A 4D Analogue Modeling Study Assessing the Effects of Transtension and Inherited Structures on Rift Interaction

Abstract:

The interaction of individual rift segments determines the evolution of a rift system and subsequent continental break-up. Inherited heterogeneities control where initial rifts will form and since these are often not properly aligned, rift segments form separately and need to interact. Another important factor affecting rift-segment interaction is the obliquity of plate divergence (transtension), which also promotes eventual continent break-up (Brune et al., 2012).

Both analogue and numerical techniques have been used to model rift interaction (e.g. Acocella et al., 1999; Allken et al., 2012) but transtension has never been applied. Here we present a first-order analogue study that elaborates upon earlier studies by assessing the effects of (1) transtension, (2) rift offset and (3) presence and geometry of inherited weak zones that link rift segments. An improved analogue set-up allows more freedom in inherited structure geometry and model analysis with X-Ray Computer Tomography (CT) techniques reveals internal structures with time (Fig. 2 and 3).

Our experiments yield the following conclusions:

1. Increasing the degree of transtension (decreasing angle α in Fig. 1) controls general rift structures: from wide rifts in orthogonal divergence settings to narrower rifts with oblique internal structures under transtensional conditions to narrow strike-slip dominated systems towards the strike-slip domain;
2. Rift linkage through transfer zones (hard linkage) is generally promoted by 1) decreasing rift offset and 2) increasing the degree of transtension. However, initial rift linkage might involve relay ramps (soft linkage) due to the interplay of divergence direction and rift offset;
3. Inherited rift-linking weak zones have little effect on rift interaction unless they are oriented ca. perpendicular to the divergence direction;
4. Since the orthogonal divergence models resemble natural examples (Fig. 3), our transtension models might predict what structures can be expected in natural transtension settings.

Work in progress involves the influence of more complex inherited structure geometries under sinistral and dextral transtension. Future work will test the effects of changing divergence rate along strike (scissorlike divergence).