Development of grabens and associated fault-drags: An experimental study

Transcription

1 Proc. Indian Acad. Sci. (Earth Planet. Sci.), Vol. 104, No. 3, September 1995, pp Printed in India. Development of grabens and associated fault-drags: An experimental study SUGATA HAZRA Department of Geological Sciences, Jadavpur Umversity, Calcutta , India Abstract. Experiments on extensional faulting were performed with semi-brittle talc-sand beds resting on a ductile clay base. The experiments show that the development of graben in the talc-sand beds is controlled by the deformation in the ductile basement. Graben-like structures form only when there is a non-uniform stretching in the basement. Uniform extension at the basement level fails to produce any such structures. Grabens initiate as large synclinal structures (sag). The sag is generated either by a downward flexing of the talc-sand bed on a ductile basement or by non-uniform thinning of beds. Listric master faults bounding the grabens intersect the basement at high angles. The master faults that initiate as curved shear planes rotate further with continued extension. At the initial stage, the graben structures are associated with normal drags, and with progressive deformation, drag patterns change from normal to a reverse one. Keywords. Fault; graben; reverse drag; basement; ductile; brittle. 1. Introduction Graben is one of the characteristic structures of extensional tectonic regime in the earth's crust. These structures have received considerable attention all over the world for their resource potential. In a graben two inwardly dipping normal faults (dip = ~ bound a wedge-shaped sedimentary prism which are often associated with different types of subsidiary structures, such as antithetic faults, drag flexures. Early works (Cloos 1929, 1930, 1931), based on experiments with clay models show the development of symmetrical grabens bound by listric normal faults and the close association of antithetic and synthetic subsidiary faults. Two types of grabens were recognised later (Cloos 1968): symmetric grabens and asymmetric grabens. Symmetric grabens are bound by two prominent faults, while in asymmetric grabens there is one prominent master fault and the reverse drag of adjacent beds (figure 1). Symmetrical grabens formed when basal plates underlying the model were pulled apart with equal velocity while asymmetric grabens developed when one overlapping plate was pulled with respect to the other. Similar experiments were also performed by Elmohandes (1981) or Dula (1991) to understand specific regional problems. The present series of experiments was carried out on a similar line to understand the role of basement tectonics in graben formation and to study the initiation, growth and the attitude of master faults. The study also investigates the evolution of normal and reverse drag flexures in the course of graben development. 489

2 490 Sugata Hazra Types of Graben 4"4. 4"4" ~4. Symmetric Graben Asymmetric Graben Figure I. Types of grabcn. 2. Experimental method Graben structures were simulated (Hubert 1937) in beds of talc and silty sand mixture. In order to obtain semi-brittle behaviour of the material, water was added to the talc-sand mixture by volume of 40%. By trial the ratio of talc to sand was chosen as 9:4 which gave the desired behaviour of a wet talc-sand bed. Models were prepared in the following manner. A metal box (20 cm x 15 cm x 10 cm) with top and bottom sides open was placed on two overlapping metal (20 cm x 12 cm) (a) Y (b) Figure 2. Experimental set-up: (a) metal box on movable plates; (b) model on a ductile clay base and (e) model on stretchable elastic bands.

3 Grabens and associated fault-drags 491 plates (figure 2a). The box was then filled with the viscous talc-sand mixture. The material was initially weak and collapsed under its own weight. So, before starting the deformation the model was kept for about 15 minutes to allow the excess water to drain out and the talc-sand bed became somewhat strong. The metal box was then carefully removed. Under tension, the thick (6-8 cm) talc-sand beds failed by shear fractures~ In the first set of experiments (type A) talc-sand beds overlay directly on the basal plates. The model was deformed by pulling the two plates away from each other. In these experiments the basal plates were either non-overlapping or overlapping to each other. I shall refer them as type A1 and type A2 experiments respectively in the later discussions. In the second set of experiments (type B) there was a viscous clay layer at the base of the talc-sand bed (figure 2b). The ductile deformation of the clay basement controlled the localization of graben formations in the overlying unit. Another set of experiments (type C) was done with elastic basal layer (figure 2c). The elastic unit was fixed to two wooden plates. The layer was stretched by pulling the wooden plates away from each other. The elastic layer thinned by homogeneous deformation and created a uniform extension in the overlying talc-sand bed. In some of the experiments the elastic layer was stretched from one side. Sets of horizontal marker lines were drawn on the vertical faces of the models. Fault displacements and the associated wall rock deformations were studied and photographed in successive stages. F~,we 3. (Continued)

4 492 Suoata Hazra Figure 3. Successive stages of development of(a) symmetric and (b aad 9 asymmetric grabens in type A experiments; (d) plan view of a graben structure. 3. Experimental results 3.1 Type A1 experiments In the initial stage of deformation an overall sag developed in the upper part of the talc-sand layer. With further extension, two conjugate shear fractures nucleated at the initial plate-junction and simultaneously propagated upward (figure 3ai). Finally a symmetric graben, bound by two pronounced master faults developed (figure 3aii). The beds in the wedge block showed normal drags adjacent to the typically concave upward (listric) faults. In plan, the fault zone described parallel fault lines (figure 3d). The fault zone width was found to be twice the thickness of the wedge block. The subsidiary shear fractures, synthetic and antithetic to the master faults were characteristically curved and steepened upward. With progressive extension, the strata in the hanging wall rotated downward against the listric faults and had a reverse drag (figure 3aiii). For larger movements, subsidiary extension zones developed at the crest of the flexure in the graben when the master faults rotated to lower dips.

5 Grabens and associated fault-drags Type A2 experiments A single master fault with vergence opposite to the movement direction formed at the base and intersected the basal detachment surface at an angle of about 50 ~ (figure 3b). The other oppositely-dipping master fault developed as an incipient antithetic fault. However, in the course of overall extension, hanging wall deformation was dominated by reverse drag flexure of beds (figure 3c). The foot wall was not affected by any faults. In some comparatively brittle talc-sand beds, a triangular gap developed at the deeper segment of the master fault (figure 7a). Such gaps did not develop in semi-brittle beds as the space was adjusted either by local rotation of layers or by antithetic faulting. The fault zone consisted of a series of parallel hinge faults. The fault throw was maximum at the central part and decreased to zero at the tips (fault relay, Larsen 1988). In plan, the fault lines were somewhat curved (figure 3d). The drag pattern systematically varied with depth. Drags in the deeper levels were dominantly reverse while those in the shallower levels were normal (figure 3b). 3.3 Type BI experiments The ductile flow in basal clay bed, that took place under the overburden weight was greatly enhanced by horizontal movement of the rigid basal plates. The ductile flow was associated with the formation of localized thinning zones (5-8 cm wide) in the clay bed, and caused a broad sag in the overlying semi-brittle beds. The overlying bed also had a tendency to sink into the viscous base and force the base materials up-doming at the flanks (figure 4ai). With the increase in downward flexure of the sand bed due to sagging, conjugate normal faults developed and gave rise to a symmetric graben structure. The graben structure overlies the zone of maximum thinning in the basal layer. Faults in the central part of the graben system were somewhat steeper than faults at the graben margin (figure 4aii). There was a close association of normal drags and reverse drags of layers in the graben. Reverse drags were characteristically common where the master faults met the thinnest zone of the basal ductile layer (figure 4aiii). Normal drags localized in shallow levels and transformed to reverse ones with progressive fault movement (figure 5). Within larger graben systems, local extension zones developed above the up-doming ductile base and subsidiary graben structures formed (figure 4aiv). This type of association of grabens of different scales is found in natural systems, e.g. Gondwana Rift System (Ghosh and Mukherjee 1985). 3.3 Type B2 experiments Thinning of the ductile base took place in a narrow zone and was associated with an overall sagging in the overlying semi-brittle bed. In the course of the sagging, faults developed. However, one set of faults was more dominant than the other and formed an asymmetric graben (figure 4bi). The fault zone consisted of numerous synthetic and antithetic listric normal faults. The faults described curved traces and had a variable throw along the strike. Dips of faults in the central part of the graben were much steeper than those at the periphery (figure 4bii). With depth fault dips decreased to about 30 ~ (figure 6).

6 494 Sugata Hazra.=. 2~ "d E 0 -d t~

7 Grabens and associated fault-drags 495 Talc sand model Viscous clay.basement r Or real drag 2 Horst Sub Graben V/.. ~ Figure 5. Development of fault structures and associated drag patterns in course of symmetnc graben formation (sketch from type B1 experiments in figure 4). 3.4 Type C experiments When the basal elastic layer was stretched by one-third of its initial length, numerous conjugate shear fractures formed uniformly over the entire talc-sand bed (figure 7b and c). Graben-like structures did not form in this type of experiments. 4. Discussion The present experimental study shows that graben formation in cover rocks greatly depends on the nature of deformation in the basement. Sub-parallel fault zones bounding symmetric or asymmetric grabens form only when the basement deformation is inhomogeneous with localization of high-rate extension zones. For uniform extension in the basement, as simulated in type C experiments, graben structures do not form in the overlying semi-brittle rocks.

8 496 Sugata Hazra ~ Sag ~ ]Talc sand ~ l model..:::'..:.':':..': '.'.'.':,': ".:'.'... ~... :. :.:- :':.'.-.:.'":-..:'.-:... Fixed side Viscous basement R e a r ~ sicle b Fil~re 6. Development of fault structures and associated drag patterns in course of asymmetric graben formation (sketch from type B2 experiments in figure 4). When the basement is brittle, significant down-throw along the normal faults can occur if the faults meet a detachment surface in the basement level. The detachment fault can adjust a large throw in a graben by horizontal displacement. The presence of detachment zones in natural graben systems have been reported from several extensional belts (for e.g. Viking Graben, Gibbs 1987). On the other hand, when the basement is ductile, the vertical movement in a graben is partly adjusted by localized deformation in the basement. In this situation the subsidence rate in the graben is likely to be controlled by the probable strain rate in the basement. The material chosen for the present experiments was capable of undergoing considerable ductile deformations during sagging and layer-drag before and after the brittle failure respectively. Such a mechanical behaviour is not unrealistic because structural associations in natural graben systems indicate both the brittle and ductile deformations of wall rocks. However, a large post-faulting ductile deformation can take place in some special geologic situations. For example, a combination of excess pore fluid pressure with slow gravity creep along the fault zone may be a probable environment for such anomalous behaviour.

9 Grabens and associated fault-draos 497 Figure 7. (a) Formation of gap at the base of the graben in brittle talc-sand beds; (b) and (e) conjugate and single set of shear fractures in type C experiments. Note that no graben-like structures have formed. Faults in experimental grabens were listric in nature. The listric nature may develop either by growth of the faults along a curved path or by syntectonic rotation of the fault plane. As both the shear stress and normal stress decrease laterally and vertically upwards, the angle between the potential fault plane and the vertical plane decreases upward. Such a spatial variation in stress supports formation of faults with a curved geometry (Hafner 1951). Displacements along listric faults and horizontal detachment surfaces create a problem with spatial compatibility when a large down-throw is envisaged. If the wall rock is considered und~qormablr a triangular gap (figure 7a) could have been generated due to the displacement incompatibility along the faults. However, in natural situations such a gap is likely to be adjusted by subsequent subsidiary faulting or bed-drags, as found in the present experiments. In the experiments with ductile basements there was no tendency of generation of such gaps. Concomitant subsidence of fault blocks into a ductile substrate appears to be a more plausible mechanism in autocratonic grabens and rift systems.

10 498 Suoata Hazra Graben structures are generally associated with different types of drags of layers (Hamblin 1965). Normal drags localized in the upper part while reverse drags at the base. This feature developed particularly where one of the normal faults was blind. The normal drag pattern of the upper part may change over to a reverse drag as the master fault propagates upward during progressive crustal extension. References Cloos Ernst 1968 Experimental analysis of Gulf coast fracture pattern; Bull. Am. Assoc. Petrol. Geol Cloos Hans 1929 Tecktoniche Experimente und die Enstehung Von Bruchlinien. 15 Cono. Geol. Int., S. Africa; Comptes Rendus 2 Cloos Hans 1930, 1931 Zur Experimentellen Tectonik I & II; Naturw~ssenschaften 18 no. 34 & 19 no. 11 Dula W F 1991 Geometric models oflistric normal faults and roll-over folds; Bull. Am. Assoc. Petrol. Geol. 75 no. 10, Elmohandcs S E 1981 Central Ethiopian Graben rifting initiated by clay modelling; Tectonophysics 73 Ghosh S K 1993 Structural Geology (London: Pergamon Press) Ghosh S K and Mukherjee A 1985 Tectonic history of Jharia basin - An intra-cratonic Gondwana basin in eastern India; Q. J. Geol. Min. Metall. Soc. India Gibbs A 1987 Development of extension and mixed mode sedimentary basins. Continental extensional Tectonics; Geol. Soc. spl. publ Hafner W 1951 Stress distribution and faulting; Geol. Soc. Am. Bull. 62 Hamblin W K 1965 Origin of reverse drag on down-thrown side of normal faults; Geol. Soc. Am. Bull Hubert M K 1937 Theory ofscale models as applied to study of geological structure; Geol. Soc. Am. Bull. 48 Larsen P H 1988 Relay structure in a lower Permian basement involved extension system, East Greenland; J. Struct. Geol