Basin Research - Volume 13, Issue 2, 2001

ABSTRACT The mechanisms driving subsidence in late orogenic basins are often not easily resolved on account of later fault reactivation and a rapidly changing stress field. Contained turbidites in such basins provide a unique opportunity of monitoring sea bed deformation and evolving bathymetry and hence patterns of subsidence during basin filling. A variety of interpretations have been proposed to explain subsidence in Neogene basins in SE Spain, including extensional, strike‐slip and thrust top mechanisms. Ponded turbidite sheets on the floor of the Neogene Sorbas Basin (SE Spain) were deposited by sand‐bearing currents which ran into enclosed bathymetric deeps where they underwent rapid suspension collapse. The structure and distribution of these sheets (and the thick mudstone caps which overlie them) act as a proxy for the containing sea bed bathymetry at the time of deposition. An analysis of the sheet architecture helps identify a trough‐axial zone of syndepositional faulting and reveals a westwards stepping of the ponding depocentre with time. Fault breaks at the sea bed influenced the position of flow arrest and the distribution of sandstone beds on the basin floor. Westward stepping of the deeper bathymetry was episodic and probably controlled by transverse faults. Re‐locations of the depocentre were accompanied by the destabilization of carbonate sand stores on the margins of the basin, resulting in the repeated emplacement of large‐volume carbonate megabeds and calciturbidites. The fill to the Sorbas Basin was shingled by the onset of compression in the east attributed to transfer of slip between intersecting strike‐slip fault strands. A sinistral fault (a splay of the Carboneras Fault System) propagated through the evolving basin fill from the east as the eastern part of the basin became inverted and the locus of subsidence migrated into the Tabernas area 20 km area to the west. The sedimentological analysis of the basin fill helps see through a late dextral overprint which ultimately juxtaposed basement rocks to the south against the inverted and upended basin, along a late slip‐modified unconformity. Conventional palaeostress analysis of fractures along the basin margin fails to see past this late dextral shearing event. Basin migration parallel to the E–W‐orientated basin axis, slip‐reversal (sinistral to dextral) and the active involvement of strike‐slip faults are now identified as important aspects of the evolution of the Sorbas Basin during the latestTortonian.

ABSTRACT Geological mapping and sedimentological investigations in the Guilin region, South China, have revealed a spindle‐ to rhomb‐shaped basin filled with Devonian shallow‐ to deep‐water carbonates. This Yangshuo Basin is interpreted as a pull‐apart basin created through secondary, synthetic strike‐slip faulting induced by major NNE–SSW‐trending, sinistral strike‐slip fault zones. These fault zones were initially reactivated along intracontinental basement faults in the course of northward migration of the South China continent. The nearly N–S‐trending margins of the Yangshuo Basin, approximately coinciding with the strike of regional fault zones, were related to the master strike‐slip faults; the NW–SE‐trending margins were related to parallel, oblique‐slip extensional faults. Nine depositional sequences recognized in Givetian through Frasnian strata can be grouped into three sequence sets (Sequences 1–2, 3–5 and 6–9), reflecting three major phases of basin evolution.

During basin nucleation, most basin margins were dominated by stromatoporoid biostromes and bioherms, upon a low‐gradient shelf. Only at the steep, fault‐controlled, eastern margin were thick stromatoporoid reefs developed. The subsequent progressive offset and pull‐apart of the master strike‐slip faults during the late Givetian intensified the differential subsidence and produced a spindle‐shaped basin. The accelerated subsidence of the basin centre led to sediment starvation, reduced current circulation and increased environmental stress, leading to the extensive development of microbial buildups on platform margins and laminites in the basin centre. Stromatoporoid reefs only survived along the windward, eastern margin for a short time. The architectures of the basin margins varied from aggradation (or slightly backstepping) in windward positions (eastern and northern margins) to moderate progradation in leeward positions. A relay ramp was present in the north‐west corner between the northern oblique fault zone and the proximal part of the western master fault. In the latest Givetian (corresponding to the top of Sequence 5), a sudden subsidence of the basin induced by further offset of the strike‐slip faults was accompanied by the rapid uplift of surrounding carbonate platforms, causing considerable platform‐margin collapse, slope erosion, basin deepening and the demise of the microbialites. Afterwards, stromatoporoid reefs were only locally restored on topographic highs along the windward margin. However, a subsequent, more intense basin subsidence in the early Frasnian (top of Sequence 6), which was accompanied by a further sharp uplift of platforms, caused more profound slope erosion and platform backstepping. Poor circulation and oxygen‐depleted waters in the now much deeper basin centre led to the deposition of chert, with silica supplied by hydrothermal fluids through deep‐seated faults. Two ‘subdeeps’ were diagonally arranged in the distal parts of the master faults, and the relay ramp was destroyed. At this time, all basin margins except the western one evolved into erosional types with gullies through which granular platform sediments were transported by gravity flows to the basin. This situation persisted into the latest Frasnian.

This case history shows that the carbonate platform architecture and evolution in a pull‐apart basin were not only strongly controlled by the tectonic activity, but also influenced by the oceanographic setting (i.e. windward vs. leeward) and environmental factors.

ABSTRACT A linear surface process model is used to examine the effect of different patterns of rock uplift on the evolution of the drainage network of the Swiss Alps. An asymmetric pattern of tectonic forcing simulates a phase of rapid retrothrusting in the south of the Swiss Alps (‘Lepontine’‐type uplift). A domal pattern of tectonic forcing in the north of the model orogen simulates the phase of the formation of the ‘Aar massif’, an external basement uplift in the frontal part of the orogenic wedge (‘Aar’‐type uplift).

Model runs using the ‘Lepontine’‐type uplift pattern result in a model mountain chain with a water divide in the zone of maximum uplift and orogen‐normal rivers. Model runs examining the effect of ‘Lepontine’‐type uplift followed by ‘Aar’‐type uplift show that the initially formed orogen‐normal river system and the water divide are both very stable and hardly affected by the additional uplift. This indifference to changes in tectonic forcing is mainly due to the requirement of a high model erosion capacity for the river systems in order to reproduce the exhumation data (high‐grade rocks in the south of the Swiss Alps point to removal of a wedge‐shaped nappe stack with a maximum thickness of about 25 km). The model behaviour is in agreement with the ancestral drainage pattern of the Alps in Oligocene and Miocene times and with the modern pattern observed in the Coast Range of British Columbia; in both cases river incision occurred across a zone of rapid uplift in the lower course of the rivers. The model behaviour does not, however, explain the modern drainage pattern in the Alps with its orogen‐parallel rivers.

When the model system is forced to develop two locally independent main water divides (simultaneous ‘Lepontine’‐ and ‘Aar’‐type uplift), a zone of reduced erosional potential forms between the two divides. As a consequence, the divides approach each other and eventually merge. The new water divide remains fixed in space independent of the two persisting uplift maxima. The model results suggest that spatial and temporal changes in tectonic forcing alone cannot produce the change from the orogen‐normal drainage pattern of the Swiss Alps in Oligocene–Miocene times to the orogen‐parallel drainage observed in the Swiss Alps today.

ABSTRACT A Tortonian to Pliocene magnetostratigraphy of the Fortuna basin supports a new chronostratigraphic framework, which is significant for the palaeogeographical and geodynamic evolution of the Eastern Betics in SE Spain.

The Neogene Fortuna basin is an elongated trough which formed over a left‐lateral strike‐slip zone in the Eastern Betics in the context of the convergence between the African and Iberian plates. Coeval with other basins in the Alicante–Cartagena area (Eastern Betics), rapid initial subsidence in the Fortuna basin started in the Tortonian as a result of WNW–ESE stretching. This led to transgression and deposition of marine sediments over extensive areas in open connection with the neighbouring basins. Since the late Tortonian, N–S to NW–SE compression led to inversion of older extensional structures. The transpressional tectonics along the NE–SW‐trending Alhama de Murcia Fault is related to the rising of a structural high which isolated the Fortuna basin from the open Mediterranean basin. The progression of basin confinement is indicated by the development of restricted marine environments and deposition of evaporites (7.8–7.6 Ma). The new basin configuration favoured rapid sediment accumulation and marine regression. The basin subsided rapidly during the Messinian, leading to the accumulation of thick continental sequences. During the Pliocene, left‐lateral shear along the Alhama de Murcia Fault caused synsedimentary folding, vertical axis block rotations and uplift of both the basin and its margins. The overall sedimentary evolution of the Fortuna basin can be regarded as a developing pull‐apart basin controlled by NE–SW strike‐slip faults. This resembles the evolution that has taken place in some areas of the Eastern Alboran basin since the late Tortonian.

ABSTRACT The intracratonic basins of central Australia are distinguished by their large negative Bouguer gravity anomalies, despite the absence of any significant topography. Over the Neoproterozoic to Palaeozoic Officer Basin, the anomalies attain a peak negative amplitude in excess of 150 mGal, amongst the largest of continental anomalies observed on Earth. Using well data from the Officer and Amadeus basins and a data grid of sedimentary thicknesses from the eastern Officer Basin, we have assessed the evolution of these intracratonic basins. One‐dimensional backstripping analysis reveals that Officer and Amadeus basin tectonic subsidence was not entirely synchronous. This implies that the basins evolved as discrete geological features once the Centralian Superbasin was dismembered into its constituent basins. Two‐ and three‐dimensional backstripping and gravity modelling suggest that the eastern Officer Basin evolved from a broad continental sag into a region of intracratonic flexural subsidence from the latest Neoproterozoic, when flexure of the lithosphere deepened the northern basin. The results from gravity modelling improve when the crust is thickened beneath the northern margin of the basin and thinned at the southern margin, as has been suggested by recent deep seismic data. The crustal thickening beneath the basin's northern margin abuts the region of greatest topographic relief and is consistent with the observed structure at the edges of many orogenic belts. If the Officer Basin evolved as a foreland‐type basin from the late Proterozoic and has retained those features to the present, then one implication is that in the absence of any significant topography, cratonic lithosphere must be able to support stresses over very long periods of geological time.