Distortion Effects of Faults on Gravity Worm Strings Robin O Leary

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1 Distortion Effects of Faults on Gravity Worm Strings Robin O Leary Problem Can upward continued gravity anomaly worm strings be used as a tool to determine the dip direction of an offsetting structure where that structure is known to cross-cut lithologies (i.e. intersect the bedding at an angle) and consequently offset stratigraphy? Furthermore, does the amount of down-dip displacement affect the geometry of the worm pattern created from the anomaly? Approach In order to test the above queries, ModelVision Pro was used to create some basic 2_D geological models over which upward continued gravity anomalies could be created. From these upward continued gravity curves a string can be drawn through the locations of maximum horizontal gradients to create an upward continued worm line. Exaggerated Density Model A linear unit of 1000 m thickness with a dip of 15 east and a density of 4 g/cm 3 was created (Figure 1a). The background density was set to a value of 0 g/cm 3 in order to obtain an exaggerated anomaly response. The offsetting structure was then activated, displacing the lithology by approximately 2000 metres (Figure 1b). Figure 1a. Unit of stratigraphy (green) dipping 15 east and approximately 1000 m thick. Axes are in metres. Figure 1b. Stratigraphy offset 2000 m by a moderately (60 ) west dipping structure. Axes are in metres.

2 Figure 2a. The 0, 1000, 2000, 4000 m upward continuations of the gravity anomaly are shown (red lines) for the linear stratigraphic unit (from Figure 1a) set at a density of 4 g/cm 3. The blue line represents the upward continued maximum horizontal gradient string. Axes are in metres. Figure 2b. The 0, 1000, 2000, 4000 m upward continuations of the gravity anomaly are shown (red lines) for the displaced (2000 m reverse slip) stratigraphic unit (from Figure 1b) set at a density of 4 g/cm 3. The blue line represents the upward continued maximum horizontal gradient string. Axes are in metres.

3 The differences in gravity anomaly patterns and amplitudes at the 1000, 2000 and 4000 m upward continuations between Figures 2a and 2b were observed to be minimal. At ground level the continuous unit of stratigraphy (Figure 2a) shows a broader west skewed distribution compared to the offset stratigraphy in Figure 2b. Figure 2b is also west skewed but with a narrower distribution since the eastern side (footwall) of the structure has been down thrown and, as a result, contributes less to the overall observed anomaly. The apparent similarity of the gravity anomalies between Figures 2a & 2b results in their worm strings being alike with both suggesting an easterly dip emulating the geometry of the modelled body. The principal differences between the two figures are; a) with displacement of the thrusting structure the worm string s steepness is increased and, b) the actual worm string is shifted west by around 2000 metres. It is evident that the worm string corresponds to the east dip of stratigraphy rather than the west-dipping offsetting structure. The thrusting displacement of the structure distorts the worm string by steepening the gradient of the worm without any actual change in the stratigraphy s dip. The offset in stratigraphy results in the origin point of the worm string being shifted in the direction of the hangingwall by approximately 2000 metres. Figure 2b demonstrated that despite the 2000 m offset along a moderately westdipping fault, the maximum gradient change dips east indicating that the influence of the stratigraphic succession on the worm string is significantly greater than that of structural variations. To test this theory further, the dip of the offsetting structure was rotated from 60 west (Figure 2b) through vertical (Figure 3a) to 60 east (Figure 3b). The resultant gravity anomalies and worm strings of the moderately west, vertical, and moderately east dipping structures (Figures 2b, 3a, & 3b respectively) are all very similar. Regardless of major changes in the style (reverse or normal), dip direction, and down dip displacement of the offsetting fault, the pattern of upward continued gravity worm points remains consistent to the broad geometry of the actual stratigraphy. A steepening worm string and a hangingwall (west) skew distortion was created by the west-dipping thrust fault in Figure 2b. The opposite occurs to the gradient of the worm string in Figure 3b, where the east-dipping normal fault distorts the observed worm string by shifting it in the direction of the footwall (west) by approximately 4000 m. It also decreases (shallows) the gradient of the worm string implying that the bedding is more gently dipping than in reality. These distortion effects have major implications for inferring actual locations and dips of structures and stratigraphy in complex terrains from worm strings.

4 Figure 3a. The 0, 1000, 2000, 4000 m upward continuations of the gravity anomaly are shown (red lines) for the displaced (2000 m vertical slip) green stratigraphic unit set at a density of 4 g/cm 3. The blue line represents the upward continued maximum horizontal gradient string. Axes are in metres. Figure 3b. The 0, 1000, 2000, 4000 m upward continuations of the gravity anomaly are shown (red lines) for the displaced (2000 m extensional slip) green stratigraphic unit set at a density of 4 g/cm 3. The blue line represents the upward continued maximum horizontal gradient string. Axes are in metres.

5 Application to Eastern Goldfields 3D Model Knowledge developed through the previous synthetic models can be applied to the Zuleika Shear area of the Yilgarn Craton (Figure 4). Gravity and magnetic edge detection over this region suggests a weak vertical to steep easterly dip which is in conflict with previously interpreted steeply west-dipping data for the Zuleika shear. Here, the Zuleika shear crosscuts a sequence of Archaean sedimentary rocks: predominantly volcaniclastic sediments and conglomerates and consequently good edge detection is difficult to obtain. It is possible to infer with knowledge of the regional geology and the above tested hypothesis that the worms in this region are expressing the geometry of the lithology rather than the Zuleika shear. This implies that in this region the maximum gradient changes are seen across the stratigraphy and not created by the offsetting shear. Figure 4. The Zuleika shear where it crosscuts an Archaean sequence of volcaniclastic sediments and conglomerates in the Eastern Goldfields of W.A.

6 Non-Exaggerated (Realistic) Density Model The difficulty in the application of worm strings in determining the geometry of faults that cross-cut stratigraphy becomes even more apparent when we apply realistic densities to the package of rocks to mirror what would be expected in this Archaean greenstone terrain. In figures 5a-d, a geologically realistic set of rock densities was applied. The purple lithological unit which was previously given a density of 4.00 g/cm 3 is given a density of 2.95 g/cm 3 to symbolise an ultramafic unit. Surrounding this ultramafic unit are mafic packages (green) with a density of 2.85 g/cm 3 and the background density is set to 2.67 g/cm 3. Figure 5a shows a gently east-dipping linear package of ultramafic and mafic rocks (similar to Figure 1a). The gravity anomaly created from this model appears as a broad high within which minimal detail can be resolved. In figure 5b-d west, vertical, and east dipping faults are applied to the stratigraphy and still no variation in the overall gravity anomaly occurs. Figure 5a. The 0, 1000, 2000, 4000 m upward continuations of the gravity anomaly are shown (red lines) for the linear ultramafic package (purple) set at a density of 2.95 g/cm 3 and the surrounding mafic bodies (green) set at a density of 2.85 g/cm 3. The background density is 2.67 g/cm 3. Axes are in metres.

7 5b. 5c.

8 5d. Figures 5b-d. The 0, 1000, 2000, 4000 m upward continuations of the gravity anomaly are shown (red lines) for the displaced (5b m reverse slip; 5c m vertical slip; 5d m normal slip) ultramafic package (purple) set at a density of 2.95 g/cm 3 and the surrounding mafic bodies (green) set at a density of 2.85 g/cm 3. The background density is 2.67 g/cm 3. Axes are in metres. Summary & Conclusions Resolving actual orientations of structures that cross-cut stratigraphy is very difficult to achieve from gravity worm strings where an independent unit is given an exaggerated density and then displaced (Figures 2 & 3) let alone in a real scenario (Figure 5). Instead the worm strings always represent the geometry of stratigraphy. Furthermore, a structure s displacement which offsets stratigraphy distorts: a) the gradient of the worm string giving it an apparent sense of geometry which is different to that of the actual underlying stratigraphy. - In an extensional regime the gradient of the gravity worm string will decrease (shallow) regardless of the initial dip direction of the stratigraphy and without any actual changes in the dip of the stratigraphy after faulting (Figures 6a & 6b). - In a contractional regime the gradient of the gravity worm string will increase (steepen) regardless of the initial dip direction of the stratigraphy and without any actual changes in the dip of the stratigraphy after faulting (Figures 6c & 6d).

9 b) the origin point of the worm string shifting it in the direction where the offset portion of the anomalous body is closer to the surface regardless of tectonic regime (normal or reverse faulting) as seen in figure 6. 6a. 6b. 6c. 6d. Figure 6. Schematic distortion effects on gravity worm strings of faults offsetting stratigraphy. 6a: a normal fault with east-dipping stratigraphy; 6b: a normal fault with west-dipping stratigraphy; 6c: a reverse fault with east-dipping stratigraphy; 6d: a reverse fault with west-dipping stratigraphy. The blue worm string represents the pattern expressed prior to fault slip. The red line represents the distortion in worm pattern after the offsetting fault is activated.