A re-assessesment of the shallow paleomagnetic inclinations of the Western Cyclades, Greece

Στις Μεσο-Μειοκαινικές λιμναίες αποθέσεις και τα δακιτικά ηφαιστειακά πετρώματα της λεκάνης Κύμης-Αλιβερίου έχουν παρατηρηθεί σταθερά μικρές εγκλίσεις σε παλαιομαγνητικές μετρήσεις οι οποίες έχουν αναφερθεί ως αποδεικτικό στοιχείο είτε για τοπική γεωμαγνητική ανωμαλία είτε για μετατόπιση προς βορρά της περιοχής του Αιγαίου. Παρουσιάζονται νέα δεδομένα μετρήσεων παλαιομαγνητισμού σε λιμναία στρώματα τα οποία δεν παρουσιάζουν μικρές εγκλίσεις και είναι συμβατά με απόθεση κοντά στο σημερινό γεωγραφικό πλάτος τους. Οι ιδιαίτερα μικρές εγκλίσεις και αποκλίσεις προς τα ανατολικά που έχουν αναφερθεί στις ηφαιστειακές διεισδύσεις του Οξύλιθου είναι αποτελέσματα ακατάλληλης διόρθωσης ως προς οριζόντιο άξονα. Οι υπερβολικά μικρές παλαιομαγνητικές εγκλίσεις που έχουν αναφερθεί στα πλουτώνια σώματα της Μυκόνου και τη Νάξου, Μέσο-Μειοκαινικής ηλικίας παραμόρφωσης, είναι συμβατές με επαναπροσανατολισμό μιας αρχικής θερμο-παραμένουσας μαγνήτισης που προέκυψε κατά την ψύξη τους κάτω από 580°C την οποία ακολούθησε ιξωδοελαστική παραμόρφωση σε θερμοκρασίες 400-500°C. Δευτερεύουσες μαγνητίσεις που παρατηρήθηκαν στα πετρώματα αυτά αντικατοπτρίζουν, πιθανότατα, τη διαδικασία μίας σταθερής -χημικής προέλευσης- παραμένουσας μαγνήτισης, παράλληλα με την προϋπάρχουσα μαγνήτιση υψηλής θερμοκρασίας, ως αποτέλεσμα της δημιουργίας μαγκεμίτη σε χαμηλή θερμοκρασία (<350°C). Κατά συνέπεια δεν μπορούμε να βρούμε ικανοποιητικές αποδείξεις είτε για τοπική μαγνητική ανωμαλία ή μετακίνηση του Αιγαίου προς βορρά είτε για περιστροφή των μικρής κλίσης κανονικών ρηγμάτων που συνθέτουν το σύστημα αποκόλλησης των Βορείων Κυκλάδων.Consistently shallow paleomagnetic inclinations measured in Early to Middle Miocene lacustrine and dacitic volcanic rocks of the Kymi-Aliveri basin have been cited as evidence for an anomalous geomagnetic field geometry or northward drift of the Aegean Sea region. We present new paleomagnetic data from the lacustrine beds that are instead not anomalously shallow and consistent with deposition near their present-day latitude as predicted by global apparent polar wander paths. Anomalously shallow inclinations and easterly declinations reported from the Oxylithos volcanics are an artifact of an inappropriate tilt correction. The excessively shallow paleomagnetic inclinations reported from the deformed Middle Miocene plutons on Mykonos and Naxos are consistent with reorientation of an original thermoremanent magnetization acquired during cooling below 580°C by subsequent ductile strain at temperatures of 400-500°C. Magnetization overprints observed in these rocks may reflect the acquisition of a stable chemical remanent magnetization lying parallel to the transposed high-temperature magnetization as the result of low-temperature (<350°C) maghemitization. We therefore find no convincing evidence for an anomalous Middle Miocene field geometry, northward drift of the Aegean, or back-tilting of the low-angle normal faults that constitute the North Cycladic Detachment System

A geological and geophysical context for the Wenchuan earthquake of 12 May 2008, Sichuan, People's Republic of China

On 12 May 2008, a magnitude 7.9 earthquake ruptured the Longmen Shan margin of the eastern Tibetan plateau. This event occurred within the context of long-term uplift and eastward enlargement of the plateau. The area has numerous geological features not typical of active convergent mountain belts, including the presence of a steep mountain front (>4 km relief) but an absence of large-magnitude low-angle thrust faults; young high topography (post ca. 15 Ma) and thickened crust but low global positioning system (GPS) shortening rates (<3 mm/yr); and no coeval foreland subsidence. In our interpretation, crustal thickening beneath the eastern Tibetan plateau occurred without large-scale shortening of the upper crust but instead is caused by ductile thickening of the deep crust in a weak (low-viscosity) layer. Late Cenozoic shortening across the Longmen Shan could be as little as 10-20 km, with folding and faulting mainly accommodating differential surface uplift between the plateau and the Sichuan Basin. The earthquake of 12 May probably reflects long-term uplift with slow convergence and right-slip, of the eastern plateau relative to the Sichuan Basin. GPS-determined rates in the vicinity of the 12 May event suggest an average recurrence interval of ∼2,000-10,000 yr

Disruption of the Hellenic arc: Late Miocene extensional detachment faults and steep Pliocene-Quaternary normal faults - Or what happened at Corinth?

Extensional faults exposed in the Peloponnesus and mainland Greece, most of which are described here for the first time, record a transition from regional extension of the Aegean domain to the modern tectonic system. The East Peloponnesus Detachment System trends north-northwest from the southern Peloponnesus to ∼30 km north of the Gulf of Corinth, dips gently northeast, and is late Miocene-early Pliocene in age. It has a minimum displacement of 25-30 km and appears to be the youngest of the regional-scale extensional systems with significant displacement that formed parallel to the Hellenic are. The partially coeval East Sterea Extensional System, which extends from the Gulf of Corinth to the Aegean Sea, contains low-angle normal faults that both crosscut and trend parallel to older structures of the Hellenic arc. Late Miocene to early Pliocene displacement within this zone disrupted the arc-parallel structures of the Hellenides. Upper Pliocene-Quaternary normal faults, which trend approximately east-west and generally dip steeply at the surface, continue the disruption of the Hellenic arc. Much of the subsidence within the Gulf of Corinth appears to be unrelated to the younger faults and is instead related to the motion on the East Peloponnesus Detachment, which crosscuts the modern graben. Copyright 2007 by the American Geophysical Union

Slab segmentation and late Cenozoic disruption of the Hellenic arc

The Hellenic subduction zone displays well-defined temporal and spatial variations in subduction rate and offers an excellent natural laboratory for studying the interaction among slab buoyancy, subduction rate, and tectonic deformation. In space, the active Hellenic subduction front is dextrally offset by 100-120 km across the Kephalonia Transform Zone, coinciding with the junction of a slowly subducting Adriatic continental lithosphere in the north (5-10 mm/yr) and a rapidly subducting Ionian oceanic lithosphere in the south (∼35 mm/yr). Subduction rates can be shown to have decreased from late Eocene time onward, reaching 5-12 mm/yr by late Miocene time, before increasing again along the southern portion of the subduction system. Geodynamic modeling demonstrates that the differing rates of subduction and the resultant trench offset arise naturally from subduction of oceanic (Pindos) lithosphere until late Eocene time, followed by subduction of a broad tract of continental or transitional lithosphere (Hellenic external carbonate platform) and then by Miocene entry of high-density oceanic (Ionian) lithosphere into the southern Hellenic trench. Model results yield an initiation age for the Kephalonia Transform of 6-8 Ma, in good agreement with observations. Consistency between geodynamic model results and geologic observations suggest that the middle Miocene and younger deformation of the Hellenic upper plate, including formation of the Central Hellenic Shear Zone, can be quantitatively understood as the result of spatial variations in the buoyancy of the subducting slab. Using this assumption, we make late Eocene, middle Miocene, and Pliocene reconstructions of the Hellenic system that include quantitative constraints from subduction modeling and geologic constraints on the timing and mode of upper plate deformation. Copyright 2011 by the American Geophysical Union

Crustal development within a retreating subduction system: The Hellenides

In retreating subduction systems, where the subduction rate is faster than the convergence rate between the upper and lower plates, the processes by which the upper plate crust is constructed have not been well understood. From our studies in the Hellenides, which formed above a retreating slab, we conclude that the external part of the Cenozoic Hellenide orogen was constructed from rocks derived from the subducting plate at least at two crustal levels. The upper crustal level within the external Hellenides consists of west-vergent thrust sheets emplaced progressively from east to west along a regional décollement from ca. 35 Ma to present. These thrust sheets consist of Mesozoic and Cenozoic strata that have been stripped from their underlying basement to form the Hellenides. The middle and lower crustal layer consists of slices of continental crust detached from the downgoing slab at depth and accreted below the upper crustal thrust sheets. These accreted slices represent ~35% (or less) of the crust belonging to the subducting lithosphere; the remainder of the crust appears to be subducted with the slab. While the process of slab rollback may be continuous at depth, the episodic detachment of crustal slices guarantees that rollback is step-like in time at the crustal level. As the subducted lithosphere rolled back beneath the Hellenides, it passed progressively from east to west through the region occupied by present-day lower crust and mantle, where there is a well-defined Moho. Any irregularities that may have been present at the base of the accreted slabs have been smoothed by processes that remain to be determined. © 2018 The Authors

Flexural response of the Venetian foreland to the SouthAlpine tectonics along the TRANSALP profile.

Abstract: The Venetian Basin was affected by flexure related to the Southalpine shortening phase during the Middle Miocene-Early Pliocene. This downbending is quantified here using a two-dimensional flexural model. A recently improved data set on basin geometry based on the bottom of the Serravallian-Tortonian clastic wedge, on palaeobathymetry and gravity anomalies is used to constrain the components of flexure and to test the importance of the initial bathymetry in evaluating the contribution of surface loads to deflection. A good fit is obtained assuming a northward broken plate configuration of the downbent Adriatic plate with an effective elastic thickness of 20 km. Results highlight that, in the studied region, flexure related to the Eastern Southern Alps is totally due to surface loads (topographic load partly replacing initial bathymetry) and that no hidden loads are required. Furthermore, the palaeobathymetry contributes up to 50% to the total flexure in the studied region