Basin Research - Volume 7, Issue 3, 1994

Tectonic subsidence of thermally generated basins is sensitive to the insulating effect of sediment. Compacting sediment reduces thermal subsidence, increases apparent stretching factors and reduces uncertainty in estimates of the breakup age. The transient effect of sediment insulation on the shape of the subsidence curve is considered by comparing model results with an exponential fit from 16 to 144 Myr after breakup. Misfits are dependent on the model parameters used, the degree of stretching, the degree of sediment compaction and the bottom boundary condition used in modelling. The magnitude of the misfit ranges up to 90 m (uncorrected for eustatic loading). These effects may alter the interpretation of backstripping results. Application to a data set from the Cambro‐Ordovician miogeocline of the Great Basin, western USA, increases apparent stretching factors and reduces uncertainty in the predicted earliest Cambrian breakup age. In this case the misfits to exponential subsidence are quite large (≅300 m) so that correction for the insulating effect of sediment does not eliminate a probable eustatic signal consistent with the Sauk sequence. If a eustatic signal is assumed, correction for model error suggests that the thermal parameters used are an improvement over those previously adopted and that the base of the lithosphere thins as sediments are added at the surface.

The formation of the Songpan‐Garzê Fold Belt and the initiation of the terrestrial Sichuan Basin are related to closing of the Palaeo‐Tethys during the Late Triassic Indosinian orogeny. The Songpan‐Garzê Fold Belt is composed of Triassic (T1‐‐T²3) turbiditic deposits and Palaeozoic greywacke‐shale, whereas the Sichuan Basin consists of Sinian to middle Upper Triassic (T²3) platform carbonates and Upper Triassic (T3X to Quaternary terrestrial elastics. Three principal deformation episodes during the Late Triassic (Norian to Rhaetian) were progressively localized towards the south‐eastern margin of the fold belt. D1 was a SW‐directed shortening event, related to continuous subduction of the Palaeo‐Tethys, and produced NW‐trending structures. Differential strain between the fold belt and the Sichuan Basin was accommodated by sinistral shearing along a NE‐trending transitional zone during D2^. D3 SE‐directed compression was the result of collision between the Cimmerian and Eurasian Continents and initiated the Longmen Mountains Thrust‐Nappe Belt and terrestrial Sichuan Basin. Post‐D3 deformation, related to SE‐directed thrusting in the Longmen Mountains, then propagated from hinterland to foreland. The Indosinian orogeny closed the Palaeo‐Tethys and terminated the marine conditions that dominated the early evolution of the intracratonic Sichuan Basin. Tectonic loading from the exhumed fold belt and Thrust‐Nappe Belt induced substantial subsidence in the Sichuan Basin, especially in the Western Sichuan Foreland Basin, resulting in the deposition of a terrestrial clastic sequence during Late Triassic (T3X to Quaternary times. The foreland basin history comprises an early stage during the Late Triassic (T3x^1–2), an over‐fill stage during the latest Triassic to Early Cretaceous (T3X³‐ K1J), and a shrinking stage from the Late Cretaceous to the Quaternary (K2J‐Q). These can be correlated with tectonic events in the Thrust‐Nappe Belt.

We present results of three sand‐box experiments that model the association between tectonic accretion and sedimentation in a forearc basin. Experimental sedimentation occurs step by step in the forearc basin during shortening of the sand wedge.

In each experiment, the development of the accretionary wedge leads to the formation of a major backthrust zone. This major deformation zone accounts for the thickening in the rear part of the wedge. In natural settings this tectonic bulge dams sediments that are transported toward the trench from mountainous terrain behind the forearc.

We test the variation of friction along the déollement and note the following: (1) shortening of a low‐friction wedge involves a mechanical balance between forethrusts and backthrust propagation and this balance is recorded by the sedimentary sequence trapped in the forearc basin. Indeed, if most of the movement occurs along the backthrust, the deepening of the basin will be larger and consequently the thickness of the sedimentary sequence will be greater. (2) Such balance does not exist in the case of a high‐friction wedge. (3) Variation of friction along the décollement during shortening of the sand wedge leads to modification in the forearc basin filling. Thus, for similar increments of convergence, the sequence deposited in the forearc basin shows relatively larger thickness when the wedge is shortened above a high‐friction décollement.

We suggest that contraction and thickening in the rear part of the wedge is an efficient mechanism to, initiate and develop a forearc basin. Thus, this kind of basin occurs in convergent settings, without collapse related to local extension or tectonic erosion. They represent a sedimentary trap on a passive basement, bounded by a tectonic bulge.

The Quaternary Hikurangi forearc basin, southeast of the North Island of New Zealand, is bounded by two actively uplifting ridges. Thus, this basin is considered to be a possible example of the basins modelled in our experiments, and we suggest that the limit between the basin and the wedge could be a complex backthrust zone.

The Northern Death Valley fault zone is a major right‐lateral structure that has accommodated 70 km or more of regional transtensional deformation in Tertiary to Recent time. Extension parallel to its north‐west transport direction in the Death Valley region of California has produced ‘pull‐apart’ structures that are responsible for opening the central Death Valley rhombochasm. In several ranges along the length of the Northern Death Valley fault zone, there is also evidence for extension directed to the south‐west, normal to strike‐slip movement. Evidence from the Funeral, Grapevine and Cottonwood Mountains suggests that a significant amount of down‐dip slip has occurred on the Northern Death Valley fault zone and parallel structures (together referred to as the Northern Death Valley fault system) coeval with the majority of right‐lateral slip and transform‐parallel extension. As a result of both these components of extension, a separate basin opened in northern Death Valley with an orientation and architecture very different from that of central Death Valley. In addition, the Northern Death Valley fault system may be responsible for the present topography of the Funeral and Grapevine Mountains. Transform‐normal extension appears to be the result of a misorientation of the Northern Death Valley fault zone within the regional stress field over the past 6 Myr, as suggested by simple geometric calculations.

Book reviewed in this article:

Sequence Stratigraphy and Facies Associations Posamentier et al. (Eds)