P.R. Cobbold

Propagating extrusion tectonics in Asia: New insights from simple experiments with plasticine

Plane indentation experiments on unilaterally confined blocks of plasticine help us to understand finite intracontinental deformation and the evolution of strike-slip faulting in eastern Asia. Several large left-lateral strike-slip faults may have been activated successively, essentially one at a time. The experiments suggest that the penetration of India into Asia has rotated (≈25°) and extruded (≈800 km) Indochina to the southeast along the then left-lateral Red River fault in the first 20 to 30 m.y. of the collision. This process can account for the opening of the South China Sea before late Miocene time. Extrusion tectonics then migrated north, activating the Altyn Tagh fault as a second major left-lateral fault and moving southern China hundreds of kilometres to the east. As this occurred, Indochina kept rotating clockwise (as much as 40°), but the sense of motion reversed on the Red River and other strike-slip faults in the south. Opening of the Mergui basin and Andaman Sea (up to the present) also appears to be a simple kinematic consequence of the extrusion. Recent rifts in northeastern China and Yunnan may be considered incipient analogs of the South China and Andaman Seas. Other Tertiary tectonic features such as the sedimentary basins of the Gulf of Thailand may be explained as collisional effects, if one uses our experiments as a guide. The experiments also suggest that a major left-lateral strike-slip fault and rift system will propagate across the Tien Shan, Mongolia, and Baikal to the Sea of Okhotsk.

Development of sheath folds in shear regimes

Abstract The development of three-dimensional fold shapes in shear regimes is studied using theoretical and experimental methods and with reference to natural examples. Theoretical studies are based on homogeneous simple shear and other valid solutions to the equations of motion of Newtonian materials. Experimental work has been done with models made from analogue materials and deformed in a simple shear machine capable of large shear strains ( γ > 10). Emphasis is placed on passive folds. Three models are presented for their development, two models invoking flow with steady stream lines, the third involving unsteadiness accompanying boudinage. Resulting sheath folds are strongly asymmetric, with curved fold hinges. Geological examples of sheath folds occur in many natural shear zones and may have formed in an essentially passive manner.

Experiments on shortening of a 4-layer model of the continental lithosphere

Davy, Ph. and Cobbold, P.R., 1991. Experiments on shortening of a 4layer model of the continental lithosphere. In: P.R. Cobbold (Editor), Experimental and Numerical Modelling of Continental Deformation. Tecronophysics, 188: l-25. We present an experimental method for modelhng lithospheric deformation. The method is based on a simplified profile of lithosphere rheology where only four layers are considered: brittle crust, ductile crust, brittle mantle and ductile mantle. The strengths of the four layers, calculated from laboratory determined flow laws, depend strongly on geothenns and slightly on strain rates and deformation regimes. The rheological profiles of the lithosphere have been recreated in experimental models built with analog materials (sand, silicone putty and golden syrup) which flow under their own weight. During lithospheric shortening, the style of deformation depends on the relative strengths of the four layers, especially those in the mantle. Of special importance is the existence, or non-existence, of a brittle mantle. The initial stages of compression often produce flexures or buckles without local isostatic compensation. Buckling exerts a strong control on the localisation of thrust faults in later stages: the thrusts tend to form at the inflexion points of buckles. The style of crustal thickening depends on the relative thicknesses and resistances of the lithospheric layers. In 2-layer lithospheric models (without resistant mantle), shortening was concentrated upon a small number of thrusts, widely spaced. In 3-layer models, shortening was concentrated into a large number of reverse faults more closely spaced and more steeply dipping than before. In Clayer models, a single thrust formed in the brittle mantle. Motion on this thrust dragged the crustal layers down, forming an asymmetric crustal root with steep imbricate slices.

Lateral extrusion in the eastern Alps, Part 1: Boundary conditions and experiments scaled for gravity

Lateral extrusion encompasses extensional collapse (gravitational spreading away from a topographic high in an orogenic belt) and tectonic escape (plane strain horizontal motion of wedges driven by forces applied to their boundaries). In the Eastern Alps it resulted from (1) an overall northerly compression (Apulia against Eurasia), (2) a strong foreland (Bohemian massif), (3) lack of constraint along a lateral boundary (Carpathian region), and (4) a previously thickened, gravitationally unstable, thermally weakened crust (Eastern Alpine orogenic belt). Six indentation experiments reproduce lateral extrusion at lithospheric scale. The models have two to four lithospheric layers, with a Mohr/Coulomb rheology for the upper and a viscous rheology for the lower crust. The lithosphere rests upon a low-viscosity asthenosphere. A broad indenter, a narrow deformable area, and a weakly constrained eastern margin fullfill as closely as possible conditions in the Eastern Alps. Indentation produces both thickening in front of the indenter and escape of triangular wedges. Lateral variations in crustal thickness become attenuated by gravitational spreading. The overall fault pattern includes domains of reverse, strike-slip, oblique normal, and pure normal faults. Strike-slip faults in conjugate sets develop serially. The narrow width of the deformable area and the strength of the foreland determine the angles between the sets. Gravitational spreading produces a rhombohedral pattern of oblique and pure normal faults along the unconstrained margin. Opposite the unconstrained margin, the indenter front shows thrusts and folds intersecting with the conjugate strike-slip sets. A triangular indenter favors spreading. High velocity of indentation favors escape. High confinement limits lateral motion, inhibits spreading, and favors thickening. Lateral extrusion in the Eastern Alps is best modeled by (1) a weak lateral confinement, (2) a broad and straight indenter, (3) a narrow width of the deformable area, and (4) a rigid foreland. Crustal thickening, lateral escape, and gravitational spreading all contribute to the overall deformation.

Segmentation of an obliquely rifted margin, Campos and Santos basins, southeastern Brazil

We make the case for Early Cretaceous transfer zones that segment the obliquely rifted Atlantic margin of southeastern Brazil. Our interpretation is based on published literature, Bouguer-corrected gravity, regional reflection seismic profiles, and well data. In the Santos and Campos basins, Neocomian rift architecture was strongly influenced by preexisting fabric and structures of the Late Proterozoic (Brasiliano orogeny). The Atlantic margin inherited an east-northeast-west-southwest orientation so that rifting was oblique to the margin. On a regional map of Bouguer-corrected gravity, a nearshore belt of positive anomalies correlates with an interpreted broad Moho uplift in the footwall of Neocomian extensional faults. Farther offshore, a second belt of positive anomalies correlates with a presalt ridge of eroded volcanic or basement anticlines covered by thin Aptian evaporites, interpreted as a failed spreading center. Intervening negative anomalies coincide with the main rift basin. All three belts show apparent offsets along linear zones trending west-northwest-east-southeast, which we interpret as transfer zones. The vergence of half rifts tends to change across transfer zones, compartmentalizing the rifted margin into subbasins. Our results have implications for the risks associated with distribution, maturation, and migration of hydrocarbons within the prolific Early Cretaceous lacustrine petroleum system of the Campos and Santos basins.

Physical models of extensional tectonics at various scales

Summary In a preliminary series of experiments, using physical models mechanical processes of extensional tectonics have been investigated at various scales. By a suitable choice of model materials, experiments were performed at low cost in a natural gravity field. Upper layers of the lithosphere were modelled using sand; lower layers, using silicone putties of two different densities; the mantle asthenosphere was modelled using honey. The models deformed under their own weight or under absolute horizontal tension. Rates of extension were controlled using a stepper motor. Surface deformation and faulting were monitored using 35 mm time-lapse photography. Lower lithosphere topography was photographed through the transparent asthenosphere. Fault patterns in models with lithosphere only, were observed by serial sectioning. Otherwise, the brittle-ductile interface was observed after suctioning off the sand. Simple experiments with uniformly extended sand layers only show that; (i) spacing of normal faults is a measure of the layer thickness; (ii) the length of fault trace increases with the amount of downthrow; and (iii) faults tend to form domino domains. Some experiments with a brittle layer on a ductile substrate show a mechanism of passive rifting where; (i) major faults occur in conjugate pairs, defining rift valleys; (ii) minor faults localize additional extension in rift-valley floors; and (iii) isostatic uplift of the viscous substrate causes uplift and tilting of rift rims. In freely floating continents, gravitational spreading leads to: (i) highly localized extension and thinning at continental margins and (ii) internal rifting.

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.

Style and pattern of salt diapirs due to thin-skinned gravitational gliding, Campos and Santos basins, offshore Brazil

Abstract Portions of seismic lines and a structure-contour map illustrate the patterns and shapes of salt diapirs and related structures in the Campos and Santos areas, off the Atlantic coast of Brazil. We interpret the structures both kinematically and mechanically, drawing on our experience with similar salt structures worldwide, with the results of recent physical modelling and with geometric restorations in section and in plan. Salt diapirs and related structures have a variety of structural styles, distributed in domains and provinces. Near the coast, there is an upper domain, 100–200 km wide, with a suite of structures that we attribute to horizontal downdip extension: these are salt rollers, in the footwalls of listric normal growth faults; salt walls of triangular cross section, beneath intersecting conjugate normal faults; turtle anticlines; and salt welds. Downslope extension started in the Albian and has continued to the present day. In the Campos area, from simple line balancing, the accumulated downslope displacement is about 100 km. Seawards of this, there is a lower domain, 100–400 km wide, with a different suite of structures, that we attribute to downdip contraction: these are growth folds of various wavelengths, in sedimentary sequences of various thicknesses; asymmetric salt walls, emplaced above reverse faults; deep basins, wedged down between conjugate reverse faults; and salt tongues above thrusts. For Campos, we estimate the total downslope contraction, accumulated since the Albian, to be about 100 km. From the balance between extension and contraction, we infer that the thin-skinned salt tectonics are gravitationally driven and independent of any basement tectonics. The structure-contour map on the top of the salt shows that structural style is variable also along strike. Seismic sections along regional contours indicate differing amounts of strain. On this basis, we distinguish five provinces, separated by NW-SE-trending lines. For the Northern Campos province, we infer radially convergent gliding; for the Campos or Cabo Frio provinces, radially divergent gliding; and for Northern Santos, divergent gliding at a large scale. The pattern in Southern Santos is complicated by right-lateral wrenching against the southern edge of the salt.

Strain heating and thermal softening in continental shear zones: a review

Abstract Strain heating results from the conversion of mechanical energy into heat during progressive deformation. As a physical phenomenon it is well known in fluid mechanics and has been studied theoretically and experimentally. This work has been extended by recent geophysical developments. Here we review (a) the fluid mechanics and geophysical work and (b) its application to continental shear zones. The models indicate that temperature rises of a few hundred degrees can be expected in major shear zones (transcurrent shear zones, the bases of thrust sheets, or the margins of large diapirs). In certain special situations (some thrust sheets and nappes), even larger rises are possible. The resulting temperature gradients should be detectable geologically, but evidence is scanty. The resulting thermal softening is sufficient to concentrate most of the deformation in narrow zones. Thus strain heating is an important crustal phenomenon which should be incorporated in models of large-scale tectonic processes. It may even contribute to local partial melting in some shear zones.

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.