Basin Research - Volume 10, Issue 3, 1998

This study constitutes a comprehensive attempt to interpret the multistoreyed benchland scenery of the South Indian shield. In South India, two rifted margins (and now competing base levels), a low‐elevation margin facing the Bay of Bengal, in existence since ≈130 Ma, and a younger (65 Ma) high‐elevation margin facing the Arabian Sea, together with a high‐standing backbone of extremely resistant granulitic rocks known as charnockites, have influenced the long‐term erosional response to rock uplift and to the eustatic shift of shorelines in a complex way.

Several tiers of flat‐lying, denudational topographic surfaces, which are indifferent to stratification, schistosity or foliation and highlighted by a characteristic weathering mantle, are identified and the conditions of their genesis discussed. General definitions and assumptions concerning the evolution of planation surfaces are presented. A major key issue is the nature of crustal uplift in cratonic regions: its relation to global tectonics, its role in the redistribution in time and space of available relief and in the deformation of pre‐existing planation surfaces, and its bearing on denudational signals which may be recorded both by isotopic measurement techniques and offshore sedimentary sequences. The proposed denudation chronology considers all these aspects, and the analysis sets out to establish a thread of geodynamic events that can account for the observed morphology. Drainage, weathering and crustal uplift patterns, geological structures and chronostratigraphic data are examined in relation to their possible connections with the migration of drainage divides and surface lowering patterns. The high number of degrees of freedom involved when considering the long‐term interplay between gravity‐driven and radiation‐driven denudation systems is one of the many problems facing this analysis, and is extensively addressed in the discussion.

Our study explores the geohydraulic history of the Acre retroarc foreland basin by gathering both spatial and temporal information from the upper 400 m of sediments. We also inquire into controls on sediment accommodation space as well as on stream vs. lacustrine domination.

The Acre basin is located in south‐west Amazonia, proximal to the Serra do Divisor which demarcates the eastern edge of the Andean fold–thrust belt. Radiocarbon ages from a range of materials indicate that the upper 50–250 m of the Solimôes Formation accumulated during the past 50 000 years. Both surficial and drill‐core sediment records show lacustrine–fluvial transitions throughout the Late Quaternary. These shifts in depositional environments are in response to episodic changes in hydrological conditions as well as to geodynamic activity, such as subsidence. Juxtaposition of lacustrine and fluvial systems in the vertical Acre basin record mimics the regional‐scale trends in the modern, upper and middle Solimôes–Amazon floodplains.

In the Acre basin record lacustrine successions are characterized by increasing calcium contents up‐section. This is also manifested, in the upper portions of lacustrine sequences outcropping at the surface, as alternating clastic and calcareous layers. The up‐section increase in carbonate content is related to increasing salinities brought about by drier hydrodynamic conditions. Desiccation cracks are typically infilled with gypsum as are cavities of fossils in bone‐beds. The latter represent isolated ponds in which the original fauna died as aridity intensified and waters became increasingly saline.

Modern trunk river systems in the Acre basin flow from south‐west to north‐east with tributaries entering from the south‐west, suggesting the influence of a domino‐style, basement, fault regime. Fault or, at least, fracture control on stream channels is also suggested throughout the greater Amazon basin in the orthogonal dispositions and asymmetric terrace systems of trunk rivers as well as of major tributaries.

The suture between two West Mediterranean crustal blocks once situated several hundreds of kilometres apart can be studied in the Vélez Rubio Corridor – Espuña area of the Eastern Betic Cordilleras. This suture, or Internal–External Zone Boundary, separates the former passive southern margin of Iberia (the External Zone) from a stack of allochthonous nappe complexes (the Internal Zone), of which the highest unit is formed by the weakly or nonmetamorphosed Malaguide Complex. Analysis of the Oligocene to middle Miocene sediments of the Vélez Rubio Corridor and the Espuña, and comparison with coeval deposits elsewhere in the Western Mediterranean shows that (a) up to the middle Miocene, the southern part of the External Zone (Southern Subbetic) was positioned some 100 km more eastward; (b) up to the early Aquitanian, the Malaguide Complex, forming part of the South Sardinian block (the southern section of a West Mediterranean continental segment) was juxtaposed to the North Sardinian block (the northern part of that continental fragment), some 400 km more eastward; (c) West European extensional rifting during the late Oligocene to earliest Aquitanian resulted in deposition of rift valley sediments (Ciudad Granada and Pliego Formations) in the Malaguide realm; (d) during the Aquitanian, the West Mediterranean segment disintegrated and the West Mediterranean oceanic basins opened, resulting in, for example, the south‐westward drift of the Internal Zone, with concomitant thrusting and thinning and deposition of submarine fans (Solana‐Algeciras Formation) along the margin; (e) in the early Burdigalian, the allochthonous Internal Zone collided with the Iberian margin, causing the disruption of the platform‐slope configuration of the External Zone; (f) after the collision a deep basin was formed upon the suture filled in with erosional products from both Internal and External Zones (Espejos–Viñuelas–Millanas Formations); (g) a strong compressive event in the late Burdigalian caused the southward thrusting of the Subbetic over the Espejos Formation, thus double‐sealing the collisional contact; (h) in the latest Burdigalian to Langhian, new strongly subsiding basins were formed in the Western Mediterranean, e.g. along the Internal–External Zone Boundary; (i) dextral strike‐slip faulting in the Serravallian resulted in a westward displacement of over 100 km of the southern Subbetic plus Internal Zone; (j) onset of a new pattern of strike‐slip faulting induced the formation of a new suite of basins in the Tortonian.

The Hexi Corridor is a Cenozoic foreland basin system at the north‐eastern margin of the Qinghai–Tibet Plateau. Active faults, having clear geomorphological expression, are mainly NW‐trending reverse‐sinistral faults and NNW‐trending reverse‐dextral faults. From the south‐west to the north‐east, landforms can be divided into three parallel zones arranged along the WNW faults. Longitudinally, three transverse uplifted blocks divide the Hexi Corridor into four subbasins. North‐eastward motion of the Qinghai–Tibet Plateau and eastward motion of the north‐east part of the Plateau resulted in compressive and shear deformations and controlled the geomorphological pattern in the Hexi Corridor.

Fission track thermogeochronology using detrital apatite and zircon from a synorogenic foreland basin on the northern margin of the Betic Cordillera Internal Zone is used to reconstruct the cooling and unroofing history of the sediment source areas in the Oligo‐Miocene mountain belt. Previously, a heavy mineral study on the same sedimentary rocks showed that progressively deeper tectonometamorphic units were being unroofed during the latest Oligocene to middle Miocene at a minimum rate of 3 km Myr⁻¹. The fission track data have further constrained the exhumation history showing that the structurally highest (i.e. shallowest) parts of the mountain belt (Malaguide Complex) cooled relatively slowly during the latest Oligocene–Aquitanian, while the deeper metamorphic units (Alpujarride Complex) cooled at much higher rates (up to 300 °C Myr⁻¹) during the Burdigalian–Langhian. These fast cooling rates from synorogenic detritus are consistent with cooling rates calculated previously for the deeper parts of the early Miocene orogenic belt, using ³⁹Ar–⁴⁰Ar dating of muscovite, biotite and amphibole from basement metamorphic rocks. Rapid cooling in the early Miocene, which commenced at ≈21 Ma, is attributed to the change in process from erosional to tectonic denudation by orogen‐scale extension within the eastern Betic Cordillera.

Many studies of critical wedges treat the interior of the wedge as continuous and do not address the manner in which it grows from the undeformed state to a typical imbricate wedge. In this paper we present a 2D kinematic–mechanical model which attempts to explain the development of a critical wedge in a fold and thrust belt in terms of both gravitational and frictional work. In the undeformed model a series of thrust faults are defined which have the potential to take up an external displacement. The active fault at a given time is that which minimizes gravitational and frictional work as a result of displacement. Displacement on the active fault causes a change in topography and deformation of other faults which may favour an alternative fault at the next time step. The model is a mixed Lagrangian–Eulerian scheme in which the upper surface, in addition to being deformed, is also subject to erosion, transport and sedimentation.

The model predicts propagation of thrust fault activity towards the foreland through time as a result of increasing topographic (gravitational) loads and frictional work on deformed hinterland faults. As the zone of fault activity progresses through the developing critical wedge several faults are active over time‐scales of ≈1 Myr. However, a simple chronology or sequence of fault activity cannot be assumed as out‐of‐sequence thrusting occurs during this overall foreland propagation. The detailed spatial and temporal activity of faults is complex and reflects the interaction between the development of topography, the contrast between basal (décollement) and internal coefficients of friction and the effects of erosion and sedimentation. In particular, rates of erosion and sedimentation are found to be important controls on fault activity both spatially and temporally. Erosion, by locally removing topography above a fault, reduces gravitational and frictional work enabling continued fault activity or reactivation. Sedimentation, conversely, acts to increase gravitational and frictional work on a fault, and therefore has the potential to blanket faults and render them inactive. Model results illustrate the complex feedbacks that can exist between tectonic and surficial mass transport processes.