Denis Gapais

Style and history of Andean deformation, Puna plateau, northwestern Argentina

Topographically, the Puna plateau of northwestern Argentina is the southern continuation of the Bolivian Altiplano. Its thickening and consecutive uplift result from the Andean orogeny. To better constrain the structural style and its progressive development, we have studied field data, topographic and satellite imagery, balanced cross sections, seismic reflection data, kinematic analysis of fault slip data, anisotropy of magnetic susceptibility (AMS), paleomagnetic data, and apatite fission track (AFT) data. Across the Puna plateau, Precambrian and Paleozoic basement ranges, bounded by high-angle reverse faults (dips ≥ 60°), alternate with Cenozoic intermontane basins. Major thrusts trend NNE-SSW and do not show a preferred vergence. Intermontane basins have various degrees of symmetry, depending on the geometries and attitudes of associated thrusts as well as on the magnitudes of their offsets. There is a close correlation between the surface expression of a basin and the amount of internal deformation. A line-balanced cross section of the Puna at 25°S has yielded a Cenozoic shortening of 10–15%, in a direction subperpendicular to the orogen. By kinematic analysis of Cenozoic fault slip data we have obtained principal directions of strain rate across the Puna. Shortening axes are subhorizontal and trend on average WNW-ESE (∼N110°), stretching axes are subvertical, and intermediate axes are subhorizontal and trend on average NNE-SSW. Strain ellipsoids are dominantly of plane strain type, and they represent dip-slip thrusting. From paleomagnetic and AMS data, shortening axes form a radial pattern around the eastern edge of the central Andes. The pattern is attributed to an inhomogeneous stress field, reflecting the eastward convex shape of the central Andean thrust front. From the history of burial and uplift, Andean shortening reached the northeastern part of the Puna in the late Eocene and the adjacent Eastern Cordillera in the late Eocene or early Oligocene. This shortening was presumably due to the Incaic phase of the Andean orogeny. In the eastern part of the orogen the onset of shortening was probably guided by preexisting Paleozoic and Mesozoic structures, so that Andean deformation propagated unevenly eastward.

Quartz fabric transition in a cooling syntectonic granite (Hermitage Massif, France)

Abstract The Hermitage Granite, situated in the northwest Massif Central (France) is a syntectonic Hercynian leucogranite emplaced along an active transcurrent shear zone. During emplacement and cooling, the progressive deformation is marked by the development of a primary homogeneous foliation gradually affected by ductile shear bands (C-S mylonites). Increase in strain heterogeneity during cooling corresponds to a change of dominant deformation mechanism of the quartz phase from grain growth and migration recrystallization to intracrystalline 〈 a 〉 slip and rotation recrystallization. Migration recrystallization is characterized by preferred orientations of c axes close to the principal extension direction. We discuss relevant deformation mechanisms and rheological implications for syntectonic plutons. In particular, we argue that the transitions between homogeneous and heterogeneous accumulation of strain cannot generally be correlated with transition between magmatic and solid-state flow.

Strain pattern in the Aar Granite (Central Alps): Orthogneiss developed by bulk inhomogeneous flattening

The central part of the Aar Granite (Aar Valley) shows lens-shaped domains of low strain separated by anastomosing domains of high strain. A comparable pattern is found on all scales of observation. Deformation gradients are outlined by a gradual change from isotropic granite to orthogneiss and locally ultramylonites. Ultramylonites are concentrated within conjugate ductile shear zones. Where moderate, the strain is concentrated in very narrow small-scale faults which affect relatively undeformed granite. Within foliated domains, finite strain measurements on deformed xenoliths indicate that (i) strain intensity increases gradually with grain size reduction and (ii) finite-strain ellipsoids are of flattening type irrespective of the degree of mylonitization. The pattern of small-scale faulting, the trends of the regional foliation and the orientation of ductile shear zones are consistent with a bulk strain field of flattening type throughout progressive deformation.

Sedimentary basins and crustal thickening

Abstract We consider the development of sedimentary basins in a tectonic context dominated by horizontal shortening and vertical thickening of the crust. Well-known examples are foreland basins; others are ramp basins and buckle basins. We have reproduced various styles of compressional basins in experiments, properly scaled for gravity. A multilayered model lithosphere, with brittle and ductile layers, floats on a model asthenosphere. A computer-driven piston provides shortening and thickening, synchronous with erosion and sedimentation. After a first stage of lithospheric buckling, thrust faults appear, mainly at inflection points. Slip on an isolated reverse fault is accompanied by flexure. Footwall flexure results in a foreland basin and becomes accentuated by sedimentation. Hangingwall flexure is less marked, but may become accentuated by erosion. Motion on a fault leads to hangingwall collapse at the surface. Either footwall sedimentation or hangingwall erosion tends to prolong the active life of a reverse fault. Slip on any pair of closely spaced reverse faults of opposite vergence results in a ramp basin. Simultaneous slip produces a symmetric ramp basin, whereas alternating slip results in a butterfly-shaped basin, with superposed foredeeps. Some well-developed ramp basins become pushed down, until bounding faults meet at the surface and the basin disappears from view. At this stage, the basin depth is equivalent to 15 km or more. Slip on any pair of widely spaced reverse faults of opposite vergence results in a pronounced central anticline, between two distinct foredeeps. In Central Asia and in Western Europe, Cenozoic crustal thickening is due to continental collision. For Central Asia (Western China, Kyrgyzstan, Uzbekistan, Tajikistan), we have compiled a regional structure-contour map on the base of the Tertiary, as well as 4 regional sections. Foreland basins and ramp basins are numerous and associated with Cenozoic thrusts. Large basins (Tarim, Junggar, Fergana, Tajik) occur around and between mountain ranges, but smaller basins (Issyk-Kul, Naryn) occur within them. In Western Europe, the Alps and Pyrenees are surrounded by foreland basins, ramp basins or intermediate styles. In the Andes and its foreland, Neogene thrusts and compressional basins are due to subduction of oceanic lithosphere. In Colombia, they account for much of the Cordillera Oriental; in NW Argentina, for the Altiplano; in West-Central Argentina, for the Sierras Pampeanas. Compressional basins are also common in other areas of older crustal thickening.

Bulk kinematics from shear zone patterns: some field examples

Abstract Geological deformations which are statistically homogeneous at bulk scale (e.g. the macroscale) are often localized into arrays of narrow shear zones at a smaller scale (e.g. the mesoscale). This paper shows that shear zone patterns can be used to estimate both a bulk finite strain ellipsoid and aspects of the bulk deformation history. We describe examples of heterogeneously deformed granitic rocks which reveal the following features. (1) Shear zones show preferred orientations. (2) There are correlations between shear zone orientations and directions and senses on shear. (3) For areas that have undergone coaxial deformation histories, shear zone patterns have orthorhombic symmetries directly related to strain ellipsoid shape. (4) For areas that have undergone non-coaxial deformation histories, shear zone patterns have a lower symmetry. (5) For areas that have undergone bulk simple shear, shear senses on shear zones are consistent with the bulk shear sense. Results are compared with predictions of kinematic models involving slip along inextensible fibres and sheets. According to these models, preferred orientations of slip surfaces track surfaces of no finite extension of the bulk strain ellipsoid, whereas slip directions track directions of large shear in the bulk strain ellipsoid. There are good correlations between preferred orientations of shear zones and fibre models.

Analogue models of laccolith formation

Abstract This paper describes dynamically scaled analogue models of laccolithic intrusions. Experiments consisted of the injection of a Newtonian fluid (low-viscosity silicone putty) into a sandpack, with or without an interbedded ductile layer of silicone putty acting as a potential decollement level. Boundary conditions were chosen to analyze the influence of the thickness of the brittle cover and of the decollement layer on the pattern of intrusion. Further experiments were made to examine the effects of an extensional regime during intrusion. Experiments showed that: (1) laccolith formation requires the occurrence of a decollement layer between two competent units, (2) the critical thickness of the decollement layer necessary for laccolith formation decreases with increasing depth, (3) laccoliths change from lenses to bell-shaped with decreasing overburden, (4) for a constant thickness of the decollement layer, the largest diameter of laccoliths is proportional to the thickness of the overburden, and (5) a syn-injection gravitational sliding regime results in an asymmetric laccolith, with amplification of the bell shape.

Tectonic significance of fault-slip data

Abstract The statistical analysis of populations of minor faults is commonly used by structural geologists working in areas of brittle rock. It is based on measurements of fault attitude, direction and sense of slip. At individual sampling localities, results are classically interpreted as indicators of stress or strain rate fields, assuming a homogeneous stress or strain rate tensor, respectively. However, fault patterns are expected to vary with time because of displacements, rigid rotations, and internal strains, which generally occur along boundaries of fault-bounded blocks as deformation proceeds. Thus, in the general situation where early faults accumulate displacements and rigid rotations, and where new faults develop during progressive deformation, fault-slip data can be rather complex and variable in space, and reflect neither local stress or strain rate tensors, nor finite strains and finite rotations in a simple way. We use two examples of faulted regions to illustrate the spatial variability of fault-slip data. This can be due to local complications at the edges of fault blocks, or to complex kinematic conditions at regional boundaries. Such complications may make it difficult to deduce a consistent and simple pattern of either stresses or strain rates. Instead, our results suggest that information contained in fault-slip data can be pertinent to finite deformation. In particular, the principal axes that we have deduced from the analysis of fault-slip data are consistent with the finite displacements at block boundaries that we have calculated by numerical restoration.

Paleomagnetic evidence for Cenozoic block rotations in the Tadjik depression (Central Asia)

This paper presents results of a paleomagnetic study of Oligo-Miocene red beds of the Tadjik depression in Central Asia. We sampled about 530 cores at 69 sites and six localities across the depression and along the western border of the Pamirs. Samples were thermally demagnetized and high-temperature components appear to predate folding of upper tertiary age. Throughout the depression, paleomagnetic inclinations are consistent with those observed on the stable Turan platform, at the western margin of the depression. However, they are shallower by about 30° than the inclination predicted from the reference apparent polar wander path. This appears to indicate a 23° difference in latitude, which is incompatible with paleogeographic reconstructions for the Tertiary. A sound interpretation of this anomaly would require a better-constrained Tertiary paleomagnetic reference for Asia. Inside the Tadjik depression, paleomagnetic declinations are all significantly rotated, counterclockwise with respect to those measured on the Turan platform. The eastern part of the depression is a domain of large rotation (52°±13° to 46°±15°), whereas smaller amounts of rotation have occurred in the western part (27°±14° to 14°±15°). The similarity between Tertiary and Cretaceous data available for the area shows that rotations have occurred since the Miocene. Little or no paleomagnetic rotations are observed in the ranges bordering the northern and western parts of the depression. Paleomagnetic and structural data suggest that block rotations in the Tadjik depression are associated with indentation of the Pamirs into stable Asia. At a larger scale, observed rotations are compatible with a model of regional sinistral wrenching, along a strip running from the Gulf of Oman to Lake Baikal.

Partitioning of transpressive motions within a sigmoidal foldbelt : the Variscan Sierras Australes, Argentina

Abstract The Sierras Australes (Buenos Aires province, Argentina) form a sigmoidal foldbelt, about 150 km long. The last major orogenic event was Variscan, as shown by synsedimentary folds in upper Palaeozoic sediments and by K-Ar or Rb-Sr ages on cleavage-forming minerals of low metamorphic grade. Folds in the Paleozoic cover are associated with cleavage, stretching lineation and kinematic indicators (shear bands and sigmoidal tails around porphyroclasts). Reworking of the granitic Precambrian basement resulted in shear zones and faults. We distinguish three structural domains within the Sierras Australes: a Northwestern Arc, a Southeastern Basin and a Central Belt. The Northwestern Arc is a simple fold-and-thrust belt, verging towards the NE. The Southeastern Basin is a foreland basin, where an underthrust towards the SW is overshadowed by a right-lateral wrench along strike. Finally, the Central Belt has resulted from a combination of right-lateral wrenching and overthrusting, both in a northerly direction. All three domains may have resulted from a uniform state of transpressive stress. Although the deformation is strongly partitioned at outcrop, it may be simpler at depth. This is suggested by experiments in which sandpacks are subjected to transpression, by reactivating a single basement fault in oblique (right-lateral reverse) slip. We describe one such experiment where fault blocks rotate counterclockwise about vertical axes, producing arcuate thrusts at the surface. By analogy, we suggest that the Sierras Australes formed in a transpressive context, as a result of oblique-slip reactivation of a deep fault zone. Strong partitioning in the upper crust was facilitated by upward splaying of faults.

Shear criteria and structural symmetry

During the last decade, it has been shown that most relevant shear criteria within ductile rocks are asymmetric structures (e.g. pressure shadows, shear bands, CS structures, fabrics, tension gashes, folds, veins). The correspondence between coaxial or non-coaxial deformation, and symmetric or asymmetric particle velocity fields, respectively accounts for the use of structural symmetry as an indicator of strain history. The application of this symmetry concept to various field examples emphasizes that: (i) the degree of symmetry of a given structural pattern reflects the bulk strain regime irrespective of the size and the mechanical behaviour of the considered system; and (ii) the strain regime can also be inferred from the order of appearance and dominance of structures which contribute to the total deformation pattern, even where the progressive deformation results in a complex pattern which cannot be directly interpreted.