Graham D. Williams

Glaciotectonized quaternary sediments at Dinas Dinlle, Gwynedd, North Wales, and their bearing on the style of deglaciation in the Eastern Irish Sea

Abstract The sedimentology and structure of Late Quaternary sediments exposed in a coastal section at Dinas Dinlle, North Wales, are described. The deposits from two small hills, the northern one being more prominent and containing northward dipping units of gravel and diamicton bounded by clay-lined thrust faults. The southern hill comprises complex folded sands and gravels overlying a basal till surface. High-resolution seismic surveys along the beach, 50 m west of the cliffs, show a roughly horizontal rockhead lying some 35 m below beach level, and demonstrate that thrust faults penetrate the entire Quaternary sequence to a detachment immediately above rockhead. Thrust units in the exposed cliff section show excellent correlation with the seismic sequence 50 m to the west, indicating a strike of 080°/260°. Balanced cross-section techniques were applied to an initial simple layered sequence comprising a lower (Irish Sea) till overlain by a coarsening upwards sand to gravel unit, followed by upper till (mixed lithology, including Welsh and northern provenance), and finally an upper gravel unit. Shortening of approximately 54% is estimated within the thrust sequence. It is concluded that the two isolated hills at Dinas Dinlle are erosional remnants of a formerly more extensive push moraine extending westwards into Caernarfon Bay which has now been truncated by coastal retreat. Deglaciation of the area was interrupted by ice sheet readvance towards 170° across a proglacial surface.

Tectonics and seismic sequence stratigraphy

A two day meeting convened by Graham Williams (University of Keele) and Angela Dobb (Mobil North Sea Ltd) was held at the Geological Society in London on 6 and 7 February 1991. The aim of the meeting was to address the problems of the interaction between eustacy, sediment supply rate and tectonics in seismically imaged basin sequences. The meeting discussed the importance of tectonics in generating recognizable sequence boundaries on both a regional and local scale. It was felt that eustacy as the dominant factor in controlling basin stratigraphy has been overplayed in the past. There were 190 participants at the conference representing the oil industry and academia. Some 21 oral presentations were given covering theoretical aspects of sequence stratigraphy, numerical modelling and field analogues. Case histories from Europe, North America, Africa and the UK were presented. Basin scale numerical modelling In the simplest case stratigraphy in extensional basins and passive margins may be subdivided into pre-, syn- and post-rift sequences representing stratigraphic fill related to a single rifting event. A widely recognized unconformity or megasequence boundary between syn- and post-rift sequences is referred to as the ‘breakup unconformity’. Numerical modelling on a basin scale in extensional regimes has been carried out by M. J. Newall and N. J. Kusznir (Shell Research, Liverpool) who concentrated on the post-rift subsidence history and palaeobathymetric evolution of Snorre, Magnus and other areas of the northern North Sea Basin. Subsidence resulting from thermal effects, compaction and sediment loading has been numerically modelled using flexural

Sedimentological response of an alluvial system to Neogene thrust tectonics, Atacama Desert, northern Chile

The Llano de la Paciencia is a thrust sheet top basin in which the sedimentological and topographic evolution can be linked to thrust tip propagation. It is an elongate gravel plain which borders the Salar de Atacama, a major intermontane basin in the Andean forearc of northern Chile. The Llano is bounded to the west by the Cerros de Purilactis a Cretaceous–Paleocene sequence uplifted by the Frontal Domeyko Thrust. The eastern margin of the Llano is formed by the Cordillera de la Sal which was uplifted by a linked back thrust-frontal thrust system. The Salar de Atacama is thus divided into a number of discrete sub-basins: the Llano de la Paciencia, the Pampa Visachita and the western sub-Llano which from east-west are bounded by the Cordillera de la Sal, the northern imbricates and the ignimbrite back thrust. Two phases of sedimentological evolution can be distinguished within the Llano on the basis of Quaternary to Recent sediment dispersal patterns. Initially, Phase 1 alluvial fan lobes prograded eastwards into the main Salar de Atacama basin. Subsequently, uplift of the Cordillera de la Sal deflected drainage systems southwards parallel to the structural strike. These Phase 2 alluvial deposits drain into the Salar de Atacama at the lateral termination of the Cordillera de la Sal frontal thrust. In places where thrust tip ramps are emergent within the Llano, gullies have been incised into the drainage pathways. This has resulted in the reworking of the early Phase 1 gravels and progradation of the Phase 2 fan lobes. The evolution of the Llano de la Paciencia illustrates the close link between topography and syntectonic alluvial drainage patterns in an arid intermontane basin.

Tectonic controls on stratigraphic evolution of the Adana Basin, Turkey

The Neogene Adana Basin of SE Turkey was initiated after thrust emplacement of the Tauride Belt to the north of the basin. Seismic reflection profiles across the southern part of the Adana Basin provide information on structural and stratigraphic evolution of the basin. Based on seismic character and the identification of unconformities, three megasequences and two mega sequence boundaries are identified. The first megasequence is partly coincident with an early Miocene rifting event and it comprises reefs that have grown on fault footwall crests with fore and back-reef sediments in intervening lows. Subsequent rapid flexurally induced subsidence generated an underfilled basin which was then passively filled by turbiditic sediments (megasequence 2) showing aggradational character on seismic profiles. Further extensional faulting and coupled sinistral wrench faulting took place after the deposition of megasequence 2. Erosional truncation of its upper part is followed by the deposition of continental and shallow marine sediments of megasequence 3 which shows localized progradational sequences characteristic of shallow water deposition. Further minor extensional faulting took place after the deposition of megasequence 3.

Displacement-distance methods in the analysis of fold-thrust structures and linked-fault systems

A new method for analysing the internal strain in thrust sheets is described. The displacement-distance diagram can be used to quantify the relative shortening (or relative stretch) between the hangingwall and footwall beds of a thrust and is a useful way of analysing commonly occurring fold-thrust structures. The stretch is a function of the relative rates of thrust propagation and thrust slip. The cumulative displacement-distance diagram described here can be used to calculate the absolute shortening or extension, when this is accommodated above a single décollement in a linked-fault system. This method offers a useful way of analysing duplex structures and large-scale, foreland thrust zones, as well as extensional fault systems and can be used as an alternative to, or in conjunction with, balanced cross-sections. Both methods can be used to quantify the variation in shortening above a single detachment zone; this variation may be mechanically significant, reflecting lithological control on thrust or normal fault growth.

Computer modelling and visualisation of the structural deformation caused by movement along geological faults

Abstract The computer modelling and visualisation of deformation caused by movement along faults has enhanced our understanding of the evolution of fault-related structures in the geological record. In particular, the development of computer software to carry out structural restoration and section balancing has provided earth scientists with an effective tool for validating structural interpretations constructed from geological and geophysical data. This paper describes both two- and three-dimensional geometric methods for modelling hanging wall deformation in response to fault movement. Equations are presented for the definition of two-dimensional fault geometries and for the determination of hanging wall geometry following movement over these faults. The Chevron and inclined shear constructions and fault-bend fold theory are described in a format to enable easy conversion into computer algorithms. The modelling of fault movement in three-dimensions is also considered in the context of the Chevron construction. Schematic models are presented which show hanging wall deformation caused by extensional, compressional and, most importantly, strike-slip movement over a complex fault surface. In addition, a new geometric technique for the restoration of deformed hanging wall surfaces is described. This technique has been called flexural flattening and involves flattening a surface represented as a mesh of triangles, back to horizontal. It has the advantages of maintaining the area of the surface before and after restoration and is relatively simple to apply in comparison to three-dimensional implementations of existing geometric methods.

Late Devensian glaciotectonic deformation at St Bees, Cumbria: a critical wedge model

Glaciotectonic deformation of the Late Devensian sediments exposed at St Bees, Cumbria is represented by minor extensional faults resulting from ice-sheet loading of the unconsolidated sediments accompanied by thrusting and folding. The highly deformed northern part of the section has numerous thrust faults associated with fold structures that verge predominantly to the SE, but with some NW-vergent (backthrust) structures. A high-resolution seismic reflection survey confirms that the thrust structures form a linked fault system that detaches at, or slightly above bed rock, which is here composed of Triassic, St Bees sandstones. In the less deformed southern part of the section, the seismic survey has imaged bedding with a southerly component of dip in the St Bees sandstone cut by steep north-dipping extensional faults which, to some extent, control rockhead topography. An integrated approach combining structural geology and high-resolution seismic reflection surveying has enabled the construction of a balanced cross-section which estimates a minimum of 115 m (22%) cumulative shortening due to glaciotectonic processes. Minor structures observed in the St Bees cliffs and larger structures interpreted from the seismic profile are compatible with a critical wedge model for deformation caused by an overlying thick ice wedge with a SE-dipping surface slope.

RESTORATION AND BALANCE OF COMPLEX FOLDED AND FAULTED ROCK VOLUMES : FLEXURAL FLATTENING, JIGSAW FITTING AND DECOMPACTION IN THREE DIMENSIONS

Abstract Techniques of two-dimensional bed length and cross-sectional area restoration are extended into true, three-dimensional (3-D) restorations using the preservation of areas of individual surfaces plus conservation of volume between surfaces. The flexural flattening technique involves the restoration of a complexly folded surface to a horizontal plane while conserving surface area and minimising finite strain. Multiple surfaces showing superimposed non-co-axial folding may be restored using the flexural flattening method applied to successively deeper surfaces. In the process of sequential restoration, volumes between the uppermost flattened surface and underlying surfaces are preserved and therefore the method is volume-balanced. The jigsaw fit of footwall and hangingwall cut-offs of each flattened surface, in map view, provides a unique restoration solution based on a unique translation and/or rotation of the hangingwall block. Flexural flattening and jigsaw fit performed sequentially on successively deeper surfaces in a three-dimensional model may incorporate removal of the uppermost layer and three-dimensional decompaction at each restoration step. The method is applied to a synthetic 3-D growth fault structure which has been produced using multiple discrete slip vectors and bulk simple shear hangingwall deformation.

North Chilean forearc tectonics and cenozoic plate kinematics

Abstract The continental forearc of northern Chile has been subjected to contemporaneous extension and compression. Here, cross-sections constructed across the forearc are presented which show that since initial shortening, deformation of the forearc has occurred in two tectonically distinct areas. These inner and outer forearc areas are separated by the strain discontinuity of the Atacama fault system and the tectonically neutral Central Depression. The outer forearc, the Coastal Cordillera, exhibits extensional tectonics, with large (up to 300 m) normal fault scarps preserved. These faults cut the earlier thrusts responsible for the elevation of Jurassic rocks at the coast above their regional elevation. The normal faults have been re-activated, displacing Quaternary salt deposits in the Salar Grande. This re-activation of the basement faults is probably due to the subduction of anomalously thick oceanic crust, producing an isostatic imbalance in the outer forearc. In the inner forearc, cross-sections through the Sierra del Medio and Cordillera de Domeyko show that structures of the Pre-Cordillera are best explained by a thick-skinned thrust system, with localized thin-skinned tectonics controlled by evaporite detachment horizons. Current forearc deformation features indicate a strong degree of correlation between subduction zone geometry and forearc tectonics. The timing of Cenozoic tectonism also fits well with established plate motion parameters, and the spatial and temporal variation in the state of stress of the forearc shows a close relationship throughout the Cenozoic to the plate kinematics and morphology of the subducting Nazca plate.

Flexural modelling of continental lithosphere deformation: a comparison of 2D and 3D techniques

Abstract The flexural isostatic response of the lithosphere in response to loading caused by continental tectonics has been modelled in 3D. The modelling approach used has been to determine hanging wall deformation following movement over a pre-defined fault surface. In addition, the lower crust is assumed to deform by a pure shear mechanism. The changes in crustal thickness resulting from these structural processes impose loads upon the lithosphere, which responds by isostatic adjustment. Algorithms have been developed to quantify the flexural isostatic response to these loads in 3D. These deflections are then superimposed upon the results from the structural modelling to generate isostatically compensated hanging wall, footwall and fault surfaces. Schematic models are presented for extensional, compressional and strike-slip deformation. Model results are dependent upon the interaction between fault geometry, displacement along the fault, which can be varied along strike, and the methodology used to quantify the flexural response of the lithosphere to loading. Emphasis has been placed upon contrasting models, which include a structural component only with those that incorporate both structural and flexural isostatic processes. Following extension, structural processes generate a relatively deep half-graben with no deformation at the basin edges. Isostatic compensation modifies this structure to produce a relatively shallow, but variable, basin depth with uplift (typically between 1 and 2 km) experienced at the basin edges. Compressional models have been generated which show the formation of large uplift structures, which are modified by isostatic compensation so that they are considerably reduced in magnitude. A regional depression (i.e. foreland basin) is also generated adjacent to the remaining uplift. Both 2D and 3D implementations of flexural isostasy have been investigated to provide insights into the validity of results provided by commonly applied 2D methods. A major advantage arising from 3D tectonic modelling is the ability to investigate the effects of oblique or entirely strike-slip components of fault movement. Strike-slip deformation has been modelled in the context of a single fault surface, which varies along strike, to show the development of pressure ridge and pull-apart basin structures. The isostatic compensation of these structures shows complex patterns of uplift and subsidence due to the interference of negative and positive loading and associated flexural deflections.