ALBERT

Description: The Roses granodiorite is a Variscan stock with well developed syn- and post-magmatic deformation structures that crops out in the Pyrenean Axial Zone. Analysis of structures reveals a continuous deformation history during and after magma cooling. The deformation history is divided on the basis of mechanical behaviour into two stages: an early one with the development of magmatic structures and a late stage with the development of mylonitic fabrics along shear zones. Both stages are separated in time by the emplacement of aplite-pegmatite dykes. Time of dyke emplacement is thought to coincide with a sudden change in rheology of the granodiorite. The abundance of quartz dioritic enclaves permits the use of shape analysis to characterize the magmatic fabric as a homogeneous deformation. Later solid-state deformation led to the development of an inhomogeneous deformation pattern with different sizes of anastomosing shear zones wrapping around lozenge-shaped domains. The displacement/width ratio measured in shear zones ranges between one and two orders of magnitude. The Roses granodiorite is thought to be a synkinematically emplaced stock which records a continuous deformatioinal history with two distinct deformation stages, both recording bulk finite strains of similar order of magnitude but with a marked difference in finite strain distribution.

Description: Cerro Quema (Azuero Peninsula, southwest Panama) is a high-sulfidation epithermal Au-Cu deposit hosted by a dacite dome complex of the Río Quema Formation (late Campanian to Maastrichtian), a fore-arc basin sequence. Mineral resource estimates (indicated + inferred) are 30.86 Mt at 0.73 g/t Au, containing 728,000 oz Au (including 76.900 oz Au equiv of Cu ore). Hydrothermal alteration and mineralization are controlled by an E-trending regional fault system. Hydrothermal alteration consists of an inner zone of vuggy quartz with locally developed advanced argillic alteration, enclosed by a well-developed zone of argillic alteration, grading to an external halo of propylitic alteration. Mineralization produced disseminations and microveinlets of pyrite and minor chalcopyrite, enargite, and tennantite, with traces of sphalerite, crosscut by late-stage base metal veins. New 40 Ar/ 39 Ar data of igneous rocks combined with biostratigraphic ages of the volcanic sequence indicate a maximum age of lower Eocene (~55–49 Ma) for the Cerro Quema deposit. It was probably triggered by the emplacement of an underlying porphyry-like intrusion associated with the Valle Rico batholith. The geologic model suggests that in the Azuero Peninsula high-sulfidation epithermal mineralization occurs in the Cretaceous-Paleogene fore arc. This consideration should be taken into account when exploring for this deposit type in similar geologic terranes.

Description: Deformation, in large parts of the middle crust, results in strained rocks consisting of grains with variable dislocation densities and microstructures which are characterized by gradual distortion and subgrain structures. Post-deformation residence of these rocks at elevated temperatures results in microstructural adjustments through static recovery and recrystallization. Here, we employ a numerical technique to simulate intragrain recovery at temperatures at or below the deformation temperature. The simulation is based on minimization of the stored energy, related to misorientation through local rotation of physical material points relative to their immediate environment. Three temperature- and/or deformation-geometry-dependent parameters were systematically varied: (1) deformation-induced dislocation types, (2) dislocation mobility and (3) size of dislocation interaction volume. Comparison with previously published in situ experiments shows consistency of numerical and experimental results. They show temperature- and dislocation-type-dependent small-scale fluctuations in subgrain-boundary misorientations and orientation variation within subgrains. These can be explained by the combined effect of increase in dislocation interaction volume and activation of climb. Our work shows microstructure can be significantly modified even if the post-deformational temperature is at or below the deformation temperature: a scenario relevant for most deformed rocks.

Description: 〈p〉Geometrical heterogeneities along layer interfaces play a key role in determining the geometries of folds developed during shortening of competent layers. We present a series of numerical simulations to investigate the influence of initial sinusoidal perturbations on the folding of single layers. Models consist of a competent viscous single layer embedded in a softer matrix, with the layer orientated parallel to the shortening direction. We first generalize the wide spectrum of sinusoidal perturbations accounting for asymmetries along and across a competent single layer, using two parameters: transversal asymmetry (〈i〉A〈/i〉') and longitudinal asymmetry (〈i〉〈/i〉). These two parameters allow the transition between classical fold shapes and pinch and swell geometries to be studied. The parameter 〈i〉A〈/i〉' describes the development of fold hinges with different geometries between the upper and lower layer interfaces, and abnormal curvatures between the outer and inner arcs of fold hinges. The parameter 〈i〉〈/i〉 induces a strong polarity on the folds, with a systematic preferred orientation of the pinch and swell regions of the layer, even if there is no shear component parallel to the layer. Our results demonstrate the importance of structural inheritance on the resulting fold geometries, and suggest that caution must be taken when using certain types of asymmetrical folds as strain markers and kinematic indicators.〈/p〉

Description: Geometrical heterogeneities along layer interfaces play a key role in determining the geometries of folds developed during shortening of competent layers. We present a series of numerical simulations to investigate the influence of initial sinusoidal perturbations on the folding of single layers. Models consist of a competent viscous single layer embedded in a softer matrix, with the layer orientated parallel to the shortening direction. We first generalize the wide spectrum of sinusoidal perturbations accounting for asymmetries along and across a competent single layer, using two parameters: transversal asymmetry ( A ') and longitudinal asymmetry ( ). These two parameters allow the transition between classical fold shapes and pinch and swell geometries to be studied. The parameter A ' describes the development of fold hinges with different geometries between the upper and lower layer interfaces, and abnormal curvatures between the outer and inner arcs of fold hinges. The parameter induces a strong polarity on the folds, with a systematic preferred orientation of the pinch and swell regions of the layer, even if there is no shear component parallel to the layer. Our results demonstrate the importance of structural inheritance on the resulting fold geometries, and suggest that caution must be taken when using certain types of asymmetrical folds as strain markers and kinematic indicators.

Description: Folded layers in rocks can be stretched again, potentially unfolding the folds back to straight layers. Little is known, however, about how to recognize partly or even entirely unfolded layers. When folded layers can unfold, what determines their mechanical behavior, and how can we recognize them in the field? In order to address these questions, we present a series of numerical simulations of the stretching of previously folded single layers and multilayers in simple shear. Layers do not completely unfold when they undergo softening before or during the stretching process, or when adjacent competent layers prevent them from unfolding. Intrafolial folds and cusp-like folds adjacent to straight layers as well as variations in fold amplitudes and limb lengths of irregular folds are indicative of stretching of a fold train.

Description: The Azuero Peninsula, located in SW Panama, is a region characterized by a long-lived intra-oceanic subduction zone. Volcanism began in Late Cretaceous time, as the result of subduction of the Farallon plate beneath the Caribbean plate. Usually, ancient volcanic arcs related to intra-oceanic subduction zones are not preserved, because they are in areas with difficult access or covered by modern volcanic arc material. However, on the Azuero peninsula, a complete section of the volcanic arc together with arc basement rocks provides the opportunity to study the sedimentation and volcanism in the initial stages of volcanic arc development. The lithostratigraphic unit which records fore-arc evolution is the "Río Quema" Formation (RQF), a volcanic apron composed of volcanic and volcaniclastic sedimentary rocks interbedded with hemipelagic limestones, submarine dacite lava domes, and intruded by basaltic-andesitic dikes. The "Río Quema" Formation, interpreted as a fore-arc basin infilling sequence, lies discordantly on top of arc basement rocks. The exceptionally well exposed arc basement, fore-arc basin, volcanic arc rocks and arc-related intrusive rocks provide an unusual opportunity to study the relationship between volcanism, sedimentation and magmatism during the arc development, with the objective to reconstruct its evolution. The "Río Quema" Formation can be divided into three groups: 1) proximal apron, a sequence dominated by lava flows, interbedded with breccias, mass flows and channel fill, all intruded by basaltic dikes. The rocks represent the nearest materials to the volcanic source, reflecting a coarse sediment supply. This depositional environment is similar to gravel-rich fan deltas and submarine ramps; 2) medial apron, characterized by a volcanosedimentary succession dominated by andesitic lava flows, polymictic volcanic conglomerates and crystal-rich sandstones with minor pelagic sediments and turbidites. These rocks were deposited from high-density turbidity currents and debris flows, directly derived from erupted material and gravitational collapse of an unstable volcanic edifice or volcaniclastic apron; 3) distal apron, a thick succession of sandy to muddy volcaniclastic rocks, interbedded with pelagic limestones and minor andesitic lavas, intruded by dacite domes and by basaltic to andesitic dikes. Bedforms and fossils suggest a quiet, relatively deep-water environment characterized by settling of clay and silt (claystone, siltstone) and by dilute turbidity currents of reworked volcaniclastic detritus. The timing of the initial stages of the volcanic arc has been constrained through a biostratigraphic study, using planktonic foraminifera and radiolarian species. The fossil assemblage indicates that the age of the "Río Quema" Formation ranges from Late Campanian to Maastrichtian, providing a good constraint for the development of the volcanic arc and volcaniclastic apron, during the initial stages of an intra-oceanic subduction zone.

Description: Partially dolomitized carbonate rocks of the Middle East and North America host large hydrocarbon reserves. The origin of some of these dolomites has been attributed to a hydrothermal mechanism. The Benicàssim area (Maestrat Basin, eastern Spain) constitutes an excellent field analogue for fault-controlled stratabound hydrothermal dolomitization: dolostone geobodies are well exposed and extend over several kilometres away from seismic-scale faults. This work investigates the main controls on the formation of stratabound versus massive dolomitization in carbonate sequences by means of two-dimensional (2D) reactive transport models applied to the Benicàssim case study. Simulation results suggest that the dolomitization capacity of Mg-rich fluids reaches a maximum at temperatures around 100 °C and a minimum at 25 °C (studied temperature range: 25–150 °C). It takes of the order of hundreds of thousands to millions of years to completely dolomitize kilometre-long limestone sections, with solutions flowing laterally through strata at velocities of metres per year (m/a). Permeability differences of two orders of magnitude between layers are required to form stratabound dolomitization. The kilometre-long stratabound dolostone geobodies of Benicàssim must have formed under a regime of lateral flux greater than metres per year over about a million years. As long-term dolomitization tends to produce massive dolostone bodies not seen at Benicàssim, the dolomitizing process there must have been limited by the availability of fluid volume or the flow-driving mechanism. Reactive transport simulations have proven a useful tool to quantify aspects of the Benicàssim genetic model of hydrothermal dolomitization.