No abstract

One of the major debated aspects of the Zagros orogenic system is the timing of onset of continental collision between Arabia and Eurasia. The Zagros hinterland in the Kurdistan region of Iraq contains a ca. 2 km-thick clastic depositional sequence of the Red Bed Series (RBS) that rests unconformably on the Arabian foreland and structurally below the Main Zagros Fault, which carries the allochthonous volcaniclastic rocks of the Walash-Naopurdan groups. Detrital zircon (DZ) U-Pb geochronology constrains both the depositional age and the provenance of the RBS and pinpoint the timing of initial arrival of Eurasian sediment on the Arabian plate. The youngest DZ U-Pb ages for the laterallyextensive (ca. 150 km) basal RBS (Suwais unit) imply a middle Oligocene (ca. 26 Ma) maximum depositional age. The provenance data reveal dominant DZ U-Pb age modes of late Paleocene (~55-60 Ma) and middle Eocene (~37-44 Ma) and, importantly, presence of ca. 10-15% DZ grains that are unequivocally derived from Eurasia, incl. Jurassic (150-200 Ma) and late Paleozoic (270-380 Ma) DZ age modes. These data suggest that the RBS deposits were mainly sourced from forearc/arc-related terranes along the SW margin and hinterland of Eurasia. We advocate that by ca. 26 Ma Neotethys oceanic crust had been consumed and that Arabia-Eurasia continental collision well was underway as indicated by deposition of strata with Eurasian provenance on the Arabian margin. These DZ U-Pb data from the RBS highlight the significance of provenance data from synorogenic deposits in revealing the timing of initial continent collision by document the earliest arrival of upper-plate sediment on the lower plate.

Cenozoic exhumation patterns in the internal and external Zagros reveal a long‐term deformation record associated with geodynamic restructuring of Arabia‐Eurasia collisional zone from continental subduction to plate suturing, which can be evaluated from thermochronometric, provenance, and subsidence analyses. Thermal modeling of zircon and apatite (U‐Th)/He ages and apatite fission track data from the Sanandaj‐Sirjan Zone (SSZ) indicates exhumation and inferred uplift along the leading edge of Eurasia starting in the Late Eocene (~35 Ma), coeval with initial foreland flexural subsidence of Arabia. Together with deceleration in Arabia‐Eurasia convergence and diminished subduction‐related magmatism, these events signal the final Neotethys closure and onset of long‐term (15–20 Myr) Arabian continental subduction beneath Eurasia, facilitated by the attenuated architecture of the precollisional Arabian margin. From 35 to 20 Ma, crustal shortening was relatively subdued and restricted to areas along the Arabia‐Eurasia plate boundary and diffuse inversion structures within continental interiors. Acceleration in SSZ cooling/exhumation rates from 19 to 16 Ma was synchronous with rapid basin subsidence and clastic progradation in the Zagros foreland. These events were contemporaneous with 20‐ to 16‐Ma surge in calc‐alkaline magmatism in central Iran and may have been linked to reorganization/deflection of Arabian plate vectors during the main phase of Red Sea rifting at 19–18 Ma. Transition from continental subduction to Arabia‐Eurasia suturing by ~12 Ma forced a transfer of strain from the subduction zone to intraplate deformational structures. This was marked by rapid outward expansion of the Zagros orogen, involving a shift in exhumation from the SSZ to Zagros fold‐thrust belt and Iranian plate interior.

In the northwestern sector of the Zagros foreland basin, axial fluvial systems initially delivered fine‐grained sediments from northwestern source regions into a contiguous basin, and later transverse fluvial systems delivered coarse‐grained sediments from northeastern sources into a structurally partitioned basin by fold‐thrust deformation. Here we integrate sedimentologic, stratigraphic, palaeomagnetic and geochronologic data from the northwestern Zagros foreland basin to define the Neogene history of deposition and sediment routing in response to progressive advance of the Zagros fold‐thrust belt. This study constrains the depositional environments, timing of deposition and provenance of nonmarine clastic deposits of the Injana (Upper Fars), Mukdadiya (Lower Bakhtiari) and Bai‐Hasan (Upper Bakhtiari) Formations in the Kurdistan region of Iraq. Sediments of the Injana Formation (~12.4–7.75 Ma) were transported axially (orogen‐parallel) from northwest to southeast by meandering and low‐sinuosity channel belt system. In contrast, during deposition of the Mukdadiya Formation (~7.75–5 Ma), sediments were delivered transversely (orogen‐perpendicular) from northeast to southwest by braided and low‐sinuosity channel belt system in distributive fluvial megafans. By ~5 Ma, the northwestern Zagros foreland basin became partitioned by growth of the Mountain Front Flexure and considerable gravel was introduced in localized alluvial fans derived from growing topographic highs. Foredeep accumulation rates during deposition of the Injana, Mukdadiya and Bai‐Hasan Formations averaged 350, 400 and 600 m/Myr respectively, suggesting accelerated accommodation generation in a rapidly subsiding basin governed by flexural subsidence. Detrital zircon U‐Pb age spectra show that in addition to sources of Mesozoic‐Cenozoic cover strata, the Injana Formation was derived chiefly from Palaeozoic‐Precambrian (including Carboniferous and latest Neoproterozoic) strata in an axial position to the northwest, likely from the Bitlis‐Puturge Massif and broader Eastern Anatolia. In contrast, the Mukdadiya and Bai‐Hasan Formations yield distinctive Palaeogene U‐Pb age peaks, particularly in the southeastern sector of the study region, consistent with transverse delivery from the arc‐related terranes of the Walash and Naopurdan volcano‐sedimentary groups (Gaveh‐Rud domain?) and Urumieh‐Dokhtar magmatic arc to the northeast. These temporal and spatial variations in stratigraphic framework, depositional environments, sediment routing and compositional provenance reveal a major drainage reorganization during Neogene shortening in the Zagros fold‐thrust belt. Whereas axial fluvial systems initially dominated the foreland basin during early orogenesis in the Kurdistan region of Iraq, transverse fluvial systems were subsequently established and delivered major sediment volumes to the foreland as a consequence of the abrupt deformation advance and associated topographic growth in the Zagros.

The genetic analysis of fold and thrust belts is facilitated by tracking the evolution of their organic endowment (petroleum tectonics). Petroleum tectonic analysis of convergent orogenic systems provides an audit of the processes that control the deformation and kinematics of orogenic belts. The distribution and deformation paths of the organic endowment intervals are key factors in determining the petroleum system evolution of fold and thrust belts. This comparison of orogenic systems illustrates the importance of flexural v. dynamic processes, orogenic wedge taper, mechanical stratigraphy and inherited architecture on the creation, preservation and destruction of petroleum accumulations. The Zagros, Pyrenees, Sevier and Beni Sub-Andean convergent systems share key characteristics of fold and thrust belts, with major differences in scale, degree of incorporation of organic endowment in evolution of the fold and thrust belt and its foreland, and preservation of fold and thrust belt wedge-top deposits. The Zagros is an orogen dominated by flexural processes that is a perfect storm for hydrocarbon generation and preservation. Its multiple stacked sources ensure continuous hydrocarbon generation while stacked detachments foster a low taper and thick wedge-top basins. The Pyrenees is also a flexurally dominated orogen, but the early consumption of its source rocks led to minimal survival of hydrocarbon accumulations during exhumation in a long lasting, high-taper orogenic wedge. The Sevier was initially a flexural orogen that was later dominated by dynamic uplift of the fold and thrust belt and distal foreland subsidence with foreland deformation. The consumption of its pre-orogenic sources during the early low-taper phase indicates a probable robust petroleum system at that time. However, the late high-taper phase exhumed and destroyed much of the early petroleum system. The addition of syntectonic foreland sources to be matured by both local and dynamic subsidence created an additional later set of petroleum systems. Post-orogenic events have left only remnants of world-class petroleum systems. The Beni segment of the Sub-Andean Orogen is a flexural system with probable dynamic overprints. Its most robust petroleum system probably occurred during its early low-taper flexural phase, with dynamic subsidence enhancement. Its late high-taper phase with possible dynamic uplift shuts down and stresses the petroleum systems. Comparison of these orogenic systems illustrates the importance of flexural v. dynamic processes, orogenic wedge taper kinematics, mechanical stratigraphy, distribution of source rocks relative to shortening and inherited architecture on the creation, preservation and destruction of petroleum accumulations in fold and thrust belts. Resource studies show that 14% of worldwide discovered hydrocarbon reserves are within the fold and thrust belts along convergent plate boundaries (Cooper 2007). Oil and gas exploration in fold and thrust belts is risky due to the complexity in tectonic style, structural a...