Inversion - Interpreting the Deformation Path - Why Does it Matter?*
By
A.D. Gibbs1
Search and Discovery Article # 40034 (2001)
*Adapted for online presentation from poster session by the author at the AAPG Convention, Denver, CO, June, 2001.
1Midland Valley Exploration Ltd, Glasgow, UK. (www.mve.com) ([email protected])
* Editorial Note: This article, which is highly graphic (or visual) in design, is presented as: (1) three posters, with (a) each represented in JPG by a small, low-resolution image map of the original; each illustration or section of text on each poster is accessible for viewing at screen scale (higher resolution) by locating the cursor over the part of interest before clicking; and (b) each represented by a PDF image, which contains the usual enlargement capabilities; and (2) searchable HTML text with figure captions linked to corresponding illustrations with descriptions.
Users without high-speed internet access to this article may experience significant delay in downloading some illustrations due to their sizes.
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Most models for inversion focus on understanding the 2D structural development of extension then compression, or occasionally compression then extension. Coupled with this are broad based models that account for the stacking of depositional and erosional packages. These are driven directly by the structural inversion. Inversion is dominantly a 3D process, whether it is driven by the extension - compression cycle, transtension or halokinetic movement. When modeled in 3D it becomes apparent that the 2D cross-sectional view under plays the importance of these systems. Many key commercial basins in Europe and elsewhere contain, or are dominated by, long lived inversion components. Therefore an understanding of the effect of these evolving geometries should be a vital concern. This poster illustrates, through a variety of inversion situations, some key influences on stratigraphy, hydrocarbon systems, and structural control of the deformation path. Key examples are illustrated by some simple 3D models. These highlight the impact of the inversion cycle but in particular the commercial impact of these situations is emphasized.
uInversion over complex faults
uInversion over complex faults
uInversion over complex faults
uInversion over complex faults
uInversion over complex faults
uInversion over complex faults
uInversion over complex faults
uInversion over complex faults
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Click here for sequence of cross sections.
Inversion
is defined as occurring when the tectonic These concepts are derived from 2D interpretations of regional systems where a change in regional stress can be demonstrated or inferred. Figure
1.2 is a cartoon of an inverted extension Inversion
also occurs without changes to regional tectonic stresses particularly where
salt movement imposes differential uplift and subsidence (Figure
1.3). Again
this leads to stacked and offset patterns of depocentres. As with These 2D cases represent very simple strain histories and the resulting geometries can be readily modeled in 2D. However, in many areas a 2D analysis can be misleading. In addition, the assumption that inversion is caused either by change in regional stress or by halokinesis is a further oversimplification. In many cases oblique faulting, salt displacement and regional changes are combined resulting in mixed inversion and non-inversion styles. Figure 1.4 shows lateral salt migration and faulting with depocentre migration and inversion of accommodation space.
Click here for sequence of A, B, and C.
Click here for sequence of A and B.
Dip slip inversion in 3D provides significant lateral changes along strike. Modelling demonstrates the importance of modelling the strike component of the system as well as the dip component. The
model in Figures 2.1, 2.2,
and 2.3 is a 3D equivalent of the simple cartoon shown in
Figure 1.2. Even small strike changes in the controlling During
extension (Figure 2.2A, 2.2B, 2.2C), Dip
slip inversion is realised in the model in two stages (Figure
2.3A-2.3B). In the
first stage (Figure 2.3A), the pre-extension phases are not completely inverted.
The cross basin influence of the change in the strike of the As shortening continues all of the depocentres become inverted (Figure 2.3B). With this model using only dip-slip extension followed by inversion, each dip section will be similar. Minor changes along strike combine to produce significant cross-basin or cross-structure elements in the sediment system as well as the structural culminations.
Inversion Over More Complex Faults
Click here for sequence of Figure 3.1A and B.
Click here for sequence of Figures 4.1A and 4.2A.
Changes
to Positioning
of the transfer or relay relative to the slip direction effects the separation
of depocentres (dark blue) both in the extension and contraction phases (Figure
3.2). Inverting on both the extension and the lateral Using the same model the effect of changing slip direction during inversion is seen in Figure 3.3. Basin parallel structures become much more segmented and lateral ramps, or relays in the earlier extension system rapidly become dominant in controlling architecture. Amplitude
of the inversion folds and rate of growth is controlled by the direction and
obliquity of slip. Offsets in the main Extension on oblique transfer controls sediment catchment areas and accommodation space development. In this model (Figure 4.1), accommodation space may be widely separated with different sediment catchment areas and sediment transfer routes. As the basin inverts, the highs may evolve in positions which are starved of the target sediments. The
3D architecture of the sediment packages during inversion provides different
traps, migration fairways, and catchment areas with time. The model (Figure
4.2)
shows two time stages (red and blue traps, with hydrocarbon migration pathways
yellow and pale blue, respectively) superimposed in the same model. This shows
changes in Structural highs migrate from yellow to blue position during inversion (Figure 4.2C).
Click here for sequence of Figure 5.1B and C.
2D models assume that salt is drawn and remains within the section; i. e., flow is radial into a dome or salt wall. This model from onshore Germany (Figure 5.1) was used to develop constraints on these assumptions in 3D. The area is composed of two downbuilt diapirs with cross section geometry similar to those shown in Figure 1.3. 3D balanced and restored models for the top salt are shown at two time stages (Figure 5.1B, C). These models show the salt catchment areas and flowage pathways for the salt at each of these time steps. Different coloured areas represent flow packages within the salt. Between the two stages the pattern of catchment areas changes with capture of the cells as the domes grow. As the salt flow pattern changes, there are time equivalent changes in the sediment accommodation space, and inversion occurs as one cell captures or migrates into the zone of influence of another through time.
Baldschuhn, R., U .Frisch, and F. Kockel, 1996, Section 37, in Tectonic Atlas of NW Germany: BGR, Hannover. The author wishes to thank the numerous colleagues and clients who have contributed to the development of this approach. The salt data is from Digitaler Geotektonischer Atlas von Nordwestdeutschland with analysis by Midland Valley. Stephen Calvert assisted in preparing the poster. |