P-T conditions of quartz-calcite boudins and vein formation within a low-angle detachment fault in Tinos Island (Aegean Sea): a fluid-inclusion study

Fluid inclusions were investigated in successive generations of quartz-calcite boudins and veins in the vicinity of, and within, the low-angle detachment fault (of Early Miocene age) exposed on Tinos Island (Cyclades, Greece). Abundant boudins, veins, tension gashes and mineral segregations were formed during various stages of ductile and brittle shear along the detachment indicating fluid flow at various crustal levels, assisted motion and slip along the detachment. Three generations of fluid inclusions were identified: (1) syn-deformation aqueous inclusions; (2) local- late-deformation H₂O-CO₂ inclusions; (3) post-slip deformation, partly decrepitated aqueous and H₂O-CO₂ inclusions. The conditions of inclusion trapping correspond to the greenschist facies, at temperatures between 300 and 450°C and pressures in the range of 1.5–4.0 kb. A gradient of pressure of about 0.5 kb from the fault zone to the rocks of upper and lower plates is suggested. The results indicate that the syn-deformation fluid was NaCl-KCl-sulfate-dominated with a salinity of about 5 wt% NaCl equiv. Downward infiltration of meteoric water is a suitable source for this fluid. During the late stage of fault activity, in the brittle field, fluid was of H₂O-CO₂ composition. Very rapid exhumation of the entire section (unrelated to motion on the investigated fault) is marked by numerous decrepitation clusters of the fluid inclusions.     

Fabric development in a weathering profile at a basement–cover interface,the sub-Cambrian peneplain,Israel: Implications for decollement tectonics

The present study examines a fossil saprock–saprolite–laterite-profile beneath the sub-Cambrian peneplain in the Pan-African Roded Granite, Israel, with regard to structure and magnetic fabrics (anisotropy of magnetic susceptibility, AMS), and image analysis of compaction. The deformed granite shows two pre-weathering foliations, S_1m (magmatic) and S_2g (gneissic). Pre-Early Cambrian weathering comprised weathering-brecciation in saprock and saprolite, and chemical weathering with clay-formation in saprolite and laterite. During subsequent Phanerozoic burial the laterite was vertically compacted to 73% of its original thickness. In the laterite, compaction produced an unconformity-parallel cleavage (S_3d) with increasing intensity towards the unconformity. Bulk susceptibility (κ_bulk) and anisotropy (P′) decrease from the unweathered granite into the saprolite, as a result of progressive magnetite breakdown, martitization and weathering-brecciation. In the laterite, an enrichment of haematite and relic Fe–Mg–mica lead to increased κ_bulk. Here, magnetic fabrics trace the compaction fabrics. The subhorizontal, compactional clay–/mica-fabric S_3d defines a structurally weak and impermeable layer. The mechanical weakness of a clay-enriched weathering horizon with an unconformity-parallel, planar shape-preferred orientation, combined with the potentially overpressured state due to the sealing character of such a zone provides a viable explanation for the abundant localization of decollement horizons at or beneath basement-cover interfaces.     

Coupled U–Pb–Hf of detrital zircons of Cambrian sandstones from Morocco and Sardinia: Implications for provenance and Precambrian crustal evolution of North Africa

U–Pb–Hf of detrital zircons from diverse Cambrian units in Morocco and Sardinia were investigated in order to clarify the sandstone provenance and how it evolved with time, to assess whether the detrital spectra mirror basement crustal composition and whether they are a reliable pointer on the ancestry of peri-Gondwanan terranes. Coupled with Hf isotopes, the detrital age spectra allow a unique perspective on crustal growth and recycling in North Africa, much of which is concealed below Phanerozoic sediments.In Morocco, the detrital signal of Lower Cambrian arkose records local crustal evolution dominated by Ediacaran (0.54–0.63 Ga) and Late-Paleoproterozoic (1.9–2.2 Ga; Eburnian) igneous activity. A preponderance of the Neoproterozoic detrital zircons possess positive ε_Hf(t) values and their respective Hf model ages (T_DM) concentrate at 1.15 Ga. In contrast, rather than by Ediacaran, the Neoproterozoic detrital signal from the Moroccan Middle Cambrian quartz-rich sandstone is dominated by Cryogenian-aged detrital zircons peaking at 0.65 Ga alongside a noteworthy early Tonian (0.95 Ga) peak; a few Stenian-age (1.0–1.1 Ga) detrital zircons are also distinguished. The majority of the Neoproterozoic zircons displays negative ε_Hf(t), indicating the provenance migrated onto distal Pan-African terranes dominated by crustal reworking. Terranes such as the Tuareg Shield were a likely provenance. The detrital signal of quartz–arenites from the Lower and Middle Cambrian of SW Sardinia resembles the Moroccan Middle Cambrian, but 1.0–1.1 Ga as well as ~ 2.5 Ga detrital zircons are more common. Therefore, Cambrian Sardinia may have been fed from different sources possibly located farther to the east along the north Gondwana margin. 1.0–1.1 Ga detrital zircons abundant in Sardinia generally display negative ε_Hf(t) values while 0.99–0.95 Ga detrital zircons (abundant in Morocco) possess positive ε_Hf(t), attesting for two petrologically-different Grenvillian sources. A paucity of detrital zircons younger than 0.6 Ga is a remarkable feature of the detrital spectra of the Moroccan and Sardinian quartz-rich sandstones. It indicates that late Cadomian orogens fringing the northern margin of North Africa were low-lying by the time the Cambrian platform was deposited. About a quarter of the Neoproterozoic-aged detrital zircons in the quartz-rich sandstones of Morocco (and a double proportion in Sardinia) display positive ε_Hf(t) values indicating considerable juvenile crust addition in North Africa, likely via island arc magmatism. A substantial fraction of the remaining Neoproterozoic zircons which possess negative ε_Hf(t) values bears evidence for mixing of old crust with juvenile magmas, implying crustal growth in an Andean-type setting was also significant in this region.     

Low-angle faults above and below a blueschist belt—Tinos Island, Cyclades, Greece

New observations from the Island of Tinos, Greece, allow a better definition of the structural position of the Alpine (Eocene) blueschist belt exposed in the islands of the Aegean Sea. These blueschists, over a significant part of the Aegean sea, are delimited from below by a low-angle thrust fault, while from above they are delimited by a low-angle, normal-type fault which omits a substantial crustal interval. Both underlying and overlying rocks were not affected by the high? metamorphism. The rapid uplift and exhumation of the high? rocks was therefore mainly the result of fault movements rather than erosion and whole-crust uplifting. The low-angle normal fault apparently had a major role in the uplift of the blueschists.     

New insights into peri-Gondwana paleogeography and the Gondwana super-fan system from detrital zircon U–Pb ages

We present a synopsis of detrital zircon U–Pb ages of sandstones from North Africa and neighboring Israel and Jordan, which allows us to identify zones with characteristic sediment provenance along the northern Gondwana margin (in present-day coordinates) in Cambrian–Ordovician times, and helps us to unravel the peri-Gondwana jigsaw puzzle. A special feature of the early Paleozoic cover sequence of North Africa is the eastward increase of 1.1–0.95 Ga detrital zircons, which become ubiquitous in the early Paleozoic sandstones of the Saharan Metacraton. Detrital zircons aged about 2.7–2.5, 2.15–1.75 and 0.75–0.53 Ga are also present. Early Paleozoic sandstones with similar provenance are known from peri-Gondwana terranes in the Eastern and Western Mediterranean and from NW Iberia. These terranes need not be transported from western Gondwana (Amazonia) as suggested previously. They were likely located to the north of the Saharan Metacraton during the early Paleozoic before they rifted off from Gondwana. Furthermore, we recognize an increase, as stratigraphic ages get younger, of ca. 1.0 Ga detrital zircons at some point between the Late Cambrian and late Middle Ordovician. We speculate that this might be linked to far-field tectonics and regional uplift in central Gondwana related to plate-tectonic reorganization along the Gondwana margin, leading to erosion of ca. 1.0 Ga basement and country rocks of the Transgondwanan supermountain and fluvial dispersal of detritus toward the Gondwana margin.     

Mass-production of Cambro–Ordovician quartz-rich sandstone as a consequence of chemical weathering of Pan-African terranes: Environmental implications

A vast sheet of mature quartz sand blanketed north Africa and Arabia from the Atlantic coast to the Persian Gulf in Cambro–Ordovician times. U–Pb geochronology of a representative section of Cambrian sandstone in southern Israel shows that these sediments are dominated by 550–650 Ma detrital zircons derived from Neoproterozoic Pan-African basement. The short time lag between magmatic consolidation of a Pan-African source and deposition of its erosional products indicates that, despite their significant mineralogical maturity, the voluminous quartz-rich sandstones on the northern margin of Gondwana are essentially first-cycle sediments.

Mass production of these voluminous first-cycle quartz-rich sandstones resulted from widespread chemical weathering of the Pan-African continental basement. We suggest that conditions favoring silicate weathering, particularly a warm and humid climate, low relief and low sedimentation rates prevailed over large tracts of Gondwana in the aftermath of the Pan-African orogeny. An unusually corrosive Cambro–Ordovician atmosphere and humid climate enhanced chemical weathering on the vegetation-free landscape. We infer that late Neoproterozoic–Cambro–Ordovician atmospheric pCO₂ rose as a consequence of widespread late Neoproterozoic volcanism, followed by an uptake of CO₂ by chemical weathering to produce the Cambro–Ordovician sandstone as a negative feedback.     

Fluid-rock interaction and thermal evolution during thrusting of an Alpine metamorphic complex (Tinos island, Greece)

This study examines the fluid-rock interaction and thermal evolution along a thrust that juxtaposes calcite-rich marbles of high P-T metamorphic unit of the Attic-Cycladic Massif (Greece) on top of a lower-grade dolomite marble unit. The Tertiary thrust represents a major phase of tectonic movement related to the decompression of the Alpine orogen in the Hellenides. The stable isotope signatures of the thrust plane and adjacent sections of the footwall and hanging wall rocks are characterized by significant carbon and oxygen isotope depletions. The depletion is most pronounced in calcite, but is almost entirely missing in coexisting dolomite. The isotopic patterns in the thrust zone can be explained by the infiltration of an externally derived water-rich H₂O-CO₂-CH₄ fluid [X _C (=X _{CO 2}+X _{CH 4})<0.05] at water-rock ratios on the order of 0.1 to 0.5 by weight. The fluid-induced calcite recrystallization is viewed as an important rheological control during thrusting. The temperature evolution of the footwall, hanging wall and mylonitic tectonic contact was determined by calcite-dolomite solvus thermometry. Histograms of calcite-dolomite temperatures are interpreted as indicating a heating of the footwall dolomite marble during the thrusting of the hotter upper plate. Conversely, the hanging wall marble unit was cooled during the thrusting. The calcite-dolomite thermometry of the thrust plane gives temperatures intermediate between the initial temperatures of the lower and upper marble units, and this leads to the conclusion that conductive heat transfer rather than fluid infiltration controlled the thermal evolution during thrusting. Received: 14 April 1998 / Accepted: 9 December 1998     

Evolution and provenance of Neoproterozoic basement and Lower Paleozoic siliciclastic cover of the Menderes Massif (western Taurides): Coupled U–Pb–Hf zircon isotope geochemistry

In the Menderes Massif (western Taurides) a Neoproterozoic basement comprising metasediments and intrusive granites is imbricated between Paleozoic platform sediments. U–Pb–Hf zircon analyses of Menderes rock units were performed by us using LA-ICP-MS. The U–Pb detrital zircon signal of the Neoproterozoic metasediments is largely consistent with a NE African (Gondwana) provenance. The oldest unit, a paragneiss, contains significant amounts (~ 30%) of Archean-aged zircons and εHf (t) values of about a half of its Neoproterozoic zircons are negative suggesting contribution from Pan-African terranes dominated by reworking of an old crust. In the overlying, mineralogically-immature Core schist (which is still Neoproterozoic), the majority of the detrital zircons are Neoproterozoic, portraying positive εHf (t) values indicating derivation from a proximal juvenile source, resembling the Arabian–Nubian Shield.The period of sedimentation of the analyzed metasediments, is constrained between 570 and 550 Ma (Late Ediacaran). The Core schist sediments, ~ 9 km thick, accumulated in less than 20 My implying a tectonic-controlled sedimentary basin evolved adjacent to the eroded juvenile terrane. Granites, now orthogneisses, intruded the basin fill at 550 Ma, they exhibit ± 0 εHf (t = 550 Ma) and T_DM ages of 1.4 Ga consistent with anatexis of various admixtures of juvenile Neoproterozoic and Late Archean detrital components. Granites in the northern Arabian–Nubian Shield are no younger than 580 Ma and their εHf (t) are usually more positive. This implies that the Menderes does not represent a straightforward continuation of the Arabian–Nubian Shield.The lower part of the pre-Carboniferous silisiclastic cover of the Menderes basement, comprises a yellowish quartzite whose U–Pb–Hf detrital zircon signal resembles that of far-traveled Ordovician sandstones in Jordan (including 0.9–1.1 Ga detrital zircons), supporting pre-Triassic paleorestorations placing the Tauride with Afro-Arabia. The detrital signal of the overlying carbonate-bearing quartzitic sequence indicates contribution from a different source: the majority of its detrital zircons yielded 550 Ma and ± 0 εHf (t = 550 Ma) values identical to that of the underlying granitic gneiss implying exposure of Menderes-like granites in the provenance.260–250 Ma lead-loss and partial resetting of the U–Pb system of certain zircons in both basement and cover units was detected. It is interpreted as a consequence of a Permian–Early Triassic thermal event preceding known Triassic granitoid intrusions.     

Origin, HP/LT metamorphism and cooling of ophiolitic mélanges in southern Evia (NW Cyclades), Greece

Basic and ultrabasic blocks within ophiolitic mélanges of the Cycladic Blueschist Unit in southern Evia provide a detailed insight into its ocean floor igneous and hydrothermal evolution, as well as the regional poly‐metamorphism occurring during Alpine orogenesis. The upper structural levels (Mt. Ochi exposures) are dominated by metamorphosed wehrlites, gabbros and highly light rare earth element (LREE)‐enriched pillow basalts, whereas the underlying Tsaki mélange consists of basic protoliths with much less fractionated REE patterns as well as mantle harzburgites. Most of the metabasites show Nb anomalies, indicative of derivation from a subduction‐affected mantle. The igneous bodies were juxtaposed and incorporated into the enclosing sedimentary sequences prior to high‐pressure/low‐temperature (HP/LT) metamorphism (M1). Glaucophane, epidote, sodic clinopyroxene and high‐Si phengite constitute the Eocene M1 assemblage, which is estimated to have formed at >11 kbar and 400–450 °C. High δ¹⁸O values of M1 minerals in Ochi metagabbros indicate that the formation of the high‐pressure assemblage was controlled by infiltration of fluids from the dehydrating host sediments. Cooling during decompression is indicated by an overprinting (M2, Early Miocene) pumpellyite–actinolite facies assemblage in metabasic rocks, calculated to have developed at P<8 kbar and T <350 °C. Possible mechanisms for such cooling include: exhumation from shallower burial levels relative to the eclogites of the NW Cyclades, accretion of colder rocks from below and extensional unroofing by low‐angle normal faults and detachments. The occurrence of sodic augite in the M2 assemblage of Tsaki metagabbros indicates that rocks at the base of the Blueschist Unit cooled faster or longer than their higher level Ochi counterparts. This suggests that differential cooling of the blueschists was enhanced by the underthrusting of colder rock units.     

Tectonic juxtaposition of blueschists and greenschists in Sifnos Island (Aegean Sea)—implications for the structure of the Cycladic blueschist belt

In the Cyclades, kilometre-thick high-pressure rock sequences, displaying blueschist- and eclogite-facies mineral assemblages, overlie a rock sequence which was thoroughly overprinted in the greenschist and amphibolite facies during its exhumation. Both rock sequences, subducted and metamorphosed at high-pressures in Eocene times, have been considered by previous workers to have been exhumed as a coherent rock unit. In contrast, it is suggested here that the preserved high-pressure rock sequences were exhumed more rapidly and prior to the underlying greenschists. Significant metamorphic, structural and geochronological discontinuities exist across the blueschist-greenschist contact and field evidence suggests that the high-pressure metamorphic rocks in Sifnos are tectonically juxtaposed above the greenschists. These two rock sequences were juxtaposed by a low-angle fault subsequent to the Oligocene-Miocene greenschist-facies overprint. Published geochronological data and petrological criteria are used to show that the high-pressure sequence cooled below 350°C when the rocks now immediately underlying it suffered a greenschist-facies overprint at temperatures of ca 450°C. The section inferred to absorb this temperature difference is now missing and it is suggested that it has been cut out by the low-angle fault.