Tertiary sequence of deformation in a thin-skinned/thick-skinned collision belt: The Zagros Folded Belt (Fars, Iran)

International audienceWe describe how thin-skinned/thick-skinned deformation in the Zagros Folded Belt interacted in time and space. Homogeneous fold wavelengths (15.8 ± 5.3 km), tectono-sedimentary evidence for simultaneous fold growth in the past 5.5 ± 2.5 Ma, drainage network organization, and homogeneous peak differential stresses (40 ± 15 MPa) together point to buckling as the dominant process responsible for cover folding. Basin analysis reveals that basement inversion occurred ∼20 Ma ago as the Arabia/Eurasian plate convergence reduced and accumulation of Neogene siliciclastics in foreland basin started. By 10 Ma, ongoing contraction occurred by underplating of Arabian crustal units beneath the Iranian plate. This process represents 75% of the total shortening. It is not before 5 Ma that the Zagros foreland was incorporated into the southward propagating basement thrust wedge. Folds rejuvenated by 3–2 Ma because of uplift driven by basement shortening and erosion. Since then, folds grew at 0.3—0.6 mm/yr and forced the rivers to flow axially. A total shortening of 65–78 km (16–19%) is estimated across the Zagros. This corresponds to shortening rates of 6.5–8 km/Ma consistent with current geodetic surveys. We point out that although thin-skinned deformation in the sedimentary cover may be important, basement-involved shortening should not be neglected as it requires far less shortening. Moreover, for such foreland folded belts involving basement shortening, underplating may be an efficient process accommodating a significant part of the plate convergence

The geologic development of the Bay d'Espoir area, southeastern Newfoundland

Bay d'Espoir on the south coast of Newfoundland exposes a section across the southeastern marginal metamorphic belt of the Newfoundland Appalachians. Two major tectonostratigraphic divisions are recognized; in the south the Little Passage Gneisses of the older division (probably Precambrian), consisting of amphibolitic and psammitic gneisses intruded by tonalite, are overlain by metavolcanic and metasedimentary cover rocks of the younger division, the Baie d'Espoir Group (possibly Ordovician). The Little Passage Gneisses have been intruded post-tectonically by a megacrystic potash feldspar granite. Both this granite and the enclosing gneisses are extensively mylonitised and reconstituted close to the contact with the cover rocks and the gneissic foliations have been largely destroyed. -- The Baie d'Espoir Group has undergone two, regionally penetrative deformations, the second of which has produced a major recumbent south-east facing anticline, the Bay d'Espoir Nappe. These deformations are the cause of the mylonitised and reconstituted zone along the contact with the Little Passage Gneisses. Garnetiferous leucocratic granite and associated aplites and pegmatites have intruded the Little Passage Gneisses, the megacrystic granite, and the Baie d'Espoir Group after its first deformation but before its second; the intrusion coincided with the metamorphic climax of the Baie d'Espoir Group. Sheets of this garnetiferous granite have been tightly folded with the reconstituted gneisses near the contact with the cover. -- It is concluded that the gneisses form the basement to the Baie d'Espoir Group and that the contact is now tectonic. Deposition of the Baie d'Espoir Group was along a continental margin which was initially of Atlantic-type but later changed to Andean-type. Deformation was probably caused by continental collision along the line of the Cape Ray Fault and the Lower Palaeozoic outcrop of central Newfoundland. Correlations with similar rocks in the Gander region, central Newfoundland, on the southwest coast of Newfoundland, and in Nova Scotia are proposed

The role of multiple weak lithologies in the deformation of cover units in the northwestern segment of the zagros fold-and-thrust belt

The geometry, kinematics and dynamics of fold-and-thrust belts are strongly influenced by the mechanical behaviour of the basal decollement. However, many fold-and-thrust belts also include mechanically weak lithologies such as evaporites and marls or mudstones at different levels within the shortened stratigraphy. The kinematics and dynamic evolution of these thrust belts are controlled by the mechanical behaviour both of the basal decollement and of the weak units embedded within the overlying stratigraphic succession. In the Zagros fold-and-thrust belt (ZFTB), the shortened sedimentary cover is between 7 and 12 km thick and mechanically weak lithologies compartmentalize the stratigraphic column at shallow and intermediate levels. In this paper, satellite, field and seismic data from the Kurdistan Region of Iraq are used to identify structures of different sizes and surface traces. The observations are used to underline the role of mechanically weak horizons within the Zagros stratigraphy and the decoupling of deformation both laterally and with depth in the belt. The decoupling between shallow and deeper structures observed in seismic profiles from the Kurdistan Region of Iraq is also reported from field observations from the Iranian part of the Zagros fold-and-thrust belt, where folds with different surface traces occur. Decoupling between shallow and deep layers by incompetent lithologies at intermediate depths (e.g. marls, mudstones and evaporites) results in the formation of disharmonic folds. The geometry, size and location of such folds may differ between outcropping and subsurface structures. Decoupling may have a significant impact on hydrocarbon exploration in different parts of the Zagros fold-and-thrust belt due to potential offsets between outcropping and subsurface structures and their associated traps