RIFT, RIFTING, TRANSTENSION AND NORMAL FAULTS:
A MINI NORTH SEA SYSTEM?
Geologic context about this 10 cm – scale duplex structure:
Rift systems (hosting horsts and grabens) are elongated and relatively narrow through of regional extension that are bounded by normal faults. Rifts are the places where the Lithosphere has broken up in response to extension. Rifting is major step of the Wilson Cycle (1968). The complex interactions between tectonics, magmatism, metamorphism, erosion and depositional processes in such context leads to assess each given study area as a particular case with its own specifities.
A in conceptual point of view, pre-rift rocks were present prior to the normal faults activity (the extension). Syn-rift deposits are synchronous with faults activity. Post-rift deposits postdates the fault activity. The reality is often much more complicated because the modalities of the extension (%, direction) are not homogeneous in both space and time (polyphased deformation).
In this study case, we provide a sample showing pre-rift deformation at the cm-scale. Using the self-similarity* concept (e.g. Mishra, and Bhatnagar, 2014), the observed geometries can be nicely compared to the North Sea rift, to the Viking Graben or for instance the Gullfaks field, in the northern North Sea (e.g. Fossen and Hesthammer, 2000, Fig.1).
Sample details:
The sample is made of a flint layer that has been affected by fracturing and normal faulting at around 3 km of depth. The fault/fracture cements are calcite.
How could we use this outcrop structural interpretation?
Seismic-scale geometries:
The main fault is located to the right of the picture. The fault zone is curved and is low-dipping into its lower part. Secondary faults and fractures are extensively developped into its hanging wall in which individualized fault blocks can be identified. Along the cross-section, faults and fractures have an almost normal fault « andersonian-style » with fault dipping around 70-80° (Anderson, 1905) whereas the surface analysis (or a kind of seismic time-slice analysis) shows at least two main fault/fractures strikes. We propose that in this case, there was not any inherited structures into the flint. This means that the deformation was either (i) poly-directional, (ii) with a strike-slip component or (iii) formed under a main stresses ratio leading to multi-directional / conjugate normal faulting (Angelier, 1989).
* »Self-similarity » refers to the idea of repeating a similar shape (often at a different scale) over and over again. In other words, a self-similar image contains copies of itself at smaller scales (Mishra and Bhatnagar, 2014).
Feel free to constructively comment this open discussion. Feel free to provide relevant references where similar structural geometries, sedimentary or diagenetic contexts, have been recognized or forecasted.
Scientific references:
Anderson, E.M., The dynamics of faulting, Trans. Edinburgh Geol. Soc., 8 (3), 387-402, 1905.
Mishra, P., Bhatnagar, G., 2014, Self-similarity: At right angles 3.2, 23-30
Wilson, T., Static or mobile Earth: The current scientific revolution. Proc. Am. Philos. Soc. 112, 309 (1968).
How could we use this outcrop structural interpretation?
Seismic-scale geometries:
The main fault is located to the right of the picture. The fault zone is curved and is low-dipping into its lower part. Secondary faults and fractures are extensively developped into its hanging wall in which individualized fault blocks can be identified. Along the cross-section, faults and fractures have an almost normal fault « andersonian-style » with fault dipping around 70-80° (Anderson, 1905) whereas the surface analysis (or a kind of seismic time-slice analysis) shows at least two main fault/fractures strikes. We propose that in this case, there was not any inherited structures into the flint. This means that the deformation was either (i) poly-directional, (ii) with a strike-slip component or (iii) formed under a main stresses ratio leading to multi-directional / conjugate normal faulting (Angelier, 1989).
* »Self-similarity » refers to the idea of repeating a similar shape (often at a different scale) over and over again. In other words, a self-similar image contains copies of itself at smaller scales (Mishra and Bhatnagar, 2014).
Feel free to constructively comment this open discussion. Feel free to provide relevant references where similar structural geometries, sedimentary or diagenetic contexts, have been recognized or forecasted.