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Normal Faults and Their Hanging-Wall Deformation: an Experimental Study1

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Normal Faults and Their Hanging-Wall Deformation: an Experimental Study1 Normal Faults and Their Hanging-Wall Deformation: An Experimental Study1 Martha Oliver Withjack,2 Quazi T. Islam,3 and Paul R. La Pointe4 ABSTRACT The observed particle paths, displacement distri- butions, bedding dips, and orientations of the prin- We have used clay models to study the effects of cipal-strain axes in our physical models with and fault shape and displacement distribution on defor- without a basal plastic sheet are compatible with mation patterns in the hanging wall of a master the assumption that homogeneous, inclined simple normal fault. The experimental results show that shear accommodates the hanging-wall deforma- fault shape influences the style of secondary fault- tion. Not all of our modeling observations, however, ing and folding. Mostly antithetic normal faults are consistent with this assumption. Specifically, form above concave-upward fault bends, whereas the observed variability with depth of the distribu- mostly synthetic normal faults form above low- tion and intensity of deformation is incompatible angle fault segments and convex-upward fault with homogeneous, inclined simple shear as the bends. Beds dip toward the master normal fault hanging-wall deformation mechanism. above concave-upward fault bends and away from the master normal fault above low-angle fault seg- ments and convex-upward fault bends. Generally, INTRODUCTION secondary faulting and folding are youngest at fault bends and become progressively older past fault For more than 60 yr, geologists have used physi- bends. cal models to simulate normal faults and their Hanging-wall deformation patterns differ signifi- hanging-wall deformation (Cloos, 1928; Cloos, cantly when a basal plastic sheet imposes a con- 1930; Cloos, 1968; McClay and Ellis, 1987a, b; Ellis stant-magnitude displacement distribution on the and McClay, 1988; McClay, 1989; Islam et al., 1991; master normal fault. In models without a plastic McClay and Scott, 1991; McClay et al., 1992; sheet, numerous secondary normal faults form in Withjack and Islam, 1993). These experimental the hanging wall of the master normal fault. Most studies have guided the structural interpretation of secondary normal faults propagate upward and, field, well, and seismic data. Additionally, they have consequently, have greater displacement at depth. provided data for testing and calibrating geometric In models with a plastic sheet, few visible sec- models of normal faults (Groshong, 1990; Dula, ondary normal faults develop. Most of these faults 1991; White and Yielding, 1991; Kerr and White, propagate downward and, consequently, have less 1992; White, 1992; Xiao and Suppe, 1992). displacement at depth. Hanging-wall folding is Physical models of normal faults differ in terms wider and bedding dips are gentler in models with- of modeling materials (wet clay vs. dry sand) and out a plastic sheet than in identical models with a experimental constraints placed on fault shape, plastic sheet. development, and displacement distribution. In physical models by Cloos (1968), the shape and development of the master normal fault and its dis- Copyright 1995. The American Association of Petroleum Geologists. All placement distribution are unconstrained (Table rights reserved. 1Manuscript received January 11, 1994; revised manuscript received 1). A master normal fault develops in clay or sand August 8, 1994; final acceptance September 7, 1994. above two diverging, overlapping metal sheets and 2Mobil Research and Development Corporation, P.O. Box 65032, Dallas, propagates upward. In physical models by McClay Texas 75265. 3Bureau of Land Management, 411 Briarwood Drive, Suite 404, Jackson, et al. (1992), the shape and development of the Mississippi 39206. master normal fault and its displacement distribu- 4Golder Associates Incorporated, 4104 148th Avenue NE, Redmond, tion are completely constrained (Table 1). A rigid Washington 98052. We thank ARCO Oil and Gas Company and Mobil Research and block and horizontal base act as the footwall of the Development Corporation for their support during the study. We also thank master normal fault, and sand represents the hang- William Brown, Sybil Callaway, Gloria Eisenstadt, Jack Howard, David Klepacki, and Eric Peterson for their careful and thoughtful reviews of the ing-wall strata. During modeling, a plastic sheet manuscript. carries the sand down the sloping surface of the AAPG Bulletin, V. 79, No. 1 (January 1995), P. 1–18. 1 2 Normal Faults and Their Hanging-Wall Deformation Table 1. Comparison of Modeling Parameters and Results Modeling Parameters Fault Modeling Fault Fault Displacement Material Shape Development Distribution Modeling Results* Wet clay Unconstrained; Unconstrained; Unconstrained sloping surface sloping surface along sloping of master of master surface; normal fault normal fault constant forms during the forms during the magnitude on experiment experiment flat surface Dry sand Dry sand Completely Completely Constant constrained: constrained magnitude 45°-, 30°-, and 0°-dipping segments Wet clay Initially Lower Unconstrained constrained: two-thirds along sloping 45°- and initially surface; 0°-dipping constrained constant segments; magnitude on 30°-, 45°–, flat surface and 0°-dipping segments; 45°-, 30°-, and 0°-dipping segments Wet clay Completely Completely Constant constrained: constrained magnitude 45°-, 30°-, and 0°-dipping footwall block and along the horizontal base. In surface of the footwall block is either planar or has these models, the rigid footwall block and horizon- a single concave-upward or convex-upward bend. tal base predetermine the shape of the master nor- Our models differ from those of Cloos (1968) in mal fault. The plastic sheet prevents the fault shape that the rigid footwall block and horizontal base from changing during modeling and imposes a define the initial shape of the master normal fault. constant-magnitude displacement distribution on Unlike the models of McClay et al. (1992), the the master normal fault. shape of the master normal fault can change dur- We have conducted our own physical models of ing modeling and the displacement distribution on normal faults to study how fault shape and dis- its sloping surface can vary in all but one of our placement distribution affect hanging-wall defor- experiments. In that experiment, a mylar sheet mation (Table 1). In our models, a rigid block and beneath the clay layer prevents the master normal horizontal base act as the footwall of the master fault from changing during the experiment and normal fault, and a layer of wet, homogeneous clay imposes a constant-magnitude displacement distri- represents the hanging-wall strata. The sloping bution on the master normal fault. Withjack et al. 3 Figure 1—Cross-sectional view of experimental apparatus. An aluminum block (black) and horizontal base (gray) act as the footwall of a master normal fault. Wet clay (white) represents the strata in its hanging wall. As shown on the right, the sloping side of the aluminum block is planar and dips 45° in experiment 1, has an upper 30˚-dipping segment and a lower 45°-dipping segment in experiment 2, and has an upper 45°-dipping segment and a lower 30°-dipping segment in experiments 3 and 4. During the experiments, the middle moveable wall and the attached aluminum sheet move toward the right, away from the aluminum block. MODELING PROCEDURE mylar sheets move away from the aluminum block. The clay, passively carried by the mylar sheet, The experimental apparatus has a horizontal moves away from the block and down its sloping base and three vertical walls (Figure 1). The outer side. The displacement rate of the moveable wall is walls are stationary, whereas the middle wall can 0.004 cm/s in all experiments. We repeat each move toward either outer wall. An aluminum experiment at least twice to verify the modeling sheet, attached to the moveable wall, covers the results. base. A 5-cm-high aluminum block overlies the To ensure geometric and kinematic similarity sheet and is attached to a fixed wall. The top sur- between physical models and actual rock deforma- face of the block is square, 25 cm wide and long. tion (assuming that inertial forces are negligible The sloping side of the block is planar and dips 45° and that the density of the modeling material and in experiment 1, has an upper 30°-dipping seg- rock are identical), the strength of the modeling ment and a lower 45°-dipping segment in experi- material and the model dimensions must be scaled ment 2, and has an upper 45°-dipping segment and down by the same factor (Hubbert, 1937). The a lower 30°-dipping segment in experiments 3 and cohesive strength of rock is about 105 times greater 4. In experiment 4, a mylar sheet, attached to the than the cohesive strength of the wet clay in the moveable wall, overlies the sloping side of the alu- physical models. The thickness of sedimentary minum block and the aluminum sheet. cover is also about 105 times greater than the clay A 7.5-cm-thick layer of clay directly overlies the thickness in the physical models. Although the cri- aluminum block and aluminum sheet in experi- teria for geometric and kinematic similarity have ments 1, 2, and 3. In experiment 4, a 5-cm-thick been satisfied, we emphasize that the physical layer of clay overlies the mylar sheet. In all experi- models are not exact scale models. Rock may ments, the clay density is 1.6 g/cm3, and its cohe- deform differently than the clay in the models. For sive strength is about 10–4 MPa (Sims, 1993). The example, rock with preexisting inhomogeneities top and sides of the clay layer are free surfaces. (e.g., faults, fractures, bedding) may behave very dif- Circular markings applied to the top and sides of ferently than the homogeneous clay in the models. the clay layer record strain during modeling. During experiments 1, 2, and 3, the moveable wall and the attached aluminum sheet move away from MODELING PARAMETERS the aluminum block. In response, the clay above the aluminum sheet moves away from the block The constraints on fault shape, development, and and down its sloping side.
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