Listric Normal Faults

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I he l\ IClil '\"l)(lJlllIil of Petroleum Bulleon \ oK, No 7 (July 19841. P. 801-815,32 rig,. Listric Normal Faults: An Dlustrated Summaryl JOHN w. SHELTON" ABSTRACT Normal faults are commonly listric, that is, the dip flat-tens with depth. Movement along this type of fault is instrumental in formation of several types of structural traps (e.g., rollover anticlines and upthrown-fault-block closures). Some listric faults are restricted to sedimentary rocks, whereas others offset basement rocks. Tbeoretical data, rock-mechanical and simulated model experiments, and foundation-engineering tests and failures suggest tbat this type of fault may occur where brittle rocks overlie duc-tile rocks in an extensional regime. In some places the duc-tile section may be thin and bounded sharply at its top. Also, the extensional regime may be locally derived within a broader stress regime of another type, as evidenced by transtension associated with strike-slip movement and arched strata in a compressive setting. The flattening of the fault reflects an increase in ductility of the rocks with depth and, in some cases, deformation of the fault due to compaction or tilting of the upthrown block. The dip angle may vary along the strike of the fault in response to changes in throw. In cross section, a listric fault may consist of relatively short, en echelon fault seg-ments. This geometry may be particularly cbaracteristic of growth faults. Sedimentary faults may sole in ductile strata, or they may represent the brittle part of a fault-flow system. Fault patterns commonly are characterized by bifurcation, some of which may occur near the ends of individual faults comprising a zone. Although unequivocal recognition of listric normal faults requires unusually extensive outcrop data, close subsurface control, or high-quality seismic data, their presence is suggested indirectly by such features as increas-ing dip with depth toward tbe controlling fault ("reverse drag"), thick progradational sandstone overlying ductile strata, and in some cases arcuate fault patterns, basins, or uplifts. Copyright1984. The American Association of Petroleum Geologists. All rights reserved. , Manuscript received, May 18, 1983; accepted, December 7, 1983. 2 ERICa, Inc., Tulsa, Oklahoma 74172. For documentary data which have not been published, the writer is indebted to Shell Oil Co. for materials from the Gulf Coast and to G. W. Hart for data and interpretations in the Arkoma basin. Laura F. Serpa and R. E. Denison pro vided information on basementinvolved faults and detached sediments, respectively. Appreciation is gratefully expressed to many colleagues and acquaintances who for more than 2 decades have stimulated thought on this subject of listric (rotational) normal faults. Kaspar Arbenz kindly reviewed the original manu script and made numerous helpful suggestions. Appreciation is also expressed to AAPG Editors M. K. Horn and Richard Steinmetz, Science Direc tor Edward A. Beaumont, and reviewers for their valuable comments. Yet the author must assume responsibility for errors or any aberration in accepted thought. David E. Brooker drafted the illustrations, and Sherry Hempel, Mildred P. Lee, anet Dianne O'Malley prepared the typescript. O'Malley also assisted in compilation of the references. S. W. Carey kindly provided the reference noted herein to his work. 801 Listric normal faults form during rifting, drifting, and evolution of passive continental margins with concomi-tant basinal denlopment. Listric faults confined to the sedimentary prism are common features on passive mar-gins, especially in progradational, post-evaporite sequences. The basement is offset by listric faults as a fun-damental element in the development of other types of basins, including those whicb formed during postorogenic extension. They also occur as secondary extensional fea-tures in an overall compressive stress regime due to plate connrgence and during transform or strike-slip faulting. INTRODUCTION A listric fault is characterized by a decreasing angle of dip with depth. It, therefore, is a curved surface which is concave upward. Apparently the concept was introduced by Edward Suess in the early part of this century (Bally et ai, 1981) as part of his description of faults in coal mines in northern France. Listric thrust faults have been recognized for a long time as a basic feature of thin-skinned tectonics, with decolle-ments. Now, as deep faults soling in the ductile crust, they are also considered an integral part of suturing during plate convergence (e.g., Thompson, 1976). Although lis-tric normal faults have been recognized as updip (or upslope) segments of gravitational slides (e.g., Reeves, 1925, 1946; Hubbert and Rubey, 1959; Wise, 1963), most commonly they have been regarded as a special feature of syndepositional faults in strongly subsident basins con-taining thick shale (with or without salt) below prograda-tional sandstone sections. This general opinion probably derives from the abundance of sedimentary faults in the northern Gulf Coast basin (Texas and Louisiana) and the common knowledge of "rotational slips" and associated failures in foundation engineering (Figure 1). Apparently little significance was given to the early work of Davis (1925) and Longwell (1933, 1945), who described listric normal faults offsetting crystalline and/or basement rocks in the western United States; to the theoretical treat-ment of Hafner (1951), who showed curved stress trajecto-ries including conditions for listric normal faults; or to the work of Carey (1958), who described listric normal faults as a major feature in development of rift valleys. It seems reasonable, therefore, to regard listric geometry as a com-mon feature of both thrust and normal faults displacing sedimentary and/or basement rocks. Wernicke and Burchfiel (1982) have grouped normal faults into two categories: rotational and nonrotational. The rotational category is divided into (a) those with rota-tion of beds along listric faults, and (b) those with rotation of beds and faults along planar or listric faults. Nonrota-tional faults have no rotation of structures along planar faults. 802 Listric Normal Faults CLAY A Figure I-Foundation failures resembling configurations of faults in sedimentary rocks. A. Rotational slip in foundation due to localized loading of uniform clay. B. Base failure due to load-ing offoundation with thin clay. After Terzaghi and Peck (1948). Wernicke and Burchfiel indicate that large-scale dis-placement on low-angle listric normal faults results in a series of tilted planar-fault blocks, forming "extensional allochthons." Both normal and thrust listric faults, along with planar faults, are of major significance to the explorationist because they are an important element in the formation of traps in faulted strata. Presently a commonly held opinion is that listric normal and thrust faults may be sequentially related (or even coincident) in some areas that undergo changes in tectonic regime. For example, listric thrust faults may be reactivated as normal faults when an earlier formed orogenic belt is subjected to extension (Bally et ai, 1966), and, conversely, normal faults may be reactivated as thrusts during the evolution of a continental margin from a passive to active phase (Cohen, 1982). Further, the location of thrusts with displacement during the active phase (after basinal subsidence) may be predetermined by buried normal faults that formed during the earlier pas-sive phase (during basinal subsidence). Listric normal faults are probably important elements in the develop-ment of many basins. Downward dip-slip movement of faulted strata in the hanging wall of a listric normal fault may result in "reverse drag" in half grabens or "rollover" (dip-direction reversal), with formation of an anticlinal feature (Figure 2). Absolute movement, with rotation of an upthrown block, may result in a tilted fault block with reverse drag. Significant variations in displacement along the strike of a fault present conditions for closure against it (Figure 3). The closure may also result from differential rotation (along the strike of a fault) of an entire block which itself is downthrown with respect to a subjacent "underlying" fault (Figure 3C), or by changes in strati-graphic thicknesses along the strike of the fault. The detailed geometry of the faults provides subtle trapping potential. For example, lateral branching or overlapping ends of faults are possible elements of subsidiary traps. Also, movement along individual faults of a fault zone may result in several traps rather than one larger trap. L __________________________________ Figure 2-Structural map of top of Wilcox Group (Eocene) in South Bancroft field, Beauregard Parish, Louisiana, showing rollover anticline. After Murray (1961). A o U -7600' -7650' 400011 -7700' 1000m A A' V= I !A' c (2) (4) Z) --------- ..... Figure 3-A. Structural map of Lower Cretaceous marker in Pleasanton field, Atascosa County, Texas, depicting tilted-fault-block trap. After Murray (1961). B. Hypothetical fault closure due to absolute movement of upthrown block. C. Fault blocks with potential for trap in each upthrown block due to rotation of that block (e.g., dip in block 2 is due to rotational movement along fault X). CLASTICS AND VOLCANICS ASTHENOSPHERE figure 4-::,chematic cross sect