scholarly journals Late orogenic extension in Hellenides

2001 ◽  
Vol 34 (1) ◽  
pp. 149
Author(s):  
Α. ΚΙΛΙΑΣ
Keyword(s):  
High Pressure ◽  
Accretionary Prism ◽  
Crustal Thickening ◽  
Plate Boundaries ◽  
Accretionary Wedge ◽  
Northern Greece ◽  
Crustal Rocks ◽  
Crete Island ◽  
Late Orogenic Extension ◽  
Back Arc

In the Hellenic orogen both typs of late orogenic extension, associated with deep crustal parts exhumation, are recognized during the Tertiare: In the areas of Olympos-Ossa and Pelion Mts in Northern Greece, as well as in the island of Crete in Southern Greece a bivergent late orogenic extension is recognized. Nappes collapse took place immediately above the cold accretionary wedge while compression was active at depth. Heer high pressure assemplages were good preserved. On the contrary, in the Rhodope and Cyclades areas an asymmetric extension dominates. Heer extensional exhumation of deep crustal rocks took place in the high thermal flow back-arc region and high pressure metamorphic rocks were highly overprinted by greenschist to amphibolite facies metamorphism. Partial melting and granitoids intrusions followed the high grade metamorphic reworking of the rocks. Tertiary late orogenic extension in the Hellenides tooke place simultaneously with successive subductions processes and crustal thickening at the front of the extended plate, forming with the associated compression a SW-ward migrated system. Extension started in the Rhodope massif during the Eocene/Oligocene to be reached in the Olympos, Ossa, Pilion and Cyclades areas in the Oligocene/Miocene and final in the Crete island at the more external Hellenides, during the Mid-Miocene. Changes in the rate of convergence between Africa and Eurasia associated with retreating plate boundaries conditions allowed the successive, extensional exhumation of the deep crustal rocks in the Hellenides. Assymetric collapse in the back-arc area was possibly favoured, because the high potential energy of the thickened crust in the active orogenic arc was counteracted by the continuing subduction along the boundaries of the converging segments of Africa and Eurasia. Symmetric collapse of the overthickened crust above the cold accretionary prism was favoured probably, due to an increasing of the upward pressure produced by the unterplating of the lithospheric slap beneath the accretionary wedge.

The crustal structure in the transition from the on land fold-and-thrust belt to the offshore accretionary prism in the Taiwan arc-continent collision

2020 ◽  
Author(s):  
Joaquina Alvarez-Marrón ◽  
Dennis Brown ◽  
Juan Alcalde ◽  
Ignacio Marzán ◽  
Hao Kuo-Chen
Keyword(s):  
Continental Margin ◽  
Seismic Tomography ◽  
Accretionary Prism ◽  
Crustal Thickening ◽  
Accretionary Wedge ◽  
Thrust Belt ◽  
Deformation Front ◽  
Coast Line ◽  
Transition Area ◽  
Fold And Thrust Belt

<p>The region of Taiwan is undergoing active, oblique arc-continent colision between the Luzon Arc on the Philippine Sea Plate and the continental margin of Eurasia. The Fold-and-Thrust Belt (FTB) in Taiwan passes southwards into a submarine accretionary wedge at the Manila subduction zone. The aim of this contribution is to examine how an on land FTB changes into a marine accretionary prism in the context of an oblique arc-continent collision. The Miocene pre-orogenic sediments of the continental margin are widespread in the FTB ca. 23° latitude while the offshore wedge is built up dominantly by Pliocene to recent syn-orogenic sediments. In the transition area from the marine accretionary wedge ca. 21° latitude to the on land FTB, the thrust wedge is climbing up the slope of the Eurasian continental margin. The deformation front is at sea floor depth of ca. 4 km in the south to less than 1 km as it reaches the coast line. Here we use the island surface geology, marine reflection seismic profiles, and seismic tomography models to construct contour maps of the basal thrust and the depth to the Moho across a transition area from near 23° to near 21° latitude. In this zone, the deformation front draws a convex curvature as the wedge widens from ca. 50 in the north and south, to more than 130 km near 22° latitude. The basal thrust surface shows a scoop shape as its dip changes from southeast near the coast line to east southward. The basal thrust reaches over 7 km deep beneath the rear of the FTB before ramping into de basement and merging into the Chaochou fault at 10 km depth. Offshore, it shows a gentler dip from 7 km to c. 10 km depth before getting steeper towards the east below the Hengchung Ridge. The basal cuts laterally along-strike through the margin’s sedimentary cover to incorporate thicker Miocene pre-orogenic sediments onto its hanging wall as it passes from the offshore wedge to the on land FTB.</p><p>In the offshore area, the Moho (we use a Vp proxy of 7.5 km/s extracted from the seismic tomography) shallows southeastward, from near 25 km depth below the shelf slope break to less than 17 km depth below the offshore wedge near 21.5° latitude before it starts to deep east towards beneath the Taiwan coast. The Moho dips northeast from near 25 km depth below the coast near Kaohsiung, to near 40 depth below the rear of the FTB at 23.5°, latitude. This complex morphology of the Moho may be related to the changes in crustal thickness and the obliquity of the collision. Because of this, crustal thickening is less pronounced beneath southern Taiwan where the thinner part of the margin is colliding with the arc.</p><p>This research is part of project PGC2018-094227-B-I00 funded by the Spanish Research Agency from the Ministry of Science Innovation and Universities of Spain.</p>

Alpine high-pressure metamorphism at the Almyropotamos window (southern Evia, Greece)

2000 ◽  
Vol 137 (4) ◽  
pp. 367-380 ◽  
Author(s):  
YONATHAN SHAKED ◽  
DOV AVIGAD ◽  
ZVI GARFUNKEL
Keyword(s):  
High Pressure ◽  
Large Scale ◽  
Structural Data ◽  
Late Eocene ◽  
Crustal Thickening ◽  
Lower Grade ◽  
The South ◽  
Northern Greece ◽  
High Pressure Metamorphism ◽  
Early Oligocene

The Alpine orogenic belt of the Hellenides has been strongly reworked by ductile and brittle extensional tectonics. Extensional structures have affected the central Aegean region and obliterated much of the original orogenic architecture since at least early Miocene times. In the area of Almyropotamos (on the island of Evia, flanking the western part of the Aegean) a unique remnant compressional nappe stack involving Tertiary metamorphic rocks has been preserved. This nappe sequence comprises a high-pressure rock unit on top of a lower grade unit. The upper unit (South Evia Blueschist Belt) is thought to be the westward continuation of the Cycladic blueschist belt metamorphosed at high-pressure conditions during Late Cretaceous–Eocene times. The underlying unit (the Almyropotamos Unit) is a continental margin sequence covered by a flysch and containing Lutetian nummulites, indicating that this unit accumulated sediments until at least late Eocene times.In the present study we analyse the petrology of the Almyropotamos nappe stack and define the P–T conditions of each of the different rock units exposed there. The presence of glaucophane, lawsonite rimmed by epidote, and jadeite (70 mol.%) suggest that peak P–T conditions in the South Evia Blueschist Belt reached approximately 10–12 kbar and 350–450 °C. Unlike previous studies, which estimated that the underlying Almyropotamos Unit reached only greenschist-facies conditions, glaucophane relics and Si-rich phengites were found by us in this unit. These indicate that high-pressure metamorphism and crustal thickening in this part of the Aegean lasted until at least the late Eocene or early Oligocene. We note that in this respect the architecture of southern Evia resembles that of northern Greece (Olympos, Ossa). Our structural data indicate that rock units in the Almyropotamos area record different folding phases, with the South Evia Blueschist Belt having a more complex fold history than the underlying Almyropotamos Unit. The entire nappe stack shares large-scale folds which are E–W trending, and locally overturned-to-the-south, and which may represent (at present coordinates) N–S contraction and nappe transport.

Pressure–temperature–deformation paths of closely associated ultra-high-pressure (diamond-bearing) crustal and mantle rocks of the Kimi complex: implications for the tectonic history of the Rhodope Mountains, northern Greece

2006 ◽  
Vol 43 (12) ◽  
pp. 1755-1776 ◽  
Author(s):  
E Mposkos ◽  
A Krohe
Keyword(s):  
High Pressure ◽  
White Mica ◽  
Temperature Deformation ◽  
Low Grade ◽  
Uhp Metamorphism ◽  
Northern Greece ◽  
Crustal Rocks ◽  
Ultra High Pressure ◽  
T Path ◽  
Static Annealing

The ultra-high-pressure (UHP) Kimi complex (uppermost eastern Rhodope Mountains) is a tectonic mixture of crustal and mantle derived associations. Pressure–temperature (P–T) paths and microtextural and geochronological data reveal that crustal and mantle parts juxtaposed against each other at a depth corresponding to ~15 kbar (1 kbar = 100 MPa) had separate ascend histories. The crustal rocks comprise amphibolitised eclogites, orthogneisses, marbles, and migmatitic pelitic gneisses. The latter document UHP metamorphism within the dehydration-melting range of pelitic gneisses, with maximum P–T conditions of >45 kbar at ~1000 °C, as determined by diamond inclusions in garnet and rutile needle exsolutions in Na-bearing garnet. Decompression was combined with only little cooling before 15 kbar, followed by more significant cooling between 15 and 10 kbar. This P–T path probably reflects ascent of UHP rocks within a subduction channel, followed by accretion in the lower crust of a thickened wedge. Although the first ascend phase was probably rapid, the overall time span for UHP metamorphism and final exhumation may have extended over more than 70 Ma. A U–Pb sensitive high-resolution ion microprobe (SHRIMP) age on zircons of ±149 Ma was suggested to date the UHP metamorphism, whereas Rb–Sr white mica and U–Pb zircon ages from syn-shearing pegmatites of ±65 Ma constrain medium- to low-grade shearing and final exhumation of UHP rocks. Mantle parts consisting of spinel–garnet metaperidotites and garnet pyroxenites reached maximum P–T conditions in the garnet-peridotite field at T > 1200 °C and P > 25 kbar. This was associated with plastic flow and followed by severe near isothermal cooling to T < 800 °C at 15 kbar and static annealing. A garnet–clinopyroxene whole-rock Sm–Nd age from a garnet pyroxenite of ±119 Ma probably reflects the age of metamorphic mantle processes (static annealing following the high P/high T strain episode), rather than constraining the age of UHP metamorphism.

High pressure, halogen-bearing melt in ultra-high temperature felsic granulites of the Central Maine Terrane, Connecticut (US)

2021 ◽  
Author(s):  
Silvio Ferrero ◽  
Jay J. Ague ◽  
Patrick J. O'Brien ◽  
Bernd Wunder ◽  
Laurent Remusat ◽  
...  
Keyword(s):  
High Pressure ◽  
Melt Inclusions ◽  
Extreme Temperature ◽  
Crustal Thickening ◽  
Deep Roots ◽  
Crustal Rocks ◽  
Melting Temperatures ◽  
Peak Metamorphism ◽  
Partial Melts ◽  
Relative Rarity

&lt;p&gt;Inclusions of relic high pressure melts provide information on the fate of crustal rocks in the deep roots of orogens during collision and crustal thickening, including at extreme temperature conditions exceeding 1000&amp;#176;C. However, discoveries of high pressure melt inclusions are still a relative rarity among case studies of inclusions in metamorphic minerals. Here we present the results of experimental and microchemical investigations of nanogranitoids in garnets from the felsic granulites of the Central Maine Terrane (Connecticut, US). Their successful experimental re-homogenization at ~2 GPa confirms that they originally were trapped portions of deep melts and makes them the first direct evidence of high pressure during peak metamorphism and melting for these felsic granulites. The trapped melt has a hydrous, granitic, and peraluminous character typical of crustal melts from metapelites. This melt is higher in mafic components (FeO and MgO) than most of the nanogranitoids investigated previously, likely the result of the extreme melting temperatures &amp;#8211; well above 1000&amp;#176;C. This is the first natural evidence of the positive correlation between temperature and mafic character of the melt, a trend previously supported only by experimental evidence. Moreover, it poses a severe caveat against the common assumption that partial melts from metasediments at depth are always leucogranitic in composition. NanoSIMS measurement on re-homogenized inclusions show significant amounts of CO&lt;sub&gt;2&lt;/sub&gt;, Cl and F. Halogen abundance in the melt is considered to be a proxy for the presence of brines (strongly saline fluids) at depth. Brines are known to shift the melting temperatures of the system toward higher values, and may have been responsible for delaying melt production via biotite dehydration melting until these rocks reached extreme temperatures of more than 1000&amp;#176;C, rather than 800-850&amp;#176;C as commonly observed for these reactions.&lt;/p&gt;

Synconvergent and coherent (ultra)high-pressure crustal rock exhumation

2021 ◽  
Author(s):  
Lorenzo G. Candioti ◽  
Joshua D. Vaughan-Hammon ◽  
Thibault Duretz ◽  
Stefan M. Schmalholz
Keyword(s):  
Numerical Models ◽  
A Priori ◽  
Western Alps ◽  
Ultrahigh Pressure ◽  
Plate Motion ◽  
Crustal Rock ◽  
Plate Boundaries ◽  
Crustal Rocks ◽  
Natural Data ◽  
The Alps

&lt;p&gt;Ultrahigh-pressure (UHP) continental crustal rocks were first discovered in the Western Alps in 1984 and have since then been observed at many convergent plate boundaries worldwide. Unveiling the processes leading to the formation and exhumation of (U)HP metamorphic crustal rocks is key to understand the geodynamic evolution of orogens such as the Alps.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;Previous numerical studies investigating (U)HP rock exhumation in the Alps predicted deep (&gt;80 km) subduction of crustal rocks and rapid buoyancy-driven exhumation of mainly incoherent (U)HP units, involving significant tectonic mixing forming so-called m&amp;#233;langes. Furthermore, these predictions often rely on excessive erosion or periods of divergent plate motion as important exhumation mechanism. Inconsistent with field observations and natural data, application of these models to the Western Alps was recently criticised.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;Here, we present models with continuous plate convergence, which exhibit local tectonic-driven upper plate extension enabling compressive- and buoyancy-driven exhumation of coherent (U)HP units along the subduction interface, involving feasible erosion.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;The two-dimensional petrological-thermo-mechanical numerical models presented here predict both subduction initiation and serpentinite channel formation without any a priori prescription of these two features. The (U)HP units are exhumed coherently, without significant internal deformation. Modelled pressure and temperature trajectories and exhumation velocities of selected crustal units agree with estimates for the Western Alps. The presented models support previous hypotheses of synconvergent exhumation, but do not rely on excessive erosion or divergent plate motion. Thus, our predictions provide new insights into processes leading to the exhumation of coherent (U)HP crustal units consistent with observations and natural data from the Western Alps.&lt;/p&gt;

Macro- and microstructural analysis of the Zhujiafang ductile shear zone, Hengshan Complex: Tectonic nature and geodynamic implications of the evolution of Trans−North China orogen

2020 ◽  
Author(s):  
Lingchao He ◽  
Jian Zhang ◽  
Guochun Zhao ◽  
Changqing Yin ◽  
Jiahui Qian ◽  
...  
Keyword(s):  
Shear Zone ◽  
North China ◽  
Shear Zones ◽  
Crustal Thickening ◽  
Slip Systems ◽  
High Grade ◽  
Ductile Shear Zone ◽  
Crustal Rocks ◽  
Ductile Shear ◽  
Lower Crustal

In worldwide orogenic belts, crustal-scale ductile shear zones are important tectonic channels along which the orogenic root (i.e., high-grade metamorphic lower-crustal rocks) commonly experienced a relatively quick exhumation or uplift process. However, their tectonic nature and geodynamic processes are poorly constrained. In the Trans−North China orogen, the crustal-scale Zhujiafang ductile shear zone represents a major tectonic boundary separating the upper and lower crusts of the orogen. Its tectonic nature, structural features, and timing provide vital information into understanding this issue. Detailed field observations showed that the Zhujiafang ductile shear zone experienced polyphase deformation. Variable macro- and microscopic kinematic indicators are extensively preserved in the highly sheared tonalite-trondhjemite-granodiorite (TTG) and supracrustal rock assemblages and indicate an obvious dextral strike-slip and dip-slip sense of shear. Electron backscattered diffraction (EBSD) was utilized to further determine the crystallographic preferred orientation (CPO) of typical rock-forming minerals, including hornblende, quartz, and feldspar. EBSD results indicate that the hornblendes are characterized by (100) &lt;001&gt; and (110) &lt;001&gt; slip systems, whereas quartz grains are dominated by prism &lt;a&gt; and prism &lt;c&gt; slip systems, suggesting an approximate shear condition of 650−700 °C. This result is consistent with traditional thermobarometry pressure-temperature calculations implemented on the same mineral assemblages. Combined with previously reported metamorphic data in the Trans−North China orogen, we suggest that the Zhujiafang supracrustal rocks were initially buried down to ∼30 km depth, where high differential stress triggered the large-scale ductile shear between the upper and lower crusts. The high-grade lower-crustal rocks were consequently exhumed upwards along the shear zone, synchronous with extensive isothermal decompression metamorphism. The timing of peak collision-related crustal thickening was further constrained by the ca. 1930 Ma metamorphic zircon ages, whereas a subsequent exhumation event was manifested by ca. 1860 Ma syntectonic granitic veins and the available Ar-Ar ages of the region. The Zhujiafang ductile shear zone thus essentially record an integrated geodynamic process of initial collision, crustal thickening, and exhumation involved in formation of the Trans−North China orogen at 1.9−1.8 Ga.

Coastal and offshore provinces of Timor-Leste — Geophysics exploration and drilling

2020 ◽  
Vol 39 (8) ◽  
pp. 543-550
Author(s):  
Roberto Fainstein ◽  
Juvêncio De Deus Correia do Rosário ◽  
Helio Casimiro Guterres ◽  
Rui Pena dos Reis ◽  
Luis Teófilo da Costa
Keyword(s):  
Continental Margin ◽  
Accretionary Prism ◽  
Stratigraphic Correlation ◽  
Accretionary Wedge ◽  
Northern Coast ◽  
Potential Field Data ◽  
The North ◽  
Banda Arc ◽  
Timor Leste ◽  
Well Data

Regional geophysics research provides for prospect assessment of Timor-Leste, part of the Southeast Asia Archipelago in a region embracing the Banda Arc, Timor Island, and the northwest Australia Gondwana continental margin edge. Timor Island is a microcontinent with several distinct tectonic provinces that developed initially by rifting and drifting away from the Australian Plate. A compressive convergence began in the Miocene whereby the continental edge of the large craton collided with the microcontinent, forming a subduction zone under the island. The bulk of Timor Island consists of a complex mélange of Tertiary, Cretaceous, Jurassic, Triassic, Permian, and volcanic features over a basal Gondwana craton. Toward the north, the offshore consists of a Tertiary minibasin facing the Banda Arc Archipelago, with volcanics interspersed onshore with the basal Gondwana pre-Permian. A prominent central overthrust nappe of Jurassic and younger layers makes up the mountains of Timor-Leste, terminating south against an accretionary wedge formed by this ongoing collision of Timor and Australia. The northern coast of the island is part of the Indonesian back arc, whereas the southern littoral onshore plus shallow waters are part of the accretionary prism. Deepwater provinces embrace the Timor Trough and the slope of the Australian continental margin being the most prospective region of Timor-Leste. Overall crust and mantle tectonic structuring of Timor-Leste is interpreted from seismic and potential field data, focusing mostly on its southern offshore geology where hydrocarbon prospectivity has been established with interpretation of regional seismic data and analyses of gravity, magnetic, and earthquake data. Well data tied to seismic provides focal points for stratigraphic correlation. Although all the known producing hydrocarbon reservoirs of the offshore are Jurassic sands, interpretation of Permian and Triassic stratigraphy provides knowledge for future prospect drilling risk assessment, both onshore and offshore.

scholarly journals Subduction of trench-fill sediments beneath an accretionary wedge: Insights from sandbox analogue experiments

Geosphere ◽  
2020 ◽  
Vol 16 (4) ◽  
pp. 953-968 ◽  
Author(s):  
Atsushi Noda ◽  
Hiroaki Koge ◽  
Yasuhiro Yamada ◽  
Ayumu Miyakawa ◽  
Juichiro Ashi
Keyword(s):  
High Pressure ◽  
Side Wall ◽  
Metamorphic Rocks ◽  
Accretionary Wedge ◽  
Seafloor Topography ◽  
Subduction Channel ◽  
Hp Metamorphism ◽  
Analogue Experiments ◽  
Topographic High

Abstract Sandy trench-fill sediments at accretionary margins are commonly scraped off at the frontal wedge and rarely subducted to the depth of high-pressure (HP) metamorphism. However, some ancient exhumed accretionary complexes are associated with high-pressure–low-temperature (HP-LT) metamorphic rocks, such as psammitic schists, which are derived from sandy trench-fill sediments. This study used sandbox analogue experiments to investigate the role of seafloor topography in the transport of trench-fill sediments to depth during subduction. We conducted two different types of experiments, with or without a rigid topographic high (representing a seamount). We used an undeformable backstop that was unfixed to the side wall of the apparatus to allow a seamount to be subducted beneath the overriding plate. In experiments without a seamount, progressive thickening of the accretionary wedge pushed the backstop down, leading to a stepping down of the décollement, narrowing of the subduction channel, and underplating of the wedge with subducting sediment. In contrast, in experiments with a topographic high, the subduction of the topographic high raised the backstop, leading to a stepping up of the décollement and widening of the subduction channel. These results suggest that the subduction of stiff topographic relief beneath an inflexible overriding plate might enable trench-fill sediments to be deeply subducted and to become the protoliths of HP-LT metamorphic rocks.

scholarly journals Dismembered Ophiolite of the Olkhon Composite Terrane (Baikal, Russia): Petrology and Emplacement

Minerals ◽  
2020 ◽  
Vol 10 (4) ◽  
pp. 305
Author(s):  
Eugene V. Sklyarov ◽  
Angrey V. Lavrenchuk ◽  
Valentine S. Fedorovsky ◽  
Evgenii V. Pushkarev ◽  
Dina V. Semenova ◽  
...  
Keyword(s):  
Siberian Craton ◽  
Regional Metamorphism ◽  
Thin Layers ◽  
Accretionary Wedge ◽  
Oblique Collision ◽  
Small Bodies ◽  
Mafic Rocks ◽  
Suprasubduction Zone ◽  
Frontal Collision ◽  
Back Arc

Dismembered ophiolites in the Early Paleozoic Olkhon terrane, a part of the Baikal collisional belt in the southern periphery of the Siberian craton, occur as fault-bounded blocks of ultramafic and mafic rocks from a few meters to hundreds of meters in size. The ultramafic rocks are mainly dunite–harzburgite peridotites with gradual transitions between the lithologies, as well as moderate amounts of enstatitite, wehrlite, and clinopyroxenite, but no lherzolite. Most peridotites have strongly depleted chemistry and a mineralogy corresponding to the harzburgite type usual for ophiolites of suprasubduction zones (SSZ). The mafic rocks are leuco- to melanocratic gabbros with different relative percentages of clinopyroxene, olivine, and plagioclase, which enclose thin layers and lenses of clinopyroxenite and anorthosite. They bear back-arc basin geochemical signatures, a setting inferred for the Neoproterozoic southern Siberian craton. The gabbroic rocks are of two geochemical groups; most of their trace-element patterns show Ta-Nb minimums and Sr maximums common to suprasubduction zone ophiolites. Judging by the Ol + Opx + Chl + Chr mineral assemblages, the Olkhon peridotites underwent low amphibolite and amphibolite regional metamorphism at 500–650 °C. The occurrence of the ultramafic and mafic bodies is consistent with formation in an accretionary wedge metamorphosed during a collisional orogeny. The mantle and crustal parts of the Olkhon ophiolite suite apparently were incorporated into the terrane during the frontal collision of perio-oceanic structures with the Siberian craton. Then, in a later oblique collision event, they became dismembered by strike-slip faulting into relatively small bodies and fault blocks exposed in the present erosional surface.

Sign in / Sign up

Export Citation Format

Share Document