Basin Research - Volume 11, Issue 2, 1999

The Vulcan Sub‐basin, located in the Timor Sea, north‐west Australia, developed during the Late Jurassic extension which ultimately led to Gondwanan plate breakup and the development of the present‐day passive continental margin. This paper describes the evolution of upper crustal extension and the development of Late Jurassic depocentres in this subbasin, via the use of forward modelling techniques. The results suggest that a lateral variation in structural style exists. The south of the basin is characterized by relatively large, discrete normal faults which have generated deep sub‐basins, whereas more distributed, small‐scale faulting further north reflects a collapse of the early basin margin, with the development of a broader, ‘sagged’ basin geometry. By combining forward and reverse modelling techniques, the degree of associated lithosphere stretching can be quantified. Upper crustal faulting, which represents up to 10% extension, is not balanced by extension in the deeper, ductile lithosphere; the magnitude of this deeper extension is evidenced by the amount of post‐Valanginian thermal subsidence. Reverse modelling shows that the lithosphere stretching

factor has a magnitude of up to β=1.55 in the southern Vulcan Sub‐basin, decreasing to β=1.2 in the northern Vulcan Sub‐basin. It is proposed that during plate breakup, deformation in the Vulcan Sub‐basin consisted of depth‐dependent lithosphere extension. This additional component of lower crustal and lithosphere stretching is considered to reflect long‐wavelength partitioning of strain associated with continental breakup, which may have extended 300–500 km landward of the continent–ocean boundary.

Cementation of quartzose sandstones is modelled assuming that the main source of silica is quartz dissolved at stylolites. The cementation process is shown to operate in one of two different regimes depending on the Damköhler number for diffusion. The regime, where diffusion of silica from the stylolites is a faster process than precipitation, is characterized by a nearly constant supersaturation between the stylolites. This regime, which spans the depth interval of quartz cementation for close stylolites, allows for approximate analytical expressions for the porosity evolution as a function of time and temperature. An expression is derived for the temperature where half the initial porosity is lost during constant burial along a constant thermal gradient. This expression is used to study the sensitivity of all parameters which enter the cementation process. The cementation process is shown to be particularly sensitive to the activation energy for quartz dissolution. The expression for the porosity decrease under constant burial is generalized to any piecewise linear burial and temperature history. The influence of the burial histories on the cementation process is then studied.

A basin evolution synthesis and an integration of geological and geophysical data, relevant as guides to the exploration of gold and lead–zinc deposits in the Itajaí Basin, are presented in this paper. The Itajaí Basin is interpreted as a collision‐related foreland basin consisting of weakly metamorphosed sediments deposited between the structural front of the Dom Feliciano fold and thrust belt and the proximal flank of the cratonic forebulge. Its sediments represent a second‐order depositional sequence deposited during a foreland transgression–regression cycle related to flexural subsidence. After deposition, the basin underwent a main late‐collisional compressional deformation phase followed by an extensional post‐orogenic relaxation. Known gold and lead–zinc deposits are associated with late‐orogenic faulting of the Itajaí Basin sediments. The gold‐bearing quartz veins are of filonean hydrothermal affiliation, while the lead–zinc deposits were formed by solution‐remobilization in a meteoric–connate–magmatic mineralizing fluid. Major trends of favourability for such deposits are recognized. The most favourable sites for lead–zinc deposits are near known mineralized areas and also along a NE‐orientated fault at the margin adjacent to the Dom Feliciano metamorphic belt. The higher favourability for gold deposits is assigned to an area along the same NE trend, and also around a small known deposit near the cratonic margin.

We examine the basin geometry and sedimentary patterns in the Muddy Creek half graben of south‐west Montana by integrating geological mapping, structural and basin analysis, ⁴⁰Ar/³⁹Ar geochronology, biostratigraphy and reflection seismic data. The half graben formed in late Middle Eocene to early Oligocene (?) time at the breakaway of a regional, WSW‐dipping detachment system. Although the structure of the half graben is that of a supradetachment basin, facies patterns and basin architecture do not conform to a recent model for extensional basins above detachment faults. The border fault, the Muddy Creek fault system, consists of three en echelon, left‐stepping normal faults separated by two relay ramps. The fault steepens southward toward each en echelon step, ranges in dip from 8 to 60° near the surface, but flattens at depths between 0 and 3 km. A broad ENE‐plunging displacement‐gradient syncline defines the central part of the half graben and is flanked by narrow SE‐and NE‐plunging anticlines to the north and south. Fine‐grained deposits of the syntectonic basin‐fill are thickest in the central syncline and interfinger with footwall‐derived conglomerate near the adjacent anticlines. These facies patterns suggest that folding was coeval with extension and sedimentation in the half graben. Pre‐extensional volcanic rocks and interbedded conglomerate filled a major ESE‐trending palaeovalley along the future axis of the Muddy Creek half graben. Synextensional sedimentary deposits include lacustrine and paludal shale, mudstone and sandstone ponded in the centre of the half graben, and a narrow (typically <1.5 km wide) fringe of coarse alluvial‐fan and fan‐delta conglomerate and sandstone derived from the footwall. Angular unconformities and rock‐slide deposits occur only locally within the syntectonic sequence. These facies patterns agree well with the half‐graben depositional model of Leeder & Gawthorpe but not with a more recent supradetachment basin model of Friedmann & Burbank despite the demonstrably low dip‐angle of the basin‐bounding normal fault. These data show that it may not be possible to differentiate between supradetachment basins and half graben with steeper border faults using the architecture of the associated basin‐fill deposits.

Although fault growth is an important control on drainage development in modern rifts, such links are difficult to establish in ancient basins. To understand how the growth and interaction of normal fault segments controls stratigraphic patterns, we investigate the response of a coarse‐grained delta system to evolution of a fault array in a Miocene half‐graben basin, Suez rift. The early Miocene Alaqa delta complex comprises a vertically stacked set of footwall‐sourced Gilbert deltas located in the immediate hangingwall of the rift border fault, adjacent to a major intrabasinal relay zone. Sedimentological and stratigraphic studies, in combination with structural analysis of the basin‐bounding fault system, permit reconstruction of the architecture, dispersal patterns and evolution of proximal Gilbert delta systems in relation to the growth and interaction of normal fault segments. Structural geometries demonstrate that fault‐related folds developed along the basin margin above upward and laterally propagating normal faults during the early stages of extension. Palaeocurrent data indicate that the delta complex formed a point‐sourced depositional system developed at the intersection of two normal fault segments. Gilbert deltas prograded transverse into the basin and laterally parallel to faults. Development of the transverse delta complex is proposed to be a function of its location adjacent to an evolving zone of fault overlap, together with focusing of dispersal between adjacent fault segments growing towards each other. Growth strata onlap and converge onto the monoclinal fold limbs indicating that these structures formed evolving structural topography. During fold growth, Gilbert deltas prograded across the deforming fold surface, became progressively rotated and incorporated into fold limbs. Spatial variability of facies architecture is linked to along‐strike variation in the style of fault/fold growth, and in particular variation in rates of crestal uplift and fold limb rotation. Our results clearly show that the growth and linkage of fault segments during fault array evolution has a fundamental control on patterns of sediment dispersal in rift basins.