E. Wallbrecher

Formation of Neoproterozoic metamorphic complex during oblique convergence (Eastern Desert, Egypt)

Major portions of the Pan-African Orogen in the Eastern Desert of Egypt were formed by island-arc accretion in the Neoproterozoic. These areas are characterized by their lack of major crustal thickening. Metamorphic core complexes occur parallel to the strike of the Eastern Desert Orogen. These domes exhibit polyphase metamorphism and deformation in contrast to the structurally overlying nappes which include ophiolitic melanges and island-arc volcanic rocks. These nappes show northwest directed, orogen-parallel thrusting in the internal parts and west to southwest directed imbrication in the external parts of the orogen. Structures related to exhumation of the metamorphic core complexes partition into different displacement paths localized within a crustal-scale wrench corridor of the Najd fault system. Northwest trending orogen-parallel, sinistral strike-slip faults define the western and eastern margins of the domes. North and south dipping low-angle normal faults developed along the northern and southern margins of the domes and form extensional bridges between them. 40Ar39Ar ages obtained from syntectonic muscovites within the shear zones gave Neoproterozoic ages of 595.9±0.5 and 588.2±0.3 Ma. The synchronous activity of strike-slip and normal faults suggests a regional east-west shortening which was accomodated by deep-level basal decollement beneath the metamorphic core complexes and a coeval northwest-southeast, orogen-parallel extension. This extension was accompanied by intramontane molasse sedimentation and emplacement of calc-alkaline plutons. Since the rapid exhumation of gneisses in the core complexes cannot be explained by thickening of the crust, the authors favour a model which calls for enhanced heat flow along the Najd fault system which would have enabled the formation of syn-extensional plutonism and triggered the exhumation of the metamorphic core complexes. Lateral buoyancy forces were concentrated within the Najd wrench corridor and enabled orogen-parallel extension.

Structural geology, single zircon ages and fluid inclusion studies of the Meatiq metamorphic core complex: Implications for Neoproterozoic tectonics in the Eastern Desert of Egypt

Abstract The Meatiq metamorphic core complex (MMCC) formed during the Precambrian as a result of multiple deformation and metamorphism in the Eastern Desert of Egypt. Structural, geochronologic, and fluid inclusion microthermometric analyses reveal a long formation/deformation history for the MMCC. This started with the break-up of Rodinia at ca. 800 Ma and continued until Pan-African collision at ca. 580 Ma. Between 800–780 Ma, rifting continued into sea floor spreading and oceanic crust formation. Synchronously, the Um Ba′anib granite intruded into an approximately 1.14 Ga old crust comprising migmatic amphibolites. Rifting was accompanied by the deposition of quartz- and mica-rich sediments. Between 660 Ma and 620 Ma, convergence between East and West Gondwanaland caused burial of sediments to a crustal depth of approximately 20 km and intrusion of calc-alkaline rocks. Subsequently, the meta-sediments were thrust across the Um Ba′anib granitoid. Deformation of both rock units took place under amphibolite-facies metamorphic conditions. Fluid inclusions with moderate density provide evidence for the retrograde stage of this metamorphic event. Continued oblique convergence between East and West Gondwanaland resulted in a transpressional regime with displacement partition. While nappe stacking continued in foreland domains, the MMCC was exhumed to a depth of 12–15 km in hinterland domains. Extension-related granitoids were emplaced between 620 and 580 Ma. Microthermometric analyses of fluid inclusions suggest a crustal depth of approximately 10–12 km for the transpressional event. Rapid exhumation was accompanied by detachment of the cover nappes and emplacement of syn-tectonic intrusions, which caused local contact metamorphism. Low-density fluid inclusions document high-T, low-P conditions for the contact metamorphism. The final stage of exhumation took place under brittle/ductile conditions at a crustal depth of approximately 3–6 km indicated by water-rich fluid inclusions. The age of this event is constrained by the intrusion of the late- to post-tectonic Arieki adamellite at approximately 580 Ma.

Neoproterozoic tectonothermal evolution of the Central Eastern Desert, Egypt: a slow velocity tectonic process of core complex exhumation

Regional cooling in the course of Neoproterozoic core complex exhumation in the Central Eastern Desert of Egypt is constraint by 40 Ar/ 39 Ar ages of hornblende and muscovite from Meatiq, Sibai and Hafafit domes. The data reveal highly diachronous cooling with hornblende ages clustering around 580 Ma in the Meatiq and the Hafafit, and 623 and 606 Ma in the Sibai. These 40 Ar/ 39 Ar ages are interpreted together with previously published structural and petrological data, radiometric ages obtained from Neoproterozoic plutons, and data on sediment dynamics from the intramontane Kareim molasse basin. Early-stage low velocity exhumation was triggered by magmatism initiated at � 650 Ma in the Sibai and caused early deposition of molasses sediments within rim synforms. Rapid late stage exhumation was released by combined effect of strike-slip and normal faulting, exhumed Meatiq and Hafafit domes and continued until � 580 Ma. We propose a new model that adopts core complex exhumation in oblique island arc collision-zones and includes transpression combined with lateral extrusion dynamics. In this model, continuous magma generation weakened the crust leading to facilitation of lateral extrusion tectonics. Since horizontal shortening is balanced by extension, no major crustal thickening and no increase of potential energy (gravitational collapse) is necessarily involved in the process of core complex formation. Core complexes were continuously but slowly exhumed without creating a significant mountain topography. 2002 Elsevier Science Ltd. All rights reserved.

Conjugate shear zones in the Southern Bohemian Massif (Austria): implications for Variscan and Alpine tectonothermal activity

Abstract Mylonitic fabrics developed within conjugate wrench ductile shear and fault systems in the Southern Bohemian Massif display both dextral (NW-SE-trending systems) and sinistral (NE-SW-trending systems) shear senses. Contrasting temperature conditions of deformation can be observed in the different shear zones. Temperatures above 650° C were reached in the Pfahl shear zone. Shearing under greenschist facies conditions took place in the Rodl and Danube shear zones. Brittle deformation dominated in the Vitis and Diendorf shear zones. 40 Ar 39 Ar dating of various size fractions of muscovite formed and/or deformed during mylonitization yield ages of ca. 287 Ma (the NW-SE-trending system) and ca. 288-281 Ma (the NE-SW-trending system). The shear zones are interpreted as a late Variscan conjugate system. 40 Ar 39 Ar age spectra of fine-grained newly grown sericite or rejuvenation of ages give evidence for post-Variscan reactivation of the shear zones. Brittle deformation within the shear zones was probably associated with maintenance of very high fluid pressure during Variscan deformation. Foreland deformation during the Alpine orogeny also played a significant role, leading not only to the development of the Ceske Budejovice Graben but also to local reactivation of the shear zones at higher crustal levels.

SHRIMP U–Pb zircon and Sm–Nd garnet ages from the granulite-facies basement of SE Kenya: evidence for Neoproterozoic polycyclic assembly of the Mozambique Belt

The Taita Hills–Galana River region is a key area to demonstrate the polycyclic nature of the Mozambique Belt in SE Kenya. On the basis of petrological and tectonic data, this area is composed of two different granulite-facies terranes, which are separated by the 20–30 km wide Galana Shear Zone. The Taita Hills and adjoining Sagala Hills exhibit a metamorphic overprint at 630–645 Ma, similar to areas in Tanzania. An emplacement age for the magmatic precursor rocks of 850–960 Ma was derived from zircon cores. Sm–Nd garnet–whole-rock analyses give an age of 585 Ma, interpreted as the cooling age after 630–645 Ma metamorphism. Nd crustal residence ages are between 1000 and 1500 Ma. The Galana Shear Zone east of the Taita Hills contains strongly deformed tonalitic migmatites with interlayered pegmatites that date a younger tectonometamorphic event at 560–580 Ma. East of the shear zone only a young metamorphic age of 550 Ma was found. The Nd model ages are c. 1500 Ma to c. 2900 Ma. In a continental configuration prior to Gondwana break-up our study area was located close to Madagascar, where several large shear zones are observed. One of these shear zones (Ranotsara Shear Zone) may be a continuation of the Galana Shear Zone.

Structural evolution within an extruding wedge: model and application to the Alpine-Pannonian system

Continental escape or lateral extrusion often results from late-stage contraction within continental collision zones when convergence is partitioned into orthogonal contraction, crustal thickening, surface uplift, and sideward motion of fault-bounded blocks. Geometrical arguments suggest that each individual fault-bounded block suffers a specific sequence of deformation. The style of deformation also depends on the location within the block. This includes: (1) initial shortening at the continental couple (future zone of maximum shortening: ZMS); (2) formation of a conjugate shear fracture system and initiation of orogen-parallel displacement of the decoupled extruding block away from the ZMS; (3) because of the changing width of the escaping block away from the ZMS the style of internal deformation changes within the extruding block: (i) shortening (thrusting, folding), surface uplift at the ZMS; (ii) strike-slip faulting along confining wrench corridors and formation of pull-apart basins at oversteps of en echelon shear fractures; (iii) extension parallel and perpendicular to the displacement vector far away from the ZMS. (4) Finally, the extruding block is gradually overprinted by general, laterally expanding contraction that starts to develop from the ZMS. This inferred sequence of deformation is tested by the Oligocene to Recent development of the Alpine-Pannonian system where late stage formation and extrusion of an orogen-parallel block started during the Oligocene. Stages 2 and 3 developed during Early to Middle Miocene, and final general contraction occurred during Late Miocene to Recent.

Estimation of the shape factor of a palaeostress ellipsoid by comparison with theoretical slickenline patterns and application of an eigenvalue method

Abstract Eigenvector analysis of the orientation matrix of slickenside striations in britteley deformed rocks is used to estimate the shape factor R = (σ 2 − σ 3 ) (σ 1 − σ 3 ) of a palaeostress ellipsoid. In a first step the displacement data are drawn with a computer program as fault slip lines following the method of Hoeppener (1955). Data which obviously do not fit into the general pattern can interactively be erased. This procedure is supported by superimposition of theoretical slickenlines calculated with the formula of Bott (1959) for a pattern of uniformly distributed theoretical fault planes. In a second step the eigenvalues of the orientation matrix of the measured striation data are calculated and compared with the eigenvalues of the calculated directions of the maximum resolved shear stress on the measured fault planes. In order to make a comparison between the eigenvalues of the measured striations and the theoretical maximum resolved shear stresses easier, all sets of eigenvalues are plotted as points in a triangle plot. The R-factor of the sample can be found by interpolation with the nearest theoretical eigenvalues. The arrangement of points which represent the theoretical eigenvalues and the position of the sample point can be used as criteria for the consistency of the data set. In a consistent data set all points, the sample point included, should lie on a straight line.