Dov Avigad

Back arc extension and denudation of Mediterranean eclogites

Eclogite-facies rocks exposed in Mediterranean back arcs are delimited from above by low-angle normal faults and detachments. Nevertheless, our work demonstrates that these extensional structures associated with back arc extension played only a limited role in removing the overburden from above the eclogites. Extension in Mediterranean back arcs began in the late Oligocene or early Miocene, but the pressure - temperature - time (P-T-t) paths of eclogite-facies rocks exposed in these areas indicate that a major part of the overburden, several tens of kilometers, has been removed from above these rocks prior to the Oligo-Miocene. We show that the time period bracketed between the peak of eclogite metamorphism (Eocene in the central Aegean, probably Upper Cretaceous in Corsica and the Betics) and the onset of back arc extension in the Oligo-Miocene was characterized by thrust faulting. In the central Aegean, Corsica, and the Betics, eclogite-bearing units were partly unroofed and then overthrusted lower-pressure units. We emphasize that, with one exception (Tinos island, Greece), the entire inventory of extensional contacts operated subsequently to the overthrusting of the eclogites above the lower-grade sequences. Thus Mediterranean back arc extension lags behind a major part of the denudation process, and is superposed on orogenic wedges that contain eclogite-facies rocks at relatively shallow structural levels. We emphasize that the mode of occurrence of eclogites in Mediterranean back arc regions involves a continuum of in-situ crustal accretion below the eclogites, widespread P-T paths that show cooling or isothermal decompression, and lower-grade rocks at the bottom of the structural pile. Thus instead of reflecting whole - crust back arc extension, the tectonic style associated with the denudation of Mediterranean eclogites better fits an active accretionary-wedge setting. This is similar to the mode of occurrence of eclogite-facies rocks in mountain belts, such as the western Alps, where decompression was synorogenic and back arc extension played no role.

Origin of northern Gondwana Cambrian sandstone revealed by detrital zircon SHRIMP dating

Voluminous Paleozoic sandstone sequences were deposited in northern Africa and Arabia following an extended Neoproterozoic orogenic cycle that culminated in the assembly of Gondwana. We measured sensitive high-resolution ion microprobe (SHRIMP) U-Pb ages of detrital zircons separated from several Cambrian units in the Elat area of southern Israel in order to unravel their provenance. This sandstone forms the base of the widespread siliciclastic section now exposed on the periphery of the Arabian-Nubian shield in northeastern Africa and Arabia. Most of the detrital zircons we analyzed yielded Neoproterozoic concordant ages with a marked concentration at 0.55‐0.65 Ga. The most likely provenance of the Neoproterozoic detritus is the Arabian-Nubian shield; 0.55‐0.65 Ga was a time of posttectonic igneous activity, rift-related volcanism, and strike-slip faulting there. Of the zircons, 30% yielded pre-Neoproterozoic ages grouped at 0.9‐1.1 Ga (Kibaran), 1.65‐1.85 Ga, and 2.45‐2.7 Ga. The majority of the pre-Neoproterozoic zircons underwent Pb loss, possibly as a consequence of the Pan-African orogeny resetting their provenance. Rocks of the Saharan metacraton and the southern Afif terrane in Saudi Arabia (;1000 km south of Elat) are plausible sources of these zircons. Kibaran basement rocks are currently exposed more than 3000 km south of Elat (flanking the Mozambique belt), but the shape of the detrital zircons of that age and the presence of feldspar in the host sandstone are not fully consistent with such a long-distance transport. Reworking of Neoproteorozoic glacial detritus may explain the presence of Kibaran detrital zircons in the Cambrian of Elat, but the possibility that the Arabian-Nubian shield contains Kibaran rocks (hitherto not recognized) should also be explored.

Uplift and exhumation of high-pressure metamorphic terrains : the example of the Cycladic blueschist belt (Aegean Sea)

The Cycladic blueschists emerged from a depth of more than 50 km during the Tertiary. These rocks are now covered only by a thin fraction of the synmetamorphic overburden (upper plate). Remnants of the upper plate include low-P crystalline rocks and sediments which have been juxtaposed onto the lower-plate blueschists in several stages since the Early Miocene. The low-angle contacts operated as normal faults which removed the overburden from above the Cycladic blueschists. Detailed investigations on Tinos island indicate that the fractured blocks of the upper plate lying above a low-angle normal fault did not rotate. This shows that the faults initiated as rather gently dipping fractures. Similar structures were also reported from other blueschist terrains. Kinematic analysis of such flat-lying faults implies that the synmetamorphic overburden should be mostly located several hundred kilometers away from the present exposure of the blueschists. Considering that the Cycladic blueschist belt exposes successively shallower units in the west, and that rocks of the upper plate have an Anatolian nature, we suggest that these blueschists were dragged from beneath Turkey. We recognize that the blueschist unit has undergone further thrusting in the final stage of the exhumation, as have its counterparts elsewhere in the Hellenides. The emergence of the blueschists in the Tertiary was concomitant with continental collision and thrusting in the Hellenides. The operation of low-angle normal faults with large extensional displacements occurred within this context of collision tectonics. This extension occurred parallel to the strike of the convergent orogen.

Provenance of north Gondwana Cambrian–Ordovician sandstone: U–Pb SHRIMP dating of detrital zircons from Israel and Jordan

A vast sequence of quartz-rich sandstone was deposited over North Africa and Arabia during Early Palaeozoic times, in the aftermath of Neoproterozoic Pan-African orogeny and the amalgamation of Gondwana. This rock sequence forms a relatively thin sheet (1–3 km thick) that was transported over a very gentle slope and deposited over a huge area. The sense of transport indicates unroofing of Gondwana terranes but the exact provenance of the siliciclastic deposit remains unclear. Detrital zircons from Cambrian arkoses that immediately overlie the Neoproterozoic Arabian–Nubian Shield in Israel and Jordan yielded Neoproterozoic U–Pb ages (900–530 Ma), suggesting derivation from a proximal source such as the Arabian–Nubian Shield. A minor fraction of earliest Neoproterozoic and older age zircons was also detected. Upward in the section, the proportion of old zircons increases and reaches a maximum (40 %) in the Ordovician strata of Jordan. The major earliest Neoproterozoic and older age groups detected are 0.95–1.1, 1.8–1.9 and 2.65–2.7 Ga, among which the 0.95–1.1 Ga group is ubiquitous and makes up as much as 27 % in the Ordovician of Jordan, indicating it is a prominent component of the detrital zircon age spectra of northeast Gondwana. The pattern of zircon ages obtained in the present work reflects progressive blanketing of the northern Arabian–Nubian Shield by Cambrian–Ordovician sediments and an increasing contribution from a more distal source, possibly south of the Arabian–Nubian Shield. The significant changes in the zircon age signal reflect many hundreds of kilometres of southward migration of the provenance.

Block rotations and continental extension in the central Aegean Sea: palaeomagnetic and structural evidence from Tinos and Mykonos (Cyclades, Greece)

Abstract The Aegean Sea, floored by an attenuated continental crust, is a Mediterranean-type back-arc basin formed above the subducting African plate. We investigated post-12 Ma block rotation in relation to back-arc extension in the area of the western Cyclades (central Aegean Sea). On Tinos island, NW-trending dacitic dikes and a Miocene monzogranite penetrated an Alpine metamorphic series. KAr analyses of 6 dike samples yielded an average age of 11.5 ± 0.4 Ma. The palaeomagnetic analyses of the dikes and the monzogranite reveal 23° vertical-axis clockwise rotation. We assume that the dacitic dikes intruded vertical (as they are today) and that no tilt correction is required. This is supported by the fact that no inclination anomaly is revealed when the palaeomagnetic data is compared with a tilt-corrected pole position from Kimi (Evia). The absence of tilting implies that post-12 Ma crustal thinning in the area of Tinos island was minor. On Mykonos, a low-angle normal fault dipping 30° to the NE juxtaposes a series of Oligo-Miocene molasse sediments above a 10–12-Ma granite. Published palaeomagnetic data revealed a considerable amount of inclination flattening and 22° clockwise rotation of the footwall granite. We show that tilting predated vertical-axis rotation, and that the (now) low-angle normal fault originated steeper, dipping ca. 54°. Planar-rotational fault geometry indicates extension of ca. 60%. In spite of the significant variations in the magnitude of post-12 Ma upper crustal extension across the Tinos-Mykonos transect, crustal thickness below the entire area changes only slightly. This indicates that upper-crustal levels were decoupled from the lower crust. The widespread Middle Miocene granitic magmatism in the Cyclades suggests that flow in the lower crust may have had a dominant role in maintaining the smooth Moho. Assuming that the western Cyclades constitute a set of NW-striking crustal blocks, it is estimated that vertical-axis rotations increased the N-S dimension of the area by ca. 31% (e.g. 25 km). The exact timing of this clockwise rotation is not well constrained, but it postdated the formation of low-angle normal faults and tilting, and may have taken place in the Late Miocene.

Geochronology of clasts in allochthonous Miocene sedimentary sequences on Mykonos and Paros Islands: implications for back-arc extension in the Aegean Sea

In the Cyclades, low-angle normal faults juxtaposed Miocene sedimentary rock units lying over Alpine blueschist- and greenschist-facies metamorphic rocks and Miocene granites. The sedimentary units in the hanging wall were deposited in fault-bounded basins while their footwalls progressively emerged through the ductile and brittle crust. The sedimentation in the basins evolved from marine turbidites in the Early Miocene to shallow/continental conglomerates during the Late Miocene. The transition from marine to fan delta sedimentation was coeval with widespread magmatism and possibly reflects true crustal uplift. It is inferred that the sequence is no younger than 8 Ma. K–Ar and 40Ar/39Ar geochronology, petrology and petrography of the clasts deposited on the hanging wall provide proof of progressive exhumation of the footwall and reveal the nature of the overburden that covered the Cycladic blueschist belt during Alpine orogeny in the Tertiary. Abundant metamorphic clasts yielding mica cooling ages between 80 and 100 Ma occur throughout the sedimentary section and probably pertain to a vast Pelagonian-type rock mass that covered the internal Hellenides from the Olympos to the central Cyclades. In addition, volcanic clasts dated at 10 Ma reveal the existence of a hitherto unknown volcanic province of that age in the central Aegean. Miocene crustal extension and exhumation of granitic plutons is recorded in the detrital sequence in a concentration of 10 Ma granitic clasts restricted to the top of the conglomerate sequence. A group of metamorphic clasts that yielded ages of 13–16 Ma possibly represents exhumation of mid crustal levels. Clasts similar to the currently-exposed Cycladic Blueschist Unit, such as 40 Ma old blueschists, eclogites and marble, were not found, thus indicating the very late exposure of these footwall rocks.

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

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 metamor- phosed 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, law- sonite 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, glau- cophane 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 north- ern 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 his- tory 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 coordi- nates) N-S contraction and nappe transport.

Detrital zircon Hf isotopic composition indicates long-distance transport of North Gondwana Cambrian–Ordovician sandstones

A voluminous Cambrian–Ordovician sequence of quartz-rich sandstones was deposited in northern Gondwana following its assembly by a series of Neoproterozoic–Cambrian orogenic events. Paleocurrent markers indicate that the sediments were carried from Gondwana hinterland toward the supercontinent margins in the north (present coordinates). Derivation from Neoproterozoic terranes is evident from the ubiquity of detrital zircons with Neoproterozoic U-Pb ages, but the exact provenance of these siliciclastic deposits remains unclear. Herein we present new Hf isotopic data from U-Pb dated detrital zircons of the Cambrian–Ordovician sandstone that tops the juvenile Neoproterozoic basement of the Arabian-Nubian Shield in Israel and Jordan. It is remarkable that the detrital zircon Hf isotopic signal is in marked contrast to the Nd and Hf isotopic signature of the underlying basement. A preponderance (61%) of the Neoproterozoic-aged detrital zircons from the Cambrian–Ordovician sandstones yielded negative e Hf(t) values incompatible with a juvenile source. Therefore, most of the detrital zircons were derived from distant terranes comprising pre-Neoproterozoic crust reworked during the assembly of Gondwana, rather than from the adjacent Arabian-Nubian Shield. Because our sampling sites are situated at the northern tip of the Arabian-Nubian Shield, sand must have been transported several thousand kilometers before deposition. This finding also implies that the Arabian-Nubian Shield and other Neoproterozoic orogens of northeast Africa were completely worn down by the onset of Cambrian deposition and that vast areas in the northern part of Gondwana were low lying at that time.

A Late Neoproterozoic (∼630 Ma) high-magnesium andesite suite from southern Israel: implications for the consolidation of Gondwanaland

Abstract The East African Orogen formed as a result of collision between portions of East and West Gondwanaland as the Mozambique Ocean closed in Late Neoproterozoic time, but it is not known exactly when. We use distinctive chemical and isotopic composition of deformed ‘schistose’ dykes in southern Israel to argue that this collision occurred after about 630 Ma, when the dykes were emplaced. These magmas had compositions of basaltic andesites and andesites but had high Mg# (100 Mg/Mg+Fe; 55–70 ppm), Ni (70–240 ppm), and Cr (100–400 ppm) indicating that the most primitive samples were in equilibrium with mantle peridotite; evolved samples suffered modest fractionation. The schistose dykes are a medium-K, calc-alkaline suite, strongly enriched in light rare earth elements and depleted in heavy rare earth elements. They are high-magnesium andesites and are similar to low-Ca type 2 boninites; similar magmas today only form over active subduction zones. The schistose dykes have non-radiogenic initial 87 Sr/ 86 Sr (0.7026–0.7033) and radiogenic 143 Nd/ 144 Nd, with ϵ Nd (630 Ma) of +2.7 to +4.9 and T DM =0.77–0.94 Ga. Hf isotopic compositions ( ϵ Hf (630 Ma)=+6.8 to +8.8) confirm the juvenile nature of these magmas. The dykes are closely related to a nearby quartz diorite, although it is not clear whether the dykes represent magma that fed into a magma body now filled with the quartz diorite, or issued from it. The generation of high-Mg andesite magma at 630 Ma involved reactive porous flow of a slab-derived melt through the mantle, requiring an active subduction zone, and strongly suggesting that young, hot seafloor – perhaps the spreading ridge of the Mozambique Ocean – was subducted. This indicates that collision between components of E. and W. Gondwana to form the East African Orogen must have occurred more recently than 630 Ma.

Mapping of hydrothermally altered rocks by the EO-1 Hyperion sensor, Northern Danakil Depression, Eritrea

An EO‐1 Hyperion scene was used to identify and map hydrothermally altered rocks and a Precambrian metamorphic sequence at and around the Alid volcanic dome, at the northern Danakil Depression, Eritrea. Mapping was coupled with laboratory analyses, including reflectance measurements, X‐ray diffraction, and petrographic examination of selected rock samples. Thematic maps were compiled from the dataset, which was carefully pre‐processed to evaluate and to correct interferences in the data. Despite the difficulties, lithological mapping using narrow spectral bands proved possible. A spectral signature attributed to ammonium was detected in the laboratory measurements of hydrothermally altered rocks from Alid. This was expressed as spectral absorption clues in the atmospherically corrected cube, at the known hydrothermally altered areas. The existence of ammonium in hydrothermally altered rocks within the Alid dome has been confirmed by previous studies. Spectral information of endmembers mineralogy found in the area (e.g. dolomite) enables a surface mineral map to be produced that stands in good agreement with the known geology along the overpass. These maps are the first hyperspectral overview of the surface mineralogy in this arid terrain and may be used as a base for future studies of remote areas such as the Danakil.