the Cenozoic tectonic shortening of the South American plate margin overriding the subducting Nazca plate. We use a coupled thermomechanical numerical modeling technique to identify factors controlling the intensity of the tectonic shortening. From the modeling, we infer that the most important factor was accelerated westward drift of the South American plate; changes in the subduction rate were less important. Other important factors are crustal structure of the overriding plate and shear coupling at the plates' interface. The model with a thick (40-45 km at 30 Ma) South American crust and relatively high friction coefficient (0.05) at the Nazca-South American interface generates >300 km of tectonic shortening during 30-35 m.y. and replicates the crustal structure and evolution of the high central Andes. The model with an initially thinner (<40 km) continental crust and lower friction coefficient (<0.015) results in <40 km of South American plate shortening, replicating the situation in the southern Andes. Our modeling also demonstrates the important role of the processes leading to mechanical weakening of the overriding plate during tectonic shortening, such as lithospheric delamination, triggered by the gabbro-eclogite transformation in the thickened continental lower crust, and mechanical failure of the sediment cover at the shield margin. [ABSTRACT FROM AUTHOR]

Proterozoic terranes in Australia record complex tectonic histories in the interval 1900-1400 Ma that have previously been interpreted by means of simple intracratonic or plate-tectonic models. However, these models do not fully account for (1) repeated tectonic reactivation (both orogenesis and rifting), (2) mainly high-temperature-low-pressure metamorphism, (3) rifting and sag creating thick sedimentary basins, (4) the nature and timing of voluminous felsic magmatism, (5) relatively large aspect ratio orogenic belts, and (6) a general paucity of diagnostic plate-boundary features. A key to understanding these histories is the observation that Australian Proterozoic terranes are characterized by an extraordinary, but heterogeneous, enrichment of the heat-producing elements. This enrichment must contribute to long-term lithospheric weakening, and thus we advocate a hybrid lithospheric evolution model with two tectonic switches: plate-boundary-derived stresses and heat-producing-element-related lithospheric weakening. The Australian Proterozoic crustal growth record is therefore a function of the magnitude of these stresses, the way in which the heat-producing elements are distributed, and how both of these change with time. [ABSTRACT FROM AUTHOR]

Banded iron formations of the ca. 2770-2405 Ma Hamersley province of Western Australia were locally upgraded to high-grade hematite ores during the Early Paleoproterozoic by a combination of hypogene and supergene processes after the initial rise of atmospheric oxygen. Ore genesis was associated with the stratigraphic break between the Lower and Upper Wyloo Groups of the Ashburton province, and has been variously linked to the Ophthalmian orogeny, late-orogenic extensional collapse, and anorogenic continental extension. Small-spot in situ Pb/Pb dating of baddeleyite by sensitive high-resolution ion microprobe (SHRIMP) has resolved the ages of two key suites of mafic intrusions, constraining for the first time the tectonic evolution of the Ashburton province and the age and setting of iron-ore formation. Mafic sills dated as ca. 2208 Ma were folded during the Ophthalmian orogeny and then cut by the unconformity at the base of the Lower Wyloo Group. A mafic dike swarm that intrudes the Lower Wyloo Group and has a close genetic relationship to iron ore is Ca. 2008 Ma, slightly younger than a new syneruptive 2031 ± 6 Ma zircon age for the Lower Wyloo Group. These new ages constrain the Ophthalmian orogeny to the period between ca. 2208 and 2031 Ma, before Lower Wyloo Group extension, sedimentation, and flood-basalt volcanism. The ca. 2008 Ma dikes present a new maximum age for iron-ore genesis and deposition of the Upper Wyloo Group, thereby linking ore genesis to a ca. 2050-2000 Ma period of continental extension similarly recorded by Paleoproterozoic terrains worldwide well after the initial oxidation of the atmosphere by ca. 2320 Ma. [ABSTRACT FROM AUTHOR]

The stable isotope composition of pedogenic and early diagenetic carbonates from the Oiyug Basin of southern Tibet allows model estimates of the paleoaltimetry of the Tibetan Plateau for the middle Miocene. Pedogenic calcium carbonate nodules have average 81soot values of -19.6‰, whereas nodular lacustrine dolomites range in composition from -7.6‰ to -5.5‰. The most negative of the carbonate isotope values can be used to constrain the oxygen isotope composition of paleoprecipitation, from which model estimates of paleoaltimetry can be made. Model results indicate that the southern Tibetan Plateau achieved elevations of ∼5200 +1370/-605 m by at least 15 Ma. Our results are identical within uncertainty to previous workers' paleoelevation estimates based on Oiyug Basin fossil floral physiognomy. This is the first time that two paleoaltimeters have been directly compared and are in accord. Collectively, these data strongly support tectonic models in which thickening of mantle lithosphere beneath the domain of crustal thickening and subsequent detachment of the mantle lithosphere plays an indiscernible role in the elevation history of this part of the Himalaya-Tibet orogenic system. [ABSTRACT FROM AUTHOR]

A lithospheric model scaled for density and viscosity was shortened obliquely in front of an indenter whose shape and orientation match those of the South Alpine indenter. Deformation was initially localized into subparallel sets of conjugate transpressive thrusts. These structures evolved into folds, which were strongly amplified and converged closer together during ongoing shortening, finally forming a series of tight antiforms. At the same time, this tightly folded structure locally accommodated significant orogen-parallel extension, attaining 100% in front of the leading edge of the indenter, but only 15% on the scale of the entire model. The internal architecture and fault pattern of the model in map view are very similar to those of the Tauern Window in the Eastern Alps, emphasizing the importance of localization of folding and shortening rather than orogen-parallel extension for its formation. [ABSTRACT FROM AUTHOR]

Upper Ordovician-Lower Devonian strata of the Meguma terrane in the Canadian Appalachians contain zircon populations, including an important Mesoproterozoic zircon population (1.0–1.4 Ga), similar to those in coeval strata of Avalonia, and strongly suggest contiguous rather than discrete histories for these terranes throughout the Paleozoic. That these terranes were juxtaposed throughout the early Paleozoic is indicated by the absence of a Cambrian-Ordovician accretionary event, the lack of intervening suture-zone ophiolitic units, and the similarity of Avalonian and Meguma basement Nd isotope signatures in early Paleozoic igneous suites. As Avalonia had accreted to Laurentia-Baltica by the Early Silurian, these data suggest that the Meguma terrane, like Avalonia, resided along the same (northern) margin of the Rheic Ocean at that time. These conclusions have implications for reconstructions of the northern Gondwanan margin in the early Paleozoic and imply that the Silurian-Devonian Acadian orogeny in Maritime Canada occurred in an Andean-type setting and was not related to collision of the Meguma terrane with the Laurentian margin. [ABSTRACT FROM AUTHOR]

The collision of a seamount or microcontinental block with an arc may produce complex strain distributions and changes in the tectono-magmatic setting. Slab rollback and associated upwelling of asthenosphere occurred ca. 440 Ma near the margin of East Gondwana in response to subduction zone lock-up following arc collision ca. 455 Ma. Worldclass porphyry copper gold deposits (e.g., Cadia) formed in the backarc during extension and magmatism associated with slab rollback. Upwelling of the asthenosphere also provided the heat engine for generation of hydrothermal fluids. The result was elevated fluid pressures and compressional brittle deformation associated with orogenic gold mineralization (e.g., Bendigo) inboard of the East Gondwana margin. [ABSTRACT FROM AUTHOR]

Discusses the mechanisms of dynamic system of tectonics and erosion. Nature of steady state; Characterization of orogenic system; Dependence of erosion rate on relief and elevation.

Examines the structural evolution of the Laurentian margin of the Appalachians. Lack of overprints on external zones; Implications for the Salinian orogeny; Transport of the crustal wedge of the upper plate.

Explains the significance of Triassic thermotectonism of the Indosinian orogeny in Vietnam to the Mesozoic accretion in Southeast Asia. Assessment of the crustal reworking based on rock samples for zircon uranium-lead dating; Mineral composition of metamorphic massifs; Impact of oblique convergence on pre-Tertiary structures.

Reports on the dominance of bidirectional, opposed, orogen-parallel extension in the detachment zones north of the More-Trondelag Fault Complex in the Scandinavian Caledonides in western Norway. Action of the fault complex as a transfer zone for the Hoybakken detachment; Magmatic activity due to extension and uplift in central Norway.

Explores alternate styles of deep lithospheric deformation during tectonic convergence in studying orogenic processes. Deformational modes of the model mantle lithosphere; Determination of the occurrence of modes in the Earth beneath collisional orogens.

Discusses the dating of the Indian continental subduction and collisional thickening in the northwest Himalaya and the early evolution of the Himalayan orogeny and the Tso Morari exhumation. Geologic setting and petrography of the Tso Morari dome; Dating the different metamorphic stages, ecologitic, amphibolitic and greenschist.

Examines the role of gravitation abody forces in the evolution of orogenes. Importance of gravitational body forces in the tectonic evolution of mountain belts; Reference to the Cenozoic Great Basin in western United States; Deformation of laramide.

Presents neodymium isotopic data to indicate that normal-thickness lithospheric mantle was preserved beneath the Cordilleran craton from Precambrian to Cenozoic time. Analogy with the South American Cordillera; Implications for Sevier-Laramide orogeny.