A mountain of folds: triangle zone in the fold-and-thrust belt of the External Pamir, Kyrgyzstan

Triangle zones are thrust sheets or stacks of thrust sheets underlain by foreland-directed thrusts and overlain by a kinematically linked “passive” roof thrust - a backthrust - directed towards the hinterland. They are not uncommon in thin-skinned fold-and-thrust belts. Most triangle zones are known from seismic data and drilling. We describe a km-scale example exposed on a flank of the Altyn Dara valley near the thrust front of the Pamir mountains in Kyrgyzstan. The External Pamir is a high-level thrust belt built from non-metamorphic strata of Permian to Neogene age. It is bounded on its internal, southern side by the Main Pamir thrust with metamorphic rocks in its hanging-wall and in the north by the Pamir Frontal thrust which juxtaposes it with undeformed foreland strata of the Alai valley.

The triangle zone has formed where the basal detachment of the External Pamir ramps up from Lower Cretaceous redbeds into a succession of Upper Cretaceous marine pelites with a few intercalated limestone horizons. The strongly deformed Upper Cretaceous strata are contained between a north-directed thrust and a south-directed backthrust, both of which carry Lower Cretaceous rocks in their hanging-walls. In stark contrast to classical models, the core of the triangle zone is occupied by a bundle of essentially unfaulted, isoclinal upright folds. The subvertical axial planes diverge slightly upwards and changing elevations of the synclinal troughs suggest an anticlinorium. This structure is exposed over a vertical distance of 1 km in the steep flank of Pik Sverdlova. The folds involve four shaly packages and three limestone horizons. The initial total thickness of this succession was about 500 m. A strong slaty cleavage is developed in the shales, but the limestones do not show marked thickness variations between the long, straight fold limbs and the tight but rounded hinges. Assuming negligible penetrative strain in the limestones, unfolding the sinuous bed length suggests 10 km of horizontal shortening accommodated by folding.

Its overall geometry suggests that the triangle zone originated as a wide zone of detachment folding above a thrust fault propagating at the base of the weak Upper Cretaceous shales. The strong contraction may indicate some kind of buttress towards the foreland such as a syndepositional fault against which the Cenomanian-Turonian succession thinned or terminated, or the backthrust itself if it initiated early on. At any rate, the highly shortened bundle of folds was at some point bypassed along a deeper detachment in Lower Cretaceous strata into which the backthrust merges.

The internal structure of the Pik Sverdlova triangle zone would be difficult to image by conventional seismic techniques. Vertical drilling would also be unlikely to fully reveal the folded architecture. We speculate that in many triangle zones folding may be a more important mechanism than incorporated in the prevailing thrust-stacking models.