S. E. Johnson

New geologic mapping and SHRIMP U-Pb zircon data in the Peninsular Ranges batholith, Baja California, Mexico: Evidence for a suture?

Geologic and SHRIMP U-Pb zircon studies in the Sierra San Pedro Martir area of northern Baja California, Mexico, suggest that the western and eastern parts of the Peninsular Ranges batholith originated as separate arcs. They are now juxtaposed along a well-exposed ductile thrust (Main Martir thrust) marked by an age discontinuity of at least 10 to 15 m.y., and sharp changes in rock type, metamorphic grade, and deformation history. The east-dipping thrust, and adjacent highly deformed rocks, were stitched by 108–97 Ma plutons that were generated in a region of crustal thickening formed during juxtaposition of the two arcs. Lack of chemical, isotopic, and geologic evidence for continentally derived rocks in the western arc may preclude an origin by rifting of the continental margin, or development of the arc on a prism of continentally derived sediments adjacent to the continental margin. Instead, the western arc may have originated as an island arc above a subduction zone outboard of North America. If so, a second, concurrent subduction zone along the North American margin must have driven convergence and suturing of the two arcs, and the Main Martir thrust may mark a nonterminal suture within a wider convergence-related deformation zone that formed ca. 115–108 Ma.

Inferring the timing of porphyroblast growth in the absence of continuity between inclusion trails and matrix foliations: can it be reliably done?

Abstract The timing of porphyroblast inclusion trails can be confidently interpreted relative to surrounding external foliations only where there is continuity between the two. Where this continuity is broken, timing is ambiguous. Where single or multiple growths of two or more different porphyroblastic minerals have occurred during a relatively complex deformation history, the risk of misinterpreting the relative timing of porphyroblast growth is high, and can lead to wrong inferences about pressure-temperature-time-deformation ( P-T-t-d ) paths. Misinterpreting porphyroblast timing can also have considerable consequences for determining rates of fabric evolution relative to changes in metamorphic conditions. The effects of porphyroblast rotation vs non-rotation (relative to an externally fixed reference frame) on inferred P-T-t-d paths are poorly understood. However, the ‘ d ’ part of the path can differ considerably, depending on whether or not porphyroblasts are inferred to have rotated. The effect on the P-T-t part of the path depends on what effect inferences about porphyroblast rotation have on the inferred sequence of porphyroblast growth.

Three-dimensional reconstruction and modelling of complexly folded surfaces using Mathematica

In this paper we provide the following three examples of how the software system Mathematica can be used to reconstruct or model the three-dimensional shapes of folded surfaces. (1) First, we revisit the reconstruction of the central inclusion surface within a garnet porphyroblast that contains spiral-shaped inclusion trails. (2) Next, we revisit the reconstruction of five foliation surfaces that define oppositely concave folds within and surrounding a plagioclase porphyroblast. (3) For the main part of this paper we model superposed folds, and the many interference patterns that can be found in two-dimensional sections through these folds. Because this special issue is accompanied by a compact disk, we have included a series of reconstructions, models and animations to illustrate these three examples. Our reconstructions and models have, in some instances, provided important constraints on the interpretations of complex or controversial microstructures, and in all instances have provided useful teaching aids.

Near-orthogonal foliation development in orogens: meaningless complexity, or reflection of fundamental dynamic processes?

Abstract Orogenic belts are geometrically complex owing to repeated deformation. Within this complexity, there is evidence that may suggest a common pattern of sequential steeply dipping and gently dipping foliations. Seven possible explanations are presented for the sequential development of these foliations, which can probably be reduced to four of general importance: (1) the passage of thrust sheets over flats and ramps; (2) switching of the maximum and minimum compressive stress orientations during orogenesis; (3) reversal of structural development owing to strong rheological anisotropy; and (4) back-rotation of crenulation hinges during crenulation cleavage development. It is suggested that all four of these reflect fundamental dynamic processes at work to build an orogen, and therefore that sequentially overprinted steeply and gently dipping foliations also reflect such fundamental processes. This illustrates that detailed geometrical analysis at the meso- and microscales can provide valuable input to dynamic models of orogenesis.

Determining finite longitudinal strains from oppositely-concave microfolds in and around porphyroblasts: a new quantitative method

Abstract This paper describes a precise new method for determining finite longitudinal strains in porphyroblastic metamorphic rocks, which makes use of oppositely-concave microfolds (OCMs) formed by heterogeneous strain of the matrix around porphyroblasts. The initial spacing between two foliation surfaces is measured inside a porphyroblast and compared to the spacing between the same two surfaces in the matrix, which results in a measure of extension ( e ) experienced by the rock during and/or after porphyroblast nucleation. A natural example is provided by the well-known ‘millipede’ plagioclase porphyroblasts from the Robertson River Metamorphics in Queensland, Australia. Twenty-four measurements were made from 22 serial thin sections cut parallel to both the X - Z and X - Y planes of finite strain, giving an average extension of 1.72 parallel to the X -direction of finite strain. The least-squares best-fit line to a plot of initial length vs change in length gives an R 2 value of 0.998. A minimum estimate of maximum shortening (negative e ) was also made by measuring the total lengths of S 1 folia that had been crenulated during OCM formation, giving a value of −0.54, which falls short of the −0.63 expected for constant-volume, plane-strain deformation. Because the OCM method is particularly suited to metapelites, results may provide new insight into mechanisms of folding and crenulation cleavage development, pressure-temperature-time-deformation histories, mass transport during deformation and metamorphism, and kinematic studies of porphyroblast behavior (rotation vs non-rotation) during ductile deformation.

Surface reconstruction from parallel serial sections using the program Mathematica : example and source code

Abstract A “package”—module of commented source code—for the computer program Mathematica has been designed to reconstruct surfaces from curves, collected in parallel serial sections through these surfaces. The package contains routines to interpolate digitized curves using continuous functions, to interpolate transversally between the resulting continuous curves, and to render the results of this later operation as a parametrized surface in three dimensions. This paper provides a listing of the package and an example of surface reconstruction using curves which were collected from serial thin sections through a garnet porphyroblast with spiral-shaped inclusion trails.

Presenting 3-D models of geological materials on the World Wide Web

Abstract In this article, we show how current World Wide Web technology can be used to present computerized 3-D models of geological materials. Unlike traditional, paper-based publishing, World Wide Web documents can incorporate several visualization techniques, some of which allow interactive user control. The concepts behind still images, animations, video sequences, object movies and virtual reality models are introduced. The advantages and pitfalls of the different visualization techniques are described, and their potential for the communication of 3-D models of geological materials is discussed. Web-resources providing the necessary browsing, playback and interaction software as well as specific authoring tools are referenced. In the HTML version of this article, which can be found on the accompanying CD-ROM, examples of 3-D macro- and microstructure reconstructions of geological materials are included as color images, animated sequences and interactive 3-D models.