Lower Tectonic Unit Research Articles

High-pressure metamorphism in gneisses and pelitic schists in the East Rhodope zone (N. Greece)

The high-alumina metapelites and the orthogneisses of the lower tectonic unit of East Rhodope underwent high P/T metamorphism followed by partial reequilibration during decompression under epidote-amphibolite/amphibolite facies to greenschist facies conditions. The high P/T mineral paragenesis in the orthogneisses is: quartz + albite + microcline + phengite (Simax = 7 atoms p.f.u.) + biotite and in the high alumina metapelites: garnet + chloritoid + chlorite + phengite (Simax. = 6.85 atoms p.f u.) + paragonite + quartz. Pressures between 14 and 15.5 kbar, for Tmin = 550°C, are estimated for the high P/T metamorphism. During continuing uplift, staurolite + chlorite, staurolite + biotite and finally andalusite + Fe-ripidolite are grown at the expense of chloritoid in metapelites, while in the orthogneisses oligoclase, still coexisting with albite, is formed; in both rock types the Si content of white K-mica decreases considerably from almost pure phengite to pure muscovite.

A Thermal-Tectonic Model for High-Pressure Metamorphism and Deformation in the Sesia Zone, Western Alps

The Sesia Zone, Western Alps, is a slice of Austroalpine continental crust that was once metamorphosed at T = 500°-560°C and P > 14-16 kbar at ~130-100 Ma B.P. Pervasive blueschist/eclogite facies assemblages record this event, and a later greenschist facies overprint is only well developed in the NW part of the zone. On the basis of thermal calculations, it is difficult to explain the preservation of high pressure assemblages in a simple model of underthrusting and subsequent uplift/erosion. An alternative model is proposed in which the Sesia Zone was emplaced by underthrusting of the Austroalpine continental margin prior to subduction of oceanic lithosphere of the Piemonte basin. Subduction of relatively cool oceanic lithosphere below the underthrust Sesia Zone then continued for a period of time (e.g., 20 Ma), thus suppressing thermal recovery of this latter unit. Exhumation of Sesia Zone rocks began during this subduction episode by uplift on the Insubric Line, erosion, and by underplating of oceanic material (Zermatt-Saas and Combin Zones) onto the base of the thickened continental crust. Pressure-temperature-time (PTt) paths can be calculated for this model which compare well with those estimated from petrological data. Differences in development of early metamorphism and deformation between upper and lower tectonic units are explained by this model on the basis of contrasting PTt paths and reaction kinetics. It is suggested that the regional pattern of greenschist overprinting was the result of differential uplift across the zone combined with the catalytic effect of deformation.

Sedimentological and structural evolution of the Arabian continental margin in the Musandam Mountains and Dibba zone, United Arab Emirates

The northernmost extremity of the Oman Mountains, the Musandam Peninsula, is composed of an allochthonous sequence of Permian to middle Cretaceous shelf carbonates. These are separated from ophiolitic rocks to the south by a northeast-southwest-trending belt known as the Dibba zone. This structurally complex belt is composed of allochthonous slope- and basin-facies sediments, Haybi volcanic rocks, “Oman Exotic” limestones, sub-ophiolitic metamorphic rocks, and ultra-mafic slices. Lithofacies correlation confirms that fragmentation and rifting of a vast east-facing carbonate platform occurred in Middle to Late Triassic time, resulting in the establishment of a shelf edge and small ocean basin. This margin was typified from northwest to southeast by ooid-skeletal lime sand shoals or small bioherms on the shelf edge, a bypass foreslope of well-laminated periplatform ooze, and basin-margin accumulations of carbonate turbidites or debris flows. The shelf edge now coincides with the northern boundary of the Dibba zone and appears to have remained stationary from Middle Triassic to Late Jurassic time. Periods of arrested carbonate sedimentation on the platform either because of exposure (Late Triassic and middle Cretaceous) or because of drowning (Late Jurassic) are represented in the basin by starved sedimentation and deposition of radiolarian cherts. A spectacular platform margin collapse occurred in the middle and Late Cretaceous, as represented by massive conglomerates, with clasts as young as Albian, above an unconformity that progressively removed all of the Lower Cretaceous. The Tethyan basinal and ophiolitic rocks of the Dibba zone were emplaced from the east-southeast during the Turonian to lower Maastrichtian. After emplacement of these allochthonous units, compressional deformation in the mid-Tertiary resulted in large-scale, open, “whaleback” folds with wavelengths as much as 15 km, generally with north-south axes. In places, these folds, which affect the complete shelf and allochthonous sequences, are overturned toward the west, and thrusting has caused previously lower tectonic units of the Late Cretaceous stacking order to be thrust over previously higher tectonic units, thus reversing the Late Cretaceous tectonostratigraphy. The maximum amount of translation on the later thrusts is in excess of 5 km on the Hagab thrust, where the complete shelf carbonate sequence of the Musandam Mountains has been thrust west-northwest over the previously higher Hawasina and Haybi thrust sheets. The Tertiary folding and thrusting can be correlated in time and space with the Zagros fold belt of southwestern Iran.

Isotopic dating of pre-Alpidic rocks from the island of Ios (Cyclades, Greece)

The lower tectonic unit of Ios provides evidence of an at least four stage metamorphic and intrusive history which well might be generalized for large parts of the internal Pelagonian.

Pre-Orogenic and Synorogenic Diagenesis and Anchimetamorphism in Lower Paleozoic Continental Margin Sequences of the Northern Appalachians in and Around Quebec City, Canada.

ABSTRACT Thermal maturation of organic matter in shales and illite crystallinity reveal a regional southeastward increase in grade from the late middle and late stages of diagenesis to epimetamorphism in early Paleozoic continental margin deposits of the Quebec Appalachians near Quebec City. Four diagenetic or low-grade metamorphic zones are distinguished: Zone I representing the late middle stage of diagenesis with a mean random reflectance (0%) of asphaltic pyrobitumen between 1.0 and 1.5 per cent and illite crystallinity (IC) between 5.5 and 8.5 mm (0.72 and 1.10°2) zone II (late diagenetic stage) with 0 varying between 1.5 and 2.8 per cent and IC between 3.2 and 5.5 mm (0.42 and 0.72°2) or up to 8.0 mm (1.04°2) for black shales, zone III (anchimetamorphic zone) with 0 varying between 2.8 and 3.8 per cent and IC between 1.8 and 3.2 mm (0.23 and 0.422) and zone IV (epizone) with 0 greater than 3.8 per cent and IC less than 1.8 mm (less than 0.23°2). In the northwest (zones I and IIA) reflectance and illite crystallinity increase, within individual nappes, as a function of age or depth of burial. Farther southeast (zones IIB to IV) no such dependence is observed; instead, diagenetic grade increases regionally in a southeastward direction. Diagenesis in zones I and IIA is either in part or entirely pre-orogenic and has been transported during nappe movement, because in the stack of nappes near Quebec City diagenetic grade decreases downward from the higher to the lower tectonic units. Depth of burial of Cambrian deep-water clastic rocks of the Chaudiere Nappe (highest tectonic unit) was estimated as 6 to 7 km on the basis of reflectance (1.71 to 2.30%, diagenetic zone IIA) and an assumed paleogeothermal gradient of 30°C/km. This burial depth may have been achieved entirely by syndepositional overburden. For the Middle Ordovician Quebec Promontory Nappe (lowest exposed tectonic unit) a reflectance of 1.01 to 1.15 per cent (diagenetic zone I) requires 4 to 5 km of overburden, part of which was probably provided by the higher tectonic nappes, because younger sedimentary rocks of sufficient thickness that could have once overlain these Middle Ordovician deposits are not known in the region. Diagenesis in zone IIA, therefore, was most likely formed entirely before orogenesis and nappe transport; in zone I it was probably raised after nappe emplacement. The grade of diagenesis and metamorphism reached in zones IIB to IV are interpreted as related to regional synorogenic heating in conjunction with the Taconic orogeny. Thermal maturation levels in zone I indicate that the rocks may not have passed the oil window, which is of interest for petroleum exploration in Quebec.