High-pressure Low-temperature Metamorphic Rocks Research Articles

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

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.

Late Carboniferous (Kasimovian) closure of the South Tianshan Ocean: No Triassic subduction

The proposed Triassic age of oceanic subduction and high-pressure/low-temperature (HP-LT) metamorphism in the South Tianshan orogen (STS) of the southwestern Central Asian Orogenic Belt needs to be re-examined on the basis of field relationship and precise age dating. Our biostratigraphic study in the Atbashi Range of southern Kyrgyzstan indicates that sedimentary strata unconformably overlie the HP-LT metamorphic rocks within the Turkestan suture zone and have a late Kasimovian (ca. 305 Ma) age. This constrains the minimum age of eclogite metamorphism in the HP-LT metamorphic complex. A Late Pennsylvanian to early Permian (Asselian) hinterland basin overlaps the margins of the Middle and South Tianshan and the ophiolitic suture between them, thus leaving no space for an oceanic basin in the northern STS in the late Permian or Triassic. In the south, overlap assemblages stitching the STS with the Tarim Craton consist of late Asselian to early Sakmarian limestones and early Permian rhyolites. Early Permian (ca. 295–265 Ma) post-collisional A-type granites and sub-alkaline plutons are widespread in the STS and are not affected by thrust deformation. These facts indicate that thrust packages of the STS were stacked together and welded to the margin of the Tarim Craton by the early Permian. The widespread occurrence of early Permian sub-alkaline granites is also not compatible with an oceanic accretionary wedge setting in the STS during the Permian or Triassic.Permian and Triassic zircon ages for subduction-related HP-LT metamorphic rocks and ophiolitic gabbro from the Atbashi and Djandjir ranges reported in two recent papers contradict geological relationships. A review of reported analytical data shows that anomalously young ages in all studied samples are based on geologically meaningless single zircon analyses and/or discordant data, which cannot be used for age calculation. The suggestion that an oceanic basin in the STS remained open and subduction continued until the Middle Triassic is erroneous because of insufficient consideration of basic regional geological relationships and incorrect interpretations of analytical results.

A ductile extensional shear zone at the contact area between HP-LT metamorphic units in the Talea Ori, central Crete, Greece: deformation during early stages of exhumation from peak metamorphic conditions

We report on an extensional ductile shear zone in central Crete at the contact area between the high-pressure low-temperature (HP-LT) metamorphic Phyllite-Quartzite unit sensu stricto (PQ s.str.) and the Talea Ori group (Plattenkalk unit). Mapping and microscopic analysis along the 20 km long contact reveal extensional shear bands, shear band cleavages (C′-type) and associated quartz veins in both units, occurring over a width of up to a kilometer. The shear offset along the shear bands is systematically perpendicular to the contact with the hanging block (PQ s.str.) being downfaulted. Abundant discordant quartz veins are associated to shear band boundaries, asymmetric boudinage and foliation boudinage, forming m-wide vein networks. These mesoscopic deformation structures together with related microstructures such as strain shadows, growth rims of albite porphyroblasts and stylolites indicate dissolution–precipitation creep as main deformation mechanism accompanied by vein formation. Temperatures during deformation are indicated to be close to peak metamorphic temperatures (≥ 300–350 °C) by the growth rims of albite porphyroblasts and micas present in the shear band cleavages, which are consistent with quartz vein microstructures showing subgrains and sutured high angle grain boundaries. Peak metamorphic temperatures inferred by the degree of graphitization of carbonaceous material are similar in the hanging wall and the footwall of the shear zone, which is consistent with fast and nearly adiabatic exhumation. Our study demonstrates the importance of ductile shear zones with high strain rate dissolution–precipitation creep and vein formation in HP-LT metamorphic rocks for the early exhumation history.

Seismic visibility of a deep subduction channel – insights from numerical simulation of high-frequency seismic waves emitted from intermediate depth earthquakes

Abstract. Return flow in a deep subduction channel (DSC) has been proposed to explain rapid exhumation of high pressure–low temperature metamorphic rocks, entirely based on the fossil rock record. Supported by thermo-mechanical models, the DSC is envisioned as a thin layer on top of the subducted plate reaching down to minimum depths of about 150 km. We perform numerical simulations of high-frequency seismic wave propagation (1–5 Hz) to explore potential seismological evidence for the in situ existence of a DSC. Motivated by field observations, for modeling purposes we assume a simple block-in-matrix (BIM) structure with eclogitic blocks floating in a serpentinite matrix. Homogenization calculations for BIM structures demonstrate that effective seismic velocities in such composites are lower than in the surrounding oceanic crust and mantle, with nearly constant values along the entire length of the DSC. Synthetic seismograms for receivers at the surface computed for intermediate depth earthquakes in the subducted oceanic crust for models with and without DSC turn out to be markedly influenced by its presence or absence. While for both models P and S waveforms are dominated by delayed high-amplitude guided waves, models with DSC exhibit a very different pattern of seismic arrivals compared to models without DSC. The main reason for the difference is the greater length and width of the low-velocity channel when a DSC is present. Seismic velocity heterogeneity within the DSC or oceanic crust is of minor importance. The characteristic patterns allow for definition of typical signatures by which models with and without DSC may be discriminated. The signatures stably recur in slightly modified form for earthquakes at different depths inside subducted oceanic crust. Available seismological data from intermediate depth earthquakes recorded in the forearc of the Hellenic subduction zone exhibit similar multi-arrival waveforms as observed in the synthetic seismograms for models with DSC. According to our results, observation of intermediate depth earthquakes along a profile across the forearc may allow to test the hypothesis of a DSC and to identify situations where such processes could be active today.

Variation of mineral composition, fabric and oxygen fugacity from massive to foliated eclogites during exhumation of subducted ocean crust in the North Qilian suture zone, NW China

Eclogites from the North Qilian suture zone are high-pressure low-temperature metamorphic rocks of ocean crust protolith, and occur in both massive and foliated varieties as individual blocks of tens to hundreds of metres in size. The massive type is weakly deformed and shows granoblastic texture characterized by a coarse-grained peak mineral assemblage of Grt1 + Omp1 + Ph + Rt ± Lws (or retrograde Cz). In contrast, the foliated type is strongly deformed and shows a fine-grained retrograde mineral assemblage of Grt2 + Omp2 + Cz + Gln + Ph. Both total FeO and aegirine contents in omphacite, as well as XFe[=Fe3+/(Fe3+ + AlVI)] in clinozoisite/epidote, increase significantly from massive to foliated eclogites. Lattice preferred orientation (LPO) of omphacite, determined by electron back-scatter diffraction analysis, is characterized by weak and strong SL-type fabrics for massive and foliated eclogites, respectively. Clinozoisite/epidote also developed SL-type fabric, but different from the LPOs of omphacite in and axes, owing to their opposite crystallographic long and short axis definitions. The transition of deformation mechanism from dislocation creep to diffusive mass transfer (DMT) creep in omphacite and the concomitant retrograde metamorphism both are efficiently facilitated when the original coarse-grained Omp1 + Grt1 + Lws assemblage is dynamically recrystallized and retrogressed into the fine-grained Fe3+-rich assemblage of Omp2 + Grt2 + Cz + Gln. The DMT process with concomitant anisotropic growth assisted by fluids is considered to be an important deformation mechanism for most minerals in the foliated eclogite. P–T estimates yielded 2.3–2.6 GPa and 485−510 °C for the massive eclogite and 1.8–2.2 GPa and 450−480 °C for the foliated eclogite. The significant increase in total Fe and Fe3+ contents in omphacite and clinozoisite/epidote from massive to foliated eclogite suggests changes in mineral compositions accompanied by an increase in oxygen fugacity during ductile deformation associated with exhumation. The LPO transition of omphacite, clinozoisite and rutile from weak SL-type in massive eclogites to strong SL-type in foliated eclogites is interpreted to represent the increment of shear strain during exhumation along the ‘subduction channel’.

Post-orogenic extension and metamorphic core complexes in a heterogeneous crust: the role of crustal layering inherited from collision. Application to the Cyclades (Aegean domain)

SUMMARY The development of metamorphic core complexes (MCC) corresponds to a mode of lithospheric continental stretching that follows collision. In most of the models that explain the formation of the MCC, high thermal gradients are necessary to weaken the lower crust and to induce its ascent. Such models fail to explain the exhumation of high pressure–low temperature metamorphic rocks in metamorphic core complex structures as observed in the Cycladic Blueschists in the Aegean domain. Besides, account for the lithological crustal stratification induced from collision has never been tested. In this paper, we use fully coupled thermomechanical modelling to investigate the impact of structural heritage and initial thermal gradient on the behaviour of the post-orogenic continental lithosphere. The models are designed and validated by petrological, structural and time data from the Cyclades. As a result, high thermal gradients (Moho temperature higher than 800 ◦ C) are neither necessary nor always sufficient to induce the development of a metamorphic core complex. At the contrary, the rheological layering of the crust inherited from collision is a first-order parameter controlling the development of extensional structures in post-orogenic settings. ‘Cold’ MCC can develop if the crust is made of a strong nappe thrust on top of weaker metamorphic cover and basement units, as observed in the Cyclades.

Tectono‐sedimentary evolution of lower to middle Miocene half‐graben basins related to an extensional detachment fault (western Crete, Greece)

AbstractCrustal extension in the overriding plate at the Aegean subduction zone, related to the rollback of the subducting African slab in the Miocene, resulted in a detachment fault separating high‐pressure/low‐temperature (HP‐LT) metamorphic lower from non‐metamorphic upper tectonic units on Crete. In western Crete, detachment faulting at deeper crustal levels was accompanied by structural disintegration of the hangingwall leading to the formation of half‐graben‐type sedimentary basins filled by alluvial fan and fan‐delta deposits. The coarse‐grained clastic sediments in these half‐grabens are exclusively derived from the non‐metamorphic units atop the detachment fault. Being in direct tectonic contact with HP‐LT metamorphic rocks of the lower tectonic units today, the basins must have formed in the period between c. 20 and 15 Ma, prior to the exposure of the HP‐LT metamorphic rocks at the surface, and juxtaposed with the latter during ongoing deformation.

Subduction of solar-type noble gases from extraterrestrial dust: constraints from high-pressure low-temperature metamorphic deep-sea sediments

Solar-type helium (He) and neon (Ne) in the Earth’s mantle were suggested to be the result of solar-wind loaded extraterrestrial dust that accumulated in deep-sea sediments and was subducted into the Earth’s mantle. To obtain additional constraints on this hypothesis, we analysed He, Ne and argon (Ar) in high pressure–low temperature metamorphic rocks representing equivalents of former pelagic clays and cherts from Andros (Cyclades, Greece) and Laytonville (California, USA). While the metasediments contain significant amounts of 4He, 21Ne and 40Ar due to U, Th and K decay, no solar-type primordial noble gases were observed. Most of these were obviously lost during metamorphism preceding 30 km subduction depth. We also analysed magnetic fines from two Pacific ODP drillcore samples, which contain solar-type He and Ne dominated by solar energetic particles (SEP). The existing noble gas isotope data of deep-sea floor magnetic fines and interplanetary dust particles demonstrate that a considerable fraction of the extraterrestrial dust reaching the Earth has lost solar wind (SW) ions implanted at low energies, leading to a preferential occurrence of deeply implanted SEP He and Ne, fractionated He/Ne ratios and measurable traces of spallogenic isotopes. This effect is most probably caused by larger particles, as these suffer more severe atmospheric entry heating and surface ablation. Only sufficiently fine-grained dust may retain the original unfractionated solar composition that is characteristic for the Earth’s mantle He and Ne. Hence, in addition to the problem of metamorphic loss of solar noble gases during subduction, the isotopic and elemental fractionation during atmospheric entry heating is a further restriction for possible subduction hypotheses.

Exhumation Paths of High-Pressure–Low-Temperature Metamorphic Rocks from the Lycian Nappes and the Menderes Massif (SW Turkey): a Multi-Equilibrium Approach

The Menderes Massif and the overlying Lycian Nappes occupy an extensive area of SW Turkey where high-pressure–low-temperature metamorphic rocks occur. Precise retrograde P–T paths reflecting the tectonic mechanisms responsible for the exhumation of these high-pressure–low-temperature rocks can be constrained with multi-equilibrium P–T estimates relying on local equilibria. Whereas a simple isothermal decompression is documented for the exhumation of high-pressure parageneses from the southern Menderes Massif, various P–T paths are observed in the overlying Karaova Formation of the Lycian Nappes. In the uppermost levels of this unit, far from the contact with the Menderes Massif, all P–T estimates depict cooling decompression paths. These high-pressure cooling paths are associated with top-to-the-NNE movements related to the Akçakaya shear zone, located at the top of the Karaova Formation. This zone of strain localization is a local intra-nappe contact that was active in the early stages of exhumation of the high-pressure rocks. In contrast, at the base of the Karaova Formation, along the contact with the Menderes Massif, P–T calculations show decompressional heating exhumation paths. These paths are associated with severe deformation characterized by top-to-the-east shearing related to a major shear zone (the Gerit shear zone) that reflects late exhumation of high-pressure parageneses under warmer conditions.

The tectonometamorphic evolution of high-pressure low-temperature metamorphic rocks of eastern Crete, Greece: constraints from microfabrics, strain, illite crystallinity and paleodifferential stress

Analysis of strain, paleodifferential stress, illite crystallinity, and microfabrics of quartz and calcite suggest that multiply alternating late Oligocene/early Miocene N–S convergence and N–S extension caused significant vertical discontinuities in the grade of high-pressure/low-temperature metamorphism of the Phyllite–Quartzite unit (PQU) of eastern Crete. Constrictional fabrics and parallelism of D2 stretching lineations and fold axes are attributed to enhanced slab pull forces at greater depth of subduction. A major D3 extensional detachment is present in the upper part of the PQU where slates with lowest metamorphic grade (diagenesis/anchizone transition) are resting on top of anchimetamorphic rocks. The highest metamorphic grade (anchi-/epizone transition) has been determined for Skythian rocks on top of pre-Alpine basement. Revived N–S convergence (D4) led to E–W-trending map-scale folds and top-to-the-S directed brittle thrusts. There is a clear correlation between D2-related metamorphic grade, finite strain, differential stress, and deformation mechanisms. At the anchi-/epizone transition, solution–precipitation and dislocation creep of quartz and calcite caused maximum finite strains (RXZ=up to 7) at high strain rates and moderate differential stresses (80–200MPa). At the diagenesis/anchizone transition, deformation was accommodated by cataclasis, dislocation glide and solution–precipitation creep resulting in relatively low finite strains (RXZ<2) at high differential stresses of up to 300MPa.

Active displacement partitioning and arc-parallel extension of the Aleutian volcanic arc based on Global Positioning System geodesy and kinematic analysis

Research Article| August 01, 2000 Active displacement partitioning and arc-parallel extension of the Aleutian volcanic arc based on Global Positioning System geodesy and kinematic analysis Hans G. Avé Lallemant; Hans G. Avé Lallemant 1Department of Geology and Geophysics, Rice University, Houston, Texas 77251-1892, USA Search for other works by this author on: GSW Google Scholar John S. Oldow John S. Oldow 2Department of Geological Sciences, University of Idaho, Moscow, Idaho 83844-3022, USA Search for other works by this author on: GSW Google Scholar Geology (2000) 28 (8): 739–742. https://doi.org/10.1130/0091-7613(2000)28<739:ADPAAE>2.0.CO;2 Article history received: 10 Jan 2000 rev-recd: 18 May 2000 accepted: 26 May 2000 first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation Hans G. Avé Lallemant, John S. Oldow; Active displacement partitioning and arc-parallel extension of the Aleutian volcanic arc based on Global Positioning System geodesy and kinematic analysis. Geology 2000;; 28 (8): 739–742. doi: https://doi.org/10.1130/0091-7613(2000)28<739:ADPAAE>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu nav search search input Search input auto suggest search filter All ContentBy SocietyGeology Search Advanced Search Abstract Global Positioning System geodesy and structural analysis of parts of the Aleutian island chain support the interpretation of forearc migration westward along right-lateral transcurrent faults. Measured displacements are essentially arc parallel in the eastern (Unalaska Island) and western (Attu Island) parts of the arc and vary from 3.1 mm/yr to 31.4 mm/yr, respectively. At the center of the arc (Adak) subduction of the Amlia fracture zone disrupts the pattern of arc-parallel displacement and records an upper plate velocity of 9.6 mm/yr parallel to plate motion. Where active, displacement partitioning accommodates 30%–50% of the strike-slip component of plate convergence. In the area of impeded subduction partitioning ceases, suggesting that the process is sensitive to the degree of coupling across the plate-boundary megathrust. The differential arc-parallel displacements observed at the ends of the arc together with structural relations indicate substantial arc-parallel extension, which may play an important role in exhumation of high-pressure–low-temperature metamorphic rocks in ancient arc systems. You do not currently have access to this article.

Stratigraphy and pre-Miocene tectonic evolution of the southwestern part of the Sivas Basin, Central Anatolia, Turkey

In central Anatolia there are several important basins developed mainly after closure of the northern branch of Neotethys. These are the Haymana, Tuzgölü, Ulukışla, Kızılırmak, Çankırı-Çorum and Sivas basins. The Sivas Basin is located in the eastern part of central Anatolia between the Central Anatolian Crystalline Complex (CACC) in the north and Taurides in the south. The basement to the southeastern part of the basin consists of recrystallized limestone and clastics of the Permian–Lower Cretaceous Bünyan Metamorphics. These units are overlain by an Upper Cretaceous ophiolitic olistostrome that is overthrust by ophiolites and high pressure–low temperature metamorphic rocks. Lower Palaeocene cover units unconformably overlie this sequence. The basement to the northwestern part is constituted by CACC that includes a high temperature–low pressure polymetamorphic succession of Palaeozoic–Mesozoic age, overthrust by ophiolites and intruded by Upper Cretaceous post-collisional granitoids and syenitoids. The uppermost Maastrichtian–Palaeocene continental to shallow marine (lagoonal) unit unconformably overlies this unit. Upper Cretaceous–Palaeocene siltstone, shale, pelagic limestone, volcaniclastic rocks and basic volcanic rock intercalations of a within-continental-plate eruptive setting have also been developed on the basement unit. These sequences represent the products of an extensional episode during Late Cretaceous–Palaeocene times in the region between the Taurides and CACC. The Middle Eocene is represented by a regional transgression which was followed by a compressional episode evidenced by thrust faults at the margins and continued regression in the central part of the basin. This compressional period continued up to the end of the Early Miocene. Units formed during this episode are overlain by Upper Miocene–Quaternary continental units intercalated with volcanic rocks formed in fault-controlled extensional basins. It is suggested that the palaeotectonic events were the result of terminal closure of the northern branch of Neotethys. However, the neotectonic events are the result of the collision of the Arabian Plate and Anatolides which causes a westward escape of the Anatolian Plate. Copyright © 1999 John Wiley & Sons, Ltd.

Miocene high-pressure metamorphic rocks of Crete, Greece: rapid exhumation by buoyant escape

Abstract The pre-Neogene thrust sheets of Crete, Greece, accreted during Oligocene and Early Miocene time, can be divided into two main groups juxtaposed by a Miocene extensional detachment. Oligo-Miocene high pressure-low temperature (HP-LT) metamorphic rocks crop out in the lower plate to the detachment, and rocks that show no evidence of Tertiary metamorphism in the upper plate. Detailed pressure, temperature and structural information from the HP-LT metamorphic rocks combined with new fission-track data from the upper plate reveal that the lower plate rocks were subducted, then pervasively deformed and metamorphosed at their maximum depth of burial (30–35 km, 300–400°C) between c. 24 and 19 Ma and then exhumed to less than 10 km depth at rates &gt;4 km Ma −1 before c. 17 Ma. Microstructural studies reveal that during exhumation the lower plate acted as a coherent block, with deformation and retrograde metamorphism localized along the extensional detachment. The rocks of the upper plate can be shown to have remained in the upper 4–7 km of the crust since at least Eocene time. After cessation of movement along the main extensional detachment, both the upper and lower plates were subjected to brittle extension and increased erosion initiated at c. 16–17 Ma. These boundary conditions imply continuous subduction retreat since at least Eocene time along the Aegean segment of the Hellenic convergent plate boundary. A tectonic model is presented where exhumation is driven by positive buoyancy of the subducted continental crust following lithospheric delamination. We propose that the subducted microcontinent was exhumed by a process of ‘oblique buoyant escape’ and entered the space created by the retreating subducting oceanic slab.

Strain partitioning and metamorphism in a deformable orogenic wedge: Application to the Alpine belt

Various processes have been proposed in the literature to explain the exhumation and preservation of blueschist and eclogitic facies rocks. Among these processes, corner flow circulation within an accretionary wedge is frequently advocated. In the Alpine collisional belt, geological, petrological and chronological evidences indicate that high-pressure low-temperature metamorphic rocks have been generated in the subduction zones which existed before the collisional event. Indeed, the internal part of the Alps have been compared to the inner zone of modern accretionary prisms. In order to discuss the corner flow circulation model and its applications to the internal part of the Alps taking into account the metamorphic and structural data, we have numerically explored in a deformable orogenic wedge: (1) the possible generation of HP-LT metamorphism in this context and the PTt paths related to the exhumation process; (2) the duration of the exhumation; and (3) the strain partitioning and the related geometries imposed by this circulation. The velocity field is computed from mass conservation law. No attempt has been made to include variable viscosity. We show that: (1) HP-LT metamorphism can be achieved in the computed conditions, PTt paths differ according to the velocity of subduction; (2) the exhumation process lasts less than 15 Ma and generates high strains which erase the initial deformation phases; and (3) metamorphic and structural data from the internal part of the Alps are compatible with the numerical constraints.

Density changes of fluid inclusions in high-pressure low-temperature metamorphic rocks from Crete: A thermobarometric approach based on the creep strength of the host minerals

The densities of fluid inclusions in quartz from high-pressure, low-temperature metamorphic rocks are generally inconsistent with the P—T conditions derived from solid-phase equilibria. Therefore, some process of change must regularly have occurred during exhumation. However, evidence of decrepitation is frequently lacking. Consideration of the rates of change of P and T in metamorphic rocks shows that very low strain rates are sufficient for volume adaptation. Dislocation creep appears to be a feasible deformation mechanism, allowing for continuous ballooning driven by a small differential pressure during decompression down to temperatures of about 300°C in quartz. Decrepitation requires a higher differential pressure. In natural rocks, it is therefore restricted to relatively low temperatures. This concept was tested on inclusions in the Phyllite—Quartzite Unit of Crete. In these rocks, cooling to below ca. 300°C at confining pressures of 3 to 4 kbar is indicated, corresponding to depths of 10 to 13 km. It is anticipated that in metamorphic rocks the isochores of all fluid inclusions in quartz that have been formed at higher temperatures and remain unaffected by decrepitation, should intersect close to the 300°C isotherm, thereby defining a point on the P—T paths rarely accessible by other methods. Extension of this concept to other minerals with known mechanical properties is straightforward, and can under favourable circumstances be used to fix several points on P—T paths.

Metamorphic core complexes of Cordilleran type in the Cyclades, Aegean Sea, Greece

The islands of Ios and Naxos in the Cycladic archipelago, Greece, contain elongate domes over which a pervasive north- to north-northeast–stretching lineation is warped. These islands may be metamorphic core complexes of Cordilleran type, involving middle-crustal rocks drawn upward and outward from underneath the sedimentary basins now 100 km to the south. From ca. 25 Ma onward, during uplift, the crystalline rocks now exposed on the islands were deformed noncoaxially in the deeper levels of a major shallow-dipping ductile shear zone. This shear zone may have connected with low-angle normal faults in its upper levels, because it is probable that its later history involved the metamorphic complexes being dragged out from under a stretching and fracturing upper plate composed mainly of unmetamorphosed sedimentary and ophiolite suite rocks. This model can be extended to explain the exhumation of the high-pressure–low-temperature metamorphic rocks on Crete, which, with the Cyclades, forms a distended paired metamorphic belt separated by an active sedimentary basin.