Estimating inversion: results from clay models

Transcription

1 Estimating inversion: results from clay models G L O R I A E I S E N S T A D T & M A R T H A O L I V E R W I T H J A C K Mobil Research and Development Corporation, PO Box , Dallas, T X 75265, USA Abstract: Physical models using wet clay show that, although both fold and fault geometries change with increasing amounts of inversion, it is difficult to estimate the magnitude of inversion. During extension, a listric main normal fault and a rollover fold cut by secondary normal faults developed above two diverging, basal plates. With 50% inversion (4cm extension, 2 cm shortening), the main normal fault underwent reverse displacement, and an asymmetric syncline formed in the pre-growth layers in the hangingwall. Secondary antithetic and synthetic normal faults passively rotated. Faults on the synclinal limb near the main normal fault rotated away from the fault, whereas those on the opposing limb rotated towards the fault. A broad anticline formed at shallow levels above the original half graben. Compressional reactivation of the main normal fault ceased between 50% and 100% inversion (4cm extension, 4cm shortening). Instead, low-angle thrust faults with small displacements accommodated most shortening. The shallow anticline above the original half graben expanded laterally and vertically. With 200% inversion (4 cm extension, 8 cm shortening), low-angle thrust faults with large displacements deformed the hangingwall and footwali of the main normal fault. Thrust faults cut the main normal fault and many of the secondary normal faults. The geometry of the hangingwall fold varied with depth. A broad anticline deformed the upper growth layers, changing to a syncline in the lower growth layers and an anticline in the pre-growth layers. Estimates of inversion from null point analysis, amount of uplift above regional datum, or line length balancing, grossly underestimate the amount of shortening in the models. In addition, both the 50% and 100% inversion models lack the large-scale deformation features typically associated with compression. These results suggest that both quantitative and qualitative techniques used to calculate inversion magnitude can significantly underestimate the amount of shortening. Furthermore, if the inversion uplift is eroded or if many small-scale deformation features are not imaged seismically, then inversion may go unrecognized. Quantifying the magnitude of shortening during inversion is a fundamental problem in basin reconstruction and modelling. The most widely cited technique for estimating inversion magnitude involves identifying a null point, which is a level of no stratigraphic offset between normal and reverse motion along a fault (De Paor & Eisenstadt 1987; Williams et al. 1989). This method assumes that all shortening occurs along a single normal fault. Therefore, the null point should reflect the relative importance of extension versus contraction (Mitra 1993). Other cited methods of estimating inversion magnitude involve measuring the amount of uplift above a regional datum (Stoneley 1982; Mitra 1993) or estimating shortening by line-length restoration techniques (Badley et al. 1989; Chadwick 1993), effectively treating inversion structures as compressional structures. Chadwick (1993) has tried to quantify the former.technique by suggesting that shortening can be calculated as a function of the uplift amplitude above the regional datum and the dip of the planar, controlling fault. In practice, most geologists make qualitative judgments to estimate inversion magnitude. These judgments are based primarily on how much the inversion geometry differs from typical extensional structures, or resembles typical compressional structures. Figure 1 shows two examples of compressional inversion structures. Cartwright (1989) suggested that the structure in the Danish Central Graben (Fig. la) represents mild inversion because of its similarity to typical extensional structures. The structure in Fig. lb, however, appears to represent greater inversion because of its resemblance to a compressional structure. In this paper we address the questions of how fault and fold geometries vary with increasing inversion and how well quantitative and qualitative methods determine the magnitude of i n - version. We approached the problem experimentally with a series of inversion models of listric normal faults using wet, layered clay as the modelling material. For comparison we also performed pure extensional and contractional From BUCHANAN, J. G. & BUCHANAN, P. G. (eds), 1995, Basin Inversion, Geological Society Special Publication No. 88,

2 ~, 120 G. EISENSTADT & M. O. WITH JACK a. twt (secs) b _:::::. --..,,-_ ~ ~ -.--:... ~ " _...-. :,- -"-=,: -- "-.% jl - _ - ~.. - ~ : " "'~.." ~. ~... ~ = _ - _ : - - _ : _ "--". - ' ~ ~ _, ~ ~ ~.:~ :.-- : ~ " ~ V --,= -" ---"..=. : km - 5 Fig. 1. Examples of inversion structures. (a) Inversion structure from Danish Central Graben, considered to be mild (after Cartwright 1989). Vertical exaggeration is approximately 1 : 1. (b) Line drawing of proprietary time-migrated seismic line showing inversion structure believed to have undergone moderate amount of inversion. Vertical exaggeration is approximately 1 : 1 at 2 seconds TWT. experiments. Although other physical models of inversion structures above listric normal faults have been reported, these models focused on hangingwall deformation using a rigid footwall. Some of those models used sand (McClay 1989; Buchanan 1991), and other models used unlayered clay that extruded sideways during inversion, causing loss of area in cross sections (Mitra 1993). In our clay models both the hangingwall and footwall could deform, the model's side surfaces were constrained to prev e n t extrusion and, because the clay was layered, we were able to examine cross sections free of edge effects. fixed wall o v e r l a p ~ ~ movable wall plates fixed wall motor Fig. 2. Diagram of experimental apparatus showing cut-away view of clay layers above thin, overlapping metal plates. The upper metal plate attaches to a fixed wall. The bottom plate is attached to a movable wall.

3 e x t e n s i o n a. b. growth layers (2) main normal fault o_,. 1 cm Fig. 3. Extensional model after 4 cm of displacement of the movable wall. (a) Photograph of section through model. Only central part of model is shown. (b) Interpreted line drawing of same cross-section showing main normal fault, rollover and secondary antithetic and synthetic normal faults. The sixth layer from the bottom is highlighted (purple) for comparison with the other models. i

4 122 G. EISENSTADT & M. O. WITHJACK Experimental procedure The experimental apparatus had three fixed walls, a movable wall, and two thin, overlapping basal metal plates (Fig. 2). The lower basal plate was attached to the movable wall and the upper basal plate was attached to the opposing, fixed wall. Clay, consisting of water (50% by weight), kaolin, powdered nepheline-syenite and powdered flint, covered the metal plates. Its cohesive strength was about 40 Pa (Sims 1993). The clay, initially 4 cm thick, 60 cm long and 24.5 to 32cm wide, was composed of twelve thin, coloured layers of identical composition. During modelling, displacement of the movable wall and the attached metal plate produced extension and/or contraction in the overlying clay. The model length remained constant while the width changed (Fig. 2). The edge of the upper plate determined the location of the main normal fault during extension and localized contractional deformation during shortening. In each model, the displacement rate of the movable wall was 4 cm/hr (0.001 cm/ s). Additional coloured clay layers were added during extension to simulate deposition and record the timing of normal faulting. Each growth layer was added such that its upper surface was fiat and horizontal. After each model run, the clay was left to dry for approximately one week. It was then serially sectioned, lacquered and photographed. Although the extent of edge effects (as evidenced by curved faults along the fixed walls) appear to only extend 2 to 4 cm from the walls, measurements of strain markers on the surface showed that the edge effects actually extended 8 to 10 cm from the fixed walls. Consequently only sections through the central part of the models were used for analysis. Extensional experiments The initial width of the extensional models was 24.5 cm, and they were extended to 28.5 cm. In these models a main normal fault formed above the edge of the upper metal plate and propagated upward through the clay (Fig. 3). The main normal fault initially formed as discontinuous fault segments along strike. With greater extension, some normal faults coalesced along strike to form a through-going main normal fault. Other normal faults, bypassed by the main normal fault, became inactive. As the experiment continued, secondary synthetic and antithetic normal faults and a rollover fold formed in the hangingwall of the main normal fault. The main normal fault and most secondary normal a. 1 cm b..,,.,,,, top of growth layers.-.- top of pre-growth layers middle pre-growth layer (6th layer) Fig. 4. Simplified line drawings of cross-sections from extensional model showing variation in fold geometry of pre-growth layers along stike. (a) Cross-section where all displacement occurred along the main normal fault, creating a rollover into the fault. (b) Cross-section where displacement occurred along several fault splays, creating a syncline in the hangingwall next to the main normal fault. faults dipped 60 to 70. The geometry of the hangingwall fold varied along strike. A hangingwall rollover formed in the lower pre-growth layers where most of the displacement occurred on the main normal fault (Fig. 4a). Where displacement was accommodated amongst the main normal fault and several fault splays, a hangingwall syncline formed instead (Fig. 4b). The extensional models resemble those published by H. Cloos (1928, 1930) and E. Cloos (1968). Unlike these published experiments, however, our models were internally layered, permitting us to analyse changes in fold and fault geometries laterally through the models. Additionally, the layers added during extension allowed us to compare structural geometries in the pre-growth and growth sections. Surface and side observation showed no apparent difference between the behaviour of the clay in layered and unlayered models, suggesting that the layering exerted no influence on the deformation. Withj ack et al. (1995) demonstrated that some boundary conditions have little effect on fold and fault geometries, whereas other boundary conditions profoundly affect resulting geometries. For example, they showed that extensional deformation patterns that form over overlapping basal plates are similar whether the footwall is rigid or deformable. In contrast, extensional physical models with a mylar sheet along the main fault surface have very different secondary fault patterns. One of the major differences is the absence of a crestal collapse graben in models without a mylar sheet.

5 ESTIMATING INVERSION 123 a. 50% inversion fixed wall distribution of shortening incremental cumulative (2 cm) (2 cm) fore-thrust zone backthrust zone main normal fault zone ;;::;~;::: : : : : : : : : : : : : : : : : : : : : : : : : : : antithetic normal fault "1~ zone ~ 1.3 cm 0.7 cm 2 cm moving wall " ~ 2jun92 b. 100% inversion fixed wall distribution of shortenin 9 incremental cumulative (2 cm) (4 cm) 0.9 cm 1.1 cm m 2 cm moving wall 2jun92 c. 200% inversion fixed wall i 2 cm moving wall 2jun92 distribution of shortening incremental (4 cm) II, cm cumulative (8 cm) 114ocm 114.Ocm Fig. 5. Map views of progressive deformation in 200% inversion model. Original main normal fault and antithetic normal fault zones shown in gray. (a) At 50% inversion, numerous small fore- and backthrusts are visible. The fore-thrust zone overlies the old main normal fault zone and accommodates twice as much shortening as the backthrust zone. An uplift develops between the fore- and backthrust zones. (b) At 100% inversion, thrusts link along strike, new thrusts develop further away from the half graben, and the uplift grows laterally and vertically. The distribution of shortening along the fore- and backthrust zones has almost equalized. (c) At 200% inversion, new thrusts form. The proximity of the moving wall hinders the development of backthrusts. Cloos (1968) showed that the overall geo m e t r y of extensional sand and clay models with overlapping basal plates is very similar; the different materials cause only minor variations. O u r preliminary tests indicate that this conclusion is not true for inversion models; clay and

6 50% inversion pre-shortening layer (1) grovv~h layers (4) /_i c,4 r~ CFfl Fig. 6. (a) Photograph of cross-section through 50% inversion model. Only central part of model is shown. (b) Interpreted line drawing: black lines are old normal faults; red lines are normal faults reactivated as reverse faults. The sixth layer from the bottom is highlighted (purple) for comparison with the other models. Z -] > -t

7 ESTIMATING INVERSION % inversion model 30apr92 section 8a Fig. 7. (a) Close-up photograph of 50% inversion model. Sketch indicates location. (b) Same photograph highlighting old normal fault reactivated as reverse fault. Note normal displacement of lower layers and reverse displacement of upper layers. sand inversion models formed over overlapping basal plates have very different fault geometries. Sand inversion models show only a minor amount of reactivation of older normal faults and develop thrust faults much sooner than clay models. Faults in clay seem to both reactivate more readily and to accommodate much more displacement than those in sand. Inversion experiments Each inversion experiment started with an extensional growth model 28.5 cm wide before shortening (except for the 200% inversion model that was 36.5 cm wide). A thin layer of clay (0.25cm thick) covered the model after extension and before shortening. Each model was initially extended 4cm, and the amount of shortening varied: the 50% inversion model was shortened 2cm; the 100% inversion model was shortened 4 cm; and 200% inversion model was shortened 8cm. The surface of the inversion models was photographed after every 0.25 cm of displacement to record fold, fault and strain patterns. Map view Figure 5 shows the progression of deformation in map view of the 200% inversion model. The first thrust faults were visible after 1.75cm of displacement of the movable wall (44% inversion). At 2cm of displacement (50% inversion), fore-thrusts and backthrusts appeared on either side of an inversion uplift (Fig. 5a). The fore-thrusts developed above the main normal fault zone of the extensional models. The backthrusts, however, were not spatially associated with the zone of antithetic normal faults. Shortening was not equally partitioned between the fore-thrust and backthrust zones, demonstrating that preferential movement occurred along the original main normal fault zone. By 4cm of displacement (100% inversion), the early-formed thrust faults had coalesced along strike and new fore-thrusts and backthrusts began to form away from the inversion uplift, causing the uplift to broaden (Fig. 5b). Shortening was confined to the fore- and backthrust zones and was almost equally partitioned between the two zones. By 8cm of displacement (200% inversion), additional foreand backthrusts developed away from the inversion uplift, causing the uplift to further broaden. At 200% inversion, shortening was equally partitioned between the fore- and backthrust zones (Fig. 5c).

8 126 G. EISENSTADT & M. O. WITHJACK 50% inversion model 30apr92 section 14a Fig. 8. (a) Close-up photograph of 50% inversion model. Sketch indicates location. (b) Same photograph highlighting rotated antithetic normal faults. Antithetic faults on the synclinal limb near the original main normal fault rotate counter-clockwise (away from the fault) and become more gently dipping, whereas antithetic faults on the opposing limb rotate clockwise (towards the fault) and become more steeply dipping as the normal fault reverses movement. 5 0 % i n v e r s i o n - c r o s s - s e c t i o n a l v i e w In cross-sectional view, the 50% inversion model lacked deformation features typically associated with compression (Fig. 6). Shortening reactivated the main normal fault and other secondary normal faults as reverse faults (compare position of highlighted layer in the extensional model (Fig. 3) and 50% inversion model (Fig. 6)). Most faults maintained a normal sense of offset, although some faults showed reverse offset in the uppermost layers (Fig. 7). Inversion produced an uplift in the upper growth and post-growth layers above the original half graben. At deeper levels, in the pre-growth layers, an asymmetric syncline formed in the hangingwall of the main normal fault. This hangingwall syncline developed in all cross sections, regardless of whether the original extensional geometry was a rollover monocline into the fault or a gentle hangingwall syncline. The development of this hangingwall syncline during inversion caused secondary normal faults on the limb near the main normal fault to rotate counter-clockwise (away from the fault). As a result the dip of synthetic faults increased and the dip of antithetic faults decreased (Fig. 8). Similarly, secondary normal faults on the opposing limb of the syncline rotated clockwise (towards the fault), causing the dip of synthetic faults to decrease and the dip of antithetic faults to increase. Only a few thrust faults can be seen in cross-sections of the 50% inversion model, all with minute displacements. Although the model surface showed two discrete zones of thrusts (Fig. 5a), these zones are not evident in cross-sectional view. The lack of correlation between surface and cross-sectional deformation may be due to few pre-existing normal faults in the uppermost growth layers. The upper growth layers accommodated the shortening along new thrusts while the interior of the original half graben accommodated the shortening by reverse motion along old normal faults % i n v e r s i o n - c r o s s - s e c t i o n a l v i e w Reverse motion along the original main normal fault stopped between 50% and 100% inversion (Fig. 9), leaving most normal faults with a normal sense of offset. The hangingwall syncline in the pre-growth layers became very gentle and those layers almost resumed their pre-extensional geometry. The syncline in the growth layers became more pronounced with increased shortening. The inversion uplift expanded laterally and vertically, and pervasive low-angle thrust faults cut the hangingwall and footwall of the original main normal fault (Figs 10 &! 1). These

9 100% inversion pre.s (D t c m Fig. 9. (a) Photograph of cross-section through 100% inversion model. Only central part of model is shown. (b) Interpreted line drawing: black lines are old normal faults; red lines are normal faults reactivated as reverse faults and newly formed thrust faults. The sixth layer from the bottom is highlighted (purple) for comparison with the other models.

10 128 G. EISENSTADT & M. O. WITHJACK 100% inversion model 23apr92 section 12 Fig. 10. (a) Close-up photograph of 100% inversion model. Sketch shows location of photograph. (b) Same photograph highlighting old normal faults, and (c) same photograph showing one of several newly formed thrust faults cutting the old main normal fault zone. thrusts verge toward the centre of the half graben. The thrust faults mapped at the surface of the 100% inversion model cannot be traced at depth in cross-sectional view. These surface thrust faults appear to acommodate the shortening in the upper growth layers (Fig. 5b) % inversion - cross-sectional v i e w By 200% inversion, low-angle thrust faults accommodated the shortening throughout the model (Fig. 12). Several thrust faults cut the main normal fault, giving it the appearance of a folded fault with a lower overall dip of about 45 (Fig. 13). With increasing shortening, the inversion uplift expanded laterally and vertically. The fold geometry changed dramatically with depth: a broad anticline developed in the uppermost layers, the syncline in the lower growth layers became more asymmetric, and an asymmetric anticline formed in the lower pregrowth layers. The central part of the half graben was uplifted and rotated towards the main normal fault. Many unusual structures exist in the 200% inversion model, although they do not all appear on every cross section. For example, on some sections small inverted grabens in the footwall of the original main normal fault have structural characteristics of flower structures (Fig. 14), and on other sections fault-propagation folds at depth appear to link upward into normal faults (Fig. 15). Influence of pre-existing structures To investigate the influence of pre-existing zones of weakness on inversion geometries, we compared two models shortened the same amount, a 100% inversion model and a contractional model. The inversion model was initially 24.5 cm wide. It was then extended 4cm, and then shortened 4cm. The contractional model was initially 28.5 cm wide, and then shortened 4cm. Comparisons of the 100% inversion model with the contractional model show that the preexisting zones of weakness associated with extension profoundly affected the fault patterns during shortenings (Figs 9 & 16). Cross-sections through the contractional model show that faulting occurred in two discrete zones on either side of a symmetrical pop-up structure (Fig. 16). In the 100% inversion experiment, however, deformation was more evenly distributed throughout the model as reactivated normal faults and newly formed thrust faults (Fig. 9). The strain distribution in map view also differed

11 ESTIMATING INVERSION % inversion model 23apr92 section 12 Fig. 11. (a) Close-up photograph of 100% inversion model. Sketch shows location of photograph. (b) Same photograph highlighting old normal faults, and (c) same photograph showing a few of the newly formed thrust faults deforming the footwall. markedly in the inversion and contractional models. In the 100% inversion model, shortening occurred unequally in two discrete zones (Fig. 5b), with most initial shortening occurring over the original main normal fault. In the contractional model, strain was initially evenly distributed throughout the model (Fig. 17a). With increasing shortening the strain then became partitioned equally into two discrete zones (Fig. 17b). It is clear from a comparison of these maps that the original extensional faults control the early distribution of shortening in the inversion models. Discussion of modelling results The layered clay models show that fold geometries change with increasing amounts of inversion (Fig. 18). In the extensional models, either a rollover or a syncline forms in the lower pre-growth layers in the hangingwall of the main normal fault (Fig. 18a). With up to 50% inversion, the main normal fault experiences reverse movement, and an asymmetric syncline develops in the pre-growth layers in the hangingwall, regardless of the original extensional geometry (Fig. 18b). A broad anticline forms at shallow levels above the original half graben. With 50% inversion, fold geometries generally resemble those associated with extension rather than compression (compare Figs 4b & 18b). Compressional reactivation of the main normal fault ceases between 50% and 100% inversion. The anticline above the original half graben becomes higher and broader, whereas the hangingwall syncline in the pre-growth layers becomes very gentle (Fig. 18c). At 200% inversion, the geometry of the hangingwall fold varies with depth (Fig. 18d). An asymmetric anticline exists in the lower pre-growth layers, a syncline affects the lower growth layers, and a broad anticline deforms the uppermost growth and post-growth layers. The models also show that fault geometries

12 200% inversion growth layers (4) (b 1 cm Fig. 12. (a) Photograph of cross-section through 200% inversion model. Only central part of model is shown. (b) Interpreted line drawing: black lines are old normal faults; red lines are normal faults reactivated as reverse faults and newly formed thrust faults. The sixth layer from the bottom is highlighted (purple) for comparison with the other models.

13 ESTIMATING INVERSION 131 I 200% inversion model 2jun92 section 17 Fig. 13. (a) Close-up photograph of 200% inversion model. Sketch shows location of photograph. (b) Same photograph showing erroneous interpretation of main normal fault being folded or having ramp/flat geometry. (c) Same photograph showing correct interpretation with old main normal fault being cut by several low-angle thrusts. 200% inversion model 2jun92 section7 Fig. 14. (a) Close-up photograph of 200% inversion model. Sketch shows location of photograph. (b) Same photograph highlighting inverted graben in the footwall resembling a flower structure. change with increasing amounts of inversion (Fig. 19). A main normal fault and secondary normal faults, having dips of 60 to 70, develop in the extensional models. With 50% inversion, secondary antithetic and synthetic normal faults passively rotate as the hangingwall reverses displacement along the main normal fault and forms a syncline in the pre-growth layers (Fig. 19b). Faults on the synclinal limb near the main normal fault rotate counter-clockwise (away

14 132 G. EISENSTADT & M. O. WITHJACK 200% inversion I model 2jun92 section11 Fig. 15. (a) Close-up photograph of 200% inversion model. Sketch shows location. (b) Same photograph showing newly formed fault-propagation fold (solid line) linking upsection to older normal fault (dashed line). from the fault), whereas those on the opposing limb rotate clockwise (towards the fault). This rotation produces anomalous high and low fault dips. By 100% inversion, reverse motion has ceased along the main normal fault. Instead, low-angle thrust faults with small displacements accommodate most of the shortening and deform both the hangingwall and footwall (Fig. 19c). These thrusts faults form on both sides of the original half graben, far from the end walls, and have a vergence towards the half graben. With 200% inversion, the hangingwall and footwall of the main normal fault are cut by low-angle thrust faults with large displacements, these faults have a vergence away from the half graben. These later thrust faults have the same dip and vergence as the thrusts formed in the pure shortening model. Some thrust faults cut the main normal fault, causing it to appear folded and more gently dipping (Fig. 19d). Techniques for determining inversion magnitude (i.e. null point analysis, line-length restoration and uplift calculations) underestimate the amount of shortening in these inversion models. Null-point analysis along the main normal fault suggested that the amount of extension was greater than the amount of shortening in the three inversion models. This prediction is correct for the 50% inversion model, but erroneous for the 100% and 200% inversion models. Null-point analysis fails because shortening during inversion is accommodated by newly formed thrust faults and reactivated secondary normal faults that are distributed throughout the model, not just by displacement along the reactivated main normal fault. In cases where the main normal fault has been cut by low angle thrust faults (200% inversion), a null point analysis becomes meaningless. Line-length restoration assumes that layer thickness remains constant during deformation and all the shortening occurs along visible thrust faults or folds. Neither assumption is valid for these inversion models and therefore the technique grossly underestimates the amount of shortening. The calculated amount of shortening for the upper layers in the 50%, 100% and 200% inversion models is less than 0.1 cm, 0.2cm, and 1.6cm, respectively. The actual amounts are 2cm, 4cm and 8cm respectively. The difference between the measured and actual amount of shortening suggests that much of the deformation is occurring at a scale too small for observation. Measuring the area of uplift provided the best estimate, although still too low, of the amount of inversion. The technique assumes that the area of uplift above the regional datum divided by the detachment depth is equivalent to the amount of shortening associated with inversion. This method suggests that the amount of shortening in the 50%, 100% and 200% inversion models was about 0.6cm, 1.6cm and 4.8cm respectively. Although area is preserved in the crosssections during inversion, uplift calculations fail to accurately predict the amounts of shortening in the physical models because both the footwall and hangingwall are elevated during inversion.

15 shortening ~ a ~ e 3 v ~ 0~ ~ 3 Fig. 16. Contractional model after 4 cm of displacement of the movable wall. (a) Photograph of centre of cross-section through model. (b) Interpreted line drawing of same cross-section showingthrust faults (red). The sixth layer from the bottom is highlighted (purple) for comparison with the other models,

16 134 G. EISENSTADT & M. O. WITHJACK a. 2 c m s h o r t e n i n g fixed wall distribution of shortenin,q incremental (2 cm) no thrusts visible at surface 2.0 cm 2 cm b. 4 c m s h o r t e n i n g 1" moving wall fixed wall 4Aug93 distribution of shortening incremental cumulative (2 cm) (4 cm) III 1.0cm II 1.0 cm 2._cm moving wall 4Aug93 Fig. 17. Map view of shortening model showing thrusts, uplift and distribution of shortening. (a) After 2 cm of displacement of the movable wall, no thrusts are visible. Shortening is distributed evenly throughout the model. (b) After 4 cm of displacement, fore- and backthrusts bound a well-developed uplift. Shortening is distributed equally along the two fault zones. Continued shortening causes new thrusts to form further away from the centre of the model, increasing both the height and width of the uplift. Thus, the interpreted regional datum is too high and the measured amount of uplift above the regional datum is too low. In real examples there is also the danger that a seemingly undeformed regional datum has been uplifted during inversion. Qualitativc judgements based on how much the geometry of an inversion structure resembles either an extensional or compressional structure would also underestimate the magnitude of inversion. For example, the structural features i n the 50% and 100% inversion models more closely resemble those in the extensional model than those in the contractional model. Consequently subtle inversion structures, like the feature in the Danish Central Graben (Fig. la), might actually have undergone significant amounts of shortening. Erosion of the uplift associated with mild inversion would leave only structures with extensional geometries. Conclusions The clay models show that although quantitative techniques fail to estimate correctly amounts of inversion, there are systematic changes in fold and fault patterns with increasing magnitudes of inversion. These changes in structural geometries, along with the recognition of smallscale deformation, may help define the magnitude of inversion in real examples. Initially shortening during inversion is accommodated along pre-existing normal faults and small-scale thrust faults. For less than 100% inversion, the main normal fault experiences reverse displacement, and an asymmetric hangingwall syncline forms in the pre-growth layers. Secondary antithetic and synthetic normal faults passively rotate. Faults on the synclinal limb near the main normal fault rotate away from the main normal fault, whereas those on the opposing limb rotate

17 ESTIMATING INVERSION i 35 Inversion Fold Geometries Inversion Fault Geometries 4 cm extension, 8 cm shortening 4 cm extension, 8 cm shortenin Jk i i 4 crn extension, 4 cm shortening 4 cm extension, 4 crn shortening e ", m 4 cm extension, 2 cm shortening 4 cm extension, 2 cm shortening L O a I 4 cm extension top of growth layers top of pre-growth layers middle pre-growth layer (6th layer) 4 cm extension normal fault ~ thrust fault... layers e " o m Fig. 18. Diagram summarizing changes in fold geometries with increasing inversion. (a) A reliever into the main normal fault is developed in the pre-growth layers during extension. (b) Characteristics of the 50% inversion model include reactivation of the main normal fault as a reverse fault creating a hangingwall syncline in pre-growth layers, and the development of a broad surface uplift. (c) Characteristics of the 100% inversion model include continued reactivation of the main normal fault as a reverse fault, continued development of the surface uplift and flattening of pre-growth layers. (d) Characteristics of the 200% inversion model include continued development of the surface uplift, development of a hangingwall anticline in the pre-growth layers, and development of a hangingwall syncline in the growth layers. Fig. 19. Diagram summarizing changes in fault geometries with increasing inversion. (a) Extensional antithetic and synthetic faults have dips of 60 to 70. (b) Characteristics of 50% inversion include reactivation of main normal fault as a reverse fault, causing rotation of secondary normal faults with anomalous high and low fault dips. (c) Characteristics of 100% inversion include continued rotation of secondary normal faults and pervasive development of fore- and backthrusts (verging towards centre of model) in both hangingwall and footwall. (d) Characteristics of 200% inversion include offset of main normal fault by low-angle thrusts (causing it to appear more gently dipping), continued uplift and rotation of secondary normal faults, and new foreand backthrusts verging away from the centre of the model. towards the fault. A broad anticline forms at shallow levels above the original half graben. Compressional reactivation of the main normal fault ceases between 50% and 100% inversion, and low-angle thrust faults with small displacements accommodate most shortening. The broad anticline above the original half graben expands laterally and grows vertically. With 200% inversion, the hangingwall and footwall of the main normal fault are cut by low-angle thrust faults with large displacements. This causes the main normal fault to appear to be more gently dipping with a ramp/flat or folded trajectory. Thrust faults cut the main normal fault and many of the secondary normal faults. The geometry of the hangingwall fold varies with depth. A tight anticline in the pre-growth layers changes to a syncline in the lower growth layers and a broad anticline in the upper growth and post-growth layers. The clay models suggest that both quantitative and qualitative methods would consistently underestimate inversion magnitude. If the footwall and hangingwall are elevated during inversion, then techniques using the amount of uplift above the regional datum would under-

18 136 G. EISENSTADT & M. O. WITHJACK estimate inversion magnitude. Similarly, if pervasive small-scale thrust faults and reactivated normal faults accommodate much of the shortening, then techniques using line-length restoration or null-point analysis would also underestimate the amount of shortening associated with inversion. A broad, gentle anticline and small-scale thrust faults are the only evidence of shortening in both the 50% and 100% inversion models. Amounts of inversion for either model would be underestimated because the large-scale faults and folds in those models are more typically associated with extension rather than compression. We thank Kris Meisling, Bruno Vendeville and Chris Banks for their thoughtful and critical reviews. Peter Hennings, Eric Peterson and Joana Vizgirda provided helpful discussions, and Charlie Wall assisted with the physical modelling. We also thank Mobil Research and Development Corporation and their management for support of this research and permission to publish. References BADLEY, M. E., PRICE, J. D. & BACKSHALL, L. C Inversion, reactivated faults and related structures: seismic examples from the southern North Sea. In: COOPER, M. A. & WILLIAMS, G. D. (eds) Inversion Tectonics. Geological Society, London, Special Publication, 44, BUCHANAN, P Geometries and Kinematic Analysis of Inversion Tectonics from Analogue Model Studies. PhD thesis, Royal Holloway and Bedford College, University of London. CARTWRIGHT, J. A The kinematics of inversion in the Danish Central Graben. In: COOPER, M. A. & WILLIAMS, G. D. (eds) Inversion Tectonics, Geological Society, London, Special Publication, 44, CHADWICK, R. A Aspects of basin inversion in southern Britain. Journal of the Geological Society, 150, CLOOS, E Experimental analysis of Gulf Coast fracture patterns. American Association of Petroleum Geologists Bulletin, 52, CLOOS, H Experimente zur inneren Tektonik. Centralblattf~r Mineralogies, Abt B, Kunstliche Gebirge, II. Natur und Museum, 60, DE PAOR, D. G. & EISENSTADT, G Stratigraphic and structural consequences of fault reversal: An example from the Franklinian Basin, Ellesmere Island. Geology, 15, MCCLAY, K. R Analogue models of inversion tectonics. In: COOPER, M. A. & WILLIAMS, G. D. (eds) Inversion Tectonics. Geological Society, London, Special Publication, 44, MITRA, S Geometry and kinematic evolution of inversion structures. American Association of Petroleum Geologists Bulletin, 77, SIMS, D The rheology of clay: a modeling material for geologic structures. Los, Transactions, American Geophysical Union, 569. STONELEV, R The structural development of the Wessex Basin. Journal of the Geological Society, 139, WILLIAMS, G. D., POWELL, C. M. & COOPER, M. A Geometry and kinematics of inversion tectonics. In: COOPER, M. A. WILLIAMS, G. D. (eds) Inversion Tectonics, Geological Society, London, Special Publication, 44, WITHJACK, M. O., ISLAM, Q. & Lg POINTE, P Normal faults and their hangingwall deformation: an experimental study. American Association of Petroleum Geologists Bulletin, 79, 1-18.