Declan G. De Paor

Strain and kinematic analysis in general shear zones

We present a unifying theory for the development and distribution of strain markers and kinematic indicators in zones of general shear, and thus provide a framework in which data previously considered contradictory may be understood. Rigid and deformable porphyroclast systems, including σ, δ and complex σ-δ grains are potential indicators of both strain and flow. The shapes and distribution of such porphyroclast systems may be used to distinguish among different tectonic regimes. General shear is divided into two fields: sub-simple shear, in which the rotational component of the strain is less than that for simple shear, and super-simple shear, in which the rotational component is greater than for simple shear. Sub-simple shear may involve narrowing or broadening of shear zones. Super-simple shear regimes are possible in local regions such as the vicinities of deformable porphyroclasts, but must be enclosed by regimes of sub-simple shear. The polar Mohr constructions for finite deformation and flow are useful to analyze general shear in theory. The hyperbolic net is employed for practical plotting of real data and derivation of the kinematic vorticity number, Wn. This number represents the relative contributions of pure and simple shear in steady flow. In nature, deformation is thought to build up and decay by processes that may invalidate the assumption of constant flow regime. We therefore introduce the concept of accelerating deformation and analyze the implications of non-steady flow for the shearing histories of deformed objects.

The Arctic Eurekan orogen: A most unusual fold-and-thrust belt

The Tertiary Eurekan orogen of northernmost North America differs from a standard fold-and-thrust belt in several respects. It lacks a metamorphic-plutonic hinterland and wedge-shaped profile; instead, 2-km-high mountains of southeast Ellesmere Island face the frontal thrust, whereas halotectonic polygons, gentle warps, and subdued topography extend from Axel Heiberg Island in the center of the system back to the Sverdrup Rim at its rear. Clastic deposits are not located in a single flexural foredeep but are distributed in topographic lows amid several thrust sheets. The age of the stratigraphy, amount of displacement, and intensity of strain all increase cratonward, and the system9s width-to-length ratio is anomalously high. The orogen is attributed to Greenland9s pivotal movement relative to North America, which formed a braided Cenozoic plate boundary in Ellesmere and Axel Heiberg Islands, not a single transform in Nares Strait as previously proposed. The three major strands of this braided system are the Parrish Glacier, Vesle Fiord, and Stolz thrusts. They die out toward a structural pole of rotation in the south, whereas to the north, movement is accommodated by a dextral transpression zone extending from Lake Hazen to northern Greenland. Field studies on Ellesmere Island combined with a regional synthesis of previous work show that Eurekan deformation style is indicative of pivotal tectonism, that the contraction necessary to accommodate Greenland9s displacement is similar in magnitude to that documented by Eurekan structures, and that the ages of continental structures are compatible with the paleomagnetic record of seafloor spreading in Labrador Sea and Baffin Bay. Previously proposed sinistral strike-slip displacement is not confirmed; rather, eastvergent thrusts and dextral wrench faults typify the system. We propose that the Eurekan orogeny marked a change from tip propagation to pivotal tectonism as the North Atlantic rift system penetrated the entire width of the Laurasian continent.

Alternative model of thrust-fault propagation

A widely accepted explanation for the geometry of thrust faults is that initial failures occur on deeply buried planes of weak rock and that thrust faults propagate toward the surface along a staircase trajectory. We propose an alternative model that applies Greteners beam-failure mechanism to a multilayered sequence. Invoking compatibility conditions, which demand that a thrust propagate both upsection and downsection, we suggest that ramps form first, at shallow levels, and are subsequently connected by flat faults. This hypothesis also explains the formation of many minor structures associated with thrusts, such as backthrusts, wedge structures, pop-ups, and duplexes, and provides a unified conceptual framework in which to evaluate field observations.

Inversion tectonics — a discussion

The term ‘inversion’ to describe an inverted basin was first used by Glennie & Boegner (1981) although inverted basins had been recognized many years before e.g. Lamplugh (1920) and Stille (1924). During this meeting it became apparent that the application of the term had broadened to such an extent that the understanding of ‘inversion’ in the petroleum industry was incompatible with much of the current usage. The discussion that follows illustrates many of the points of disagreement, perhaps the most contentious of which is the use of the term ‘negative inversion’ although this was also introduced by Glennie & Boegner (1981). Most of the discussion was presented verbally at the meeting and was recorded, transcribed and returned to speakers for their corrections. In addition, a number of written contributions were received. All contributions to the discussion have been edited as gently as possible so as to retain the exact meaning intended by the contributor. All contributors are included as co-authors in this discussion article but clearly this does not mean that individuals necessarily accept all the points made by other contributors. The editors have identified individual contributions. The discussion commenced with some proposals by the editors which are briefly reproduced here. This discussion article concludes with a considered revision of the proposals on nomenclature which aims to satisfy some of the shortcomings identified in the discussions. Our initial definition of inversion relied on the concept of regional elevation. The regional elevation of a marker horizon is the structural elevation of the horizon

Rf/øf strain analysis using an orientation net

Abstract Since the classical work of Cloos, deformed distributions of elliptical objects such as ooids or pebbles have been recognized as an extremely important category of geological strain marker. However, elliptical objects are not easily analyzed, especially where primary sedimentary fabrics are tectonically imbricated. This paper demonstrates that previously published analytical techniques generally address only specific aspects of deformed ellipse distributions; as research tools, they are like a stereonet with great circles only or small circles only. All of the above methods can be combined with the aid of a new orientation net which is as convenient to use in the field as a standard stereonet. Uniform and imbricate fabrics are evaluated with equal ease and assumptions are subjected to statistical testing.

Volume loss and tectonic flattening strain in granitic mylonites from the Blue Ridge province, central Appalachians

Abstract Granitic mylonites from the Blue Ridge province in the central Appalachians demonstrate the contribution of various deformation mechanisms to the strain geometry that developed during greenschist facies deformation in thrust-related shear zones. Strain accumulated through cataclasis in feldspar, crystal plastic processes in quartz, and a significant contribution of pressure solution in both feldspar and quartz. Strain analysis was performed using R f / φ f techniques on quartz grain shapes and by measuring stretched and boudinaged feldspars. Petrographic and geochemical evidence indicates that deformation ranged from isovolumetric to upwards of 60% bulk-rock volume loss. Quartz and feldspar strain values plot within the field of apparent flattening. Feldspars in both XZ and YZ sections are boudinaged and separated by transverse veins and record a true tectonic flattening strain. Strain data indicate that the bulk deformation path in shear zones throughout the Blue Ridge province significantly deviated from plane strain.

The digital revolution in geologic mapping

Geologic field data collection, analysis, and map compilation are undergoing a revolution in methods, largely precipitated by global positioning system (GPS) and geographic information system (GIS) equipped mobile computers paired with virtual globe visualizations. Modern, ruggedized personal digital assistants (PDAs) and tablet PCs can record a wide spectrum of geologic data and facilitate iterative geologic map construction and evaluation on location in the field. Spatial data, maps, and interpretations can be presented in a variety of formats on virtual globes, such as Google Earth and NASA World Wind, given only a basic knowledge of scripting languages. As a case study, we present geologic maps assembled in Google Earth that are based on digital field data. Interactive features of these maps include (1) the ability to zoom, pan, and tilt the terrain and map to any desired viewpoint; (2) selectable, draped polygons representing the spatial extent of geologic units that can be rendered semi-transparent, allowing the viewer to examine the underlying terrain; (3) vertical cross sections that emerge from the subsurface in their proper location and orientation; (4) structural symbols (e.g., strike and dip), positioned at outcrop locations, that can display associated metadata; and (5) other data, such as digital photos or sketches, as clickable objects in their correct field locations. Google Earth–based interactive geologic maps communicate data and interpretations in a format that is more intuitive and easy to grasp than the traditional format of paper maps and cross sections. The virtual three-dimensional (3-D) interface removes much of the cognitive barrier of attempting to visualize 3-D features from a two-dimensional map or cross section. Thus, the digital revolution in geologic mapping is finally providing geosci entists with tools to present important concepts in an intuitive format understandable to the expert and layperson alike.

Mohr circles of the First and Second Kind and their use to represent tensor operations

Abstract Since their introduction to the geological literature by Brace (1959, 1960, 1961), Mohr circles for large irrotational deformations have proved valuable as aids to our understanding of deformation geometry. However, confusion persists regarding sign conventions. We show that there are two basic kinds of Mohr circles, each with its distinct set of sign conventions. These two divisions, which we call Mohr circles of the First and Second Kind, are not merely reflections of one another in Mohr space. They represent two distinct aspects of the relationship between the space of tensor components (Mohr space) and the space of geological structures (geographical space). The distinction between Mohr circles of the First and Second Kind is critical when the circles are drawn in off-axis positions for asymmetric tensors. Constructions in Mohr space are described which correspond to various standard tensor operations including transposition, inversion, addition and various kinds of multiplication. For some of these operations Mohr circles of one kind or the other offer advantages.

Practical analysis of general shear zones using the porphyroclast hyperbolic distribution method: An Example from the Scandinavian Caledonides

The porphyroclast hyperbolic distribution (PHD) method of kinematic vorticity analysis draws heavily upon the pioneering work of Ghosh and Ramberg (1976), among others. In general shear zones, rigid ellipsoidal grains may rotate either forwards (with the sense of shear) or backwards (counter to the sense of shear) according to their axial ratios and orientations. Grains of a critical shape have their forward motion exactly countered by backward motion in response to the pure shear component of deformation. When plotted on the hyperbolic net, the axial ratios and orientations of these easily recognizable ‘stable end’ grains define a hyperbola asymptotic to the eigenvectors of flow.

Balanced Section in Thrust Belts Part 1: Construction

Balanced geological cross sections are an important aid to understanding thrust-belt structures and to estimating their hydrocarbon-bearing potential. Two approaches to section balancing have been taken in the past: construction of retrodeformable sections from raw data, and modification of previous interpretations after an evaluation procedure. These approaches may be unified by use of a Langrangian grid. Projection of datum locations, dips, and stratigraphic thicknesses onto the section plane is performed directly by calculation, or graphically, using plunge lines. Spline interpolation and parallel-fold modeling are the simplest ways of filling gaps in the sectional data, but isogon interpolation yields rheologically more realistic results. Faults are interpolated using cutoff geometries as constraints. Fault tips are located from distance-displacement plots, whereas the depth to detachment is obtained by a modified Chamberlin construction. Interpolations made on the section are directed back to the geologic map via plunge lines. Although these techniques of section construction do not guarantee a balance, especially when complexities such as growth structures, diapirs, or strike-slip faults are present, they eliminate many potential errors.