Fractures often form during folding..
Fractures often form as a result of the stresses generated during folding. These local stresses are different to the regional stresses responsible for the formation of the fold itself. There are a few models which can be used to predict distribution of fold-related fractures. The Stearns model [1] is shown on Fig. 1.
Fig. 1. Stearns model of fractures that are caused by folding. Resulting from work carried out on the Teton Anticline in Wyoming (Stearns & Friedman, 1972). Lower shows extension (yellow) and compressive (blue) movements related to Type 2 and Type 1 fractures respectively.
Our example is a hand specimen fold..
The fold in Fig. 2 is a hand specimen (about 8 cm across) from the Sao Francisco Basin, onshore Brazil [2]. Alternating carbonate - shale layers have been folded into a tight (chevron) fold. Note how the shales are thicker in the hinge zone than the more competent carbonate layers. This is because a void is created as a result of the more competent units more or less retaining their thickness. The void then gets filled by the more incompetent shales [3].
Fig. 2. Late Precambrian Bambui Group carbonates of the Sao Francisco Basin onshore Brazil. Folded during the Cambrian. Sample is about 8 cm across. The limestones are also laminated; possibly part algal in origin. Credit: Andy Racey.
And the fractures ?
I’m particularly interested in the fracture network on the limbs of this anticline. On Fig. 3b. we are looking at the limb on the right hand side of Fig. 3a. We interpret 2 fracture sets (Fig 3c); the red set runs from top right to bottom left and the pink set from top left to bottom right. There is no dominant set. We can interpret these as belonging to Stearn’s Type 2 category (Fig 1) resulting from local extension during folding. This hand speciment fold can be used as an analogue for fold-related fracturing in tight folds of similar geology (mechanical stratigraphy).
Fig. 3. View of right hand side fold limb (middle) and interpretation of 2 fracture sets in red and pink (right). Photos: Andy Racey.
In conclusion..
Fractures often form as a result of folding. Their distribution is related to fold - related stresses, not those that cause the folding in the first place. The hand-specimen example here is ideal to study the resulting fracture network for a tight fold. These observations can be used together with outcrop work to predict the distribution of fold-related fractures.
Reference
[1] Stearns D.W. & Friedman, M. 1972. Reservoirs in fractured rock. In: Stratigraphic oil & gas fields – classification, exploration methods and case histories. R.E. Kring (ed). AAPG Memoir, 16, 82-106.
[2] Martins-Neto, M.A., Pedrosa-Soares, A.C., Lima, S.A.A. 2001. Tectono-sedimentary evolution of sedimentary basins from Late Paleoproterozoic to Late Neoproterozoic in the Sao Francisco craton and Aracuai fold belt, eastern Brazil. Sedimentary Geology 141 - 142, 343 - 370.
[3] Ramsay, J. G., 1974, Development of chevron folds: Geological Society of America Bulletin, v. 85, p. 1741-1754.