Large Clockwise Rotation Research Articles

Interference between shallow and deep-seated structures in the Sicilian fold and thrust belt, Italy

Abstract: The fold and thrust belt in western Sicily is characterized by the presence and interference of shallow and deep-seated compressional structures, which were generated and developed at different structural levels. The shallow structures consist of imbricated thrusts and asymmetric folds, with a typical wavelength of 2 km, involving relatively thin deep-water units. These units are superimposed on thick platform carbonate units, along a wide and originally almost flat floor thrust. The axial trend of the folds is variable, as multi-phase folding often occurred, producing a characteristic interference pattern, reflecting continuous variations of the apparent transport direction during the emplacement (i.e. rotation of the allochthonous thrust sheets). The deep-seated structures consist of large, double-verging pop-up structures, with a typical wavelength of 5–10 km, involving thicker platform carbonate successions. The deep-seated structures are characterized by large folds, with vertical to overturned limbs, caused by high-angle, transpressive ramps that reactivated previous (i.e. Mesozoic) synsedimentary normal or transtensional faults. The floor thrust of the shallow structures was passively deformed by the subsequent growth of the underlying, younger deep-seated structures. Large clockwise rotations of the tectonic units occurred during the compressional deformation, and the amount of rotation is apparently related to the timing and amount of the tectonic transport.

Palaeomagnetism of the Upper Cretaceous Sânpetru Formation (Haţeg Basin, South Carpathians)

This paper reports on the palaeomagnetism of 10 localities from Upper Cretaceous sediments of the Sânpetru Formation exposed in the Haţeg Basin (South Carpathians, Romania). The Sânpetru Formation is one of the best exposed latest Cretaceous continental successions of Eastern Europe being famous for its unique assemblage of dinosaurs, mammals and other vertebrates. A total of 169 samples were collected from 66 strata through an 860-m-thick section. For most of the samples, the remanences were soft and/or noisy, forcing us to reject 63% of the collection. In the rest of the collection, both polarities are observed and inclination-only analysis indicates that pre-tilting remanence is preserved for many samples. These data, along with published palaeomagnetic results from the Haţeg Basin, pass a regional fold test. The palaeomagnetic study has provided an acceptable magnetostratigraphy which shows that the studied part of the Sânpetru Formation was deposited between the beginning of the Chron 32n.1n and the end of Chron 31n. These first palaeomagnetic results obtained from the Upper Cretaceous sedimentary rocks from the South Carpathians confirm the previous palaeomagnetic results obtained from magmatic rocks: the subtropical palaeolatitude during Late Cretaceous and the large clockwise rotation during the Tertiary.

The origin of Neogene tectonic rotations in the Galatean volcanic massif, central Anatolia

A paleomagnetic study was carried out on Neogene volcanic rocks at 30 sites within the Galatean massif (40.4°N, 31.5°E) to determine possible block rotations due to stress variations. Two phases of rotation could be characterized as the result of Neogene volcanic activity. We suggest that the first stage of rotation was isolated in Early Middle Miocene calc-alkali rocks, with a relative counterclockwise rotation of R ± ΔR = −20.2 ± 9.3° with respect to Eurasia. This accommodates the south-westward rotational collapse of the Western Anatolia peninsula across a pole on the Bitlis suture. In the neotectonic period, on other hand, a relative clockwise rotation of R ± ΔR = 27.3 ± 6.4° with respect to Eurasia is predicted. In contrast to the uniform clockwise rotations, extremely large clockwise rotations up to 264° are restricted in a narrow zone between two dextral faults. We believe that the second stage rotations support the idea of individual microblock rotations due to deformation along the North Anatolian Fault zone.

Palaeomagnetic confirmation of Palaeozoic clockwise rotation of the Famatina Ranges (NW Argentina): implications for the evolution of the SW margin of Gondwana

SUMMARY Palaeomagnetic results from Palaeozoic volcanic and sedimentary units of the Famatina Ranges, in NW Argentina (28.7°S, 67.8°W) are reported. A late Early to late Middle Ordovician palaeomagnetic pole was obtained from a pre-tectonic remanence carried by magnetite and isolated in volcanics of the Molles Formation and the Cerro Morado Group (MCM1, 16.7°S, 357.2°E, A95= 6.5°, K= 38.5, N= 14 sites). This pole position is rotated 39° clockwise respect to the coeval reference pole for Gondwana but it is consistent with previous Early Ordovician poles from the Famatina belt and the Faja Eruptiva Oriental in the Puna region of NW Argentina. The sedimentary layers of the Molles Formation, however, present a secondary magnetization carried by hematite, which is interpreted of Permian age and yields a pole position (MCM2) at 78.7°S, 330.8°E (A95= 7.2°, K= 16.1, n= 27 samples). Two additional independent palaeomagnetic poles were obtained from the Permian De La Cuesta Formation, exposed at two different localities in the same area. While one consisted in a exclusively reverse polarity magnetization and a pole position (LC1, 76.9°S, 345.2°E, A95= 6.0°, K= 21.1, n= 29 samples) compatible with the late Early to early Late Permian palaeomagnetic poles from South America, the other presented only normal polarities and a pole position (LC2, 74.5°N, 275.4°E, A95= 2.0°, K= 258.3, n= 21 samples) suggestive of a Cretaceous remagnetization. These new palaeomagnetic results confirm on a much more robust database previous proposals that the Ordovician rocks of the Famatina belt have undergone a large clockwise rotation. They also constrain the rotation to pre-Permian times. Different tectonic models involving the Late Ordovician docking of a large para-authochthonous terrane or a pattern of systematic large-scale rotations in the Early Palaeozoic continental margin of Western Gondwana are discussed.

Impact of India–Asia collision on SE Asia: The record in Borneo

Borneo occupies a central position in the Sundaland promontory of SE Asia. It has a complex Cenozoic geological history of sedimentation and deformation which began at about the same time that India is commonly suggested to have started to collide with Asia. Some tectonic reconstructions of east and SE Asia interpret a large SE Asian block with Borneo at its centre which has been rotated clockwise and displaced southwards along major strike–slip faults during the Cenozoic due to the indentation of Asia by India. However, the geological history of Borneo is not consistent with the island simply forming part of a large block extruded from Asia. The large clockwise rotations and displacements predicted by the indentor model for Borneo are incompatible with palaeomagnetic evidence and there is no evidence that the major strike–slip faults of the Asian mainland reach Borneo. Seismic tomography shows there is a deep high velocity anomaly in the lower mantle beneath SE Asia interpreted as subducted lithosphere but it can be explained just as well by alternative tectonic models as by the indentor model. Very great thicknesses of Cenozoic sediments are present in Borneo and circum-Borneo basins, and large amounts of sediment were transported to the Crocker turbidite fan of north Borneo from the Eocene to the Early Miocene, but all evidence indicates that these sediments were derived from local sources and not from distant sources in Asia elevated by India–Asia collision. The Cenozoic geological history of Borneo records subduction of the proto-South China Sea and Miocene collision after this ocean lithosphere was eliminated, and a variety of effects resulting from long-term subduction beneath SE Asia. There is little to indicate that India–Asia collision has influenced the Cenozoic geological record in Borneo.

Relocation of the tectonic boundary between the Raukumara and Wairoa Domains (East Coast, North Island, New Zealand): Implications for the rotation history of the Hikurangi margin

Paleomagnetic studies of Neogene marine sediments have documented large clockwise rotations of the Hikurangi margin (East Coast, North Island) during the Neogene, with the exception of the Raukumara Peninsula, which is unrotated with respect to the Australian plate. Immediately south of the Raukumara Peninsula, the Wairoa region has been rotated clockwise by 50–60°; the boundary between these domains is associated with a change in regional structural trends. However, a declination of 70 ± 14° reported from Otaian (19–22 Ma) sediments in the Rakauroa area is located to the north of this change. Characterisation of how differential rotations have been accommodated along the Hikurangi margin has been frustrated by this apparent mismatch between paleomagnetic and structural data. Paleomagnetic analysis of two new Rakauroa localities has yielded declinations of 16 ± 7° and 19 ± 9°, consistent with expected values for the Australian plate. This region is therefore not part of the Wairoa Domain. A strong viscous magnetic overprint was observed in many samples, the incomplete removal of which resulted in the misidentification of a large declination anomaly in the previous study. The paleomagnetically defined boundary between the Raukumara and Wairoa Domains now coincides with the area where regional structural trends alter. Reassignment of the Rakauroa area to the Raukumara Domain also results in a revised rotation history for the Wairoa Domain, suggesting rotation rates of 4–5°/m.y. since the late Miocene (5–10 Ma), and potentially no earlier rotation. No reliable record of early and middle Miocene vertical axis rotation on the Hikurangi margin now exists north of Marlborough; further studies are required to properly constrain the rotation history for this time interval.

Tectonic reconstruction of the northern Andean blocks: Oblique convergence and rotations derived from the kinematics of the Piedras–Girardot area, Colombia

A detailed kinematic study in the Piedras–Girardot area reveals that approximately 32 km of ENE–WSW oblique convergence is accommodated within a northeast-trending transpressional shear zone with a shear strain of 0.8 and a convergence factor of 2. Early Campanian deformation is marked by the incipient propagation of northeast-trending faults that uplifted gentle domes where the accumulation of sandy units did not take place. Maastrichtian unroofing of a metamorphic terrane to the west is documented by a conglomerate that was deformed shortly after deposition developing a conspicuous intragranular fabric of microscopic veins that accommodates less than 5% extension. This extensional fabric, distortion of fossil molds, and a moderate cleavage accommodating less than 5% contraction, developed concurrently, but before large-scale faulting and folding. Paleogene folding and southwestward thrust sheet propagation are recorded by syntectonic strata. Neogene deformation took place only in the western flank of this foldbelt. The amount, direction, and timing of deformation documented here contradict current tectonic models for the Cordillera Oriental and demand a new tectonic framework to approach the study of the structure of the northern Andes. Thus, an alternative model was constructed by defining three continental blocks: the Maracaibo, Cordillera Central, and Cordillera Oriental blocks. Oblique deformation imposed by the relative eastward and northeastward motion of the Caribbean Plate was modeled as rigid-body rotation and translation for rigid blocks (derived from published paleomagnetic and kinematic data), and as internal distortion and dilation for weak blocks (derived from the Piedras–Girardot area). This model explains not only coeval dextral and sinistral transpression and transtension, but also large clockwise rotation documented by paleomagnetic studies in the Caribbean–northern Andean region.

The accretionary history of southern South America from the latest Proterozoic to the Late Palaeozoic: some palaeomagnetic constraints

Abstract It is now accepted that southern South America was formed from several terranes of diverse origin and evolution. However, a detailed history of the accretionary processes has not been unravelled yet. Palaeomagnetism can play an important role in such an endeavour. Palaeomagnetic constraints on the tectonic evolution of this region in the Proterozoic and Palaeozoic are reviewed and discussed. Data from the Rio de la Plata craton suggest that this block was already attached to most major Gondwana blocks by the end of the Proterozoic and may have formed a single continental mass with Congo-Sao Francisco, West Nile and Arabia throughout most of the Vendian. A large ocean separating these cratons from Amazonia and West Africa, prior to Gondwana assembly, is supported by available palaeomagnetic data. To the west of the Rio de la Plata craton is the Pampia terrane. Despite lack of palaeomagnetic data, geological evidence supports a model of Early Cambrian collision between these blocks. An Early Ordovician magmatic arc, the Famatina-Eastern Puna belt, which had developed on the western margin of the already accreted Pampia terrane, shows a systematic pattern of large clockwise rotation that has been interpreted as representative of the whole terrane. The favoured tectonic model portrays a continental magmatic arc with a back-arc basin to the east that was closed when the terrane rotated. There is little doubt of a Laurentian origin for the Cuyania (Precordillera) terrane, given the amount and diversity of evidence, including palaeomagnetism. The tectonic mechanism for accretion and its timing are still controversial. New palaeomagnetic data from Late Ordovician rocks of Cuyania support the ‘Laurentian plateau’ hypothesis, which suggests that Cuyania was still linked to Laurentia well into the Ordovician. Nevertheless, these new data do not rule out the more generally favoured ‘microcontinent model’. To the west of Cuyania is the Chilenia terrane, separated by a belt of ophiolites of Late Ordovician age. Very little is known about this terrane, although some U-Pb ages and Nd model ages point to a Laurentian origin for its basement. Lack of palaeomagnetic data precludes determining its kinematic evolution. The Arequipa-Antofalla block may actually be a composite terrane. Palaeomagnetic data obtained so far come exclusively from the southern Antofalla block. Recently acquired data in the western Puna of Argentina confirm the originally proposed distribution of Early Palaeozoic palaeomagnetic poles, which, despite several uncertainties, delineate a pattern of significant counterclockwise rotations with a possible anomaly in palaeolatitude for the late Cambrian. The data suggest a major tectonic discontinuity between the Eastern and Western Puna of Argentina in the Early Palaeozoic. Four palaeomagnetic poles of Devonian to Permian age from the North Patagonian Massif are consistent in position and age with the Gondwana apparent polar wander path, suggesting that both continental masses have not experienced major relative displacement since the Devonian. The data do not, however, rule out a restricted separation of Patagonia orthogonal to its northern boundary in the Early or Middle Palaeozoic and subsequent collision in the Late Palaeozoic.

Revision of the timing, magnitude and distribution of Neogene rotations in the western Aegean region

To evaluate the dimension and the timing of the clockwise rotating domain and the nature of the structures that accommodate the rotating domain in the western Aegean region, paleomagnetic analyses were carried out in northern, western and southwestern Greece. The results show that the rotating domain covers an area including the external Albanides, western mainland Greece including Evia and probably at least partly the Peloponnesos. Smaller clockwise rotations on the order of 30–40° were reported previously from the Chalkidiki peninsula and the islands of Skyros and Limnos. Previously, two phases of approximately 25° of rotation were suggested, the last one during the Plio-Pleistocene. Our analysis shows that the western Aegean domain rotated approximately 40° clockwise between 15–13 and 8 Ma, followed by an additional 10° after 4 Ma. The rotating domain is accommodated in the north by deformation associated with the Scutari–Pec fault zone and in the west by the Ionian thrust and the Hellenic subduction zone. To the south, no rocks older than ∼10 Ma are available so no conclusive data are obtained. To the east of the rotating domain, extensional detachment systems of the Cyclades and Rhodope areas were active during the rotation phase and may explain at least part of the differences in finite amounts of rotation between nonrotation or counterclockwise rotations observed in northern and eastern Greece and the large clockwise rotations in western Greece.

New Palaeomagnetic Data from the Betic Cordillera: Constraints on the Timing and the Geographical Distribution of Tectonic Rotations in Southern Spain

A palaeomagnetic investigation has been carried out at 14 sites on Jurassic red nodular limestones from the central and eastern part of the External Zones of the Betic Cordillera (Subbetic and Prebetic Zones). Progressive thermal demagnetisation of samples from the Subbetic Zone reveals the presence of two stable magnetic components of the natural remanent magnetisation: 1) a secondary Neogene syn-folding component and 2) the original Jurassic magnetisation. As similar characteristics have been reported in Jurassic limestones from the western Subbetic Zone, a widespread remagnetisation event took place within < 106 years in the entire Subbetic region during Neogene times. In contrast, in the Prebetic region, no evidence for a secondary overprint has been detected. Palaeomagnetic Jurassic declinations indicate variable and locally very large clockwise rotations (35°-140°), but the two sites in the north-westernmost part of the investigated region are not rotated. The use of both components of magnetisation and the incremental fold-test results allowed the timing of block rotations in the Subbetic Zone to be constrained. Rotations in the western Subbetic occurred after the acquisition of the secondary overprint, whereas in the central part of the Subbetic Zone they were completed by the time of the remagnetisation event.

New Moscovian palaeomagnetic pole from the Edjeleh fold (Saharan craton, Algeria)

A palaeomagnetic study was carried out in the Moscovian (∼305Ma) formation in the Edjeleh anticline, the only area where important dips can be observed in the Upper Palaeozoic series of the Illizi basin (Algeria). This study shows the existence of three magnetization components. Two of them are interpreted as Cenozoic and Permian remagnetizations; their poles are 88.8°N, 164.0°E, K=262, A95=3.3° and 43.4°S, 61.7°E, K=93, A95=5.9°, respectively. The third component is determined by both well-defined ChRMs and remagnetization circles analysis. Its associated fold test is positive and significant. Because the folding started before or during the Autunian, this third component was acquired very early and is very probably the primary magnetization. Its corresponding palaeomagnetic pole (28.3°S, 58.9°E, K=157, A95=4.2°) is close to the poles from the Saharan platform obtained from neighbouring periods. The positive fold test of this study thus validates these previous Upper Palaeozoic poles. This new result is in agreement with the geodynamical model (Matte 1986; Henry et al. 1992), which proposed the occurrence of a large clockwise rotation of Africa during the Carboniferous; such a motion agrees with the Permian Pangaea A2 reconstruction.

Tertiary paleomagnetic data from northwestern Yunnan, China: further evidence for large clockwise rotation of the Indochina block and its tectonic implications

Abstract Eocene red sandstones were collected at 15 sites for paleomagnetic study from two areas (Lanping and Yunlong) of the western part of Yunnan province, China, in the northern part of Indochina. The high-temperature magnetization component with an unblocking temperature of around 680°C is isolated after stepwise thermal demagnetization. Characteristic directions from nine accepted sites reveal a positive fold test at the 99% confidence limit, indicating that the high-temperature magnetization component is possibly of Eocene age. A tilt-corrected mean direction from nine accepted sites is D=266.1°, I=−39.8° with α95=11.2°, corresponding to a paleopole at 14.5°N, 169.7°E with A95=10.9°. A comparison with previously reported Tertiary paleomagnetic data indicates that the Lanping basin of the northern part of the Indochina block rotated clockwise through 91° (±15°) with respect to the Yangtze craton. This large magnitude clockwise rotation is probably caused by the superimposition of regional and local rotations. Our previous middle Cretaceous paleomagnetic results from the Yunlong area indicate that about 40° of clockwise rotation out of 90° is explained by tectonic rotation due to a relatively rigid body rotation of the Simao Terrane during indentation of India. The remaining component of rotation is probably ascribed to further Eocene/post-Eocene rotational motion of the tectonically independent small blocks within the Lanping basin associated with fault movement.

Large clockwise rotations in an extensional allochthon, Alboran Domain (southern Spain)

Palaeomagnetic, structural and geochronological analysis of a suite of Oligocene mafic dykes intruded into the Malaguide allochthon in the Internal Zone of the Betic Cordillera demonstrate that at least 134° clockwise vertical-axis rotation of a large region occurred, probably during early Miocene thrusting onto the External Zones. Initial results from Ar/Ar whole rock analysis suggests that the dykes were probably intruded over a period of time between 23 Ma and 30 Ma. Results of demagnetization experiments demonstrate that the remanence in many of the dykes is composed of one to three components. The lowest temperature component is poorly defined. The intermediate temperature (IT) component, carried by pyrrhotite, has reversed polarity, a declination of 314° and an inclination of −48°. This component was probably acquired during cooling after greenschist-facies metamorphism. The high temperature (HT) component is carried by magnetite and was probably acquired at or shortly after intrusion. This component has both reversed and normal polarity, with an average declination of 320° and an inclination of −13°. Rotation of approximately 35° about a horizontal NNE-trending axis, in present-day coordinates, brings the inclination of the HT component into statistical agreement with both the IT component and the expected inclination for the Oligocene. This rotation leaves the dykes in a near-vertical orientation but brings the foliation in the surrounding schists to approximately horizontal. We suggest that the original regional foliation may have been sub-horizontal and related to the ductile phase of late-orogenic extension in the Betic Cordillera. The dykes were intruded sub-vertically into the schists with an original NW–SE trend. The region was then tilted and rotated clockwise by 23°±15 before acquiring the IT component after which it underwent 134°±10° of clockwise rotation about a vertical axis.

Paleomagnetic data from the Caborca terrane, Mexico: Implications for Cordilleran tectonics and the Mojave‐Sonora megashear hypothesis

Two ancient magnetizations have been isolated in rocks of the Caborca terrane, northwest Mexico. The characteristic magnetizations of Neoproterozoic and Paleozoic miogeoclinal shelf‐strata, arc‐derived Lower Jurassic marine strata, and Jurassic volcanic and volcaniclastic rocks are of dual polarity and east‐northeast declination (or south‐south‐west) and shallow inclination. Magnetizations in Neoproterozoic and Paleozoic miogeoclinal strata are interpreted as secondary (J*) and to be of similar age to those observed in Lower and Middle Jurassic rocks. Remanence acquisition is bracketed between about 190 and 160 Ma. The overall mean (D=15.0°, I=8.5°; n=38 sites; six localities; k=19.1, α95=5.5°) suggests a moderate to large clockwise rotation of 12 to 50° (depending on reference direction assumed) of the Caborca terrane, and rocks of the Sonoran segment of the Cordilleran volcanic arc, with respect to the North America craton. When compared with expected inclinations, observed values are not anomalously steep, arguing against statistically significant southward latitudinal displacement of the Caborca block after remanence acquisition. Late Cretaceous intrusions yield primary, dual‐polarity steep inclination “K” magnetizations (D=341.4°, I=52.3°; n=10 sites; five localities; k=38.3, α95=7.9°) and have locally remagnetized Neoproterozoic and Jurassic strata. When present, secondary (K*) magnetizations in Neoproterozoic strata are of higher coercivity and higher unblocking temperature than the characteristic (J*) magnetization. Importantly, the regional internal consistency of data for Late Cretaceous intrusions suggests that effects of Tertiary tilt or rotation about a vertical axis over the broad region sampled (∼5000 km2) are not substantial. Late Cretaceous primary (K) magnetizations and secondary (K*) magnetizations yield a combined mean of D=348.1°, I=50.7° (N=10 localities; 47 sites; k=53.5, α95=6.7°), indicating at most small (&lt;∼10°) clockwise rotation of the Caborca region with respect to the craton. Permissible post‐Late Cretaceous latitudinal displacement is near or below the detection limit of paleomagnetism (&lt;∼300 km). Limited data from Lower Cretaceous strata of the Bisbee Group (D=339.9°, I=47.9°; n=4 sites) suggest that the modest clockwise rotations inferred on the basis of J* magnetizations in Jurassic and older strata occurred in Jurassic time. Together, the lack of evidence for southward displacement, yet evidence for statistically significant clockwise rotation, and the overall similarity of Jurassic magnetizations in the Cordilleran arc with those of the Caborca block, despite the fact that some of them are clearly secondary, are not consistent with the Mojave‐Sonora megashear hypothesis of Late Jurassic left‐lateral strike‐slip motion of the crust of northern Mexico.

A preliminary paleomagnetic pole for mid-Cretaceous rocks from Tobago: further evidence for large clockwise rotations in the Caribbean-South American plate boundary zone

The island of Tobago occupies the eastern end of the central (igneous) belt of the Caribbean-South American plate boundary zone. Volcaniclastic sediment of the Albian (∼ 100 Ma) Tobago Volcanic Group and dikes of similar age within it were sampled in two homoclinal sections with different attitudes. The mean of virtual geomagnetic poles for 12 sites (25.4°N, 24.1°E, A95 = 4.2°) is well defined, pre-tilting and apparently reliable, yet is far removed from a similar age reference pole for South America. Five other paleomagnetic studies of Cretaceous rocks from widely separated localities farther west in the plate boundary zone yield pole positions that are remarkably similar to the Tobago pole. Poles obtained from the Guajira Peninsula of Colombia, the islands of Aruba and Bonaire, and the Caribbean Mountains of Venezuela are among those that agree with the Tobago result. The paleolatitudes for study areas within the plate boundary zone are consistent with an origin on or near South America, yet the poles throughout the zone are rotated roughly 90°. Dextral relative motion between the Caribbean and South American plates was probably responsible for the rotation.

Origin and motion history of the Philippine Sea Plate

The Philippine Sea Plate is the one major plate whose Tertiary motion is poorly constrained and whose origin is problematical. Its southern boundary is the Sorong Fault system which is part of a major left-lateral fault system at the northern margin of the Australian plate. The southern part of the plate in eastern Indonesia has been neglected in most syntheses but includes some of the oldest rocks within the plate which are separated from remnant arcs of the Daito Ridge province of the northern Philippine Sea by the West Philippine Central Basin. The east Indonesian islands of the Halmahera-Waigeo region contain a good Mesozoic and Tertiary stratigraphic record indicating a long arc history for the southern part of the plate. New palaeomagnetic data from these islands define two sub-areas: an area forming part of the Philippine Sea Plate north of the Sorong Fault, and an area within the Sorong Fault system. The area north of the fault records a long-term clockwise rotation history whereas that within the fault zone records local rotations interpreted as due to deformation at the plate edge. Rocks of Philippine Sea Plate origin within both areas record similar latitudinal shifts. The rotation of the area north of the Sorong Fault is considered to represent the motion of the southern part of the Philippine Sea Plate. The new data indicate large Tertiary clockwise rotations similar to earlier suggestions for other parts of the plate but record a discontinuous and more complex motion history than previously suggested. For the southern part of the plate there was 40° rotation with northward translation between 0 and 25 Ma, no significant rotation between 25 and 40 Ma, and there was 50° rotation with southward translation between 40 and 50 Ma. We show that the new palaeomagnetic data form part of a single set with earlier palaeomagnetic data from elsewhere in the plate. The translation history of the southern part of the plate in eastern Indonesia can be reconciled with northward motions recorded elsewhere and can be used to determine rotation poles for the plate (15°N, 160°E for the interval 5–25 Ma, and 10°N, 150°E for the interval 40–50 Ma). Reconstructions based on these poles predict that at ∼ 45 Ma the Palau-Kyushu Ridge had a WNW-ESE orientation which is very different from that postulated by many models used to explain the widespread boninite volcanism in the Izu-Bonin-Marians forearc at this time. The long arc history of the southern part of the plate and the reconstructions based on the rotation poles calculated from the palaeomagnetic data favour an origin for the West Philippine Basin by spreading in a backarc basin.

Paleomagnetic evidence for clockwise rotation of the Simao region since the Cretaceous: A consequence of India-Asia collision

Samples were collected for paleomagnetic analysis from 36 sites from six formations west of the Red River Fault in southwestern Yunnan Province, China on the Simao block (23.4°N, 100.8°E) in an attempt to examine the effects of the India-Asia collision on the Asian continent. Paleomagnetic results from rocks ranging from Cretaceous to Miocene in age indicate a large clockwise rotation of these sites with respect to stable Eurasia since the Late Cretaceous. The tectonic-corrected data from Cretaceous samples (mean direction of two formations) are D = 112.6°, I = 34.7°, α 95 = 7.5°, k = 9.1, n = 45, for Eocene samples the data are D = 84.7°, I = 38.9°, α 95 = 7.6°, k = 12.0, n = 32, and for Miocene samples they are D = 21.1°, I = 35.5°, α 95 = 7.1°, k = 15.3, n = 29. The samples from the Paleocene and Pliocene are either remagnetized or too weak to provide stable directions. The Cretaceous sites were sampled from folded beds and these sites pass the fold test. The amount of rotation seen in this study ( R = 86.1 ± 9.9° as a mean value for the Cretaceous, R = 76.8 ± 11° for the Eocene, and R = 14.7 ± 10.0° for the Miocene) is significantly larger than that documented in previous studies from the Lower Cretaceous of northwestern and central Yunnan Province and may indicate that differential rotation has taken place in the region. This rotation can be explained by the northward motion and collision of India with Asia in the Paleocene, which caused continuous deformation of southwestern China in the form of strike-slip faulting and clockwise rotation. It is concluded that the rotation and deformation resulting from this collision probably continued from the Paleocene until at least the Miocene.

Palaeomagnetism of the Balantak ophiolite, Sulawesi

Palaeomagnetic data from the East Arm of Sulawesi provide constraints on the Cretaceous and subsequent history of a fragment of the tectonic mosaic of a complex region close to the junction between SE Asia, the Indo-Australian plate and the western Pacific microplates. Primary thermoremanence has been demonstrated in samples from 23 sites in a tilted Cretaceous-Palaeogene lava sequence in the Balantak ophiolite at Batusimpang on the eastern tip of the East Arm of Sulawesi. Its direction ( D = 60.5°, I = −32.1°, α 95 = 5.5°) indicates formation at 17 ± 4°S and ∼ 60° of subsequent clockwise rotation. The primany mode of remanence is deduced from NRM/TRM comparisons; low-temperature cycling and other rock magnetic tests point to SD magnetite as the principal remanence carrier. Supporting evidence for the palaeolatitude and northward movement of the East Arm is derived from other lavas at Binsil, and from the Boba Cherts, but lack of tectonic control limits the interpretation. The contrast between these results and the subequatorial origin of contemporary rocks on nearly Halmahera is consistent with subduction of Indian Ocean lithosphere beneath the Sunda margin in the late Mesozoic and early Tertiary. Large differences in declination of remanence in the E Sulawesi rocks indicate large clockwise and anticlockwise rotations of tectonic blocks only tens of kilometres across.

Miocene paleomagnetic results from southeastern Korea

Paleomagnetic samples were collected from 25 sites of Late Oligocene to Middle Miocene age from southeastern South Korea. The mean direction obtained is D = 39.4°, I = 46.2° with α 95 = 9.5°, which is statistically distinguishable from the present axial geocentric dipole field direction. When combined with the previous reported results of eight sites from this region, we get a mean direction ( D = 40.1°, I = 47.4°, α 95 = 7.9°) for 33 sites of Late Oligocene to Middle Miocene age. This direction yields an early Neogene virtual geomagnetic pole of 56.5°N, 215.2°E with an α 95 of 7.9°, discordant from the Cretaceous reference pole for Korea. The paleomagnetic data suggest that portions of southeastern Korea underwent a large clockwise rotation of about 40° during the Neogene. Two large NE-trending right-lateral faults (the Yangsan Fault and the Dongnae Fault, Fig. 1) cut the study areas. Both these faults show evidence for recent activity. In addition, minor faults with similar trends to these major faults occur. The observed deflections in paleomagnetic declinations were probably caused by rigid block rotations of crustal blocks in a right lateral shear system during the Neogene. The paleomagnetic directions for Miocene aged rocks from southeastern Korea and southwestern Japan are similar in direction and magnitude. This suggests that the cause of the rotations in both areas may be related to a similar tectonic event in this area during the Neogene.

Timing of rotational motion of Apuseni Mountains (Romania): paleomagnetic data from Tertiary magmatic rocks

Paleomagnetic results are reported from 8 sites in Eocene to Oligocene magmatic rocks and 27 sites in Middle Miocene to Upper Miocene volcanic rocks of the Apuseni Mountains. Following AF demagnetization 29 sites were selected for tectonic interpretation. These results, together with previously published data imply a northward displacement of the area studied between the Late Cretaceous and Oligocene, followed by a large clockwise rotation prior to the Middle Miocene, after which no significant movement has occurred. These results agree with other data from the Carpatho-Pannonian domain, showing a period of great mobility during the Early Miocene.