Structure of the Mountain Front Flexure along the Anaran anticline in the Pusht-e Kuh Arc (NW Zagros, Iran): insights from sand box models

Abstract

The Mountain Frontal Flexure shows a single step along the front of the Pusht-e Kuh Arc with about 3 km of structural relief. This front has been interpreted as being formed by a basement monocline above a blind crustal-scale and low-angle thrust with a ramp–flat geometry (the ramp dips 12–15° towards the inner part of the orogen and cuts the entire crust). The Anaran anticline on top of the Mountain Frontal Flexure shows an irregular geometry in map view and consists of four segments with diverse directions of which the SE Anaran, the Central Anaran and the NW Dome are culminations. The North–South Anaran segment may form a linking zone developed during the rise and amplification of single culminations, the NW Dome and the Central Anaran, above the Mountain Frontal Flexure. The asymmetric Anaran anticline is characterized by the existence of multiple normal faults, some of them with significant dip-slip displacements of up to 1000 m. These faults limit grabens located along the crests of the anticline segments. Cross-cutting relationships show that the normal faults along the Central Anaran are older than along the North–South Anaran, reinforcing the temporal constraints on the later growth of this segment of the anticline. The geometry of the Anaran anticline is asymmetric with the subvertical forelimb very little exposed. This forelimb is cut above and below by a thrust system that seems to develop along the fold hinges. The lower thrust, with a ramp–flat geometry, carries the entire anticline towards the foreland on top of slightly deformed rocks in the footwall. The thrust flattens in the Gachsaran evaporitic level forming a typical triangular zone filled with evaporites, which produce a strong fold disharmony between the overburden (Passive Group) and the underlying rocks (Competent Group). The growth of the Anaran anticline lasted for about 6 Ma and was the consequence of detachment folding that was subsequently thrust, rotated and uplifted above the Mountain Frontal Flexure with coeval reactivation of earlier crestal layer-parallel extension normal faults to accommodate the large increase of structural relief between the foreland and the tectonic arc. Three main results from analogue modelling have been combined with field data to resolve the geometry of the Anaran anticline as well as its evolution: (1) a thickening of intermediate evaporites (Gachsaran Formation) is produced above the flat segment of the thrust carrying the anticline on top of foreland strata; (2) growth strata deposited in the adjacent syncline modify the geometry of the anticline by increasing the dip and the length of its forelimb; (3) coeval erosion to anticline growth, as well as thick growth strata deposition, increases fold amplification rather than foreland propagation of deformation. The proposed fold model may be applied to other anticlines on top of this major basement-related thrust, such as the Siah Kuh and Khaviz anticlines in the Pusht-e Kuh Arc and Dezful Embayment domains.

Crustal-scale cross-sections across the NW Zagros belt: implications for the Arabian margin reconstruction

“…Similarly to other regions of the Zagros, the present-day seismicity (Berberian, 1995;Engdahl et al 2006;Tatar, Hatzfeld & Ghafori-Ashtiany, 2004) and topography strongly point to the involvement of the whole crust in the deformation of the Pusht-e Kuh arc (e.g. Falcon, 1961;Emami et al 2010;Hatzfeld & Molnar, 2010;Leturmy, Molinaro & Frizon de Lamotte, 2010). Our calculations along the geological cross-section show less than 1 • of topographic slope and 0 • for basal detachment angle, in agreement with other similar estimations (McQuarrie, 2004;Ford, 2004).…”

“…6). We propose a thrust system that consists of a basal low-angle and NE-dipping crustal-scale thrust (T4), which is in agreement with the geometry of the Mountain Front Flexure and the high and fairly constant topography and structural relief of the entire Pusht-e Kuh arc (Emami et al 2010). The geometry of the basal thrust is comparable with the one proposed by Mouthereau, Lacombe & Meyer (2006).…”

“…Detailed work carried out in the Lurestan region for individual anticlines shows 13.5 % shortening in the Kabir Kuh anticline ) and 11.5 % shortening in the Anaran anticline (Emami et al 2010) (see locations in Fig. 3).…”

“…The boundary between these two domains is the Mountain Front Flexure (MFF; also known as Mountain Front Fault), a major morphotectonic discontinuity that uplifts the Simply Folded Belt by a few kilometres with respect to the foreland (Falcon, 1961;Berberian, 1995;Sepehr & Cosgrove, 2004;Emami et al 2010). The Mountain Front Flexure is marked by intense seismic activity occurring at depths between 10 and 20 km (Engdahl et al 2006), and displays a sinuous trace that defines two salients (Lurestan or Pusht-e Kuh arc and Fars arc) and two reentrants (Kirkuk Embayment and Dezful Embayment) (Fig.…”

“…Cross-sections presented in Figure 4 illustrate some of these problems, which were dealt with in two different ways: (a) assuming that shortening is constant through single anticlines, such as the Anaran (Emami et al 2010) and Kabir Kuh anticlines ) (Figs 4a and 4b, respectively); and (b) assuming that shortening is different above and below some of the main detachments and thus must be transferred to the front or to the back by thrusting (e.g. Blanc et al 2003;Casciello et al 2009) (Fig.…”