Investigating the kinematics of local thrust sheet rotation in the limb of an orocline: a paleomagnetic and structural analysis of the Esla tectonic unit, Cantabrian–Asturian Arc, NW Iberia

The Esla tectonic unit lies along the southern boundary of the Cantabrian–Asturian Arc, a highly curved foreland fold-thrust belt that was deformed during the final amalgamation of the Pangea supercontinent. Previous structural and paleomagnetic analyses of the Cantabrian–Asturian Arc suggest a two-stage tectonic history in which an originally linear belt was bent into its present configuration, creating an orocline. The Esla tectonic unit is a particularly complex region due to the interaction of rotating thrust sheets from the southern limb of the arc and the southward-directed thrusts of the Picos de Europa tectonic domain during late-stage north–south shortening and oroclinal bending. These structural interactions resulted in intense modification of early-phase thin-skinned tectonic structures that were previously affected by a deeper out-of-sequence antiformal stack that passively deformed the early thrust stack. A total of 75 paleomagnetic sites were collected from the Portilla and Santa Lucia formations, two carbonate passive-margin reef platform units from the middle Devonian. Similar to other regions of the Cantabrian–Asturian Arc, Esla Unit samples carry a secondary remanent magnetization that was acquired after initial thrusting and folding of Variscan deformation in the late Carboniferous. Protracted deformation during late-stage oroclinal bending caused reactivation of existing thrust sheets that include the Esla and younger Corniero and Valbuena thrusts. When combined with existing structural data and interpretations, these data indicate that the present-day sinuosity of the Esla Unit is the consequence of both secondary rotation of originally linear features in the western Esla exposures (e.g., frontal thrusts), and secondary modification and tightening of originally curvilinear features in the eastern Esla exposures (e.g., hanging-wall lateral/oblique ramps). Differences in structural style between the Esla and other tectonic units of the arc highlight the complex kinematics of oroclinal bending, which at the orogen-scale buckled an originally linear, north–south (in present-day coordinates) trending Cantabrian–Asturian thrust belt during the final stages of Pangea amalgamation.     

Clockwise paleomagnetic rotations in northeastern Iran: Major implications on recent geodynamic evolution of outer sectors of the Arabia-Eurasia collision zone

In this study, an extensive paleomagnetic sampling (70 sites) was carried out in north-eastern Iran with the aim of reconstructing the rotation history of the outer margin of the Eurasia-Arabia collision area represented by the Ala-Dagh, Binalud and Kopeh-Dagh mountain belts. We sampled the red beds units from the Lower Cretaceous Shurijeh Fm. and from the Middle-Upper Miocene Upper Red Fm (URF). Paleomagnetic results from all the sampled areas show a homogeneous amount of CW rotations measured in the above-mentioned Formations. These paleomagnetic results suggest that the oroclinal bending process that caused the curvature of Alborz mountain belt in north Iran after the Middle-Late Miocene, also extended to the Ala-Dagh, Binalud and Kopeh-Dagh mountain belts, at the north-eastern border of the Arabia-Eurasia deforming zone.Based on our paleomagnetic results and on GPS, seismological, geomorphological and structural data available in the area, a hypothesis of tectonic evolution of the northern Iran-South Caspian Basin area, from Middle-Late Miocene to Present, is here proposed. In this model, the initiation of the oroclinal bending processes in northern Iran occurred about 6–4 myr ago, related to the impinging of North Iran between the South Caspian block and the southern margin of the Turan platform, driven by the northward subduction of the South Caspian basement under the Aspheron-Balkhan Sill. As paleomagnetic results from this study show a pattern of vertical axis rotations that is inconsistent with the present-day kinematics of the northern Iranian blocks as described by seismicity and GPS data, we suggest that the tectonic processes responsible for the bending of northern Iran mountain chains are no longer active and that the westward motion of the South Caspian basin, and therefore the initiation of opposite strike-slip motion along the Ashk-Abad and Shahrud faults, occurred very recently (∼2 My ago). We therefore propose that initiation of the northward subduction of the South Caspian basin below the Apsheron-Balkhan Sill and the westward extrusion of the South Caspian block did not occur at the same time, with the former occurring between the late Miocene and the Pliocene, and the latter during the Pleistocene.     

Does the Manning Orocline exist? New structural evidence from the inner hinge of the Manning Orocline (eastern Australia)

The map-view structure of the southern New England Orogen in the eastern Gondwanan margin is characterised by four tight orogenic-scale curvatures: Texas, Coffs-Harbour, Manning and Nambucca oroclines. Here we focus on the geometry of the Manning Orocline and examine whether the inner-arc area of the oroclinal structure is expressed within the accretionary wedge rocks of the Tablelands Complex. Our observations from the Tablelands Complex (Armidale–Walcha area) show that rocks were subjected to penetrative deformation (D₁), which resulted in a regional slaty cleavage (S₁) and related isoclinal folds. This was followed by subsequent deformation (D₂) associated with minor gentle folds. In a larger scale, the steeply dipping S₁ structural fabric shows a continuous map-view curvature, thus defining a macroscopic fold structure. We interpret this macroscopic fold as the expression of the Manning Orocline within the accretionary wedge complex. This interpretation is consistent with the contorted spatial distribution of other tectonic elements (serpentinite belt, forearc basin terranes and early Permian granitoids), which independently define the structure of the Manning Orocline. Our new structural data support the existence of the Manning Orocline and the quadruple oroclinal geometry of the whole southern New England Orogen. The origin of these oroclines is attributed to multiple stages of bending, possibly associated with an earlier phase of curvature during slab rollback (in the early Permian), followed by a subsequent (middle-late Permian) episode of contractional deformation that tightened the oroclinal structure.     

四川盆地南缘早三叠世古地磁结果及其构造意义

四川盆地南缘早三叠世古地磁研究表明,川东和川东南地区一系列北北东、北东向褶皱的形成,并非同造山期形成的弧形弯曲构造,它是受深部断裂控制形成的简单褶皱。这一结果再次表明,华北与扬子地块的拼合晚于早三叠世,可能于早/中侏罗世完成拼合过程。     

Primary vs. secondary curved fold axes: Deciphering the origin of the Aït Attab syncline (Moroccan High Atlas) using paleomagnetic data

The Aït Attab syncline, located in the Central High Atlas, displays a curved geometry in plan view, and is considered as one of the most spectacular fold shapes in the Central High Atlasic belt. We conducted a paleomagnetic study in Jurassic-Cretaceous red beds to investigate the origin of this geometry. The Natural Remanent Magnetization (NRM) is dominated by a secondary magnetization carried by haematite with unvarying normal polarity that has been dated at about 100 Ma. The regional fold test performed in both limbs of the syncline is positive and the paleomagnetic vectors (after tectonic correction) are parallel throughout the curvature, indicating a negative oroclinal bending test. These results are inconsistent with previous works that consider the bent geometry of this syncline to result from subsequent distortion of originally NE–SW trending structures by rotation about a vertical axis. We interpret the NRM data to demonstrate that the changing trend of the Aït Attab syncline is a primary feature, resulting from the influence of pre-existing, NE–SW and E-W-striking extensional faults that developed during a strike-slip regime. Paleomagnetic results also reveal that the tilting observed in the sampled red beds is post Albian, probably linked to the Cenozoic inversion of the High Atlasic belt.     

Kinematics of orocline-parallel faults in the Texas and Coffs Harbour oroclines (eastern Australia) and the role of flexural slip during oroclinal bending

The Texas and Coffs Harbour oroclines are defined by a Z-shaped curvature in the southern New England Orogen (eastern Australia), but the geometry and kinematics of faults around these oroclines, as well as their possible role during oroclinal bending, have hitherto not been understood. Using aeromagnetic and open file seismic data, as well as field observations, the pattern, geometry and kinematics of fault systems, have been investigated. Fault traces with a strike-slip component are oriented parallel to the curved magnetic and structural fabrics of the Texas and Coffs Harbour oroclines. Our observations show evidence for sinistral or sinistral-reverse, dextral (or dextral-reverse) and normal kinematics along NW-striking faults. The dominant kinematics along NNE- and NE-striking faults is dextral or dextral-reverse. The timing of faulting is not well constrained, but the ubiquitous recognition of orocline-parallel faults may suggest that a flexural slip mechanism operated during oroclinal bending in the early–middle Permian (ca 299–265 Ma). Our observations indicate that many of the orocline-parallel faults, with strike-slip separation, were reactivated during the Mesozoic and Cenozoic, as indicated by the recognition of displaced Triassic granitoids, Mesozoic sedimentary rocks and Cenozoic basalts.     

The Eocene Southern Vancouver Island Orocline — a response to seamount accretion and the cause of fold-and-thrust belt and extensional basin formation

A deflection of the fault controlled southwestern coastline of Vancouver Island suggests the presence of a minor orocline, with a Southern Crustal Block (south of Barkley Sound–Alberni Inlet) rotated 20° counterclockwise relative to a Northern Fixed Crustal Block about a pole of rotation located northeast of Port Alberni. In this paper two models of orocline development, one of pure block rotation and one of pure bending, are proposed. The predictions of these models are tested against available geological maps, structural orientation data, identified regions of extension and contraction, and paleomagnetic data. Structural orientation and paleomagnetic data are consistent with 18° of post-Late Cretaceous counter clockwise rotation of the Southern Crustal Block relative to the Northern Fixed Crustal Block. A southward increase in the magnitude of rotation evident in the structural orientation data argues for a model of bending. Both bending and block rotation models predict the development of a zone of contraction along the northeast margin of the Southern Crustal Block, coincident with the location of the Eocene Cowichan fold-and-thrust belt, that diminishes northward toward the pole of rotation. As predicted, the fold-and-thrust belt is characterized by a northerly decrease in the amount of shortening, from >30% at the south end of the thrust belt, to 0% shortening north of Port Alberni. The northerly decrease in shortening is complemented by a north to south change in structural style from cylindrical to conical folds, and finally to planar, undeformed strata. The model of block rotation predicts the presence of a zone of extension extending southwest from the zone of rotation, coincident with the location of Eocene extensional structures within Barkley Sound and with horst and graben structures in the offshore Eocene to Miocene Tofino basin. Extension is less than predicted by a model of pure block rotation and suggests that much of the oroclinal rotation was accommodated by bending. Timing constraints indicate that orocline development was coeval with, and resulted from, the Eocene accretion of seamounts of the Crescent terrane. These findings demonstrate that oroclinal orogeny, the buckling of a linear crustal beam about vertical axes of rotation, can significantly impact the geometry, structure and character of an orogenic belt, even where the buckles are minor (<20° of rotation).     

Cryptic tectonic domains of the Klamath Mountains, California and Oregon

Numerous fragments of oceanic crust and island arcs make up the Klamath Mountains province. These fragments were joined together (amalgamated) in an oceanic setting during Paleozoic and Mesozoic collisional events and were accreted to North America as a composite unit during latest Jurassic or earliest Cretaceous time. The roughly arcuate and concentric distribution of the terranes of the Klamath Mountains does not now seem to be a result of simple oroclinal bending as earlier believed. Although commonly described as a west-facing arcuate structure, the province is cut diagonally by a vaguely defined NW-trending zone of discontinuity, or hinge line, that divides the province into NE and SW tectonic domains. The zone of discontinuity is marked by a number of lithic and structural anomalies, and particularly by the distribution of a remarkable series of belts of plutonic rocks. The terranes, regional structures, and plutonic belts of the NE domain trend NE and are generally wider and more coherent than the narrow NW-trending terranes and plutonic belts of the SW domain. Most plutonic belts of the NE domain do not have equivalents in the SW domain. Paleomagnetic evidence suggests that all the plutonic belts, except possibly the youngest (the earliest Cretaceous Shasta Bally belt), were emplaced before the Klamath Mountains terranes finally accreted to North America.     

Eocene paleomagnetism of the Caucasus (southwest Georgia): oroclinal bending in the Arabian syntaxis

The Caucasus is very important for our understanding of tectonic evolution of the Alpine belt, but only a few reliable paleomagnetic results were reported from this region so far. We studied a collection of more than 300 samples of middle Eocene volcanics and volcano-sedimentary rocks from 10 localities in the Adjaro–Trialet tectonic zone (ATZ) in the western part of the Caucasus. Stepwise thermal demagnetization isolates a characteristic remanent magnetization (ChRM) in 19 sites out of 31 studied. ChRM reversed directions prevail, and a few vectors of normal polarity are antipodal to the reversed ones after tilt correction. The fold test is positive too, and we consider the ChRM primary. Analysis of Tertiary declinations and strikes of Alpine folds in the Adjaro–Trialet zone and the Pontides in Northern Turkey shows a large data scatter; Late Cretaceous data from the same region, however, reveal good correlation between paleomagnetic and structural data. Combining Late Cretaceous and Tertiary data indicates oroclinal bending of the Alpine structures which are locally complicated with different deformation. The overall mean Tertiary inclination is slightly shallower than the reference Eurasian inclination recalculated from one apparent polar wander path (APWP), but agrees with other. This finding is in accord with geological evidence on moderate post-Eocene shortening across the Caucasus. We did not find any indication of long-lived paleomagnetic anomalies, such as to Cenozoic anomalously shallow inclinations further to the east in Central Asia.     

A unified kinematic model for the evolution of detachment folds

Detachment folds represent a major structural element in a number of fold belts. They are common in the Jura Mountains, the Zagros fold belt, the Central Appalachian fold belt, the Wyoming fold-belt, the Brooks Range, the Parry Islands fold belt, and parts of the SubAndean belt. These structures form in stratigraphic packages with high competency contrasts among units. The competent upper units exhibit parallel fold geometries, whereas the weak lower unit displays disharmonic folding and significant penetrative deformation. Two distinct geometric types, disharmonic detachment folds, and lift-off folds have been recognized. However, these structures commonly represent different stages in the progressive evolution of detachment folds. The structures first form by symmetric or asymmetric folding, with the fold wavelength controlled by the thickness of the dominant units. Volumetric constraints require sinking of units in the synclines, and movement of the ductile unit from the synclines to the anticlines. Continuing deformation results in increasing fold amplitudes and tighter geometries resulting from both limb segment rotation and hinge migration. Initially, limb rotation occurs primarily by flexural slip folding, but in the late stages of deformation, the rotation may involve significant internal deformation of units between locked hinges. The folds eventually assume tight isoclinal geometries resembling lift-off folds. Variations in the geometry of detachment fold geometry, such as fold asymmetry, significant faulting, and fold associated with multiple detachments, are related to variations in the mechanical stratigraphy and pre-existing structure.     

柴达木盆地东缘早古生代弯山构造

位于中国中央造山带内部的柴达木盆地周缘出露有代表原特提斯洋盆的蛇绿岩带、指示大洋俯冲与大陆深俯冲的高压-超高压变质带以及不同性质的早古生代花岗岩带。根据这些构造单元的空间展布形态及其综合地质年龄分布,表现为一条环绕柴达木盆地东缘的连续而弯曲的加里东期造山带。造山带内发育一系列右行走滑断裂和韧性剪切带,与古地磁资料所揭示的柴达木地块在早古生代的相对逆时针旋转息息相关。本文提出,柴达木盆地周缘造山带为一弯山构造。它是在原特提斯洋向南斜向俯冲闭合过程中,诱发的大型走滑断裂和柴达木地块逆时针旋转牵引造山带发生弯曲所致。     

Unraveling the early–middle Paleozoic paleogeography of Kazakhstan on the basis of Ordovician and Devonian paleomagnetic results

It is a common concept that different tectonic units in the western part of the Central Asian Orogenic Belt were united into the landmass of the Kazakhstania continent in the Paleozoic but many important details of its history remain enigmatic and controversial. Recently published paleomagnetic data from this region demonstrate that the ~ 2000 km long horseshoe-shaped Devonian Volcanic Belt was created by oroclinal bending of an originally rectilinear active margin of Kazakhstania. Still, the Silurian and Devonian paleomagnetic results which this interpretation is based upon are limited and unevenly spread along the belt, and additional middle Paleozoic data are highly desirable. Accordingly, we studied three mid-Paleozoic objects from different segments of this volcanic belt. Two of the three new objects yielded paleomagnetic directions that fit perfectly into the oroclinal scenario, whereas the third one provided no interpretable data. The earlier history of Kazakhstania, however, remains misty. We obtained three new Ordovician results in north–central Kazakhstan and found similar inclinations but widely dissimilar declinations. Previously published data show a large scatter of Ordovician declinations in South Kazakhstan and Kyrgyzstan as well. We analyzed all seven Middle–Late Ordovician paleolatitudes and came to the conclusion that a nearly E–W trending active margin of the Kazakhstania landmass had existed at low (~ 10°S) latitudes at that time. We hypothesize that this margin of the Kazakhstania landmass collided with another island arc, called Baydaulet–Akbastau, and with the Aktau–Junggar microcontinent by the Ordovician–Silurian boundary. As a result of this collision, subduction ceased, and regional deformation, magmatism, and rotations of crustal fragments took place in most of Kazakhstania. In Silurian time, Kazakhstania moved northward crossing the equator and rotating clockwise by ~ 45°. This changed the orientation of the Kazakhstania to NW–SE, and thereby established the (rectilinear) predecessor of the modern curved Devonian Volcanic Belt.     

准噶尔南缘新生代断裂的形成机制

准南有三条走向东西右列的新生代冲断褶皱带,是天山北麓右行走滑兼走逆冲断层的尾端冲断扇构造。各冲断褶皱带的西端与天山北麓断层相接触处形成最早,在中新世中期开始形成,主体在中新世晚期约10Ma开始形成,其末端在第四纪才开始形成,表现出挤压的构造动力和变形自南向北扩展。准南逆冲构造带的初始时间比天山南麓的库车逆冲构造带晚约8Ma,说明天山造山带因为塑性较高,构造动力传播是耗时的,这与塔里木盆地刚性高、瞬时传递的特征形成对照。瞬时传递构造应力和耗时传递构造动力在空间上的交替出现是印藏陆陆碰撞导致陆内变形传播形式的基本原因。     

Primary curvature in the Mid-Continent Rift: Paleomagnetism of the Portage Lake Volcanics (northern Michigan, USA)

Rocks of the North American Mid-Continent Rift (MCR) in the Keweenaw Peninsula of northern Michigan display a change in structural trend from east to west, varying in strike from 100° in the east to 35° farther west, before returning to a more east–west trend of 80° near Wisconsin. In general, curvature can be described either as of primary origin, meaning that the arc developed in its present curved state, or as of secondary origin, when the arc formed from an initially straighter geometry. A powerful tool in evaluating the origin and degree of curvature in any deformed belt is paleomagnetism, where coincident change in declination and strike is evidence for secondary curvature. Several paleomagnetic studies have been completed on rift-related sedimentary rocks, as well as lava flows, from the MCR in the Lake Superior region. Paleomagnetic directions for the sedimentary rocks vary much more in declination than in inclination, possibly reflecting a vertical axis rotation. If secondary rotation has affected the sedimentary rocks, then underlying volcanic rocks should also demonstrate a similar rotation.Thirty-one sites were collected from the Portage Lake Volcanics in the most highly curved part of the Keweenaw Peninsula in the Upper Peninsula of Michigan. Sites were chosen to maximize the variation in structural trend. Thermal demagnetization results showed two components in samples from most sites, a lower temperature A component (< 580 °C) as well as a higher temperature B component (> 580 °C). Both components were tested for primary remanence using a conglomerate test. The A component, carried by magnetite, passes the conglomerate test at a 95% confidence level, and is therefore considered a primary remanence. The B component, carried by hematite, fails at the 95% level and is considered a secondary magnetization. Most importantly, declination of the primary (A) magnetization between sites shows no correlation with strike, demonstrating that vertical axis rotations cannot explain the curvature of the Mid-Continent Rift. We conclude that curvature in the Lake Superior Region is a primary feature, likely reflecting a pre-existing zone of weakness that was exploited during rifting.     

Oroclinal bending of the Middle and Late Paleozoic volcanic belts in Kazakhstan: Paleomagnetic evidence and geological implications

Paleomagnetic data on Middle- and Late-Paleozoic rocks from the central part of the Ural-Mongolian Belt in Kazakhstan are considered. The primary remanences in the Permian rocks and secondary magnetization components of the same age in pre-Permian rocks of central and northern Kazakhstan are not rotated relative to the East European Platform. In southern Kazakhstan adjoining the Tien Shan almost all data point to large, up to 90°, counterclockwise rotation of blocks. These rotations, related to the regional wrench fault zone, must be subtracted from older paleomagnetic data to ensure their correct interpretation. The paleomagnetic declinations in Upper Carboniferous rocks coincide more or less over all of Kazakhstan, whereas the Silurian and Early Devonian declinations in the north and south of Kazakhstan differ approximately by 180°. It can be suggested that the Devonian volcanic belt, having a horseshoe outline, was initially an almost rectilinear NW-trending feature. Its oroclinal bending took place in the Devonian and Early Carboniferous and completed by the Late Carboniferous. We compared the model of the Kazakh Orocline based on paleomagnetic data with the geological events in this territory. It turned out that a slow bending of an initially rectilinear subduction zone is consistent with lateral migration of active volcanism and folding inside a developing loop, whereas extension outside the loop was accompanied by subsidence and rifting. In general, the proposed model connects the main tectonic events in Kazakhstan with the movements established from paleomagnetic data.     

Internal deformation of Sikhote Alin volcanic belt, far eastern Russia: Paleocene paleomagnetic results

We present paleomagnetic results of Paleocene welded tuffs of the 53–50 Ma Bogopol Group from the northern region (46°N, 137°E) of the Sikhote Alin volcanic belt. Characteristic paleomagnetic directions with high unblocking temperature components above 560 °C were isolated from all the sites. A tilt-corrected mean paleomagnetic direction from the northern region is D=345.8°, I=49.9°, α₉₅=14.6° (N=9). The reliability of the magnetization is ascertained through the presence of normal and reversed polarities. The mean paleomagnetic direction from the northern region of the Sikhote Alin volcanic belt reflects a counterclockwise rotation of 29° from the Paleocene mean paleomagnetic direction expected from its southern region. The counterclockwise rotation of 25° is suggested from the paleomagnetic data of the Kisin Group that underlies the Bogopol Group. These results establish that internal tectonic deformation occurred within the Sikhote Alin volcanic belt over the past 50 Ma. The northern region from 44.6° to 46.0°N in the Sikhote Alin volcanic belt was subjected to counterclockwise rotational motion through 29±17° with respect to the southern region. The tectonic rotation of the northern region is ascribable to relative motion between the Zhuravlevka terrane and the Olginsk–Taukhinsk terranes that compose the basements of the Sikhote Alin volcanic belt.     

Late orogenic, large-scale rotations in the Tien Shan and adjacent mobile belts in Kyrgyzstan and Kazakhstan

Most of Kazakhstan belongs to the central part of the Eurasian Paleozoic mobile belts for which previously proposed tectonic scenarios have been rather disparate. Of particular interest is the origin of strongly curved Middle and Late Paleozoic volcanic belts of island-arc and Andean-arc affinities that dominate the structure of Kazakhstan. We undertook a paleomagnetic study of Carboniferous to Upper Permian volcanics and sediments from several localities in the Ili River basin between the Tien Shan and the Junggar–Alatau ranges in southeast Kazakhstan. Our main goal was to investigate the Permian kinematic evolution of these belts, particularly in terms of rotations about vertical axes, in the hope of deciphering the dynamics that played a role during the latest Paleozoic deformation in this area. This deformation, in turn, can then be related to the amalgamation of this area with Baltica, Siberia, and Tarim in the expanding Eurasian supercontinent. Thermal demagnetization revealed that most Permian rocks retained a pretilting and likely primary component, which is of reversed polarity at three localities and normal at the fourth. In contrast, most Carboniferous rocks are dominated by postfolding reversed overprints of probably “mid-Permian” age, whereas presumably primary components are isolated from a few sites at two localities. Mean inclinations of primary components generally agree with coeval reference values extrapolated from Baltica, whereas declinations from primary as well as secondary components are deflected counterclockwise (ccw) by up to  90°. Such ccw rotated directions have previously also been observed in other Tien Shan sampling areas and in the adjacent Tarim Block to the south. However, two other areas in Kazakhstan show clockwise (cw) rotations of Permian magnetization directions. One area is located in the Kendyktas block about 300 km to the west of the Ili River valley, and the other is found in the Chingiz Range, to the north of Lake Balkhash and about 400 km to the north of the Ili River valley. The timing of the ccw as well as cw rotations is clearly later than the disappearance of any marine basins from northern Tarim, the Tien Shan and eastern Kazakhstan, so that the rotations cannot be attributed to island-arc or Andean-margin plate settings — instead we attribute the rotations to large-scale, east–west (present-day coordinates), sinistral wrenching in an intracontinental setting, related to convergence between Siberia and Baltica, as recently proposed by Natal'in and Şengör [Natal'in, B.A., and Şengör, A.M.C., 2005. Late Palaeozoic to Triassic evolution of the Turan and Scythian platforms: the pre-history of the palaeo-Tethyan closure, Tectonophysics, 404, 175–202.]. Our previous work in the Chingiz and North Tien Shan areas on Ordovician and Silurian rocks suggested relative rotations of  180°, whereas the Permian declination differences are of the order of 90° between the two areas. Thus, we assume that about 50% of the total post-Ordovician rotations are of pre-Late Permian age, with the other half of Late Permian–earliest Mesozoic age. The pre-Late Permian rotations are likely related to oroclinal bending during plate boundary evolution in a supra-subduction setting, given the calc-alkaline character of nearly all of the pre-Late Permian volcanics in the strongly curved belts.     

Peri-equatorial paleolatitudes for Jurassic radiolarian cherts of Greece

Radiolarian-rich sediments dominated pelagic deposition over large portions of the Tethys Ocean during middle to late Jurassic time as shown by extensive bedded chert sequences found in both continental margin and ophiolite units of the Mediterranean region. Which paleoceanographic mechanisms and paleotectonic setting favored radiolarian deposition during the Jurassic, and the nature of a Tethys-wide change from biosiliceous to biocalcareous (mainly nannofossil) deposition at the beginning of Cretaceous time, have remained open questions. Previous paleomagnetic analyses of Jurassic red radiolarian cherts in the Italian Apennines indicate that radiolarian deposition occurred at low peri-equatorial latitudes, similar to modern day deposition of radiolarian-rich sediments within equatorial zones of high biologic productivity. To test this result for other sectors of the Mediterranean region, we undertook paleomagnetic study of Mesozoic (mostly middle to upper Jurassic) red radiolarian cherts within the Aegean region on the Peloponnesus and in continental Greece. Sampled units are from the Sub-Pelagonian Zone on the Argolis Peninsula, the Pindos–Olonos Zone on the Koroni Peninsula, near Karpenissi in central Greece, and the Ionian Zone in the Varathi area of northwestern Greece. Thermal demagnetization of samples from all sections removed low-temperature viscous and moderate-temperature overprint magnetizations that fail the available fold tests. At Argolis and Koroni, however, the cherts carry a third high-temperature magnetization that generally exhibits a polarity stratigraphy and passes the available fold tests. We interpret the high-temperature component to be the primary magnetization acquired during chert deposition and early diagenesis. At Kandhia and Koliaky (Argolis), the primary declinations and previous results indicate clockwise vertical-axis rotations of  40° relative to “stable” Europe. Due to ambiguities in hemispheric origin (N or S) and thus paleomagnetic polarity, the observed declinations could indicate either clockwise (CW) or counterclockwise (CCW) vertical-axis rotations. Thus at Adriani (Koroni), the primary declinations indicate either CW or CCW rotations of  95° or  84°, depending on paleomagnetic polarity and age. The primary inclinations for all Peloponnesus sites indicate peri-equatorial paleolatitudes similar to those found for coeval radiolarian cherts exposed in other Mediterranean orogenic belts. Our new paleomagnetic data support the interpretation that Mesozoic radiolarites within the Tethys Ocean were originally deposited along peri-equatorial belts of divergence and high biologic productivity.     

Paleomagnetism of the Central Iberian curve's putative hinge: Too many oroclines in the Iberian Variscides

The Variscan mountain belt in Iberia defines a large “S” shape with the Cantabrian Orocline in the north and the Central Iberian curve, an alleged orocline belt of opposite curvature, to the south. The Cantabrian Orocline is kinematically well constrained, but the geometry and kinematics of the Central Iberian curve are still controversial. Here, we investigate the kinematics of the Central Iberian curve, which plays an important role in the amalgamation of Pangea since it may have accommodated much of the post-collisional deformation. We have performed a paleomagnetic study on Carboniferous granitoids and Cambrian limestones within the hinge of the curve. Our paleomagnetic and rock magnetic results show a primary magnetization in the granitoids and a widespread Carboniferous remagnetization of the limestones. Syn-kinematic granitoids show ca. 70° counter-clockwise rotations consistent with the southern limb of the Cantabrian Orocline. Post-kinematic granitoids and Cambrian limestones show consistent inclinations but very scattered declinations suggesting that they were magnetized coevally to and after the ~ 70° rotation. Our results show no differential rotations between northern, southern limb and the hinge zone. Therefore, we discard a late Carboniferous oroclinal origin for the Central Iberian curve.     

The role of salt in fold-and-thrust belts

The style of deformation in thin-skinned fold-and-thrust belts is critically dependent upon the resistance to sliding along the detachment between the mass of deforming sediments and the underlying rocks. Evaporites can provide an extremely weak horizon within which a basal detachment can form and along which only a relatively small shear traction can be supported. Fold-and-thrust belts that form atop a salt layer, such as the Appalachian Plateau, the Franklin Mountains in northwestern Canada, and the Jura of the Alps, among others, share several readily observable characteristics. As predicted by a simple mechanical model for fold-and-thrust belts, a detachment in salt permits a thrust belt to have an extremely narrow cross-sectional taper. In addition, predicted orientations of the principal stress axes over a salt décollement are consistent with the commonly observed lack of a consistently dominant vergence direction of structures within the thrust belt. Other common attributes of salt-basal thin-skinned deformation include the presence of several widely but regularly spaced folds and abrupt changes in deformational style at the edge of the salt basin.