Large Counterclockwise Rotation Research Articles

Palaeomagnetic evidence of the deformation of the Pontides during the closure of the Intra-Pontide Ocean in the early Cretaceous

SUMMARY Several models exist concerning the deformation history of the Pontides in North Anatolia during the Cretaceous period, which vary depending on the positions of the Istanbul and Sakarya zones, the consumption of the northern branches of the Neotethys ocean and the rifting of several sub-basins. Notably, the early Cretaceous tectonic history of the Pontides involved the closure of the northern Neotethys ocean (Intra-Pontide ocean), and the collision between the Istanbul and Sakarya zones, producing thrust structures along the collisional front. The lack of palaeomagnetic data providing evidence for this deformation pattern demonstrates that further investigation is required, particularly focusing on the Lower Cretaceous strata in the Pontides. Thus, this study aimed to examine samples from a total of 78 sites from the Lower–Upper Cretaceous sedimentary rocks, and Middle Eocene to Middle Miocene sedimentary and volcanic rocks. Results of this study indicated large counterclockwise rotations up to R±ΔR = −73.9°±9.1°, and small clockwise rotations of R±ΔR = 14.2°±12.2° in the Istanbul and Sakarya zones, during the Early Cretaceous and Late Cretaceous periods. These rotation patterns are accompanied by the closure of the Intra-Pontide ocean, and the collision between the Istanbul and Sakarya zones during the Early and Late Cretaceous periods. On the other hand, in the Middle Eocene, small counterclockwise rotations of R±ΔR = −6.4°±13.9° and 4.6°±12.9° along the western coastline of the Pontides indicated that the northern margin of the Pontides was stable during this period.

Paleomagnetism in the Cambrian Urda-Los Navalucillos Limestone (Montes de Toledo, Spain): Implications for late-Variscan kinematics and oroclinal bending in the Central Iberian Zone

A magnetic and paleomagnetic study was carried out on weakly metamorphic Cambrian limestones from the southern limb of the Central Iberian Arc (CIA), a late-Variscan orocline located to the south of the Ibero-Armorican Arc (IAA). >270 cores were obtained from 32 sites located at 5 outcropping structures in the Urda-Los Navalucillos Formation of Montes de Toledo (Central Iberian Zone, Spain), in an area close to the hinge zone of the CIA. These outcrops are affected by two regional-scale Variscan folding phases, namely C1 and C3, which developed interference patterns. A characteristic paleomagnetic component was isolated in 19 sites at 4 of the structures, which shows different temporal relationships with C3 folds, ranging from possibly syn-folding to clearly post-folding. The resulting mean directions of the magnetic vector, in geographic coordinates, always show northward to north-western declinations and negative, low inclinations, suggesting acquisition during some normal polarity event(s) when Iberia was located in the southern hemisphere, i.e. prior to the geomagnetic reverse polarity Kiaman superchron. While the inclination of the paleomagnetic mean directions is coherent between structures, the declination ranges from N to NW, suggesting a vertical axis rotation synkinematic to C3 folding, and older than 318 Ma. These directions suggest that the southern limb of the CIA first underwent a 42° clockwise rotation during the late Carboniferous that can be related to the development of the CIA, and which has been differentially recorded by the paleomagnetic directions of the different structures. This was later followed by a large counterclockwise rotation related to the formation of the IAA, which affects to the entire paleomagnetic dataset.

Paleomagnetism and Geochronology of the Early Cretaceous Dipilto Batholith (NW Nicaragua): Chortís Block Large Rotation With Respect to SW North America

AbstractThe Dipilto Batholith is one of the largest plutonic complexes (~1,200 km2) of the Chortís block. It is located at northwestern Nicaragua in Central America. We present paleomagnetic, rock magnetic, and petrographic results from 34 sites (269 samples) of this batholith and some associated dikes. Four U‐Pb (zircon) isotopic ages were obtained from samples collected through the intrusive body in order to gain insight into the timing of silicic magmatism in the southern Chortís block and its possible tectonic and paleogeographic connections with the southwestern North America Plate during the Early Cretaceous. The paleomagnetic analyses of 16 selected sites indicate a large counterclockwise rotation (~101°), with a related paleolatitude of 28.7°N, and isotopic ages ranging from 119.08 ± 0.37 to 112.69 ± 0.44 Ma. Our results imply an updated tectonic model resulted from an inherited peri‐Gondwanan position and open the discussion of at least three possible tectonic scenarios for the Chortís block during the Early Cretaceous: (1) Chortís block with an angle of ~45° with respect to the southwestern margin of North America. This would imply the removal of an in‐between continental or oceanic crust segment by subduction or subduction erosion; (2) Chortís block immediately juxtaposed to the southwestern margin of North America, being part of extension‐compression events of the Guerrero Terrane‐Arperos basin; or (3) Chortís block juxtaposed to North America, but with intraplate deformation resulting in overestimated counterclockwise rotation for a tectonic decoupled block, called herein the “Dipilto block.”

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.

Late Cretaceous–early Eocene counterclockwise rotation of the Fueguian Andes and evolution of the Patagonia–Antarctic Peninsula system

The southernmost Andes of Patagonia and Tierra del Fuego present a prominent arc-shaped structure: the Patagonian Bend. Whether the bending is a primary curvature or an orocline is still matter of controversy. New paleomagnetic data have been obtained south of the Beagle Channel in 39 out of 61 sites. They have been drilled in Late Jurassic and Early Cretaceous sediments and interbedded volcanics and in mid-Cretaceous to Eocene intrusives of the Fuegian Batholith. The anisotropy of magnetic susceptibility was measured at each site and the influence of magnetic fabric on the characteristic remanent magnetizations (ChRM) in plutonic rocks was corrected using inverse tensors of anisotropy of remanent magnetizations. Normal polarity secondary magnetizations with west-directed declination were obtained in the sediments and they did not pass the fold test. These characteristic directions are similar to those recorded by mid Cretaceous intrusives suggesting a remagnetization event during the normal Cretaceous superchron and describe a large (>90°) counterclockwise rotation. Late Cretaceous to Eocene rocks of the Fueguian Batholith, record decreasing counterclockwise rotations of 45° to 30°. These paleomagnetic results are interpreted as evidence of a large counterclockwise rotation of the Fueguian Andes related to the closure of the Rocas Verdes Basin and the formation of the Darwin Cordillera during the Late Cretaceous and Paleocene.The tectonic evolution of the Patagonian Bend can thus be described as the formation of a progressive arc from an oroclinal stage during the closure of the Rocas Verdes basin to a mainly primary arc during the final stages of deformation of the Magallanes fold and thrust belt. Plate reconstructions show that the Antarctic Peninsula would have formed a continuous margin with Patagonia between the Early Cretaceous and the Eocene, and acted as a non-rotational rigid block facilitating the development of the Patagonian Bend.

The Cimmerian accretionary wedge of Anarak, Central Iran

The occurrence in Iran of several ophiolite belts dating between Late Palaeozoic to Triassic poses several questions on the possible existence of various sutures marking the closure of the Palaeotethys ocean between Eurasia and this Gondwana-derived microplate. In this scenario, the Anarak region in Central Iran still represents a conundrum. Contrasting geochronological, paleontological, paleomagnetic data and reported field evidence suggest different origins for the Anarak Metamorphic Complex (AMC). The AMC is either interpreted, as: (1) relict of an accretionary wedge developed at the Eurasia margin during the Palaeotethys subduction as part of the Cimmerian suture zone of NE Iran, displaced to Central Iran by a large counter-clockwise rotation of the central Iranian blocks; (2) autochthonous unit forming a secondary branch of the main suture zone.Our structural, petrographic and geochemical data indicate that the AMC consists of several metamorphic units also including dismembered “ophiolites” which display different tectono-metamorphic evolutions. Three main ductile deformational events can be distinguished in the AMC. The Morghab and Chah Gorbeh complexes preserve a different M1 metamorphism, characterized by blueschist relics in the S1 foliation of the former unit, and greenschist assemblages in the latter. They share a subsequent similar D2 deformational and M2 metamorphic history, showing a prograde metamorphism with syn- to post-deformation growth of blueschist facies mineral assemblages on pre-existing greenschist facies associations. High pressure, low temperature (HP/LT) metamorphism responsible for the growth of sodic amphibole has been recognized also within marble lenses at the contact between the Chah Gorbeh Complex and serpentinites. Evidence of HP/LT metamorphism also occurs in glaucophane-bearing meta-pillow lavas and serpentinites, which contain antigorite and form most of the “ophiolites” within the AMC. Structural relationships show that the Chah Gorbeh and Morghab units and the “ophiolites” were tectonically coupled within an accretionary wedge before the D2 folding stage. The other units of the AMC lack evidence of HP metamorphism in the area around Anarak, especially the Lakh Marble, a large thrust sheet that occupies the uppermost structural position in the AMC. Available radiometric ages of trondhjemite dikes and stocks that intruded the accretionary wedge, as well as our new data, constrain the subduction event at the end of the Carboniferous, before 290Ma. These data suggest that the AMC is part of an allochthonous crustal fragment belonging to the Variscan belt developed along the southern Eurasian margin before the Cimmerian collision of Iran. Subsequent deformational events that occurred during the Mesozoic and the Cenozoic, up to the Miocene and possibly later, resulted in folding, thrusting and faulting that dismembered the original structure of the wedge accompanying its displacement to the present day position.

Tectonic rotations and transcurrent deformation south of the Abancay deflection in the Andes of southern Peru

We report new paleomagnetic results from 55 out of 76 sites sampled at different localities along a transect from Nazca to Cuzco where the general structures of the Peruvian Andes are strongly offset across the Abancay deflection. Nine new 39Ar/40Ar ages better constrain the timing of volcanism along the western edge of the Western Cordillera at the latitude of Nazca. A mean paleomagnetic result from 22 sites in the lower Miocene volcanics does not show significant rotation (R = −2.3° ± 7.7°) of the western margin of the Central Andean Plateau since the early Miocene. Within the Western Cordillera we sampled three structural blocks bounded to the north by the Abancay fault system. In the westernmost block, a large counterclockwise rotation (R = −65.0° ± 11.1°) is found in Mesozoic limestones and Paleocene‐Eocene red beds. Magnitude of rotation decreases toward the east from (R = −35.6° ± 12.8°) in the central block to (R = −4.5° ± 8.4°) south of the town of Cuzco. The anisotropy of magnetic susceptibility (AMS) recorded by the red beds sediments is the consequence of compaction and tectonic strain during the early stages of deformation. We show that the magnetic lineations were also rotated counterclockwise as the remanent magnetizations. The present study confirms results from the Peruvian fore arc, showing that rotations are not older than circa 40 Ma and likely not younger than circa 20 Ma. The spatial variation in the amount of counterclockwise rotation suggests a large component of shear along the Abancay deflection concomitant with a broad late Eocene–Oligocene oroclinal deformation in southern Peru.

Counterclockwise rotation of the western Alps since the Oligocene: New insights from paleomagnetic data

A paleomagnetic study of Penninic units in the southern part of the Alpine arc has been carried out. More than 200 samples (23 sites) were collected in Briançonnais Ammonitico rosso limestones of the high Ubaye valley and the Ligurian Alps. A characteristic component of magnetization of reverse polarity was isolated on most of the sites. This component does not pass the fold test and is interpreted as a Tertiary overprint related to Alpine metamorphism. Mean directions in geographic coordinates are D = 121°, I = −52°, α95 = 11°, and D = 72°, I = −48°, α95 = 15° for the Ubaye and the Liguria localities, respectively, indicating large counterclockwise rotations about vertical axis of 68° and 117° relative to stable Europe. These rotations, in agreement with a previous study conducted in the Briançon area, together with other paleomagnetic data from the western Alps, show that the internal Alps suffered a large but nonhomogeneous counterclockwise rotation since the Oligocene. The rotations are in agreement with the combination of earlier separately proposed processes: the rotation of the Adriatic plate accounts for about 25° of rotation, remaining rotation and southward gradient would be related to left‐lateral shear accommodating the displacement between Adria and Europe at the southern border of the western Alps. Furthermore, the southward extrusion of the western Alps south of the Simplon fault zone may account for up to 10° of rotation. Rotations appear therefore as a major process accommodating deformation in the western Alps since the Oligocene.

Palaeomagnetic study of Vendian and Early Cambrian rocks of South Siberia and Central Mongolia: was the Siberian platform assembled at this time?

A palaeomagnetic study of Vendian and Early Cambrian sediments from the Angara block of the Siberian platform: Shaman (52.08°N, 108.83°E) and Minya (58.0°N, 110.0°E) Formations, and the Tuva-Mongolian block: Tsagan-Olom and Bayan-Gol Formations (46.76°N, 96.37°E) isolated three different components of magnetization through thermal demagnetization. The stable high-temperature characteristic remanence directions show both normal and reverse polarities. The mean palaeopoles computed after these high-temperature components are: 32.0°S/71.1°E (dp/dm=6.9°/13.8°) for the Vendian Shaman Formation (10 sites, 80 samples), 33.7°S/37.2°E (dp/dm=8.6°/14.7°) for the Vendian Minya Formation (12 samples), 22.8°S/28.4°E (dp/dm=10.8°/21.6°) for the Vendian Tsagan-Olom Formation (4 sites, 25 samples) and 21.4°S/167.1°E (dp/dm=9.6°/19.1°) for the Early Cambrian Bayan-Gol Formation (6 sites, 49 samples). From a compilation of Vendian and Early Cambrian palaeopoles from the Anabar, Angara and Aldan blocks of the Siberian platform and Tuva-Mongolia block, we propose a model where these blocks were situated in an equatorial to low south palaeolatitude position, with their present-day southern boundaries facing the north pole. From the analysis of the scatter of these poles, we conclude that the Siberian platform might not have fully amalgamated by this time, and that significant rotations occurred after the Early Cambrian. Our new palaeopoles for the Tuva-Mongolia block, together with previously published ones, show that this block was already adjacent to Siberia by the Vendian and Early Cambrian. We propose that the large counterclockwise rotation of the Tuva-Mongolia block with respect to Angara block could mark the end of the closure of the part of the Palaeo-Asian ocean separating these two blocks, and could account for the occurrence of Vendian-Early Cambrian ophiolites in the region.

Paleomagnetic evidence for a very large counterclockwise rotation of the Madre de Dios Archipelago, southern Chile

A paleomagnetic study of Late Paleozoic exotic units exposed in the Madre de Dios Archipelago and Diego de Almagro island, southern Chile, is reported. These units correspond to the Tarlton Limestone and the metabasaltic pillow lavas of the Denaro Complex. Nineteen sites were drilled (14 in limestone and five in lavas). Standard demagnetization procedures show that the primary remanence is not present in the investigated rocks, preventing the determination of the original latitudinal position of these rocks. However, a post-tectonic secondary magnetic component was isolated in eight sites at the Madre de Dios Archipelago, in both limestone and basaltic pillow lavas. Rock magnetic experiments and microscopic observations indicated that the carrier of this remanence is secondary titanomagnetite in the limestone and titanomagnetite–titanomaghemite in the lavas. The most likely event to produce this secondary magnetization is the nearby intrusion of the south Patagonian batholith in the Early Cretaceous. Mean direction from eight sites shows a very large (117.7±29.9°) counterclockwise rotation of the study area with negligible paleolatitude anomaly. This rotation is only constrained as post-Early Cretaceous. Restoration of the structures to their pre-rotation position indicates that accretion of the exotic block took place from the northwest and not from the south as previously proposed. This is more consistent with a low latitude origin for the limestone and the general paleogeographic framework. Different tectonic mechanisms and ages for this large rotation are possible.

Palaeomagnetism of Late Miocene to Quaternary volcanics from the eastern segment of the Trans-Mexican Volcanic Belt

A systematic palaeomagnetic study in the eastern part of the Trans-Mexican Volcanic Belt includes 39 Miocene, Pliocene and Quaternary volcanic rocks in the southeastern Mexico Basin (Sierra Nevada and Sierra de Rı́o Frı́o), the Altiplano area, and the Palma Sola Massif. A total of 430 samples have been selectively demagnetized using mostly alternating field demagnetizing methods, supplemented by thermal analyses. Most characteristic remanences are carried by low-Ti titanomagnetites, with occasional titanohematites or slightly maghemitized low-Ti titanomagnetites, of similar direction. Seven sites were discarded because they presented intermediate directions, hydrothermal alteration or were remagnetized by lightning strikes. The mean directions of 32 sites, together with 24 sites from Sierra de las Cruces in the western Mexico basin, indicate rocks older than 2 Ma are rotated some 10° counterclockwise with respect to Quaternary rocks, whereas there is no rotational difference between Miocene and Pliocene rocks. Statistical analyses between different regrouped populations confirm that the rotational pattern is due to the age of the volcanics rocks but not to their spatial distribution. The Quaternary mean direction from the three Mexico Basin ranges is consistent with the geographical reference pole. In contrast, the Pliocene mean direction from volcanic rocks of the Altiplano area and the Sierra de Las Cruces is slightly rotated some 10° westwards with respect to the reference direction from North America. No significant rotations have been observed in the eastern TMVB (from the western Mexico Basin to the border of the Altiplano), between late Miocene and late Pliocene time. It suggests that a very small, counterclockwise vertical-axis rotation may have been taken place in this segment of the TMVB between late Pliocene and Quaternary times. Comparisons of these results with a summary of the available palaeomagnetic data in the area indicate that the previously reported Quaternary rotations are of questionable reliability, and that the large counterclockwise rotations, reported in Cretaceous to Miocene rocks, probably took place before the late Miocene. These new palaeomagnetic data support the idea that the eastern TMVB since the late Miocene, has been a zone of extension with a little, left-lateral shear component.

Large counterclockwise rotation of the Inner West Carpathian Paleogene Flysch—Evidence from paleomagnetic investigations of the Podhale Flysch (Poland)

We are reporting the first paleomagnetic results from the Podhale Flysch, which crops out in the area between the Pieniny Klippen Belt and the Tatra Mts., where claystones and mudstones were drilled at 10 localities, mainly from subhorizontal strata. In all cases, the magnetic fabric was found to be typical of undeformed sediments, with well developed magnetic lineation (aligned with the sedimentary transport direction) at some of the localities; the dominant magnetic mineral was identified as magnetite, accompanied by iron sulphides. For six of the localities, with one exception for those with poorly developed lineation, we obtained statistically well-defined paleomagnetic mean directions, on AF or on combined AF and thermal demagnetization.The overall-mean paleomagnetic direction is D=298° 1=53° k=121, a95=6°, in tectonic coordinates. Similar direction was observed for Inner Carpathian flysch from the Levoča basin (Slovakia). We conclude, that the flysch of the two basins must have travelled a few hundred kilometres to the North, after the early Miocene tectonic phase: this displacement was accompanied by about 60° counterclockwise rotation with respect to Stable Europe.

New palaeomagnetic data from Central Iran and a Triassic palaeoreconstruction

New pole positions for Triassic and Cretaceous times have been obtained from volcanic and sedimentary sequences in Central Iran. These new results confirm the general trend of the Apparent Polar Wander Path (APWP) of the Central-East-Iran microplate (CEIM) from the Triassic through the Tertiary as published by Soffel and Forster (1983, 1984). Two new palaeopoles for the Triassic of the CEIM have been obtained; limestones and tuffs from the Nakhlak region yield a mean direction of 094.0°/25.0°, N=12, k=4.1,α95=24.7°, after bedding correction, corresponding to a palaeopole position of 310.8°E; 3.9°S, and volcanic rocks from the Sirjan regions yield a mean direction of 114.5°/35.1°, N=44, k=45.9,α95=3.2° after bedding correction and a palaeopole position of 295.8°E; 10.3°N. Combining these with the two previously published results yields a new palaeopole position of 317.5°E; 12.7°N, for the Triassic of the CEIM, thus confirming that large counterclockwise rotations of the CEIM have occurred since the Triassic time. New results have also been obtained from Cretaceous limestones from the Saghand region of the CEIM. The mean direction of 340.7°/26.3°, N=33, k=44.3,α95=3.8°, and the corresponding palaeopole position of 283.1°E; 64.4°N, is in agreement with previously determined Cretaceous palaeopole positions of the CEIM. Furthermore, results have also been obtained from Triassic dolomite, limestone, sandstone and siltstone from the Natanz region, which is located to the west of the CEIM. A total of 161 specimens from 44 cores taken at five sites gave a mean direction of the five sites at 033.3°/25.1°, N=5, k=69.0,α95=9.3° and a palaeopole position of 167.2°E; 53.7°N. They pass the positive fold test of McElhinny (1964) on the level of 99% confidence. This pole position is in fairly good agreement with the mean Triassic pole position of the Turan Plate (149°E; 49°N). It indicates that the area of Natanz has not undergone the large counterclockwise rotation relative to the Turan plate since the Triassic, which has been shown for the CEIM. A Triassic palaeogeographic reconstruction of Iran, Arabia (Gondwana) and the Turan Plate (Eurasia) is also presented.

Borneo: a stable portion of the Eurasian margin since the Eocene

Samples collected from sixteen paleomagnetic sites of Plio-Pleistocene basaltic rocks from southeastern Borneo yield mean direction of D m = 180.4°, I m = −13.8° (α 95 7.8° k = 23.6) and a paleomagnetic pole of λ = 82.7° N, σ = 120.5° E . Samples collected from nine sites, Eocene-Early Miocene basalt, andesite, and shale, yield a mean direction of D m = 182.0°, I m = −2.2° (α 95 = 11.0° k = 22.8) and a mean Eocene-Early Miocene pole of λ = 87.8° N, σ = 173.5° E . These Eocene-Early Miocene directions pass a fold test. Rock magnetic studies suggest that the carrier of remanence in the samples is magnetite. These Eocene to Pleistocene paleomagnetic results from southeastern Borneo suggest the island has been at its present position with respect to Eurasia since Eocene. Our results are consistent with other studies showing no evidence for latitudinal displacement; however, these results do not concur with the previous reports of a large Tertiary counterclockwise rotation. Our data are in general agreement with the rotations predicted by the extrusion tectonic model for southeast Asia and argue against the numerous Cenozoic tectonic reconstructions of the region which require a large counterclockwise rotation of Borneo.

Paleomagnetism of Mesozoic rocks from the central Andes of southern Peru: Importance of rotations in the development of the Bolivian orocline

A paleomagnetic study has been undertaken in Jurassic volcanics and Cretaceous intrusions from coastal southern Peru. A secondary magnetization in the Jurassic volcanics was identified at 21 sites from three main localities along the southern coast of Peru (mean virtual geomagnetic pole: 59.5°N, 190.1°E, k=111, α95=3.0). This secondary magnetization was acquired during a tectonomagmatic event that may be related to the emplacement of the Peruvian coastal batholith and/or to the first Andean phase of compressional deformation in Late Cretaceous. If we assume that the remagnetization is of Late Cretaceous age, a regional counterclockwise rotation of about 20° is recorded by the forearc of southern Peru from 16°S to 18°S. Primary remanent magnetizations recorded by the volcanics and the batholith reveal a more complex tectonic history of block rotations. Counterclockwise rotations of 50° to 80° are documented near the localities of Chala (eight sites), Oconà (two sites) and Arequipa (four sites). Preliminary results based on three sites of Late Permian to Early Triassic sediments from northern Bolivia also show a large counterclockwise rotation. The observed differences between the primary and the secondary magnetization indicate that large block rotations occurred prior to the acquisition of the secondary magnetization. Perhaps, this first episode of deformation partially structured the Andean chain in the Central Andes and induced an incipient orocline. During the Cenozoic, a differential shortening along the chain gave the present‐day shape of the Bolivian deflection and resulted in a counterclockwise rotation of the forearc system by about 20° as indicated by the remagnetization.

Paleomagnetic evidence for a large counterclockwise rotation of northern Greece prior to the Tertiary clockwise rotation

AbstractThis study aims at unravel the geotectonic evolution of northern Greece prior to the already established Tertiary clockwise rotation. Therefore, Mesozoie sediments, Early Mesozoie ophiolites and Carboniferous granites were sampled. While the metamorphosed and/or too weakly magnetized limestones had to be rejected, the gabbros and serpentinites of the 80 km long Chalkidiki belt (40.4°N, 23.3”E), and the granites of the northern Pelagonian zone (40.8°N, 21.2°E) have yielded similar results interpretable in terms of geoleetonies. In both areas the demagnetizing process has revealed a poh phased magnetic evolution.The oldest magnetizations, labelled M (D=311°, I=20°, a95, = 15°; VGP: 37°N, 272.5°, for the ophiolites; D=320.5°, I = 26°, a95 =11°; VGP : 46°N, 264.5”E, for the granites) are interpreted as overprints acquired in Late Jurassic-Cretaceous times. The younger magnetizations, called C2 (D = 66°, I = 28°, a95 = 9°; VGP : 28°N, 117°E, in the ophiolites ; D=64°, I = 2° a95, = 11°; VCP : 20°N, I28...

Paleomagnetic results from Luzon and the central Philippines

Samples were collected from 86 paleomagnetic sites from the islands of Luzon, Marinduque, Mindoro, Panay, Negros, Cebu, and Mindanao in the Philippine Arc. The sampling sites range in age from Pleistocene to Jurassic. Characteristic directions of magnetization of the samples were determined by the use of vector plots. Curie temperature determinations, thin section studies, and hysteresis studies showed that remanence of these samples is carried by fine‐grained (pseudo‐single domain) magnetite. Positive fold tests from Miocene data from Panay, Jurassic data from Mindoro, and Cretaceous data from Cebu suggest that the magnetization of these regions was acquired prior to folding. Rotations reported below are measured with respect to the axial goecentric dipole field. The Plio‐Pleistocene data set shows no resolvable rotation for the 22 sites. This data set suggests that the various terranes that make up the Philippine Arc have behaved as a single unit during the past 5 m.y. or that deformation has been below the limits of resolution. The inclination data from the Plio‐Pleistocene sites have anomalously shallow inclination and are consistent with other Plio‐Pleistocene data from Vietnam, Taiwan, and the Marianas. These data support earlier suggestions for a late Neogene offset dipole effect. The late Miocene sites fall into two separate groups. Ten sites from western Luzon show evidence for around 20° of clockwise rotation. In contrast to this, late Miocene samples from the Bicol region, Negros, Marinduque, and Mindanao are not rotated. The cause of the postlate Miocene clockwise rotation of Luzon is unknown, but a Pliocene collision of the North Luzon Arc with Taiwan is suggested. Early Neogene results also separate into two different populations. The population from Marinduque shows evidence for a large counterclockwise rotation. The second early Neogene population comes from Panay, Cebu, and Mindanao and clearly shows evidence for a clockwise rotation. The validity of this rotation is further supported by a fold test and a reversal test. These early Neogene data sets are consistent with a middle to late Miocene collision of the Palawan Continental Terrane and the Central Philippine Arc. Data from six dikes of possible Oligocene age from the Zambales Ophiolite are highly discordant from the present field, being rotated approximately 60° clockwise. The directions from these dikes are similar to a direction reported earlier from late Oligocene sediments also from the Zambales region. These two data sets support the interpretation that the Eocene direction from Zambales is recording a large clockwise rotation of the region. Data from the Mesozoic sites are from two regions. Data from the Cretaceous Pandan formation of Cebu are discordant with data from the Upper Jurassic from Mindoro. The presence of a fold test from each region and a reversal test from Mindoro supports the interpretation that each of these data sets is reliable. The VGP of Mindoro is displaced southward from the Late Jurassic VGP of South China, suggesting a post‐Jurassic southward migration of Mindoro.

Paleomagnetic constraints on the late Cretaceous and Cenozoic tectonics of southeastern Asia

Many features of the Cenozoic tectonic history of central and southeastern Asia can be understood as direct consequences of the thrust and penetration of India into Asia. Recent indentation experiments with plasticine (Tapponnier et al. [7]) have extended this idea and have led to the prediction of a pattern of large rotations and displacements of continental blocks that can be tested by paleomagnetism. The available Cretaceous and Cenozoic paleomagnetic data from this part of the world have been reviewed and a new APWP for Eurasia has been constructed for reference. The negligible rotation of South China and large clockwise rotation of Indochina are consistent with the model, i.e., with an history of large-scale left-lateral strike-slip motion along the Altyn Tagh and Red River faults. Data from Malaya and Borneo can be reconciled with the model, although in a less straightforward fashion. The large counter clockwise rotation of South Tibet implies that it rotated in sympathy with India during the collision and suggests that future indentation experiments should include this feature. Finally a middle Cretaceous reconstruction of the south margin of Asia is proposed. One interesting result is the restored continuity of geological features in Tibet and Indochina, with active subduction of oceanic (Indian plate) crust taking place to the south at subtropical latitudes.