Counterclockwise Vertical Axis Rotation Research Articles

Translation, collision and vertical‐axis rotation in the Organyà and Montsec minibasins (South‐Central Pyrenees, Spain)

AbstractThis paper presents a sequentially restored cross‐section of the Organyà and Montsec minibasins based on geological mapping, new field observations and available borehole data. The main objective was to describe the geometry and evolution of both basins in terms of salt tectonics and minibasin mobility. To this end, a comprehensive palaeomagnetic database has been used to constrain vertical‐axis rotations potentially related to minibasin translation and pivoting. The Organyà minibasin constitutes an asymmetric depocentre formed during the Upper Jurassic‐Lower Cretaceous by translation above a southerly inclined salt layer. Salt evacuation and minibasin touchdown induced salt accumulation on the northern side of the basin that culminated in the development of the major Santa Fe unconformity during the late Albian—early Cenomanian. Indicative of salt quiescence is the following isopachous Cenomanian to lower Santonian sequence Salt tectonics resumed during the late Santonian—Palaeocene, with the Montsec minibasin downbuilding coinciding with the onset of Pyrenean convergence. Changes of the base‐salt topography reflects regional‐scale geodynamic processes. The acceleration of crustal thinning in the North Pyrenean zone during the late Albian‐early Cenomanian favoured uplift in the Axial Zone, increasing slope and triggering salt mobilization in the Southern Pyrenees. Likewise, the onset of contraction renewed the downslope gliding of the Organyà and Montsec minbasins, and supports the idea that the early stages of basin inversion were governed by gravity tectonics. The kinematic reconstruction suggests that the more that 30° counterclockwise vertical axis rotation records pivoting during the suprasalt translation of the Organyà minibasin rather than solely the Iberian microplate rotation.

The Upper Jurassic Garedu Red Bed Formation of the northern Tabas Block: elucidating Late Cimmerian tectonics in east-Central Iran

The Garedu Red Bed Formation (GRBF) of the northern Tabas Block (Central-East Iranian Microcontinent, CEIM) is a lithologically variable, up to 500-m-thick, predominantly continental unit. It rests gradually or unconformably on marine limestones of the Esfandiar Subgroup (Callovian–Oxfordian) and is assigned to the Kimmeridgian–Tithonian. In the lower part, it consists of pebble- to boulder-sized conglomerates/breccias composed of limestone clasts intercalated with calcareous sandstones, litho-/bioclastic rudstones and lacustrine carbonates. Up-section, sharp-based pebbly sandstones and red silt-/fine-grained sandstones of braided river origin predominate. Palaeocurrent data suggest a principal sediment transport from west to east and a lateral interfingering of the GRBF with marine greenish marls of the Korond Formation at the eastern margin of the Tabas Block. Westwards, the GRBF grades into the playa deposits of the Magu Gypsum Formation. Red colours and common calcretes suggest arid to semi-arid climatic conditions. The onset of Garedu Red Bed deposition indicates a major geodynamic change with the onset of compressive tectonics of the Late Cimmerian Tectonic Event (LCTE), being strongest at the eastern margin of the northern Tabas Block. When traced southwards, the same tectonic event is expressed by extension, indicating a shift in tectonic style along the boundary fault between the Tabas and Lut blocks. The complex Upper Jurassic facies distribution as well as the spatio-temporal changes in tectonic regime along the block-bounding faults are explained by the onset of counterclockwise vertical-axis rotation of the CEIM in the Kimmeridgian. The block boundaries accommodated the rotation by right-lateral strike slip, transpressional in today’s northern and transtensional in today’s southern segments of the block-bounding faults. Rotation occurred within bracketing transcurrent faults and continued into the Early Cretaceous, finally resulting in the opening of narrow oceanic basins encircling the CEIM. Palaeogeographically, the GRBF is part of a suite of red bed formations not only present on the CEIM, but also along the Sanandaj-Sirjan Zone (NW Iran), in northeastern Iran and beyond, indicating inter-regional tectonic instability, uplift and erosion under (semi-)arid climatic conditions across the Jurassic–Cretaceous boundary. Thus, even if our geodynamic model successfully explains Late Jurassic tectonic rotations, fault motions and facies distribution for the CEIM, the basic cause of the LCTE still remains enigmatic.

Dramatic Pliocene–Quaternary subsidence of the southern Rhodes Basin and concomitant north-tilting and uplift of the Anaximander Mountains, the junction of Hellenic and Cyprus arcs, eastern Mediterranean Sea

The Anaximander Mountains and the Rhodes Basin at the junction of Cyprus and Hellenic arcs are one of the tectonically most complex regions in the eastern Mediterranean. Harmonically-tilted Messinian and the uppermost Messinian–Pliocene successions across the northern and eastern margins of the Rhodes Basin showed that these regions experienced dramatic subsidence exceeding ~1500–2500 ms (~1125–1875 m @ 1500 m s−1) during the upper Pliocene–Quaternary. Tilted upper Pliocene–Quaternary growth strata wedges showing apparent north-directed downlap along the northern foothills of the Anaximander Mountain suggested that this region experienced ~1000–2500 ms (~750–1125 m) concomitant uplift. The absence of faults showing extensional stratigraphic separations and the relative subsidence rates calculated using amount of tilting collectively suggest that the Rhodes Basin experienced sagging of its deeper basinal regions, which is attributable to the loading effect of the building of the Tauride Mountains modified by the vertical axis rotations of the Beydağları Block and the Island of Rhodes, and the horizontal axis rotation of the Anaximander Mountain.Messinian and uppermost Messinian–Pliocene were tectonically relatively quiet as shown by the harmonically structured and isopachous successions across the Rhodes Basin and Anaximander Mountain. This interval coincides with the paleomagnetically-determined 9° counterclockwise vertical axis rotation of the Island of Rhodes. The dramatic subsidence observed within the Rhodes Basin and the concommitant uplift of the Anaximander Mountain and its western extension occurred broadly coincident with southeast tilting of the Island of Rhodes and its associated 500–600 m subsidence between 2.5 and 1.8 Ma, the northwest tilting between 1.5 and 1.1 Ma, and the associated re-emergence of the southeastern coast. The large counterclockwise rotation of the Island of Rhodes since 0.8 Ma corresponds with the enormous subsidence observed across the northeastern sector of the Rhodes Basin, which brought the shelf-edge deltas from the near-sea level to their present-day depth of 1200–1500 m.

Clockwise vertical-axis rotation in the West Vardar zone of Serbia: tectonic implications

The Vardar-Tethyan mega-suture between the Eurasian and Adria (Gondwana) margins comprises remnants of oceanic lithosphere of Neotethys and distal parts of the adjacent continental margins including unconformable Late Cretaceous sedimentary cover and Cenozoic igneous rocks of post-obduction age. This study provides kinematic constraints for displacements and rotations affecting the Serbian part of the West Vardar and Jadar-Kopaonik units after the closure of the Neotethyan (= Sava) Ocean. The targets of the paleomagnetic research were Late Cretaceous sediments representing an overstep sequence and a wide variety of Oligocene-Earliest Miocene igneous rocks. The studied 48 sampling points were distributed within two segments of a narrow and 160 km long strip of the Western Vardar zone, namely the wider Rudnik Mts. area to the north and the Kopaonik Mts. area to the south. The results of standard laboratory processing and statistical evaluation suggest that both studied segments of the Western Vardar zone were affected by 30°–46° clockwise vertical-axis rotation which must have taken place after 20 Ma. It is proposed that clockwise vertical-axis rotation documented in the present study on one hand, and the previously documented counterclockwise vertical-axis rotation of Adria, on the other hand are related to large scale extension in the southern Pannonian basin.

Paleomagnetic constraints on the timing and distribution of Cenozoic rotations in Central and Eastern Anatolia

Abstract. To quantitatively reconstruct the kinematic evolution of Central and Eastern Anatolia within the framework of Neotethyan subduction accommodating Africa–Eurasia convergence, we paleomagnetically assess the timing and amount of vertical axis rotations across the Ulukışla and Sivas regions. We show paleomagnetic results from ∼ 30 localities identifying a coherent rotation of a SE Anatolian rotating block comprised of the southern Kırşehir Block, the Ulukışla Basin, the Central and Eastern Taurides, and the southern part of the Sivas Basin. Using our new and published results, we compute an apparent polar wander path (APWP) for this block since the Late Cretaceous, showing that it experienced a ∼ 30–35° counterclockwise vertical axis rotation since the Oligocene time relative to Eurasia. Sediments in the northern Sivas region show clockwise rotations. We use the rotation patterns together with known fault zones to argue that the counterclockwise-rotating domain of south-central Anatolia was bounded by the Savcılı Thrust Zone and Deliler–Tecer Fault Zone in the north and by the African–Arabian trench in the south, the western boundary of which is poorly constrained and requires future study. Our new paleomagnetic constraints provide a key ingredient for future kinematic restorations of the Anatolian tectonic collage.

Paleomagnetism in Extremadura (Central Iberian zone, Spain) Paleozoic rocks: extensive remagnetizations and further constraints on the extent of the Cantabrian orocline

The winding Variscan belt in Iberia, featuring the Cantabrian orocline (NW Iberia) and the Central Iberian curve, is a foremost expression of the late Carboniferous amalgamation of Pangea, which produced remagnetizations spanning almost the entire globe. Also in Iberia, late Carboniferous remagnetizations are widespread often hindering paleomagnetic interpretations in terms of pre-Pangean geologic history. In contrast, such remagnetizations facilitated the kinematic study of the Cantabrian orocline. Immediately to its south is located the Central Iberian curve whose geometry and kinematics are under debate. Recent studies suggest that this putative structure cannot have formed in the same process as the Cantabrian orocline. Here we present a paleomagnetic and rock magnetic study from Extremadura, a region in the utmost west of the southern limb of the Central Iberian curve. Our new results show two distinct remagnetization events in Paleozoic rocks in Extremadura: (1) Mesozoic or Cenozoic remagnetization occurring in dolomitized limestones and (2) late Carboniferous remagnetization in limestones, characterized by consistent shallow inclinations, but largely scattered declinations indicating a counter clockwise (CCW) vertical axis rotation. Pyrrhotite is documented as magnetic carrier in the limestones which testifies a remagnetization under anchimetamorphic conditions, i.e. during the Variscan orogeny. We interpret the declination scattering as a remagnetization coeval to the vertical axis rotation. The described CCW rotations are those expected for the southern limb of the Cantabrian orocline and are in disagreement with a late Carboniferous secondary origin for the Central Iberian bend, extending the Cantabrian orocline to at least most of the Iberian peninsula.

Preliminary Magnetostratigraphic and Isotopic Dating of the Ngwa Formation (Dschang Western Cameroon)

A magnetostratigraphic study has been carried out to constrain the age of the volcano-sedimentary Ngwa formation in the eastern part of the Dschang region. A stratigraphic section of about 80 meters thick corresponding to 26 sites has been sampled, and it is composed mainly of fine-grained sandstones, clays, lignite, volcanic sediment and tuffs. A magnetic study conducted on 56 samples shows one or two components of magnetization carried either by titanomagnetite, magnetite and Fe-sulphide. The section that was sampled shows one normal polarity and one reversed polarity. In the lower part of the section, a K-Ar radiometric dating was performed on the plagioclase minerals isolated from the tuffs level situated about 15 meters above the lignite seam, and gave an age of 20.1 ± 0.7 Ma. Constrained by this age, the observed polarity zones can be readily correlated with chrons C6An.1n-C6An.1r of the GPTS. This study suggests that the age of the lignite is comprised between 20.04 Ma and 20.21 Ma. The mean direction of the characteristic remanent magnetization documents a counterclockwise vertical axis rotation of about 8° with respect to the expected Lower Miocene direction derived from the Africa polar wander curve.

Magnetostratigraphy of the Xiaolongtan Formation bearing Lufengpithecus keiyuanensis in Yunnan, southwestern China: Constraint on the initiation time of the southern segment of the Xianshuihe–Xiaojiang fault

The late Cenozoic extensional basins in Yunnan Province (southwestern China), which are kinematically linked with the regional strike-slip faults, can provide meaningful constraints on the fault activity history and tectonic evolution of the southeast margin of the Tibetan Plateau (SEMTP), and further on the geodynamic evolution of the Tibetan Plateau. However, this has been severely impeded by the lack of precise age constraints on the timing of fault activity. To better constrain the timing of fault activity and the tectonic rotation of SEMTP, we undertook a high-resolution magnetostratigraphic study on the Xiaolongtan Formation in the Xiaolongtan Basin, which is located at the southern tip of the Xianshuihe–Xiaojiang fault and is well-known by the presence of hominoid Lufengpithecus keiyuanensis. Rock magnetic experiments indicate that magnetite is the main remanence carrier. Correlation to the geomagnetic polarity timescale was achieved by combining magnetostratigraphic and biostratigraphic data. Our correlation suggests that the Xiaolongtan Formation sedimentary sequence spans from Chron C5Ar.1r to Chron C5n.2n, which indicates that the age of the Xiaolongtan Formation ranges from ~10Ma to 12.7Ma, and that the ages of the two sedimentary layers possibly bearing the hominoid L. keiyuanensis are ~11.6Ma or ~12.5Ma. The basal age of the sediments is 12.7Ma, which indicates that the activation of the southern Xianshuihe–Xiaojiang fault was initiated at this time. The overall mean paleomagnetic direction (D=353.2°, I=34.2°, α95=2.1°, n=166) documents a counter-clockwise vertical axis rotation of −8±3° with respect to Eurasia, which is the response to the activity of the left-lateral Xianshuihe–Xiaojiang Fault.

One or two oroclines in the Variscan orogen of Iberia? Implications for Pangea amalgamation

The supercontinent Pangea formed in the late Carboniferous as a result of the Gondwana-Laurussia collision, producing the strongly sinuous Variscan–Alleghanian orogen. Iberia is interpreted to comprise two Variscan bends, forming an S-shaped orogenic belt: the Cantabrian orocline to the north and the Central Iberian bend to the south. Coeval formation of both oroclines, however, requires significant north-south shortening (in present-day coordinates) during Pangea’s amalgamation. In contrast to the Cantabrian orocline, neither the kinematics nor geometry of the Central Iberian bend is well constrained. We provide paleomagnetic data from the southern limb of the Central Iberian bend, showing ∼60° counterclockwise vertical axis rotation during the late Carboniferous to early Permian, comparable to that determined for the southern limb of the Cantabrian orocline. This result is incompatible with the hypothesized S-shaped bend in the Iberian Variscides. We argue that Central Iberia, if really bent, must have acquired its curvature before the Cantabrian orocline, the curvature being an inherited structure. We propose a new mechanism of Pangea formation, compatible with the geology, geochronology, and paleomagnetism, in which a clockwise rotation of Gondwana produces the necessary change in the stress field to form the late Variscan Cantabrian orocline.

Did Adria rotate relative to Africa?

Abstract. The first and foremost boundary condition for kinematic reconstructions of the Mediterranean region is the relative motion between Africa and Eurasia, constrained through reconstructions of the Atlantic Ocean. The Adria continental block is in a downgoing plate position relative to the strongly curved central Mediterranean subduction-related orogens, and forms the foreland of the Apennines, Alps, Dinarides, and Albanides–Hellenides. It is connected to the African plate through the Ionian Basin, likely with Lower Mesozoic oceanic lithosphere. If the relative motion of Adria versus Africa is known, its position relative to Eurasia can be constrained through a plate circuit, thus allowing robust boundary conditions for the reconstruction of the complex kinematic history of the Mediterranean region. Based on kinematic reconstructions for the Neogene motion of Adria versus Africa, as interpreted from the Alps and from Ionian Basin and its surrounding areas, it has been suggested that Adria underwent counterclockwise (ccw) vertical axis rotations ranging from ~ 0 to 20°. Here, we provide six new paleomagnetic poles from Adria, derived from the Lower Cretaceous to Upper Miocene carbonatic units of the Apulian peninsula (southern Italy). These, in combination with published poles from the Po Plain (Italy), the Istrian peninsula (Croatia), and the Gargano promontory (Italy), document a post-Eocene 9.8 ± 9.5° counterclockwise vertical axis rotation of Adria. Our results do not show evidence of significant Africa–Adria rotation between the Early Cretaceous and Eocene. Models based on reconstructions of the Alps, invoking 17° ccw rotation, and based on the Ionian Basin, invoking 2° ccw rotation, are both permitted within the documented rotation range, yet are mutually exclusive. This apparent enigma could possibly be solved only if one or more of the following conditions are satisfied: (i) Neogene shortening in the western Alps has been significantly underestimated (by as much as 150 km); (ii) Neogene extension in the Ionian Basin has been significantly underestimated (by as much as 420 km); and/or (iii) a major sinistral strike-slip zone has decoupled northern and southern Adria in Neogene time. Here we present five alternative reconstructions of Adria at 20 Ma, highlighting the kinematic uncertainties, and satisfying the inferred rotation pattern from this study and/or from previously proposed kinematic reconstructions.

Paleomagnetic results from the eastern Caliente-Enterprise zone, southwestern Utah: Implications for initiation of a major Miocene transfer zone

The Miocene Caliente-Enterprise zone (CEZ) in southwestern Utah (USA) is a 20–50-km-wide east-northeast–trending left-lateral transfer zone that displaces north-south–trending crustal blocks of the eastern Basin and Range Province to the west. Previous paleomagnetic results from the central and western CEZ show significant counterclockwise vertical axis rotations of strike-slip–bounded fault blocks, and these rotation estimates vary in magnitude both across and along the strike of the zone. Results of recent detailed geologic mapping and new geochronologic data in the area east of previous studies allow us to extend paleomagnetic studies into the easternmost CEZ. New paleomagnetic results include data from 4 regionally extensive latest Oligocene to early Miocene (ca. 24–22 Ma) ignimbrite sheets and from 3 ca. 22–20 Ma Iron Axis laccoliths. These data reveal significant magnitudes and similar spatially variable components of counterclockwise vertical axis rotation. Rotation estimates from the ignimbrites are assessed relative to what we interpret to be a nonrotated to only minimally rotated reference section just north of the Colorado Plateau (Grass Valley) and from several low-extension areas in southeast Nevada. Accepted tilt-corrected paleomagnetic data from sites away from the reference areas are discordant in declination from the expected individual ash-flow tuff directions, with rotation (R) and flattening (F) estimates that range from R = −2° to −84° and F = +15° to −14°. Many of the rotation estimates are large and statistically significant. For example, site P-18 from the Bauers Tuff yields an R = −61.1° ± 5.3° and F = −0.6° ± 5.0°. Relative to the expected Miocene direction, in situ paleomagnetic data from the Iron Axis laccoliths, specifically the Three Peaks laccolith, yield a mean that is discordant in declination, with estimated R = −22.2° and F = −8.8° values. These rotation and flattening estimates, although consistent with the overall data set from volcanic rocks, must be considered of lesser quality, as we are unable to accurately correct these data for possible effects of local tilting. If the rotation estimate is viable, then we suggest that this component of deformation involves much of the upper crust, and we furthermore propose that the boundary of the eastern CEZ extends farther east than previously envisioned, to within a few kilometers of the breakaway with the Colorado Plateau. The transitional zone between the eastern CEZ and Colorado Plateau is therefore abrupt and occurs within a narrow zone near Cedar City, Utah.

Timing and structural evolution in the limb of an orocline: The Pisuerga–Carrión Unit (southern limb of the Cantabrian Orocline, NW Spain)

Abstract Oroclines are the largest scale folds on Earth, and the process of oroclinal formation is a key topic in tectonics. However, most studies of oroclines have focused on the hinge areas, where the changes in strike, and therefore the orocline shape, are most obvious. In this paper, we investigate the deformation mechanisms, the timing, and the structural and tectonic evolution of the Pisuerga–Carrion Unit, situated on the southern limb of the Cantabrian orocline at the NW of the Iberian Peninsula. The Cantabrian Orocline located in the Variscan Belt of Western Europe has been recently defined as a secondary orocline, constraining kinematics and deformation timing. Our study in the Pisuerga–Carrion Unit reveals that an out-of-sequence thrust system developed and reactivated existing structures by a flexural-slip mechanism that was diachronous with respect to oroclinal formation. Joint analysis of unconformity-bounded rock sequences provide a late Moscovian age for oroclinal initiation (ca. 308 Ma), at least locally. Additionally, comparing those joint sets found in different series we quantify a minimum of 40° counterclockwise vertical axis rotation for the Pisuerga–Carrion Unit during the Late Pennsylvanian.

Paleomagnetic and inner diapiric structural constraints on the kinematic evolution of a salt-wall: The Bicorb-Quesa and northern Navarrés salt-wall segments case (Prebetic Zone, SE Iberia)

Paleomagnetism together with an analysis of the internal structure of the Bicorb-Quesa and northern Navarrés salt-wall segments (Prebetic Zone in SE Iberia) were used to constrain their kinematics and driving mechanisms. Paleomagnetic data from Upper Triassic red beds of the selected salt-related structures and from the Miocene rocks belonging to adjacent syn-diapiric half-grabens reveal 15–30° counter-clockwise vertical-axis rotations of the salt-wall rocks and a 20° clockwise rotation of the Jurassic-Miocene cover block located south of the salt-wall. This, together with the salt-wall structure, indicates that the origin of the salt-wall was linked to the motion of a late Miocene thin-skinned extensional fault system, which detached on the Upper Triassic evaporites. Specifically, the salt-wall formed by the south-southwest displacement with a 20° clockwise rotation component of a cover block bounded northwards by the detachment disruptions generated by the motion of pre-existent basement faults. The Upper Triassic detachment level was first affected by a counter-clockwise vertical axis rotation and, during the Paleogene-earliest Miocene building of the Iberian Chain, by tight WNW-trending folds and SSE-directed minor thrusts. This study also shows that Paleomagnetism together with the analysis of the internal structure can successfully depict the geometry and kinematic evolution of complex salt-wall structures.

Palaeomagnetism of the upper volcanic supergroup, southern part of the Sierra Madre Occidental, Mexico

Ash flow tuffs, or ignimbrites have been recently proposed to be a good material for palaeointensity determination. In this paper, we present a multidisciplinary study, combining geochronology, petrology, rock magnetism and palaeomagnetism, carried out on Oligocene to Early Miocene ignimbrites and related flows from the southern part of the Sierra Madre Occidental. Two new 40Ar/39Ar ages were determined for ignimbrites; 20.4 ± 0.2 Ma (the youngest age obtained so far in this area) and 29.2 ± 0.5 Ma. Density measurements, as a proxy for welding, proved to be extremely useful to estimate the emplacement temperature and the origin of the magnetizations carried by the ignimbrites. After alternating field and thermal demagnetizations, the mean palaeomagnetic pole (Lat = 66.8°N; Long = 180.5°E; Kappa = 142; A95 = 6.3°), calculated for the period 28–31 Ma, is in close agreement with our only Miocene determination. Comparison with the North America Synthetic Apparent Polar Path indicates a net counter-clockwise vertical axis rotation of about 10 ± 4° compared to stable North America, which occurred likely during the last extensional episode in the Late Miocene (ca. 12–9 Ma). Palaeointensity estimates, obtained with the Thellier–Coe method, are mainly questionable and should not be used for global interpretation. Therefore, these ignimbrites are not a viable material for reliable palaeointensity determinations.

Neogene tectonic evolution of the Gibraltar Arc: New paleomagnetic constrains from the Betic chain

New paleomagnetic results from Neogene sedimentary sequences from the Betic chain (Spain) are here presented. Sedimentary basins located in different areas were selected in order to obtain paleomagnetic data from structural domains that experienced different tectonic evolution during the Neogene. Whereas no rotations have been evidenced in the Late Tortonian sediments in the Guadalquivir foreland basin, clockwise vertical axis rotations have been measured in sedimentary basins located in the central part of the Betics: the Aquitanian to Messinian sediments in the Alcalà la Real basin and the Tortonian and Messinian sediments in the Granada basin. Moreover, counterclockwise vertical axis rotations, associated to left lateral strike-slip faults have been locally measured from sedimetary basins in the eastern Betics: the Middle Miocene to Lower Pliocene sites from the Lorca and Vera basins and, locally, the Tortonian units of the Huercal-Overa basin. Our results show that, conversely from what was believed up to now, paleomagnetic rotations continued in the Betics after Late Miocene, enhancing the role of vertical axis rotations in the recent tectonic evolution of the Gibraltar Arc.

Preliminary Magnetostratigraphic Study of the Late Cretaceous Dinosaur Site from Villeveyrac-Mèze Basin, Southern France

A magnetostratigraphic study was performed on continental Upper Cretaceous beds in the L'Olivet quarry, Villeveyrac-Mèze Basin (Hérault, France). Nearly all specimens subjected to progressive thermal demagnetization procedures exhibit opposite polarities for the low- and high-temperature magnetization components. The high-temperature component shows reverse polarity, characteristic of the Late Cretaceous magnetization, which is well isolated between 200° and 450°C. The overall mean paleomagnetic direction (incl. = -39.1°, decl. = 160.7°, k = 15, a95 = 10) suggests a counterclockwise vertical axis rotation of about 15.1° ± 8.3 with respect to the expected Late Cretaceous direction calculated from the Eurasian polar wander curve (incl. = -49.0°, decl. = 174°). Combined with local biostratigraphic data, the L'Olivet sequence can be correlated with polarity chron C33.1r, corresponding to the Early Campanian with an age of 83.5 to 79.5 Ma

The role of crustal heterogeneity in controlling vertical coupling during Laramide shortening and the development of the Caribbean-North America transform boundary in southern Mexico: insights from analogue models

AbstractAnalogue models of polyphase deformation involving crustal differences in strength, thickness and density give insights into lateral and vertical strain propagation during Late Cretaceous shortening and Early Tertiary left-lateral shearing related to the early development of the North America-Caribbean plate boundary in southern Mexico. Analogue models reproduce a two-phase deformation characterized by a first stage of compression orthogonal to the plate boundary, simulating deformation induced by the Laramide orogeny, followed by a later stage of left-lateral transpression associated with the transfer of the Chortis block from the North American to the Caribbean plate during the early stage of development of the new plate boundary in Early Tertiary times. Based on detailed structural observations in the Guerrero-Morelos platform and the western part of the Mixteco terrane of southern Mexico, we document that a transpressive regime affected continental red bed sequences of Early Paleocene to Late Eocene, and rotated and refolded Laramide structures during this second phase. Our model ends before the transtensional regime that affected the region, which is marked by a volcanic episode of Late Eocene-Oligocene. This change in the deformation regime records the passage of the NW tip of the Chortis block (North America-Cocos-Caribbean triple junction), when subduction replaced transform faulting along the southern Mexico margin. The models focus on the structures formed around the flanks of a thicker/more rigid crustal block that simulates the rock assemblages of the Palaeozoic orogens of southern Mexico (Mixteco-Oaxaca-Juarez block, MOJB). The comparison of the mechanism of deformation of three different analogue models with the natural prototype explains most of the structures observed around the MOJB. Counterclockwise vertical-axis rotations of pre-existing structures in the western flank of the MOJB observed in the Guerrero-Morelos platform are consistent with the modelled structures. Vertical movements of the modelled MOJB induced by the transpressive regime can explain the Papalutla thrust and the basement upheaval and gravitational sliding of the cover in the Tentzo Ranges observed at the western and northern margins of the MOJB, respectively. The growth and propagation of thrusting controlled by the geometry of the block along the eastern margin also correlates with the Vista Hermosa fault. The propagation of strain to the north increases with higher contrast in strength of the thick block with respect to the adjacent modelled crust. Analogue modelling failed to reproduce all the structural details of southern Mexico and, specifically, the structures observed inside the MOJB. The latter, however, are controlled by pre-existing discontinuities, which are not simulated in the model. As a whole, the results demonstrate that crustal heterogeneity in a developing left-lateral plate boundary zone produces a stronger vertical coupling between ductile and brittle crust and a widening of the deformation zone along the margin of the North America plate in southern Mexico.

Geodynamic implications of paleomagnetic data from Tertiary sediments in Sakhalin, Russia (NW Pacific)

N‐S trending right‐lateral strike‐slip faults, which were active in the Tertiary, transect Sakhalin, Russia, while Mesozoic forearc and accretionary rocks testify to an earlier period of subduction. Several kinematic models have been proposed for the region, but the details required to constrain these models, such as the timing of the transition from subduction to strike‐slip tectonics in Sakhalin, are still unknown. Even first‐order tectonic features, such as the boundaries of the plates with which Sakhalin evolved during the Tertiary, are poorly known. Paleomagnetic results from around Sakhalin were obtained to constrain the geodynamic evolution of the region. Comparison of paleomagnetic inclination data with the apparent polar wander paths for the Eurasian, Pacific, and North American Plates suggests that Sakhalin probably evolved with the North American Plate, although a history including the Eurasian Plate cannot be ruled out. Paleomagnetic declination data suggest that significant clockwise vertical axis rotation has occurred in Sakhalin since the mid‐Paleocene. It is likely that this rotational deformation was accommodated by Tertiary activity on right‐lateral strike‐slip faults, which may be associated with the opening of the Japan Sea, Tatar Strait, and Kuril Basin. These data contradict a published kinematic model for eastern Sakhalin, where counterclockwise vertical axis rotations were predicted for Neogene basins in the East Sakhalin Mountains. Agreement is better, however, with published paleomagnetic data from southern Sakhalin, where clockwise vertical axis rotations were documented.

Magnetostratigraphy of the Middle Miocene continental sedimentary sequences of the Mae Moh Basin in northern Thailand: evidence for counterclockwise block rotation

A magnetostratigraphic study has been performed on the Mae Moh basin, well known for its lignite mining in the Lampang province. Paleomagnetic samples were collected from 66 stratigraphic levels. Rock magnetic investigations indicate the presence of low coercivity minerals. Specimens subjected to progressive thermal (or alternating field) demagnetization procedures show that nearly all of them exhibit a low temperature magnetization component, and a high temperature characteristic component (ChRM), with either positive or negative virtual geomagnetic pole latitudes and opposite polarity, considered as the characteristic Miocene magnetization. The overall mean paleomagnetic direction (incl.=22.2°, decl.=358.3°, κ=15, α 95=4) documents a counterclockwise vertical axis rotation of about 13°±1.32 with respect to the expected Miocene direction derived from the Eurasian polar wander curve (incl.=42°, decl.=11°). Our paleomagnetic results are not consistent with the previously reported paleomagnetic data. The rotation observed in the Mae Moh basin can be the response to local tectonics. A section sampled for magnetostratigraphy reveals a polarity sequence of nine magnetozones that can reliably be correlated to the geomagnetic polarity time scale. According to biochronological constraints, the magnetostratigraphic results from the mammal-bearing succession correlate with chron C5ABn–C5An2n, between 13.5 and 12.1 Ma. According to the mean sedimentation rate of about 17.5 cm/ka, ages of 12.5 and 12.8 Ma are proposed for the fossiliferous levels (J5 and K1, K2 lignite zones) were the mammal remains were found.

Magnetochronology of the Upper Cenozoic strata in the Southwestern Chinese Tian Shan: rates of Pleistocene folding and thrusting

The southwestern Chinese Tian Shan of Central Asia is an actively deforming part of the Indian–Asian collision system. Paleomagnetic investigations of two Plio–Pleistocene terrestrial successions provide the first detailed magnetostratigraphy for the upper Cenozoic strata in this region. Paleomagnetic samples were collected from 358 sites within the Atushi Formation and Xiyu Formation across the Atushi–Talanghe anticline near Atushi. Thermal demagnetization behavior, reversal and fold tests of paleomagnetic stability indicate that the characteristic remanence directions were acquired before tilting and folding of the strata. A composite magnetostratigraphic section for this sequence correlates with the Mammoth subchron to the Jaramillo subchron, between 3.3 and 1.07 Myr, of the geomagnetic polarity timescale of Cande and Kent [J. Geophys. Res. 100 (1995) 6093–6095]. The mean declination (349±3°) in the Boguzihe section indicates a counterclockwise vertical-axis rotation (−11±2°) during the past 1.4 Myr. The conglomeratic Xiyu Formation is time-transgressive along its progradational contact with the underlying Atushi Formation across a map distance of ∼6 km; the basal contact ranges from less than 1.0 Myr in the Ganhangou section to 1.9 Myr in the Boguzihe section to the southwest, and to ∼2.8 Myr on the northwest limb of the anticline. Northeastward lateral propagation and growth of the Atushi–Talanghe anticline initiated at ∼1.4 Myr in the Boguzihe, and ∼1.2 Myr in the Ganhangou. The cross section in the Boguzihe provides a conservative (maximum) estimate of shortening rate of ∼3.3 (4.4) mm/yr and uplift rate of ∼3 mm/yr. This rate represents about half of the geodetically determined shortening rate between the northern Tarim Basin and the Kyrgyz Tian Shan [Wang et al., Acta Seismol. Sin. 22 (2000) 263–270; Reiger et al., Earth Planet. Sci. Lett. 191 (2001) 157–165]. During fold growth over the past 1.4 Myr, the crest of the Atushi–Talanghe anticline eroded at an average rate of 2.6–2.7 mm/yr.