Structural geology

Stressed rocks

Credit: ROB BUTLER

When subjected to stress, the Earth's crust responds by breaking or bending, depending on how hot the going gets. When the crust breaks, it develops ruptures, and rock masses are displaced along these faults. Where exactly and how faulting will occur depends to a large extent on the nature of the stresses. But pre-existing flaws within the crust could also make it more susceptible to breaking.

To find out how important the pre-existing flaws are, Rob Butler of the University of Leeds and his colleagues headed to the remote Nanga Parbat region of Pakistan (Jour. Geol. Soc. 165, 449–452; 2008), an ideal spot to observe the Earth's crust under stress because the Indian subcontinent is thrusting into Eurasia here at a speed of a few centimetres per year. Moreover, until about a million years ago, the Nanga Parbat rocks lay at depth within the crust. When they popped up to the surface, they lacked significant faults or fractures. Therefore, all faults that we see today formed under the same stress regime.

The researchers measured the orientations of smaller faults that formed in the vicinity of the Liachar Thrust — a major fault where a massive slab of rock is being shoved upon another. They also investigated the direction of movement along these smaller associated faults, along with the pre-faulting fabric of the rocks and the nature of the present-day stress regime.

It turns out that even the Nanga Parbat rock mass was not entirely flawless prior to the latest faulting episode, which began about a million years ago. Deformation at high temperatures in the geologic past had forced it to flow in a ductile manner. The minerals within the rock aligned in response to this flow, resulting in numerous parallel weak planes, or foliation. Butler and colleagues found that during the more recent brittle faulting related to the Liachar Thrust, the foliation has affected faulting on either side of the thrust differently.

The orientations of faults in the rock mass above or in the hanging wall of the Liachar Thrust match those of the foliation closely, and most faults are parallel to each other. Here, the foliation is gently inclined and aligned favourably with the stress regime and was therefore easily reactivated as faults. Contrarily, in the rock mass constituting the footwall of the Liachar Thrust, the more steeply inclined foliation is aligned almost perpendicular to the direction of the regional stresses and thus escaped reactivation. Faults that developed in the footwall cut across the foliation, intersect each other and are randomly oriented.

Faults are important conduits for water, oil and mineralizing fluids. The different dispositions of the faults above and below the Liachar Thrust could give clues on the pattern of fluid flow in these masses.

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Bondre, N. Stressed rocks. Nature Geosci 1, 293 (2008). https://doi.org/10.1038/ngeo190

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