Fernando O. Marques

Topography as a major factor in the development of arcuate thrust belts: insights from sandbox experiments

Abstract We have used sandbox experiments to investigate and to illustrate the effects of topography upon the development of arcuate thrust belts. In experiments where a sand pack shortened and thickened in front of an advancing rectilinear piston, the geometry of the developing thrust wedge was highly sensitive to variations in surface topography. In the absence of erosion and sedimentation, the surface slope tended to become uniform, as predicted by the theory of critical taper. Under these conditions, the wedge propagated by sequential accretion of new thrust slices. In contrast, where erosion or sedimentation caused the topographic profile to become irregular, thrusts developed out of sequence. For example, erosion throughout a hinterland caused underlying thrusts to remain active and inhibited the development of new thrusts in the foreland. Where initial topography was irregular in plan view, accreting thrusts tended to be arcuate. They were convex towards the foreland, around an initially high area; concave towards the foreland, around an initially low area. Initial plateaux tended to behave rigidly, while arcuate thrust slices accreted to them. Thrust motions were radial with respect to each plateau. Within transfer zones to each side, fault blocks rotated about vertical axes and thrust motions were oblique-slip. At late stages of deformation, the surface slope of the thrust wedge tended towards a uniform value. Initial mountains of conical shape (representing volcanoes) also escaped deformation, except at depth, where they detached. Arcuate thrust slices accreted to front and back. Where a developing thrust wedge was subject to local incision, accreting thrust slices dipped towards surrounding areas of high topography, forming Vs across valleys. Arcuate structural patterns are to be found around the three highest plateaux on Earth (Tibet, Pamirs and Altiplano) and around the Tromen volcanic ridge in the Neuquen Basin of northern Patagonia. We infer that these areas behaved in quasi-rigid fashion, protected as they were by their high topography.

Rotation of rigid elliptical cylinders in viscous simple shear flow: analogue experiments

We used experiments to investigate the behaviour of rigid elliptical cylinders embedded in a viscous fluid of finite thickness, and for very small ratios (S) between shear zone width (z) and the shortest principal axis (e2) of the ellipse that constitutes the base of the elliptical cylinder (3.5≥ S = z/e2 ≥1.2). Former theoretical and experimental work on the rotation of rigid inclusions immersed in a viscous matrix considered infinite shear zone thickness and/or a very large ratio between shear zone thickness and inclusion size (S >> 1), and concluded that inclusions rotate continuously and synthetically with the applied bulk simple shear flow (except for inclusions with infinite axial ratio). Our results depart greatly from the analytical predictions. Experiments were carried out at constant shear zone thickness, but variable S and degree of coupling at the inclusion/matrix interface. The results show that: (1) confined inclusion rotation always deviates from theoretical predictions for infinite shear zone thickness, even for synthetic rotation; (2) the deviation in the angular velocity and/or in the sense of rotation increases as S approaches 1, and antithetic rotation is possible from a position with ellipse longest axis parallel to the shear plane; and (3) a slipping inclusion/matrix interface greatly enhances deviation from theoretical predictions, with antithetic rotations to as much as 20°.

Polyphase Variscan emplacement of exotic terranes (Morais and Bragança Massifs) onto Iberian successions: Evidence from 40Ar/39Ar mineral ages

Abstract The Braganca and Morais Nappe Complexes comprise internally imbricated continental tectonic units (including schists, deformed metagranite, granulite and local eclogite) and a structurally underlying ophiolite nappe. High-pressure assemblages in some of the continental sequences were variably retrogressed during an amphibolite facies metamorphic overprint. By contrast, rocks within the ophiolite nappe display prograde amphibolite facies assemblages. Hornblende concentrates from both tectonic units exposed in the Braganca Nappe Complex record 40 Ar/ 39 Ar plateau and isotope correlation ages of c. 385–390 Ma. A similar date is recorded by hornblende within mylonitic amphibolite (ophiolite protolith) along a ductile thrust which separates the two major nappe units. Muscovite within blastomylonitic metagranite (Lagoa augen gneiss) in upper structural levels of the Morais Massif records 370–375 Ma 40 Ar/ 39 Ar plateau ages. Muscovite within phyllinite developed along a ductile thrust separating the gneiss from the structurally underlying ophiolite nappe records a 375 Ma age. Together the 40 Ar/ 39 Ar data suggest that a regionally significant, early Variscan amphibolite facies metamorphism occurred prior to c. 385–390 Ma (Early Devonian). This was associated with ductile imbrication of continental and ophiolite structural units with resultant formation of a composite nappe complex. The composite nappe complex must have been maintained at relatively high structural levels throughout the remaining Variscan evolution because there is no evidence for post-370 Ma rejuvenation of intracrystalline argon systems in any of crystalline structural units examined. This is in marked contrast to a significant Late Devonian (c. 330 Ma), locally high pressure metamorphism which is recorded in structurally underlying elements of the Lower Allochthon (part of a foreland thrust complex composed of metasedimentary units with Iberian protolith affinities). Together available chronologic constraints indicate emplacement of the previously assembled composite nappe complex into its present relative structural position at c. 330 Ma with resultant deformation and metamorphism of structurally underlying components of the Iberian thrust complex.

Plastic deformation of silicate garnets: II. Deformation microstructures in natural samples

Abstract We have used Transmission Electron Microscopy to characterize the deformation microstructures in natural garnets from several localities: eclogites from Sesia Lanzo (Alps), eclogites and garnet amphibolite from Braganca (Portugal), garnet pyroxenite from Lherz (France) and eclogites from Yakutia pipe (Siberia). Two characteristic microstructures have been identified. The first, found in eclogites from the Alps, consists of microplasticity associated with microcracking that suggests brittle behavior. The remaining samples show a microstructure characteristic of dislocation creep with recovery (sub-grain boundaries). The transition between these deformation regimes occurs at ≈600°C, i.e., at the boundary between group C and group B eclogites. The deformation microstructures suggest that the dislocations experience strong lattice friction below ≈600°C whereas at high temperature, diffusion assists dislocation glide and climb (recovery). We observed the following glide systems in the whole temperature range: 1/2〈111〉{110}, 1/2〈111〉{112}, 1/2〈111〉{123}, 〈100〉{010}, and 〈100〉{011}. No correlation could be established between the dislocation microstructure and the hydrous component or the chemistry of the garnets among the pyralspites. Comparison of the natural deformation microstructures and those generated in high-pressure experiments [Voegele et al., 1998 (Part I)] validates the extrapolation of experimental data to nature.

Geodynamic evolution of the Continental Allochthonous Terrane (CAT) of the Bragança Nappe Complex, NE Portugal

The Braganca Massif, in particular its Continental Allochthonous Terrane (CAT), is a keystone in the comprehension of the pre-Variscan and Variscan geodynamic evolution of the Eurasian plate, in particular of the high-grade metamorphic evolution. The Braganca CAT comprises three types of granulites (recording quite distinct metamorphic histories) and paragneisses with enclosed eclogite boudins. It has undergone five characterizable, main deformation phases during two orogenic cycles, separated by the extensive intrusion of mafic/ultramafic layered bodies (types 2 and 3 granulites) into a pervasively deformed granulitic lower continental crust (type 1 granulites). Geochronological data obtained on high-grade metamorphic rocks from Braganca and other similar rocks in the Variscan fold belt have been typically interpreted in two different ways: polycyclical evolution, Precambrian and Paleozoic (Variscan), or monocyclical evolution (Variscan). This second hypothesis is in clear contradiction with field evidence, with tectonometamorphic data, and even with some geochronological data, as shown in this article. Thus we suggest that the tectonometamorphic evolution of the Braganca CAT, and, by comparison, of other similar CAT in the Variscan fold belt, took place during two orogenic cycles, one Precambrian (Grenvillian? Pan-African/Cadomian?) and another one Paleozoic (Variscan).

Large-scale active slump of the southeastern flank of Pico Island, Azores

We report evidence for ongoing lateral slump of part of the southeastern flank of the Pico volcanic ridge in the Azores. Data from a high-resolution digital elevation model, field work, GPS, and radar interferometry show that: (1) the slumping sector is several cubic kilometers in size; (2) the structure involves several curved scars with normal fault kinematics; (3) the central part is undergoing little horizontal displacement toward the southeast (1.6 ± 1.3 mm/yr), but significant downward movement (5–12 mm/yr); and (4) the outer part of the southeastern flank of Pico is subsiding faster than the inner parts; this likely reflects recent individualization of a steep seaward-dipping fault in the moving mass. The slump shares similarities with active slumps recognized elsewhere, although the studied area may represent only the proximal part of a much larger complex potentially affecting the deep submarine base of the island. Displacement of the subaerial part of the southeastern flank of Pico seems to be accommodated by the movement and rotation of large blocks along listric normal faults.

The Late-Variscan fault network in central–northern Portugal (NW Iberia): a re-evaluation

Abstract The fault network in central–northern Portugal, especially the fault system with mean strike N25°, has been used to help deduce the late Palaeozoic dynamics of Western Europe. On the other hand, the N80° strike–slip fault system was recognized in previous works as Late-Variscan and left lateral, but was scarcely mapped and its importance neglected. This study shows that the N25° faults were dextral in the late stages of the Variscan Orogeny and sinistral during the Alpine Cycle due to pervasive reactivation. Fault rocks and intrusions are clearly different, according to age: mostly high temperature quartz infillings, but locally muscovite, tourmaline, and aplite dykes in Late-Variscan times, and low temperature cataclasites, fault gouges and Mesozoic mafic dykes in younger Alpine times. We dated dextral N45° segments of the N25° fault system because they are less prone to reactivation by the NNW–SSE Alpine compression, and can thus preserve the Variscan record. We used K–Ar in muscovite concentrates and obtained a minimum age of the analysed faults of ca. 312 Ma, which sets a lower limit to the so-called Late-Variscan wrench-faulting period. The present study shows that the N80° fault set is pervasive and sinistral in central–northern Portugal, and therefore, that it only admits one brittle dextral conjugate, the N25° fault system. Both were generated by a maximum compressive stress bearing between N50° and N55° in azimuth. We did not find evidence of a Variscan sinistral strike–slip movement in the N25° fault system, and therefore, this kinematics is believed to represent only the displacements accommodated during the Alpine Cycle.

Sheath folds formed by drag induced by rotation of rigid inclusions in viscous simple shear flow: nature and experiment

Abstract Natural examples of fold patterns associated with rotated boudins occur in the Ossa–Morena Zone (S Portugal) and triggered the present study. In ductile marbles with embedded competent mafic boudins, the folds seem to have originated by rotation of these bodies and show differential development, from gentle deflections to sheath folds. This suggests a dependence of the structure type on the distance of the metamorphic layering to the rigid body. The experimental work presented here was performed with rotating inclusions in bulk simple shear and introduces a new variable, the distance of the marker layer from the rigid inclusion (d), expressed in terms of E, the ratio between the spacing between marker layers (D) and the greater principal dimension of the inclusion (a). Contrary to previous experimental work in the literature, we used planar marker layers without concomitant boudinage. Our results show new processes of development of sheath folds from planar layers, and also that some non-cylindrical layer deflections are transient and, thus, that sheath folds do not originate from them. When d is small and the inclusion starts with its longest axis normal to the shear plane, sheath folds develop by drag pull processes on originally planar marker layers. Transient non-cylindrical deflections develop on originally planar marker layers when the distance between the marker layers is smaller than the longest principal dimension of the rigid particle. The monoclinic symmetry of the drag sheath fold patterns associated with rotating rigid inclusions indicate non-coaxial flow and can be used as a shear sense criterion (quarter structure).

2-D shape preferred orientations of rigid particles in transtensional viscous flow

Ghosh and Ramberg [Tectonophysics 34 (1976) 1–70] studied the two-dimensional (2-D) rotational behaviour of individual rigid particles embedded in a viscous medium subject to simple shear and transpression (Sr>0), at low/medium shear strains. We now extend this theoretical study to transtension (Sr<0) by deriving a new analytical solution, to non-interacting populations of rigid particles and to high shear strains. We used different initial orientations of particles in the same graph to simulate populations of particles in an originally isotropic rock. Our results show that: (1) shape preferred orientations (SPOs) developed in viscous simple shear flow are transient and cyclical, except for particles with aspect ratio=∞ (or material lines), which tend to the shear direction; (2) stable SPOs can develop in transpression; they dip towards the shear sense but, when defining an S/C structure with the shear foliation, this composite fabric is always antithetic and, thus, indicates the wrong sense of shear; (3) stable SPOs can also develop in transtension; they dip opposite to the shear sense but, when defining an S/C structure with the shear foliation, this composite fabric is always synthetic and, thus, indicates the correct sense of shear; (4) SPOs produced in transpression or transtension can be used as vorticity gauges only when they represent stable orientations, which must be demonstrated a priori.

Palaeomagnetic study of a subaerial volcanic ridge (Sao Jorge Island, Azores) for the past 1.3 Myr: evidence for the Cobb Mountain Subchron, volcano flank instability and tectonomagmatic implications

We present a palaeomagnetic study on 38 lava flows and 20 dykes encompassing the past 1.3 Myr on S. Jorge Island (Azores ArchipelagoNorth Atlantic Ocean). The sections sampled in the southeastern and central/western parts of the island record reversed and normal polarities, respectively. They indicate a mean palaeomagnetic pole (81.3 degrees N, 160.7 degrees E, K= 33 and A95= 3.4 degrees) with a latitude shallower than that expected from Geocentric Axial Dipole assumption, suggesting an effect of non-dipolar components of the Earth magnetic field. Virtual Geomagnetic Poles of eight flows and two dykes closely follow the contemporaneous records of the Cobb Mountain Subchron (ODP/DSDP programs) and constrain the age transition from reversed to normal polarity at ca. 1.207 +/- 0.017 Ma. Volcano flank instabilities, probably related to dyke emplacement along an NNWSSE direction, led to southwestward tilting of the lava pile towards the sea. Two spatially and temporally distinct dyke systems have been recognized on the island. The eastern is dominated by NNWSSE trending dykes emplaced before the end of the Matuyama Chron, whereas in the central/western parts the eruptive fissures oriented WNWESE controlled the westward growth of the S. Jorge Island during the Brunhes Chron. Both directions are consistent with the present-day regional stress conditions deduced from plate kinematics and tectonomorphology and suggest the emplacement of dykes along pre-existing fractures. The distinct timing and location of each dyke system likely results from a slight shift of the magmatic source.