ATLAS: 3D ANALOGUE MODELLING OF EXTENSIONAL FAULT STRUCTURES PLUS FIELD APPLICATIONS


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Authors: Drs Nick M. Lemon and Tariq Mahmood

SUMMARY

Analogue modelling in a sandbox of modest construction and proportion can imitate the style of structures imaged by seismic. Models were constructed to simulate deformation in a sedimentary sequence above simple listric, ramp-flat-ramp and complex 3D detachment surfaces. A new 3D technique has been devised to simulate progressive deformation above complex detachment morphologies. This technique a lso has ability to simulate geometry and kinematics of extensional structures in a complex polyphase area. The technique has proven particularly useful in the understanding of structures developed in regions where the sedimentary sequence has been subject to more than one period of extension, each with a different orientation. The technique can also be applied to simulate structures of extensional basins that underwent subsequent compression.

Two modelling paths were followed. Initially, forward models were produced, including those which speculated on the types of fault-to-fault geometries and the angles of extension, concentrating on areas where major fault detachments perpendicular to the currently active extensional force may interact with previous faults at various angles to the current main detachment. The second modelling mode i nvolved the determination of the detachment surface from seismic interpretation and building that into a model box. The subsequent deformation was then used to check the accuracy of the initial interpretation.

Hangingwall geometries were simulated above single detachment morphologies in all extensional sandbox models developed prior to the start of our experiments. The previous models were unable to demonstrate structural patterns where pre-existing faults with a range of orientations became incorporated in extension. A wide variety of detachment shapes and their combinations have been constructed in th e model box. Schematic experiments have been conducted to demonstrate the hangingwall geometries developed above complex morphologies with varying orientations of side ramps. The models were extended in a series of successive 10 per cent increments and surface fault patterns have been monitored during each extensional stage. Analysis of the structures produced has been achieved by marrying sequent ial plan view photographs of the model surface with serial vertical sections of the bulk of the model. The longitudinal, transverse and oblique sections have been interpreted and illustrated. These models are valuable in the prediction of deformation in areas of wide-spread 2D seismic, in areas of poor seismic quality, in the confirmation of seismic interpretation and in the delineation of prospec ts.

These models provide the first understanding of the areal distribution of fault patterns and associated subsidiary troughs in areas of complex detachments. The models show the distribution of pre-rift and syn-rift sediments in extensional terranes. Computerised Tomography (CT Scanning) has been used to determine compaction and porosity variation in the hangingwall of a 3D model. These data give information on fluid migration pathways through sediment packages during extensional deformation. This technique provides a non-destructive sandbox analysis of hangingwall geometries.

Pre-existing faults can commonly occur in areas like the North West Shelf of Western Australia, where the direction of extension has changed through time. Interpretations of a number of seismic lines along the main detachments in the Carnarvon Basin were depth converted and the shape of the detachment was built into a scaled 3D model box. The box was loaded with coloured sand in thicknesses equiva lent to the known stratigraphy of the area and extended by amounts equivalent to that indicated on the depth converted sections. The basic aim of this work was to provide control on fault linkages and the structure of key horizons in tectonically complex areas with relatively sparse seismic coverage, and to verify existing interpretations in such areas. These models provide interpretive templates for the evolution of extensional fault structures and show the orientation of faults which may logically be linked in a widely spaced 2D survey.

ATLAS CHAPTERS

The basic aim of this atlas is to provide a better understanding of structures developed in regions where a sedimentary sequence has been subject to more than one period of extension each, with different orientations, thereby assisting seismic interpretation in complex extensional terranes. Two modelling paths were followed. Initially, forward models were produced, including those which speculate d on the types of fault-to-fault geometries and the angles of extension, concentrating on areas where major fault detachments perpendicular to the currently active extensional force may interact with previous faults at various angles to the current main detachment. The second modelling mode involved the determination of the detachment surface from seismic interpretation and building that into a mo del box. The subsequent deformation was then used to check the accuracy of the initial interpretation.

The 2D modelling section of this atlas is a repetition of experiments presented in the literature but presents new results. This section demonstrates sequential fault evolution and illustrates fault reactivation and nucleation during progressive stages of deformation. Analysis of particle displacement sheds some light on the accuracy of fault reconstruction techniques. Modified Chevron Con struction and Inclined Shear Construction, which imply movement parallel to the detachment or along inclined shear planes are considered most likely to be accurate. Our analysis shows that even those are inadequate. This section also describes how a particle motion diagram can be used as a template and placed over a scaled geological section to perform predictive deformation.

The third section of this atlas comprises three-dimensional sandbox models. A new 3D technique devised at NCPGG is being discussed. This section provides understanding of areal distribution of fault patterns and associated subsidiary troughs in areas of complex detachments. These forward models investigate hangingwall deformation and fault patterns in areas where a listric extensional faul t is modified by the presence of an existing fault at an angle to the currently active detachment. These models show the distribution of pre-rift and syn-rift sediments in extensional terranes. This section provides a wide variety of models using complex detachment morphologies. Most are likely to occur in nature but one or two may be improbable. This section also illustrates the influence of side ramp orientations on the hangingwall geometries. Some models display structures developed in response to extension above combinations of detachment geometries and varying degrees of extension, as well as extension and compression in situ. The application of this section has been described in the section titled Field Applications.

The fourth section describes the X-Ray Computerised Tomography (CT) technique. This technique provides non-destructive sandbox analysis of hangingwall geometries. As well as exciting new results on compaction and porosity variation in the hangingwall of models, the data can be used to determine fluid migration pathways through sediment sequences during extensional deformation.

The fifth section shows three dimensional graphical display of sandbox models implemented on computer. Serial sections were digitised with the threshold of Digitizer v2.1 a third party digitisation software module.

The final section of this atlas provides examples of the techniques applied to the North West Shelf, Western Australia. The techniques discussed in this atlas have wide implications and can be applied to any extensional terrane. Pre-existing faults can commonly occur in areas like the North West Shelf of WA, where the direction of extension has changed through time. Seismic section along t he major detachments has been selected. Depth conversion of the seismic sections has allowed construction of scaled models incorporating complex detachments. The accuracy of these techniques to simulate hangingwall geometry has been clearly illustrated in this section, as well as providing new concepts on seismic interpretation and petroleum exploration.