Gordon S. Lister

S--C Mylonites

Two types of foliations are commonly developed in mylonites and mylonitic rocks: (a) S-surfaces related to the accumulation of finite strain and (b) C-surfaces related to displacement discontinuities or zones of relatively high shear strain. There are two types of S-C mylonites. Type I S-C mylonites, described by Berthe et al., typically occur in deformed granitoids. They involve narrow zones of intense shear strain which cut across (mylonitic) foliation. Type II S-C mylonites (described here) have widespread occurrence in quartz-mica rocks involved in zones of intense non-coaxial laminar flow. The C-surfaces are defined by trails of mica ‘fish’ formed as the result of microscopic displacement discontinuities or zones of very high shear strain. The S-surfaces are defined by oblique foliations in the adjacent quartz aggregates, formed as the result of dynamic recrystallization which periodically resets the ‘finite-strain clock’. These oblique foliations are characterized by grain elongations, alignments of segments of the grain boundary enveloping surfaces, and by trails of grains with similar c-axis orientations. Examples of this aspect of foliation development in mylonitic rocks are so widespread that we suggest the creation of a broad class of S-C tectonites, and a deviation from the general tradition of purely geometric analysis of foliation and time relationships. Kinematic indicators such as those discussed here allow the recognition of kilometre-scale zones of intense non-coaxial laminar flow in crustal rocks, and unambiguous determination of the sense of shear.

The origin of metamorphic core complexes and detachment faults formed during Tertiary continental extension in the northern Colorado River region, U.S.A.

Metamorphic core complexes form as the result of major continental extension, when the middle and lower continental crust is dragged out from beneath the fracturing, extending upper crust. Movement zones capable of producing such effects evolve in space as well as with time. Deforming rocks in the footwall are uplifted through a progression of different metamorphic and deformational environments, producing a characteristic sequence of (overprinted) meso- and microstructures. The movement zone is folded as the result of the bowing upwards of the lower crust to form a broad basement culmination, as the result of isostatic rebound due to tectonic denudation, but most likely also as the result of local isostatic adjustments due to granite intrusion in the middle crust. A succession of splays branch off from the master detachment fault at depth, excising substantial portions of the lower portions of the upper plate as successive detachment faults eat upwards through it. At the same time, detachment faults incise into progressively deeper levels of the lower plate, although the amount of incision is limited, because the locus of movement remains at approximately the same level in the lower plate. The detachment faults presently observed in the metamorphic core complexes are relatively young features, formed late in the geological evolution of these bodies, and are only the last in a succession of low-angle normal faults that sliced through the upper crust at the upward terminations of major, shallow-dipping, ductile shear zones in the extending Cordilleran orogen. Excisement of listric fault bottoms can explain some of the enigmatic domino-like fault blocks, and other structural relations observed in these terranes. Evidence in support of this model is illustrated from detachment terranes in the northern Colorado River region of southern Nevada, southeastern California and western Arizona, where multiple generations of detachment faults have produced remarkable excisement and incisement geometries.

Detachment faulting and the evolution of passive continental margins

Major detachment faults play a key role in the lithospheric extension process in the Basin and Range province and may also be important in other continental extension terranes. Such detachment faulting leads to an inherent asymmetry of extensional structure and of uplift/subsidence patterns. Detachment models developed for the formation of metamorphic core complexes can also be applied to the formation of passive continental margins. We therefore suggest the existence of upper-plate and lower-plate passive margins. These give rise to a complementary asymmetry of opposing margins after continental breakup. Transfer faults offset marginal features and allow margins to switch from upper-plate to lower-plate characteristics along strike.

Metamorphic core complexes of Cordilleran type in the Cyclades, Aegean Sea, Greece

The islands of Ios and Naxos in the Cycladic archipelago, Greece, contain elongate domes over which a pervasive north- to north-northeast–stretching lineation is warped. These islands may be metamorphic core complexes of Cordilleran type, involving middle-crustal rocks drawn upward and outward from underneath the sedimentary basins now 100 km to the south. From ca. 25 Ma onward, during uplift, the crystalline rocks now exposed on the islands were deformed noncoaxially in the deeper levels of a major shallow-dipping ductile shear zone. This shear zone may have connected with low-angle normal faults in its upper levels, because it is probable that its later history involved the metamorphic complexes being dragged out from under a stretching and fracturing upper plate composed mainly of unmetamorphosed sedimentary and ophiolite suite rocks. This model can be extended to explain the exhumation of the high-pressure–low-temperature metamorphic rocks on Crete, which, with the Cyclades, forms a distended paired metamorphic belt separated by an active sedimentary basin.

Relative motions of Africa, Iberia and Europe during Alpine orogeny

Abstract A revised kinematic model for the motions of Africa and Iberia relative to Europe since the Middle Jurassic is presented in order to provide boundary conditions for Alpine–Mediterranean reconstructions. These motions were calculated using up-to-date kinematic data predominantly based on magnetic isochrons in the Atlantic Ocean and published by various authors during the last 15 years. It is shown that convergence of Africa with respect to Europe commenced during the Cretaceous Normal Superchron (CNS), between chrons M0 and 34 (120–83 Ma). This motion was subjected to fluctuations in convergence rates characterised by two periods of relatively rapid convergence (during Late Cretaceous and Eocene–Oligocene times) that alternated with periods of slower convergence (during the Paleocene and since the Early Miocene). Distinct changes in plate kinematics are recognised in the motion of Iberia with respect to Europe, indicated by: (1) a Late Jurassic–Early Cretaceous left-lateral strike–slip motion; (2) Late Cretaceous convergence; (3) Paleocene quiescence; (4) a short period of right-lateral strike–slip motion; and (5) final Eocene–Oligocene convergence. Based on these results, it is speculated that a collisional episode in the Alpine orogeny at ca. 65 Ma resulted in a dramatic decrease in the relative plate motions and that a slower motion since the Early Miocene promoted extension in the Mediterranean back-arc basins.

The simulation of fabric development during plastic deformation and its application to quartzite: the influence of deformation history

The effect of deformation history on the development of crystallographic preferred orientation in quartzities has been simulated using a computer program based on the Taylor-Bishop-Hill analysis. Model quartzities with different combinations of glide systems have been subjected to various coaxial and non-coaxial deformation histories. It is possible to obtain information from the fabrics that develop during simple histories; for example, the location of the axis of extension is generally associated with a pole free area on a c-axis plot, and progressive axial shortening, plane strain and axial shortening produce characteristic fabrics. In progressive simple shear the fabric skeleton becomes asymmetric relative to the sense of shear and a-axes preferentially align in the flow plane parallel to the flow direction. However, this example illustrates that the fabric orientation and characteristics are controlled by the kinematic framework and bear only an indirect relationship to the finite strain accumulated to that point in the history. The imprint of the closing stages of deformation limits to some degree the use of crystallographic fabrics as a tool for structural geologists, but in favourable circumstances data can be obtained concerning characteristics of the deformation history, on the scale of the hand-specimen, for the last part of this history.

The partitioning of deformation in flowing rock masses

Abstract Most geological structures owe their development to heterogeneous and/or non-steady flow. Flow partitioning refers to the division of the instantaneous velocity field into components related to translation, strain and rotation. Flow partitioning during the development of a geological structure usually changes with time, and different histories of flow are involved at different points in the developing structure. Heterogeneity leads to the development of flow domains with different characteristics, notably the degree of non-coaxiality of deformation, and different partitionings of the rotational component of flow into shear-induced vorticity and spin. During non-steady flow there is often continual repartitioning of vorticity between the spin and shear-induced components. Spin is an important aspect of flow leading to the development of structure. This paper discusses the partitioning of deformation in a flowing rock mass. Coaxial flow tends to occur in isotropic materials in regions near weakly constrained boundaries. Non-coaxial flow is favoured by suitably oriented material anisotropy and/or the presence of well-defined boundary constraints involving parallel relative displacements. A developing anisotropy (e.g., a crystallographic fabric in a shear zone) can induce a change from coaxial to non-coaxial flow. Other effects are related to the scale of approximation, for example bulk shear-induced vorticity can be locally converted to spin. Flow in the crust and mantle of the earth produces geological structures on all scales and involves many different types of rheological response. The factors that affect flow partitioning (e.g., spin versus non-coaxial deformation) are therefore of interest.

Detachment models for the formation of passive continental margins

The inherent asymmetry of extension by detachment leads to contrasting and conjugate classes of passive margins. Upper-plate margins comprise crust above a deeper detachment. Lower-plate margins comprise the footwall of the detachment, overlain by faulted upper plate remnants. Such margins have distinctive architectures, structural styles, uplift-subsidence paths and thermal histories. The wide range in structural styles on passive margins is predicted by five models which incorporate detachment faults linked to flat ductile shear zones, and ductile stretching of the thermal lithosphere below the shear zones. These models provide explanations for enigmatic structural and morphological features of passive margins such as marginal plateaux, outer highs, unstructured synrift sag basins, and perched rift basins. Numerical modelling of isostatic uplift-subsidence histories shows that different patterns of uplift-subsidence behaviour can be explained by variations in detachment geometry and change in the amount of lithospheric stretching. Voluminous igneous underplating is predicted if anomalously hot asthenosphere is uplifted. The arrival of such mantle derived melts may cause significant additional uplift. Upper-plate margins undergo thermally induced uplift, with permanent uplift due to igneous underplating. This uplift may be the origin of passive margin mountains in the adjacent hinterlands. Marginal plateaux are emergent or very shallowly submerged throughout the extension history, with postrift subsidence to intermediate water depths. The lithosphere is extended below a midcrustal detachment, but with little extension of the upper-plate. The pattern of subsidence on an Atlantic margin requires an extended upper plate superimposed on progressively more stretched subdetachment lithosphere. Conjugate margins are described from the Tasman Sea, the Atlantic Ocean and the Great Southern Ocean, illustrating both the principle of complementary asymmetry and the different patterns of uplift or subsidence on opposing passive margins.

Neogene and Quaternary rollback evolution of the Tyrrhenian Sea, the Apennines, and the Sicilian Maghrebides

Reconstruction of the evolution of the Tyrrhenian Sea shows that the major stage of rifting associated with the opening of this basin began at similar to10 Ma. It involved two episodes of back arc extension, which were induced by the rollback of a west dipping subducting slab. The first period of extension (10-6 Ma) was prominent in the northern Tyrrhenian Sea and in the western part of the southern Tyrrhenian Sea. The second period of extension, mainly affected the southern Tyrrhenian Sea, began in the latest Messinian (6-5 Ma) and has been accompanied by subduction rollback at rates of 60-100 km Myr(-1). Slab reconstruction, combined with paleomagnetic and paleogeographic constraints, indicates that in the central Apennines, the latest Messinian (6-5 Ma) arrival of a carbonate platform at the subduction zone impeded subduction and initiated a slab tear and major strike-slip faults. These processes resulted in the formation of a narrow subducting slab beneath the Ionian Sea that has undergone faster subduction rollback and induced extreme rates of back arc extension.

Vorticity and non-coaxiality in progressive deformations

A measure of the non-coaxiality involved in progressive deformation histories is proposed in the form of the kinematical vorticity number, Wk. This number is a measure of the relative effects of rotation of material lines (relative to the instantaneous stretching axes) and of stretching of these material lines. As such, Wk, is a measure of the instantaneous degree of non-coaxiality. A detailed example is first presented in the form of a progressive simple shearing in which the shear plane rotates relative to an external coodinate system. This is followed by examples of more complicated deformation histories. Three specific types of progressive, isochoric (constant volume) deformation histories are recognized. Those for which 0 ≤ Wk 0 is a special case of these corresponding to a coaxial history. Histories with Wk > 0 are non-coaxial. Those histories with Wk = 1 correspond to progressive simple shearing. Those histories with 1 < Wk < ∞ are pulsating and lines that have been extended may be shortened in future increments.