Derek J. Blundell

A dual doubly vergent orogen in the Banda Arc continent-arc collision zone as observed on deep seismic reflection profiles

New deep seismic reflection profiles across the Banda Arc of Indonesia reveal reflectors in the uppermost 50 km of the lithosphere. Combined with existing earthquake hypocenter locations and focal mechanisms, the new structural geometries inferred from the reflectors yield a more complete analysis of deformation during the past 10 m.y. and provide insights into how strain is partitioned across an orogenic belt in which volcanic arcs and continents converge. A clearly defined Wadati-Benioff zone, and recent deformation of shelf sediments observed on older shallow seismic profiles, indicated to previous workers that substantial convergence occurred at the Timor Trough. A few focal mechanisms, seafloor escarpments, and recent geodetic surveying indicate that convergence at ∼7 cm yr−1 currently (last 200 kyr?) occurs at the northern margin of the now inactive volcanic arc, the Wetar Thrust zone. Reflectors on the new seismic profiles are interpreted as thrust faults and folds that occur throughout the crust and within the uppermost mantle between the Timor Trough and Wetar Thrust. Specifically, basement reflectors beneath the toe of the accretionary complex have reverse-sense offsets that imply blind thrusts. The whole crust is horizontally shortened, not only the sedimentary cover rocks that previously deformed into duplexes above a decollement. Reflectors dipping away from both margins of the forearc basin and at the northern margin of the volcanic arc are interpreted as evidence of thrusting. Thus each arc represents a doubly vergent fold-and-thrust belt, but only the northern one is currently active. Crustal thicknesses inferred from seismic velocities, reflectors, and gravity anomalies are consistent with the merging of a thinned continental shelf margin with oceanic lithosphere to form an orogenic belt with at present 3–4 km of topographic relief in the region of eastern Timor.

Salt tectonics: some aspects of deformation mechanics

This volume is dedicated to studies of the deformation of evaporite rocks in pillows and diapirs, and the surrounding sedimentary overburden rocks and sediments. Salt diapirs have become the focus of attention in the last forty years, because of their strategic importance in controlling hydrocarbon reserves, and their unique physical properties enable storage of hydrocarbons and toxic waste. Their economic importance is unique on the Earth’s surface, as evaporites in the Middle East are responsible for trapping up to 60% of its hydrocarbon reserves (Edgell). Salt also produces some of the most complex and beautiful deformation features on the Earth’s surface, although few of these surface exposures have been examined in detail. The first section of this volume consists of analyses of outcrop, cave, mine and borehole information which add to our general understanding of the internal diapir deformation patterns and overburden tectonics. This is followed by papers mainly based on seismic reflection profile interpretation, which provide accurate documentation of salt tectonics in the NW Europe Zechstein Basin (Buchanan et al., Stewart et al., Zirngast), Persian Gulf (Edgell), and the Angolan Margin (Spathopoulos). Salt diapirism is principally controlled by the rheology of the overburden rocks and physical models have given us important new insights into the deformation of overburden. Deformation experiments are presented on overburden around salt structures (Alsop), salt sheet segmentation (Koyi), and development of giant counter-regional faults on the Brazilian continental margin (Szatmari et al.). Petersen & Lerche investigate the influence of salt sheets and diapirs on thermal

Physical modelling of overburden deformation around salt diapirs

Abstract Salt diapirs produce highly complex deformation patterns in the surrounding overburden which are difficult to image seismically or model numerically. To further the understanding of deformation around salt structures we have used physical models with brittle granular overburdens, and simulated sedimentation accompanying diapiric rise. Rigid linear indentors produce a crestal horst above the indentor with two flanking graben. Large amounts of indentation produce predominantly reverse faults along the margins of the diapir and extensional faults over the crest. In contrast hemispherical indentors produced inward-dipping concentric reverse faults. Forefully intruded polymer diapirs produce a narrow crestal graben above the diapir crest. Downbuilt polymer diapirs produce extensional faults on the flanks of the diapirs which offset the diapir walls. No clear secondary rim synclines were developed as polymer is withdrawn evenly from the whole of the polymer layer across the model. Strong and anisotropic layered overburden were simulated using cohesive clay and mica respectively. Detachments were formed along mica layers which suppressed faults with large throws. Cohesive layer deformation began with tensile fractures, permitting rotation of fractured rafts around the sides of the diapir. These progressively slip off the crest and create large gaps in the stratigraphy.

Cenozoic inversion and uplift of southern Britain

Abstract Whereas significant exhumation of northern Britain took place during Paleocene time, probably as a consequence of uplift caused by a mantle plume, Paleogene basin inversion and uplift in southern Britain appears to be a consequence of Alpine tectonism. Recent publications demonstrate that inversion of Mesozoic basins across southern Britain was accompanied by subsidence of flanking basins in areas that had previously remained stable. Structures observed on seismic sections across the Weald Basin in SE England reveal that inversion occurred locally by north-directed reverse movements on pre-existing normal faults that cut down at a low angle deep into the basement. The overall effect of inversion of the Weald Basin, however, is a bulk deformation that produced a domal uplift, flanked by subsidence of the London and Hampshire-Dieppe basins. A 2D finite element thermomechanical model of continental lithosphere containing a region of reduced strength in the crust simulates Jurassic-Early Cretaceous extension to form the Weald Basin, followed by compression during the Tertiary to produce its inversion and the flanking basins. The timing of tectonic events across southern Britain correlates with times when Alpine stresses were transmitted into the foreland to the north sufficiently well to link them. Through most of Tertiary time, the landscape of southern England was of relatively low elevation and low-energy surface processes. However, late Neogene uplift, generally greater in the west, appears to have been part of a larger-scale uplift of land areas with hard rock at surface, which has no obvious tectonic explanation.

Synthetic seismic reflection profile through the Ivrea zone–Serie dei Laghi continental crustal section, northwestern Italy

A geologic cross section, restored to its original horizontal orientation in Permian-Triassic time, has been constructed for the middle and lower continental crustal rocks of the Ivrea-Verbano zone and the adjacent Serie dei Laghi of northwestern Italy. Seismic P-wave velocities of a representative suite of rock samples were measured to high-pressure and high-temperature conditions. A synthetic seismic reflection profile, ∼76 km long and 30 km thick, was computed to compare what can be deduced from the seismic profile with what is known in much more detail from geologic mapping. Imaged features correspond closely to those seen on many present-day profiles, and the broad features of the tectonic evolution would be correctly interpreted, but important recumbent fold structures would be missed, and relationships between intrusive bodies and their country rocks would be unclear.

DEEP SEISMIC STRUCTURE AND THERMO-MECHANICAL MODELLING OF CONTINENTAL COLLISION ZONES

Abstract A series of 2D thermo-mechanical models are presented which quantify crustal thickening and the resulting temperature perturbations associated with thrusting in an attempt to constrain the existence of temperatures responsible for the generation of anatectic granites in a compressional environment. Deformation is assumed to occur in the crust by a simple thrusting mechanism, the Moho acting as a detachment surface, as has been observed on deep seismic reflection data, while the more ductile sub-crustal lithosphere is assumed to behave elastically and deform in a pure shear manner. Investigated are the relative effects of the rate of crustal shortening, the distribution of radioactive elements within the crust, the reduced heat flow from the mantle, isostasy and erosion. Our results show that previous oversimplified 1D models significantly overestimate the elevated temperatures in lower crustal regions caused by radiogenic heating, and that this mechanism will only have a significant effect in regions of relatively thin lithosphere, or where there are unusually high rates of radiogenic heating in the crust. Generation of synthetic P-T-t data for locations deep down in the lower crust enables us to quantify the temperatures produced following a period of compressional deformation, providing insight into the time scales involved for the generation of crustal melt granites within an orogenic belt.

The timing and location of major ore deposits in an evolving orogen: the geodynamic context

Abstract Although it is possible to identify the potential controls on mineralization, the problem remains to identify the critical factors. Very large mineral deposits are rare occurrences in the geological record and are likely to have resulted from the combination of an unusual set of circumstances. When attempting to understand the mineralization processes that occurred to form a major ore deposit in the geological past, especially the reasons why the deposit formed at a particular time and location within an evolving orogenic system, it is instructive to look at mineralization in modern, active subduction complexes. There it is possible to measure and quantify the rates at which both tectonic and mineralizing processes occur. In a complex subduction system, regions of extension develop. For example, subduction hinge retreat is a process that creates extension and generates heat from the upwelling of hot asthenosphere ahead of the retreating slab, producing partial melting, magmatism and associated mineralization. Seismic tomography not only images mantle as it is now, but subduction slab anomalies can be interpreted in terms of the past history of subduction. This can be used to test tectonic plate reconstructions. Tectonic and magmatic events occur rapidly and are of short duration so that many are ephemeral and will not be preserved. Furthermore, they can be diachronous as is the case with the lithospheric slab tear clockwise around the Carpathian Arc during the Neogene. If the tectonic setting is paramount in determining the onset of the mineralization process and generation of mineralizing fluids, the fluid transport system that localizes the mineralization in space and time and concentrates the metal charge is the key to finding when and where the ore deposits occur. Fault and fracture networks in the crust provide various mechanisms for the localized expulsion of fluid in pulses of short duration. Excess surface water flow following large earthquakes in the Basin and Range region of USA offers a modern analogue to quantify fluid flow related to extensional faulting. Evidence from the Woodlark basin, east of Papua New Guinea, suggests that similar conditions pertain in the oceanic environment. Whilst there are limits to the use of regions of active tectonism as modern analogues to explain the mineralization of ancient orogenic systems, they do provide the best opportunity to understand the mechanisms of mineral processes and the controls on the location and timing of major ore deposits.

Seismic Modeling of lower and mid-crustal structure as exemplified by the Massiccio Dei Laghi (Ivrea-Verbano zone and Serie Dei Laghi) crustal section, northwestern Italy

An aim of seismic reflection profiling of the present-day deep continental crust is to infer the geologic structure and hence to infer the tectonic processes that led to the formation of that structure. An important test of the validity of such interpretations lies in comparison with exposed geologic sections that once lay at deep crustal levels. The Massiccio dei Laghi (Lakes Massif) of Northern Italy provides such a section, on a scale comparable with that of contemporary deep seismic profiles, yet illustrates heterogeneity on a wide range of scales. On account of its importance as providing a substantial composite cross section through continental crustal rocks, coupled with its relative accessibility in the inner arc of the Western Alps, the region has been subjected to an extraordinary degree of structural, petrological, geochemical and petrophysical study by geoscientists during the past few decades. The rocks of the region record Paleozoic accretion, metamorphic and magmatic processes, the effects of the Hercynian orogeny,post-orogenic magmatic underplating and associated lithospheric stretching and thinning, Mesozoic extension and effects associated with the position of the region in Alpine tectonism (see review by Handy et al., 1999). The assembly of the rock units probably dates from Permo-Triassic time onwards.Their relative positions in outcrop are likely close to how they that might be taken as a model for a magmatically underplated and extended crustal section. Such a section can be compared with interpretations drawn from present-day seismic reflection profiles taken from regions of extended lithosphere that have not had the misfortune(good fortune?) to be upended and exposed during a subsequent phase of collisional orogeny.