Orogenic Curvature Research Articles

The bending of a supra-subduction zone produced crustal thickening and arc migration of the Mongolian Orocline

Orogenic curvatures have been widely recognized along global convergent plate boundaries. Understanding the impact of such curvatures on the tectonic evolution of orogens and their three-dimensional architecture has been challenging. Here we address this issue by studying magmatism around the tightly curved Mongolian Orocline in Central Asia. Our results show that during the Permian–Triassic, arc magmatism around the inner hinge of the orocline became younger towards the core of the orocline. During the same period, the crust was thickened, as indicated by Lanthanum-Ytterbium ratio proxy. These findings, together with the observation that the present-day hinge zone of the Mongolian Orocline is wider, indicate that this zone was subjected to significant crustal-scale contraction. This contractional deformation accounts for the relatively thicker crust around the inner hinge of the Mongolian Orocline, and offers a novel perspective on the formation of elevated topography around the hinge of curved plate boundaries.

The Geometry and Kinematics of the Latest Paleozoic Allatoona Fault, One of the Youngest Thrusts in the Southernmost Appalachian Hinterland, Alabama and Georgia, U.S.A.

The Allatoona thrust fault in the southernmost hinterland of the Appalachian Blue Ridge-Piedmont megathrust sheet is among the latest structures in the kinematic sequence of events along the west flank of the orogen. It is an out-of-sequence, craton-directed thrust fault that cuts metamorphic isograds and earlier thrusts, and it has a nearly linear trace of ≥280 km, making it one of the major thrust faults in the orogen. On the northwest, the fault cuts Pennsylvanian or younger(?) regional cross antiforms that cause significant orogenic curvature of older underlying thrust sheets and is likely Permian in age. To the southeast, however, units within the fault hanging wall maintain a nearly constant width resulting in a significant change in the regional structural architecture of the orogen. In the central segment of the fault, where it marks the western/eastern Blue Ridge domain boundary, a ~20 km-long eyelid window (Mulberry Rock window) framed by three amphibolite facies thrust sheets overlying the greenschist facies Talladega belt allochthon, allows a 3-D view into the structural architecture, kinematics, and trajectories of the regional thrusts. Two earlier thrusts within the window (Mulberry Rock and Burnt Hickory Ridge thrusts, with a combined minimum horizontal net slip component of 27 km) are cut by the Allatoona fault, which is a ~15 m-wide high strain zone with top-to-the-northwest displacement, and a >17.2 km horizontal net slip vector. Structural branch points between the Allatoona and Mulberry Rock thrusts indicate that the Mulberry Rock allochthon is a large north-trending horse beneath the Allatoona fault, centered on the Mulberry Rock window, which is likely the result of oblique ramp thrusting over the massive Mulberry Rock Gneiss. The Allatoona fault cuts down obliquely into the tectonostratigraphy progressively deeper both to the northeast and northwest, locally approaching underlying foreland thrust sheets, and cutting older regional structures. To the northeast, the Allatoona fault lies at the base of the Dahlonega gold belt, becoming an internal eastern Blue Ridge thrust at Dawsonville, Georgia. Although that sequence extends another 120 km into North Carolina, continuation of the Allatoona fault that additional distance is in debate. Regardless, the Allatoona is one of the kinematically latest and longest faults in the southern Appalachian orogen.

Strain analysis of the Xuefengshan Belt, South China: From internal strain variation to formation of the orogenic curvature

The Early Mesozoic Xuefengshan Belt, located in the central part of the South China Block, East Asia, developed as an intracontinental belt during the Middle Triassic. Analyses of structural and strain patterns in Neoproterozoic conglomerates constrain the internal deformation and three-dimensional kinematic evolution of this fold-and-thrust belt. Finite strain data (RXZ) range from 1.1 to 3.2 and generally increase eastwards towards the deep units of the orogenic core. Our results also show that at basal décollement zone the D1 schistosity-forming deformation has a gradient of stronger fabrics, higher strain ratios, and a dominant thrust-normal flattening mechanism. High tectonic levels exhibit lower strain ratios and later structural overprinting that locally modified the D1 strain patterns. Parallel to the belt, enhanced strain and tectonic uplifting along the northern and southern sections of the belt indicate an along-strike structural variation that corresponds to the formation of the orogenic curvature. During the thrusting propagation, large igneous intrusions escaped deformation due to their stiffness and facilitated strain accumulation on the peripheries, whereas stress in the central part migrated progressively and formed a tectonic salient towards the west without significant strain localization.

Devonian Crustal Stretching in the Northern Tasmanides (Australia) and Implications for Oroclinal Bending

AbstractThe Tasmanides in eastern Australia exhibit a number of orogenic curvatures (oroclines), and possibly, a continental‐scale bend that defines the continuation of the Delamerian Orogen with the Thomson Orogen. We provide an insight into the geodynamic processes associated with the origin of this orocline. We present interpretations of seismic reflection profiles and potential field data from the Thomson Orogen, which provide information on the crustal architecture and unravel major structures and kinematic relationships. Results show that a large area in the northern Tasmanides is underlain by thinned crust, bounded in the north and south by ~E‐W trending geophysical features with apparent sinistral and dextral sense of kinematics, respectively. Within the highly extended crust of the Thomson Orogen, there is evidence for widespread Devonian basins bounded by normal faults. In stark contrast to the southern Tasmanides, where rocks show evidence for an earlier (Silurian) episode of extension and Devonian contractional deformation, no evidence for Silurian synrift sedimentation is observed in the Thomson Orogen. Evidence for ~E‐W trending sinistral and dextral crustal‐scale shear zones in the northern and southern boundaries of the Thomson Orogen, respectively, may represent tear faults, which were active during the Early Devonian and were possibly accompanied by tear‐related magmatism. We suggest that crustal stretching in the northern Tasmanides was associated with Devonian back‐arc extension in response to trench retreat, bounded by zones of slab‐tearing and crustal segmentation that ultimately led to the development of the Delamerian‐Thomson Orocline.

Effective elastic thickness in the Central Andes. Correlation to orogenic deformation styles and lower crust high-gravity anomaly

Global studies have assessed the importance of elastic thickness (Te) on orogenic evolution, showing that the style and nature of upper crustal shortening are influenced by the inherited lithospheric strength. Thus, pioneer works have identified that the upper crustal deformation style in the easternmost sector of the Central Andes in South America are related to the elastic thickness (Te). There, the thick-skinned and pure-shear style of Santa Bárbara system was initially related to the existence of low Te values. In contrast, the thin-skinned and simple-shear style of deformation in the Subandean system involves high Te values. However, more recent Te studies in the Central Andes present conflicting results which lead to question this straightforward relation. Results from these studies show a strong dependence on the applied methodology hampering the general understanding of the lithospheric thermo-mechanical state of the Central Andes. To contribute to this issue, we perform a high-resolution Te map, using forward modeling by solving flexural equation of infinite plate model in two dimensions. To achieve this, the crust-mantle interface was calculated using a high-resolution gravity anomaly dataset which combines satellite and terrestrial data, and an average density contrast. Additionally, the gravity anomaly and the foreland basin depth in the Central Andes were best predicted by considering that lower crustal rocks fill the space deflected downward in the plate model. The obtained Te values show an inverse correlation with previous heat flow studies, and a strong spatial correlation with the styles and mechanisms of deformation in the easternmost sector of the Central Andes. In the Santa Bárbara system Te values less than 10 km predominate, whereas in the Subandean system high Te values were observed. Such high values correlate with the orogenic curvature and with an shallower gravity Moho zone, which breaks the regional trend of the Central Andes. This shallower gravity Moho is linked to a high-gravity anomaly located in the east part of the Eastern Cordillera and Subandean system. These results are also correlated with a high-velocity zone in the upper mantle previously found by receiver functions studies. This correlation could indicate changes in the properties of the lower crustal rocks that justify the shallower gravity Moho zone and explain in part the highest Te values.

Controls of structural inheritance on orogenic curvature and foreland basin sedimentation: Insights from the Przemyśl area, Western Carpathians

Orogenic curvatures can have various origins and are widely debated worldwide. In the Poland-Ukraine border area, the Outer Western Carpathians are characterized by a marked curvature. The origin of this curvature was analysed by integrating stratigraphic information with structural constraints and anisotropy of the magnetic susceptibility (AMS) data. Hangingwall frontal ramp domains are characterized by a relatively simple deformation dominated by layer-parallel shortening and folding around a regional NW-SE trending axis, recorded by an AMS lineation with a similar trend. On the other hand, the N-S trending hangingwall oblique ramp domain is characterized by maximum AMS axes recording transpressional strain either dominated by simple shear (sub-horizontal AMS lineation) or pure shear (steeply plunging AMS lineation) components. Early Miocene basin inversion with two distinct depocentres created a number of different detachment surfaces and thickness variations for the sedimentary successions involved in thrusting. The main depocentre of the Lower-Middle Miocene foredeep was originally located in the recess area of the curved Carpathian front. On the other hand, the occurrence of a salient to the west resulted in the axial zone of the foreland flexure being filled with allochthonous units, thereby dramatically reducing the accommodation space for foredeep sediments in this area. Our results suggest that thrust-belt geometry was controlled by the inherited Mesozoic extensional basin architecture.

Geometry, kinematics and tectonic models of the Kazakhstan Orocline, Central Asian Orogenic Belt

The Central Asian Orogenic Belt (CAOB) is one of the largest accretionary orogens on Earth and is characterized by the occurrence of tight oroclines (Kazakhstan and Tuva-Mongolian oroclines). The origin of these large-scale orogenic curvatures is not quite understood, but is fundamentally important for understanding crustal growth and tectonic evolution of the CAOB. Here we provide an outline of available geological and paleomagnetic data around the Kazakhstan Orocline, with an aim of clarifying the geometry, kinematics and geodynamic origin of the orocline. The Kazakhstan Orocline is evident in a total magmatic image, and can be traced by the continuation of high magnetic anomalies associated with the Devonian Volcanic Belt and the Late Devonian to Carboniferous Balkhash-Yili arc. Paleomagnetic data show ∼112–126° clockwise rotation of the northern limb relative to the southern limb in the Late Devonian to Early Carboniferous, as well as ∼15–28° clockwise rotation of the northern limb and ∼39–40° anticlockwise rotation of the southern limb relative to the hinge of the orocline during the Late Carboniferous to Permian. We argue that the Kazakhstan Orocline experienced two-stage bending with the early stage of bending (Late Devonian to Early Carboniferous; ∼112–126°) driven by slab rollback, and the later stage (Late Carboniferous to Permian; 54–68°) possibly associated with the amalgamation of the Siberian, Tarim and Baltic cratons. This new tectonic model is compatible with the occurrence of rift basins, the spatial migration of magmatic arc, and the development of large-scale strike-slip fault systems during oroclinal bending.

Sedimentological evidence for rotation of the Early Permian Nambucca block (eastern Australia)

The Early Permian tectonic history of eastern Australia led to the formation of several orogenic curvatures termed the New England oroclines. How these oroclines formed is a controversial issue that is crucial for understanding the paleo-Pacific subduction dynamics at the Gondwanan margin and the formation of curved orogenic belts in general. Here we present new constraints on the role of vertical-axis block rotations in the New England oroclines using paleocurrent indicators from the core of the oroclinal structure (the Nambucca block). Focusing on the lower sedimentary succession within the Nambucca block (Kempsey beds), we recognize two facies associations. Facies association A comprises conglomerate and gravelly sandstone with minor sandstone, collectively interpreted as the deposits of coastal to subaqueous marine fans. Facies association B is made of heterolithic intervals of sandstone and mudrock that are interpreted as the products of deposition on a marine continental slope. Younging directions suggest that facies association A represents the basal part of the succession that is overlain by the more heterolithic association. The paleogeographic position of the Nambucca block, in conjunction with its stratigraphy and geochronological provenance, suggests that it formed as part of a large, deep-marine backarc basin. Paleocurrent and paleoslope directions are north to northeast, inconsistent with the present understanding of the Permian paleogeography that involved an approximately north-south–oriented continental margin (in present coordinates) and an eastward-deepening marine surface. This supports previous paleomagnetic interpretations of counterclockwise rotations of adjacent blocks. In conjunction with recently published structural, paleomagnetic, and geochronological constraints, our data suggest that counterclockwise rotations occurred between 285 and 275 Ma in the course of the formation of the southern segment of the New England oroclines (Manning orocline). The rotation incorporated both continental and marine plate margin segments of eastern Gondwana, thereby deforming the deep backarc basin that is partially represented by the Nambucca block. Our data thus provide constraints both on the kinematics and on the timing of the much-debated southern segment of the New England oroclines.

Structural inversion of the Tamworth Belt: Insights into the development of orogenic curvature in the southern New England Orogen, Australia

The middle to late Permian Hunter Bowen Event is credited with the development of orogenic curvature in the southern New England Orogen, yet contention surrounds the structural dynamics responsible for the development of this curvature. Debate is largely centred on the roles of orogen parallel strike-slip and orogen normal extension and contraction to explain the development of curvature. To evaluate the dynamic history of the Hunter Bowen Event, we present new kinematic reconstructions of the Tamworth Belt. The Tamworth Belt formed as a Carboniferous forearc basin and was subsequently inverted during the Hunter Bowen Event. Kinematic reconstructions of the Tamworth Belt are based on new maps and cross-sections built from a synthesis of best-available mapping, chronostratigraphic data and new interpretations of depth-converted seismic data. The following conclusions are made from our study: (i) the Hunter Bowen Event was dominantly driven by margin normal contraction (east–west shortening; present-day coordinates), and; (ii) variations in structural style along the strike of the Tamworth Belt can be explained by orthogonal vs. oblique inversion, which reflects the angular relationship between the principal shortening vector and continental-arc margin. Given these conclusions, we suggest that curvature around the controversial Manning Bend was influenced by the presence of primary curvature in the continental margin, and that the Hastings Block was translated along a sinistral strike-slip fault system that formed along this oblique (with respect to the regional east–west extension and convergence direction) part of the margin. Given the available temporal data, the translation of the Hastings Block took place in the Early Permian (Asselian) and therefore preceded the Hunter Bowen Event. Accordingly, we suggest that the Hunter Bowen Event was dominantly associated with enhancing curvature that was either primary in origin, or associated with fault block translation during the Early Permian. This model differs to previously proposed reconstructions where curvature largely formed by orogen parallel strike-slip transportation during the Hunter Bowen Event.

The tectonic history of the Niğde‐Kırşehir Massif and the Taurides since the Late Mesozoic: Paleomagnetic evidence for two‐phase orogenic curvature in Central Anatolia

AbstractThe Niğde‐Kırşehir Massif, known also as the Central Anatolian Block, is bordered by the sutures of the Neotethys Ocean. The massif suffered several deformation phases during and after the consumption of the surrounding oceans and the postcollisional events of the continental pieces of Anatolia in latest Cretaceous to Miocene. Previous paleomagnetic studies on the Niğde‐Kırşehir Massif and its surroundings displayed either insufficient data or have claimed large rotations and/or remagnetization. In order to understand the tectonic history of the Niğde‐Kırşehir Massif and its adjacent blocks we have sampled 147 different sites in the age range of Upper Jurassic to Miocene from the Niğde‐Kırşehir Massif throughout its W/SW and E/SE boundaries and the central‐southeastern Taurides. The results display that except the limestones in central Taurides, all rocks examined carry a primary magnetization. Among these an important finding is that rotations between the massif and the central‐eastern Taurides indicate an oroclinal bending with counterclockwise rotation of R = 41.1° ± 7.6° in the SE and clockwise rotation of R = 45.9° ± 9.3° in the central Taurides from Upper Cretaceous rocks with respect to the African reference direction. Paleomagnetic rotations in the SE Taurides are compatible with the vergent direction of the thrusts generated from consumption of the Intra‐Tauride Ocean prior to postcollisional convergence between Taurides and the massif. In the central Taurides it has been shown that the clockwise rotation of 45.9 ± 9.3 started in Middle Eocene, because of a remagnetization in Upper Cretaceous limestones. The deformation was linked to the final closure of the southern Neotethys and the collision between the African and Eurasian plates. In the Niğde‐Kırşehir Massif counterclockwise rotation up to 25.5° ± 7.3° is recognized during Middle Eocene and interpreted in terms of block rotation together with the Taurides. After the Miocene a counterclockwise rotation of 16.8° ± 3.9° along the Eastern Taurides shows that this area was mostly affected by the westward movement of Anatolia despite the Niğde‐Kırşehir Massif and its SW/W area—the central Taurides—which is recognized as stable with counterclockwise rotation less than 10°.

The tectonic significance of lower Permian successions in the Texas Orocline (Eastern Australia)

The Texas Orocline is a prominent orogenic curvature that developed during the early Permian in the southern New England Orogen. Outliers preserving lower Permian sedimentary successions (Bondonga, Silver Spur, Pikedale, Terrica, Alum Rock and Ashford beds) approximately outline the oroclinal structure, but the tectonic processes responsible for the development of these basinal successions, and their relationships to the Texas Orocline, are unclear. Here we address this shortcoming by providing new U–Pb detrital and primary zircon ages from these successions, as well as detailed stratigraphic and structural data from the largest exposed succession (Bondonga beds). Field observations and U–Pb geochronological data suggest that the lower Permian successions in the Texas Orocline are remnants of a single, formerly larger basin that was deposited after ca 302 Ma. Time constraints for formation of this basin are correlative with constraints from the lower Permian Nambucca Block, which was likely deposited in response to regional back-arc extension during and/or after the development of the Texas Orocline. The conclusion that the lower Permian sedimentary basins in the Texas Orocline belong to this back-arc extensional system supports the suggestion that oroclinal bending in the New England Orogen was primarily controlled by trench retreat and associated overriding-plate extension.

Paleomagnetic data from the New England Orogen (eastern Australia) and implications for oroclinal bending

Orogenic curvatures (oroclines) are common in modern and ancient orogens, but the geodynamic driving forces of many oroclines remain controversial. Here we focus on the New England oroclines of eastern Australia, the formation of which had been previously broadly constrained to the Early–Middle Permian. This time interval encompasses periods of both back-arc extension (at ~300–280Ma) and subsequent contractional deformation (Hunter–Bowen Orogeny) that commenced at ~270Ma along the paleo-Pacific and Gondwanan subduction plate boundary. We present new paleomagnetic data from volcanic rocks that were extruded during the transition from extension to contraction (at ~272Ma), and we show that the oroclinal structure must have formed prior to the emplacement of the volcanic rocks. Our results thus indicate that oroclinal bending in the southernmost New England Orogen has been completed prior to the onset of Middle Permian contractional deformation. It is therefore concluded that the oroclines have likely formed during back-arc extension, and that a major contribution to the orogenic curvature was driven by trench retreat.

Simultaneous batholith emplacement, terrane/continent collision, and oroclinal bending in the Blue Mountains Province, North American Cordillera

The North American Cordillera is a classic example of accretionary orogen, consisting of multiple oceanic terranes attached to the western margin of Laurentia during the Mesozoic times. Although the Cordillera is linear for most parts, terrane boundaries are at a high angle to the overall structural grain in several segments of the orogen, which has been a matter of longstanding controversy as to how and when these orogenic curvatures formed. This paper discusses mechanisms, kinematics, and timing of initiation of one of these major curvatures, the Blue Mountains Province in northeastern Oregon. Here magmatic fabric patterns and anisotropy of magnetic susceptibility in the Wallowa batholith record three phases of progressive deformation of the host Wallowa terrane during Early Cretaceous. First is terrane-oblique ~NE-SW shortening, interpreted as recording attachment of the amalgamated oceanic and fringing terranes to the continental margin during dextral convergence at ~140 Ma. Deformation subsequently switched to pure shear-dominated ~NNE-SSW shortening associated with crustal thickening, caused by continued impingement of the amalgamated Blue Mountains superterrane into a presumed westward concave reentrant in the continental margin at ~135–128 Ma. Upon impingement (at ~126 Ma), the northern portion of the superterrane became “locked,” leading to reorientation of the principal shortening direction to ~NNW-SSE while its still deformable southern portion rotated clockwise about a vertical axis. We thus propose oblique bending as the main mechanism of the orocline formation whereby horizontal compressive forces resulting from plate convergence acted at an angle to the terrane boundaries.

Kinematic reconstruction of the Bolivian orocline

Orogenic curvature is a common feature in many mountain belts and is strongly linked to the magnitude, direction, and mechanics of crustal shortening. Determining how formation of the Bolivian orocline influenced crustal deformation in the central Andes has direct implications for geodynamics of the high-elevation Altiplano plateau. This study presents new reconstructions of the Bolivian orocline constrained by shortening estimates, thermochronology, regional paleomagnetic data, and strain data from lat 12°S to 22°S. The reconstructions investigate paleomagnetically permissible orocline limb rotations of 0°, 6°, and 13° on the kinematic compatibility of shortening constraints. Deformation was restored in 5 m.y. steps from 50 to 0 Ma, and kinematic compatibility was quantified based on the area of map-view overlap at each step. No limb rotation resulted in 14,000 km 2 of overlap, while 13° limb rotations and 50 km of orogen-parallel displacement on known strike-slip faults reduce overlap to 3000 km 2 . The preferred model builds on these results by imposing additional rotations at the orocline core and displacement on the Cochabamba fault. This model reduces overlap to 1600 km 2 but predicts map-view shortening estimates 70–90 km greater in the northern limb and 20–30 km greater in the southern limb than determined from cross sections. Of the modeled increase, ∼20 km is due to limb rotation, while the remaining 50–70 km is due to transpressional shortening on the Cochabamba and Rio Novillero faults. Total shortening in the preferred model is 370 km in the northern limb, 380 km at the orocline core, and 300–350 in the southern limb.

Crustal and upper mantle responses to lithospheric segmentation in the northern Apennines

Lithospheric tear faults are expected to develop in response to along-strike variations in the rates of slab rollback. However, the exact geometry of such structures and their crustal and upper mantle expressions are still debated. We present an analysis of seismic, structural and morphological features that possibly represent the expression of lithospheric segmentation in the northern Apennines. Geophysical observations show evidence for the existence of a discontinuity in the lithospheric structure beneath the northern Apennines, characterized by a change in the spatial distribution of intermediate-depth seismicity, along-strike variations in the pattern of crustal seismicity, and a bend in the Moho topography. The near-surface expression of this discontinuity is associated with an abrupt change in the morphology and exhumation history of the northern Apennines in the proximity of the Livorno-Sillaro Lineament. We interpret these features as evidence for incipient tearing of the lithospheric slab beneath the northern Apennines, marking the boundary between domains that underwent contrasting styles of lithospheric deformation, which are either associated with different rates of slab rollback or a transition from underplating to retreat. We suggest that similar types of structures may play a crucial role in the evolution of convergent plate boundaries, allowing segmentation of orogenic belts and facilitating the development of orogenic curvatures, Ultimately, further tearing along such structures could potentially lead to the occurrence of tear–related magmatism and the formation of slab windows.

Provenance of the Early Permian Nambucca block (eastern Australia) and implications for the role of trench retreat in accretionary orogens

The New England orogen of eastern Australia is characterized by tight orogenic curvatures (oroclines). Oroclinal bending commenced in the Early Permian during a period of extension that involved crustal melting, widespread emplacement of S-type granitoids, high-temperature metamorphism, exhumation of metamorphic complexes, extensional faulting, and development of rift basins. One of these basins is the Early Permian Nambucca block, which is situated in the “core” of the oroclinal structure, but its origin and time of deposition are poorly constrained. Here, we present new U-Pb ages of detrital zircons from the Nambucca block, which include age populations as young as 299 and 285 Ma, confirming the Early Permian deposition of the succession. Additional Devonian–Carboniferous and Precambrian ages indicate that detritus was mainly derived from the New England subduction complex and cratonic Gondwana. The range of ages suggests that the Nambucca Basin received detritus from both arc and continent and that deposition occurred in a back-arc setting. Given the coeval formation of the Nambucca Basin and the New England oroclines, we propose that this back-arc extensional basin was controlled by trench retreat, which resulted in “Mediterranean-style” orogenic curvatures along the plate boundary of eastern Gondwana. The recognition of a genetic link between oroclinal bending and back-arc extension may explain how accretionary orogens, such as the eastern Australian Tasmanides, were able to obtain an anomalous width without a substantial contribution of accreted exotic terranes. A similar mode of tectonism may have played an important role in other accretionary orogens.

Dating of lithospheric buckling: 40Ar/39Ar ages of syn-orocline strike–slip shear zones in northwestern Iberia

Orogenic curvature is a ubiquitous feature of mountain belts, and the plate tectonic and geodynamic setting responsible for the development of curved orogens is a subject of debate. In order to distinguish between different models of orocline formation it is necessary to tightly constrain the absolute timing of oroclinal development. However, determining the absolute (isotopic) timing of oroclinal bending is challenging. The most direct method available to constrain the time interval during which an orocline formed is to obtain absolute age constraints on fabrics generated within syn-orocline strike–slip shear zones that accommodated rotations around a vertical axis during the development of crustal or lithospheric scale orogenic curvature. Here we present a data set of 40Ar/39Ar ages obtained from five shear-zones, some of which display curved traces parallel to the Cantabrian Orocline structural grain in NW Iberia and are interpreted to have been generated coevally to it. The 40Ar/39Ar ages were obtained by laser-rastering induced step-heating on single muscovite crystals that grew synkinematically during shearing. All five samples yielded ages that cluster tightly at 308±3Ma providing direct evidence for the age of oroclinal bending. This age is consistent with the age constraints obtained from paleomagnetic and structural data that place the onset of oroclinal bending in Moscovian times. Our 40Ar/39Ar age determinations therefore provide a reliable absolute constraint on the age of buckling of the Variscan orogen around a vertical axis and provide further evidence that oroclinal bending is unrelated to Variscan convergence/collision or the subsequent extensional collapse of the mountain belt.

Geodynamics of oroclinal bending: Insights from the Mediterranean

The Alpine Orogen in the Mediterranean region exhibits a series of orogenic curvatures (oroclines). The evolution of these oroclines is relatively well constrained by a plethora of geophysical and geological data, and therefore, their origin can inform us on the fundamental processes controlling oroclinal bending. Here, a synthesis of the geometry of Mediterranean oroclines, followed by a discussion on their geodynamic origin is presented. The geometrical synthesis is based on a new classification of Mediterranean oroclines, which defines a first-order orocline (Adriatic Orocline) by the general northward-convex shape of the Alpine Orogen from Cyprus to Gibraltar. Superimposed on the limbs of this orocline, are second-, third- and fourth-order oroclines. The major process that led to the formation of the Adriatic Orocline is the indentation of Adria into Europe, whereas second- and third-order oroclines (e.g., Western Mediterranean and Gibraltar oroclines, respectively) were primarily controlled by a combination of trench retreat and slab tearing. It appears, therefore, that the geodynamics of Mediterranean oroclines has been entirely dependent on plate boundary migration and segmentation, as expressed in the interlinked processes of indentation, trench retreat and slab tearing. The relative contribution of specific geodynamic processes, and their maturity, could be inferred from geometrical characteristics, such as the amplitude-to-width ratio, the orientation of the curvature (convex or concave) relative to the convergence vector, and their geometrical relationship with backarc extensional basins (e.g., in the concave side of the orocline). Based on the information from the Mediterranean oroclines, it is concluded that oroclinal bending commonly involves lithospheric-scale processes, and is not restricted to thin-skinned deformation. However, contrary to previous suggestions that assume that the whole lithosphere can buckle, there is no clear evidence that such processes occur in modern tectonic environments.

The Ainsa Fold and thrust oblique zone of the central Pyrenees: Kinematics of a curved contractional system from paleomagnetic and structural data

AbstractIntegration of structural, stratigraphic, and paleomagnetic data from the N–S trending structures of the Ainsa Oblique Zone reveals the kinematics of the major thrust salient in the central Pyrenees. These structures experienced clockwise vertical axis rotations that vary from 70° in the east (Mediano anticline) to 55° in the west (Boltaña anticline). Clockwise vertical axis rotations of 60° to 45° occurred from early Lutetian to late Bartonian when the folds and thrusts of the Ainsa Oblique Zone developed. This vertical axis rotation stage resulted from a difference of about 50 km in the amount of displacement on the Gavarnie thrust and an accompanying change in structural style at crustal scale from the central to the western Pyrenees, related to the NE–SW trending pinch out of Triassic evaporites at its basal detachment. A second rotation event of at least 10° took place since Priabonian, as a result of a greater displacement of the Serres Marginals thrust sheet with respect to the Gavarnie thrust sheet above the Upper Eocene‐Oligocene salts. The deduced kinematics demonstrates that the orogenic curvature of the central Pyrenees is a progressive curvature resulting from divergent thrust transport direction. Layer parallel shortening mesostructures and kilometer‐scale folds also developed by a progressive curvature related to divergent shortening directions during vertical axis rotation. Rotation space problems were solved by along‐strike extension which triggered the formation of transverse extensional faults and diapirs at the outer arcs of structural bends.

The contorted New England Orogen (eastern Australia): New evidence from U‐Pb geochronology of early Permian granitoids

A series of sharp bends (oroclines) are recognized in the Paleozoic to early Mesozoic New England Orogen of eastern Australia. The exact geometry and origin of these bends is obscured by voluminous magmatism and is still debated. Here we present zircon U‐Pb ages that confirm the lateral continuation of early Permian (296–288 Ma) granitoids and shed new light on the oroclinal structure. Orogenic curvature is defined by the alignment of early Permian granitoids parallel to the structural grain of the orogen, as well as the curved geometry of sub‐vertical deformation fabrics, forearc basin terranes, and serpentinite outcrops. Alternative geometrical interpretations may involve two bends (Texas and Coffs Harbour Oroclines), three bends (+Manning Orocline), or even four bends (+Nambucca Orocline). We argue that the model involving four bends is most consistent with available data, although further kinematic constraints are required to confirm the existence of the Manning and Nambucca Oroclines. A subsequent phase of younger magmatism (<260 Ma) cuts across the curved structural grain, providing a minimum age constraint for orocline development. Assuming a structure of four oroclines, we suggest a tentative tectonic model that involves an early stage of subduction curvature during slab rollback at 300–285 Ma, followed by bending associated with dextral transpression. A final tightening of the curved structures was possibly obtained by E‐W shortening during the late Permian to Triassic (265–230 Ma) Hunter‐Bowen orogeny.