Cees W. Passchier

Extensional collapse and uplift in a polymetamorphic granulite terrain in the Archaean and Palaeoproterozoic of north China

Abstract The Archaean granulite terrain in the Datong-Huai An area, north China, comprises a basement complex of felsic and mafic granulite, overlain by a sedimentary sequence dominated by metapelite and metapsammite. Both lithological associations are separated by a tectonic contact, and structural, textural and stratigraphic data indicate that the terrain was affected by two prominent tectono-thermal cycles (C A and C B ), associated with two granulite facies events, M1 and M2, which probably occurred ∼2500 Ma and 1800 Ma ago, respectively. Of these, M1 events were dominant and associated with all major structures in the area. In the basement gneiss, M1 reached peak metamorphic conditions of 12–14 kbar and 800–900°C during a foliation-forming event, D 2Ba , which transposed an earlier S 1Ba foliation of unknown age and origin. In the allochthonous cover sediments M1 peak-conditions of about 8 kbar and 800°C were reached after a prograde thickening event, D 1Co , associated with the formation of a bedding-parallel foliation, while large amounts of S-type granites were emplaced. The localised D 2Ba and D 1Co structures are interpreted as spatially distinct responses at different crustal levels to one metamorphic event. Juxtaposition of basement and allochthonous cover along a large, low-angle normal shear zone occurred during D 3 . This detachment zone accommodated extension after crustal thickening, and allowed unloading of the footwall resulting in a rise of the basement rocks through 4–6 kbar as deduced from syn-D 3 decompression textures. Coeval cooling textures in the allochthonous cover sequence indicate that lateral rather than vertical displacements occurred in the hanging wall of the detachment. During C B , M2 peak-conditions of about 4–6 kbar and 650–700°C were reached coeval with the development of discrete, left-lateral strike-slip zones (D 4 ) that transect both basement and allochthonous cover, and accommodated some uplift. The deduced C A and C B P - T paths have equivalents in many Precambrian granulites. In general decompressional paths like the one recorded in the basement rocks, are linked to processes involving crustal thickening followed by extensional collapse. In the Datong-Huai An area it can be shown that low-angle detachments may have played an important role in the uplift and cooling history of granulite belts as long ago as the Archaean-Proterozoic boundary.

Modelling of mantled porphyroclasts using non-Newtonian rock analogue materials

Abstract The development of δ-type porphyroclasts was modelled experimentally in a circular transparent shear rig using crystalline rock analogue materials. Rectangular camphor objects embedded in a matrix of octachloropropane were deformed in simple shear flow. The camphor deformed into δ-objects with progressive deformation up to a shear strain of 100. The development of these δ-objects and of gradients of flow parameters in the matrix was followed from step to step during the deformation. The δ-objects in this experiment show stair-stepping of wings that did not occur in earlier experiments with Newtonian fluids. Analysis of the flow pattern indicates that this is due to an unusual flow perturbation geometry around the relatively rigid camphor objects which rotate more slowly than equivalent objects in Newtonian flow. The non-Newtonian rheology of the experimental materials may be responsible for this deviant behaviour. This implies that non-Newtonian fluids are better analogues than Newtonian fluids to study the development of structures in rocks with non-Newtonian rheology, since the geometry of the resulting structures can be different.

Structural geology across a proposed Archaean terrane boundary in the eastern Yilgarn craton, Western Australia

Abstract The Eastern Goldfields Province of the Archaean Yilgarn craton, Western Australia, can be divided into several NW—SE- trending terranes with different stratigraphic signature. At least two main phases of penetrative ductile deformation affect the terranes. D1 is a phase of nappe-type transport, mainly to the north, along gently dipping ductile shear zones. It is interpreted as a phase of regional extension contemporaneous with intrusion of massive granitoid plutons and possibly contemporaneous with deposition of felsic volcanics and metasediments in the greenstone belts. Terrane boundaries were probably formed during D1. D2 gave rise to steep NW—SE- to N—S-trending foliations and upright folds. D2 is strongly partitioned over the area for strain and vorticity. The pattern of partitioning seems to be dependent on the position of granitoid plutons which give rise to development of D2 strain shadows. D1 structures have been preserved mainly in these strain shadows. Terrane boundaries are transected by boundaries of D2 structural domains. Minor intrusion of granitoids continued during and after D2. D1 is possibly associated with arrival of a mantle plume below the Yilgarn craton, and D2 may be a phase of regional shortening in a thinned lithosphere weakened by the thermal event.

The analysis of progressive deformation in rock analogues

Abstract Two-dimensional deformation experiments using analogue materials such as octachloropropane and camphor have proven to be a powerful tool in the investigation of microstructural development, since they make it possible to observe the specimen throughout its deformation history. The addition of dispersed marker particles makes it possible to trace the movement of material points during deformation. From this information the flow and deformation can be interpolated for any point within the specimen at any time. The geometry of the flow and deformation can then be visualized in the form of grids, contour-plots, strain ellipses and other methods. Despite this apparently ideal setting for studying flow and deformation, the need to use marker particles to trace material points introduces unexpected problems in the analysis. We present a computer program to analyse the movement of marker particles. Digitization of the positions of the particles is improved by the use of video input and semi-automatic digitization. The use of polynomials to describe the complete path of marker particles and least-squares best-fit solving improves the analysis of the data.

The kinematic interpretation of obliquely-transected porphyroblasts: an example from the Trois Seigneurs Massif, France

Abstract Obliquely-transected porphyroblasts (OTPs) contain a straight inclusion pattern that is continuous with the external foliation, but oblique in orientation. OTPs are interpreted to form by porphyroblast growth between two phases of deformation. The obliqueness of the inclusion pattern and external foliation is due to the second deformation phase and has been variously explained by porphyroblast rotation in the kinematic reference frame of bulk flow, or by foliation rotation around a stationary porphyroblast. The two models imply opposite sense-of-vorticity (also loosely referred to as sense of shear) to produce the same geometry and their relevance should be known if OTPs are to be used as sense-of-vorticity indicators. Andalusite-OTPs in the Trois Seigneurs Massif, French Pyrenees, have a spool-shape in three dimensions that is interpreted to result from passive overgrowth of andalusite on a spaced S 2 cleavage. Subsequent non-coaxial D 3 flow led to development of an S 3 crenulation cleavage and the OTP geometry. Although the porphyroblasts rotated, rotation of the foliation in the kinematic reference frame of bulk flow was mainly responsible for the OTP geometry. This example shows that OTPs can indeed develop by foliation rotation in non-coaxial flow. Similar looking OTPs can apparently develop in flows with opposite sense-of-vorticity depending on whether foliation or porphyroblast rotation is dominant; the dominance of either development mechanism depends on the importance of flow partitioning and solution transfer during progressive deformation.

Ductile reactivation of Proterozoic brittle fault rocks; an example from the Vestfold Hills, East Antarctica

Abstract Brittle fault rocks, usually in the form of minor pseudotachylyte veins have been reported from most high-grade metamorphic Archaean and Proterozoic terrains. In analyses of local crustal evolution such brittle structures are often ignored; brittle faults commonly reflect the very last phases of local deformation during final uplift of the basement to shallow crustal levels, and as such they are thought to be of minor or no importance in large-scale Archaean and Proterozoic tectonics. New data from Archaean and Proterozoic high grade terrains in Australia and Antarctica indicate that minor fault veins may in some cases represent significant early low-grade brittle events which have been overprinted and largely erased by subsequent phases of ductile deformation. This paper describes such a brittle event in the Vestfold Hills, East Antarctica, which may represent an important phase of mid-Proterozoic uplift and crustal extension.

The sliding-scale Mohr diagram

Abstract Mohr diagrams for the position gradient tensor are a useful tool in problems of finite deformation, for example to determine the angle between deformed lines and the angle of rotation of finite strain axes. The conventional Mohr diagram is less suitable, however, to visualise progressive deformation or deformation paths. A sliding-scale Mohr diagram is introduced which serves this purpose better. It consists of a Mohr circle of fixed diameter and a mobile reference frame origin that maps the deformation path. The sliding-scale Mohr diagram can be used as a visualisation tool for teaching purposes and for research on kinematics of progressive deformation.