A critical reappraisal of paleomagnetic evidence for Philippine Sea Plate rotation

SHAvdL and DJJvH were funded by NWO Vici grant 865.17.001 to DJJvH. DPG is funded by a ‘Ramón y Cajal’ Fellowship (RYC2019-028244-I) and the grant KiTSuNE (PID2021-128801NA-I00) both funded by ‘ MCIN/AEI/ESF Investing in your future’. We thank Fagan Matthys and CJ Paulino for their help in the field.The kinematic history of the Philippine Sea Plate (PSP) is crucial for interpreting its geological record related to subduction initiation processes and the paleogeography of the junction between the Paleo-Pacific and Tethyan oceanic realms. However, reconstructing PSP's kinematic history is difficult because the plate has been surrounded by subduction zones for most of its history. In absence of marine magnetic anomalies to constrain PSP's motion relative to its neighboring plates, paleomagnetic data may be used as quantitative constraints on its motion. Previous paleomagnetic studies interpreted easterly deflected declinations to infer clockwise rotations of up to 90° since the Eocene. However, rotations inferred from these datasets may also reflect local block rotations related to plate margin deformation. We here re-evaluate to what extent paleomagnetic data from the PSP unequivocally demonstrate plate motion rather than local rotation. To this end, we provide new data from Guam, in the Mariana forearc, and reassess published paleomagnetic data. Our new data from Guam come from two localities in the Eocene, two in the Oligocene, and two in the Miocene. Our compilation assesses data quality against recently defined criteria. Our new results demonstrate that in Guam, declination differences of up to 35° exist in rocks of Eocene age, indicating local rotations. Our compilation identifies both clockwise and counterclockwise rotations from the plate margins, with little confidence which of these would reflect plate-wide rotation. We compiled paleolatitude data from igneous rocks, which we correct for microplate rotation constrained by intra-PSP marine magnetic anomalies and show a northward drift of the PSP of ∼15° since the Eocene, but without a paleomagnetic necessity for major vertical axis rotation. Hence, with the currently available data, rotations of the PSP may be permitted, but are not required. Plate motion is currently better reconstructed from geological constraints contained in circum-PSP orogenic belts.Nederlandse Organisatie voor Wetenschappelijk Onderzoek 865.17.001‘Ramón y Cajal’ Fellowship RYC2019-028244-IKiTSuNE (PID2021-128801NA-I00)MCIN/AEI/ESF Investing in your futur

Provenance analysis of the Paleozoic sequences of the northern Gondwana margin in NW Iberia: Passive margin to Variscan collision and orocline development

The Cantabrian Zone of NW Iberia preserves a voluminous, almost continuous, sedimentary sequence that ranges from Neoproterozoic to Early Permian in age. Its tectonic setting is controversial and recent hypotheses include (i) passive margin deposition along the northern margin of Gondwana or (ii) an active continental margin or (iii) a drifting ribbon continent. In this paper we present detrital zircon U–Pb laser ablation age data from 13 samples taken in detrital rocks from the Cantabrian Zone sequence ranging from Early Silurian to Early Permian in depositional age. The obtained results, together with previously published detrital zircon ages from Ediacaran– Ordovician strata, allow a comprehensive analysis of changing provenance through time. Collectively, these data indicate that this portion of Iberia was part of the passive margin of Gondwana at least from Ordovician to Late Devonian times. Zircon populations in all samples show strong similarities with the Sahara Craton and with zircons found in Libya, suggesting that NW Iberia occupied a paleoposition close to those regions of present-day northern Africa during this time interval. Changes in provenance in the Late Devonian are attributed to the onset of the collision between Gondwana and Laurussia. Additionally, the Middle Carboniferous to Permian samples record populations consistent with the recycling of older sedimentary sequences and exhumation of the igneous rocks formed before and during the Variscan orogeny. Late-Devonian to Permian samples yield zircon populations that reflect topographic changes produced during the Variscan orogeny and development of the lithospheric scale oroclinal buckling

Iberian late-Variscan granitoids: Some considerations on crustal sources and the significance of “mantle extraction ages

A suite of post-tectonic granitoids (mostly peraluminous, broadly I-type granodiorites and monzogranites) and mafic rocks from NWIberia with crystallization ages between ca. 309 and 290 Ma has been investigated for Sm–Nd isotopes and inherited zircon content in order to constrain the nature of their source rocks. εNd values (at 300 Ma) vary from −0.2 to −5.9 and TDM values range from 1.01 to 1.58 Ga. Inherited (xenocrystic) zircons yielded ages ranging from 458 to 676 Ma, with 90% of data between 490 and 646 Ma, corresponding to Neoproterozoic(mostly Ediacaran), Cambrian andOrdovician ages. Only three highlydiscordant analyses yielded ages older than 650 Ma. Based on the data reported herein and relevant data fromthe literaturewe contend that post-tectonic granitoids of the Iberian Variscan Belt (with exception of the scarce anatectic S-type granitoids) were derived mostly from metaigneous lower crustal sources which in turn were ultimately derived from a subcontinental lithospheric mantle enriched between ca. 0.9 and 1.1 Ga. I-type granitoids and mantle-derived mafic rocks both underwent varying degrees of contamination by ametasedimentary lower crust depleted in pre-650 Ma zircon (through previousmelting episodes) with a time-integrated Sm–Nd evolution different to that of the metaigneous lower crust. Participation of this metasedimentary crust in the genesis of these granitoids may account for Nd isotopic variability and Nd model ages well in excess of 1.2 Ga

Whence come detrital zircons in Siluro-Devonian rocks from Iberia?

Seven Silurian and Devonian samples from the Cantabrian and Central Iberian zones of the Variscan belt have been investigated for paleogeographic purposes using detrital zircon U-Pb ages. A total of 764 analyses were performed. All samples contain four main age populations in variable relative proportions: Ediacaran–Cryogenian (ca. 0.55–0.8 Ga), Tonian–Stenian (0.85–1.2 Ga), Paleoproterozoic (ca. 1.8–2.2 Ga) and Archean (ca. 2.5–3.3 Ga). The two first groups constitute ca. 60–80% of the total population in all samples. In addition, 5 samples contain very minor Paleozoic (Cambrian) zircons and 6 samples contain minor but significant zircons of Middle and Early Mesoproterozoic age (Ectasian–Calymmian). These data, used in conjunction with detrital zircon U-Pb data of underlying Ordovician and Ediacaran strata constrain the evolution of the northern margin of west Gondwana, highlighting the transition from an arc environment (Cadomian-Avalonian arc orogeny) to a stable platform following the opening of the Rheic Ocean and the drift of Avalonian terranes. Variations in detrital zircon populations in Middle–Late Devonian times reflect the onset of Variscan convergence between Laurussia and Gondwana. The abundance (up to ca. 50%) of zircons of Tonian–Stenian age in Devonian sedimentary rocks, that could not have been recycled from the underlying strata, may be interpreted in different ways: a) the existence of a large Tonian–Stenian arc terrane exposed in the NE African realm (in or around the Arabian-Nubian shield) that was progressively exhumed throughout the Paleozoic, b) the participation from Ordovician times onwards of a more easterly alongshore provenance of Tonian–Stenian zircons. In this scenario, the South China block could have furnished Tonian– Stenian zircons to the Ordovician and Siluro-Devonian basins of Iberia, c) increase in the relative proportion of Tonian–Stenian zircons with respect to the Ediacaran– Cryogenian population (arc-derived zircons) due to the drift of the Avalonian-Cadomian ribbon continent following the opening of the Rheic Ocean.Peer Reviewe

Buckling an orogen: The Cantabrian Orocline

The Paleozoic Variscan orogeny was a large-scale collisional event that involved amalgamation of multiple continents and micro-continents. Available structural, geological, geochemical, and geophysical data from Iberia are consistent with a model of oroclinal bending at the lithospheric scale of an originally near-linear convergent margin during the last stages of Variscan deformation in the late Paleozoic. Closure of the Rheic Ocean resulted in E-W shortening (in present-day coordinates) in the Carboniferous, producing a near linear N-S–trending, east-verging orogenic belt. Subsequent N-S shortening near the Carboniferous-Permian boundary resulted in oroclinal bending, highlighted by the formation of the Cantabrian Orocline. Together, these data constrain oroclinal bending in Iberia to have occurred during the latest Carboniferous over about a 10-million-year time window, which agrees well with recent geodynamical models and structural data that relate oroclinal bending with lithospheric delamination in the Variscan. This late-stage orogenic event remains an enigmatic part of final Pangaea amalgamation

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Presentació

Análisis de deformación finita en cinturones de pliegues y cabalgamientos (Talas Ala Tau, Kirguistán y zona cantabríca, NW de Iberia)

[ES] En este trabajo de grado se recoge la situación geológica de las zonas estudiadas, Talas Ala Tau, Kirguistán y zona cantabríca, NW de Iberia y se describen la metodología empleada para llevar a cabo el análisis de deformación finita en cinturones de pliegues y cabalgamientos,la interpretación de los resultados y las conclusiones que se obtienen de los mismos

Evolución geodinámica del Oroclinal Ibero Armoricano. Geología estructural, modelización análoga y geocronología

[EN] The Ibero-Armorican orocline is a central component of the Western European Variscan Belt, a complex continental-scale orogen (1000 km wide and 8000 km long) that formed through a series of protracted collisional events extending from ca.420 Ma to 320 Ma. Variscan deformation represents the closing of at least two – and possibly four – oceans between Laurentia, Baltica, Gondwana, and several micro-continents during the Paleozoic amalgamation of the Pangea supercontinent. The Ibero-Armorican orocline is characterized by the arcuate structural trend that traces an arc from Brittany across the Cantabrian Sea into western Iberia, where it is truncated by the Cenozoic Betic-Alpine front in southern Spain. New studies consider the Ibero-Armorican orocline as part of a coupled bend together with the southern Central-Iberian arc. The Ibero-Armorican orocline is a curved orogenic system characterized by a 180o bend of the Variscan structural grain. The Ibero-Armorican orocline has been the object of many studies, especially at its core. The aforementioned studies have attempted to decipher the curved mountain belt kinematics, and a wealth of different hypotheses have been proposed: a primary arc inherited from a Neoproterozoic embayment; a progressive arc resulting from indentation of a point-shaped block situated either in Gondwana or in Avalonia, an oblique collision producing a non-cylindrical orogen, a thin-skinned origin produced by a progressive change in the transport direction of the thrust units similar to a photographic iris, a large scale trans-continental shear zone, and more recently a true orocline formed by the rotation around a vertical axis of an originally linear orogen. In this PhD thesis the kinematics and dynamics of the Ibero-Armorican orocline have been studied at a lithospheric scale through structural analysis, analogue modelling and detrital zircon geochronology. With the data presented in this thesis and all the previous data published, a plausible overall interpretation is that the Variscan orogen was folded around a vertical axis during the Pennsylvanian during a period that lasted about 10 m.y during the Late Pennsylvanian. The structures developed during the formation of Iberian-Armorican orocline buckling suggest that this process occurred due to a large change in the stress field from E-W to N-S (in present day coordinates), which implies that the folding of the orogen was produced by the mechanism of buckling. The buckling process affected the whole lithosphere, which would have been deformed by a dominant mechanism of longitudinal-tangential strain. According to the experimental analogue models, the root formed in the lithospheric-mantle beneath the core of oroclinal was probably caused by lithospheric folding. This root became gravitationally unstable at around the Carboniferous-Permian boundary. At that time it could begin to develop a Rayleigh-Taylor instability ending with the detachment and sinking of the lithosphericmantle in the asthenospheric-mantle. This process of lithospheric-mantle detachment would have produced an inversion of the topography as recorded by the detrital zircons[ES] El oroclinal Ibero Armoricano se sitúa en la cadena Varisca del suroeste de Europa, que es un orógeno a escala continental -con dimensiones de 8000 km de largo y 1000 de ancho- que se formó como consecuencia de una colisión continental producida durante el Devónico y el Carbonífero. La deformación Varisca representa la clausura de entre dos y cuatro océanos situados entre los continentes de Laurencia, Báltica, Gondwana y varios microcontinentes durante la amalgamación de Pangea. El oroclinal Ibero Armoricano se caracteriza por presentar un patrón estructural con forma de arco de 180º que se puede seguir desde la península de Bretaña, a través del mar Cantábrico y el oeste de la península Ibérica, donde desaparece bajo el frente alpino del Sur de Iberia. Las interpretaciones más recientes consideran al oroclinal Ibero Armoricano parte de un sistema doble de oroclinales con forma de “S” que continúa hacia el sur de la península en el arco Centroibérico. El oroclinal Ibero Armoricano ha sido objeto de muchos estudios, especialmente en su núcleo, que pretendían resolver la cinemática de su curvatura. De estos estudios se han desprendido varias hipótesis distintas para su formación: un arco primario procedente de un golfo Neoproterozoico, un arco progresivo resultado de la indentación de un bloque puntiagudo situado en Gondwana o Avalonia, una colisión no cilíndrica, un origen epidérmico basado en el cambio de la dirección de transporte de los mantos, una cizalla transcontinental y más recientemente un oroclinal verdadero formado por la rotación de un orógeno linear alrededor de un eje vertical. En esta tesis se ha estudiado la cinemática y dinámica del oroclinal Ibero Armoricano a través de la geología estructural, la modelización análoga y la geocronología de circones detríticos. De los datos presentados en este trabajo y publicados anteriormente se puede interpretar que el orógeno Varisco fue plegado alrededor de un eje vertical durante el Pensilvaniense superior en un proceso de 10 millones de años de duración. Las estructuras que se desarrollaron durante la formación del mismo, sugieren que el proceso de plegamiento se produjo debido a un cambio en el régimen de esfuerzos de este-oeste a norte-sur (en coordenadas actuales) lo que implicaría que el plegamiento se produjo por pandeo (buckling). Este proceso de plegamiento habría ocurrido a escala litosférica, que presumiblemente se deformó mediante el mecanismo de deformación longitudinaltangencial. De acuerdo con la modelización análoga este plegamiento produciría una raíz litosférica bajo el núcleo del oroclinal producida posiblemente por plegamiento litosférico. Esta raíz habría dejado de ser estable gravitacionalmente en el límite Carbonífero-Pérmico, momento en el que comenzaría una inestabilidad tipo Rayleigh-Taylor que acabaría con el desprendimiento y hundimiento de la raíz litosférica en el manto astenosférico. Un proceso dedesprendimiento litosférico como el descrito habría producido una inversión de la topografía como la que ha sido registrada en los circones detrítico

From supercontinent to superplate: Late Paleozoic Pangea's inner deformation suggests it was a short-lived superplate

Thorough and constructive reviews by J. Brendan Murphy and an anonymous reviewer improved greatly this paper. I would like to acknowledge (in alphabetical order) Mark Dekkers, John Geissman, Gabriel Gutierrez-Alonso, Stephen Johnston, Cor Langereis, Patrick Meere, Tatsuki Tsujimori, and Rob van der Voo for endless discussions about the reconstruction. This paper is a contribution to UNESCO's IGCP 648: Supercontinent Cycles & Global Geodynamics project and it is dedicated to the career of Rob van der Voo. This work has been funded by a post-doctoral fellowship from The Netherlands Research Centre for Integrated Solid Earth Sciences (ISES), a fellowship for overseas researchers from the Japan Society for Promotion of Science (JSPS) (grant P16329) and a MEXT/JSPS KAKENHI Grant (JP16F16329) and a Ramon y Cajal Fellowship from the Spanish Ministry of Science and Innovation. I especially thank all the beats Charles Robert Watts has ever made. The author declares no competing interest.The supercontinent cycle explains how landmasses amalgamate into supercontinents that dismember after a ~ 100 Myr tenure in a quasi-periodic manner. Supercontinents are thought to be rigid superplates whose formation controls many of the Earth's secular variations, from long-term climate trends to global mantle circulation. Pangea, the latest continental superplate, formed ~330 Ma, began to rift ~240 Ma, finally broke-up ~200 Ma, is generally considered the template for all previous supercontinents. The existence of Pangea as a superplate at ~330 Ma is inconsistent with: (i) the kinematic constraints of the continent-continent collision that became progressively younger westwards and it only ended in the early Permian times in its westernmost side; (ii) the widespread ‘post-orogenic’ magmatism in the core of Pangea and the hot high-pressure metamorphism in the Paleotethys; and (iii) the global paleomagnetic database that shows paleolatitudinal overlaps between the participating continents, and significant vertical axis rotations in the core of Pangea between 330 and 270 Ma, which suggests >1500 km of shortening and extension. Here I present a tectonic reconstruction that reconciles the paleomagnetic and geological discrepancies. In this model, after the initial amalgamation of Pangea as a landmass, the comprising plates kept on interacting between each other and the asthenosphere during the late Carboniferous and early Permian (320–270 Ma) instead of being a rigid plate. After that and concomitant with a plate reorganization, Pangea finally established as a superplate for a brief period of <70 Myr. This superplate tenure might be, following most recent models, too short to control the global mantle circulation.Netherlands Research Centre for Integrated Solid Earth Sciences (ISES)Ministry of Education, Culture, Sports, Science and Technology, Japan (MEXT) Japan Society for the Promotion of Science P16329 Ministry of Education, Culture, Sports, Science and Technology, Japan (MEXT) Japan Society for the Promotion of ScienceGrants-in-Aid for Scientific Research (KAKENHI) JP16F16329 Spanish Governmen