Insights in the exhumation history of the NWZagros frombedrock and detrital apatite fission-track analysis: evidence for a long-lived orogeny

We present the ¢rst ¢ssion-track (FT) thermochronology results for theNWZagrosBelt (SWIran) in order to identify denudation episodes that occurred during the protracted Zagros orogeny. Samples were collected from the two main detrital successions of the NWZagros foreland basin: the Palaeocene^early Eocene Amiran^Kashkan succession and theMiocene Agha Jari and Bakhtyari Formations. In situ bedrock samples were furthermore collected in the Sanandaj-Sirjan Zone. Only apatite ¢ssion-track (AFT) data have been successfully obtained, including 26 ages and11track-length distributions. Five families of AFTages have been documented from analyses of in situ bedrock and detrital samples: pre-middle Jurassic at 171 and 225Ma, early^late Cretaceous at 91Ma, Maastrichtian at 66Ma,middle^late Eocene at 38Ma and Oligocene^early Miocene at 22Ma.The most widespread middle^late Eocene cooling phase, around 38Ma, is documented by a predominant grain-age population in Agha Jari sediments and by cooling ages of a granitic boulder sample. AFTages document at least three cooling/denudation periods linked to major geodynamic events related to the Zagros orogeny, during the lateCretaceous oceanic obduction event, during the middle and late Eocene and during the earlyMiocene. Both late Cretaceous and early Miocene orogenic processes produced bending of the Arabian plate and concomitant foreland deposition. Between the two major £exural foreland episodes, the middle^late Eocene phase mostly produced a long-lasting slow- or nondepositional episode in the inner part of the foreland basin, whereas deposition and tectonics migrated to theNE along the Sanandaj-Sirjan domain and its Gaveh Rud fore-arc basin. As evidenced in this study, the Zagros orogeny was long-lived and multiepisodic, implying that the timing of accretion of the di¡erent tectonic domains that form the Zagros Mountains requires cautious interpretation

Timing and spatial distribution of deformation in the Newfoundland Appalachians: a "multi-stage collision" history

The Newfoundland Appalachians have been interpreted as an area where Lower Paleozoic plate convergence culminated in collision between an Ordovician volcanic chain and the North American craton hi Middle Ordovician times. Closure of the intervening proto-Atlantic (Iapetus) ocean was considered incomplete. Subsequent deformation gave rise to regional folding and faulting.Recent studies in the Newfoundland Dunnage zone have revealed that the deformation history is far more complex than previously recognized. Large-scale thrusting, folding and faulting occurred in Silurian-Devonian times. Furthermore, it has been suggested that the Dunnage zone is an allochthonous terrane underlain by dominantly continental crust rather than representing remnants of a "rooted" ocean basin.In view of these results a revision of tectonic scenarios and zonal subdivision is warranted and a "multi-stage collision" history will be discussed, with emphasis on the spatial distribution and significance of Silurian-Devonian deformation in central Newfoundland.Subduction in Lower Paleozoic times gave rise to the formation of a volcanic terrane; concurrently, to the southeast a marginal sea was formed (Mariana-type subduction). In Middle Ordovician times the volcanic terrane collided with the North American craton ("first-stage collision") and back-arc spreading terminated. Continued crustal shortening resulted in the formation of a Silurian accretionary terrane (telescoped marginal sea), and its subsequent deformation ("second-stage collision"). Devonian (-Carboniferous?) strike-slip faulting represents the third stage in the collision history.The model is applicable to large tracts of the Caledonian-Appalachian chain. Its main characteristics are: 1. (a) the revised zonal subdivision of the area is based on characteristics of Silurian and older rocks, rather than Middle Ordovician and older rocks only;2. (b) the central part of the orogen represents a telescoped marginal sea that formed to the southeast of the Ordovician volcanic chain, rather than a remnant of the incompletely closed Iapetus ocean;3. (c) the earliest deformation is progressively younger toward the southeast;4. (d) the Appalachian collision history is a result of the activity of a single deformation regime over a long period of at least 75 Ma.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/26738/1/0000289.pd

Evidence for pre-folding vein development in the Oligo-Miocene Asmari Formation in the Central Zagros Fold Belt, Iran

International audienceIn order to understand the interplay between vein development and folding in the carbonates of the Oligo-Miocene Asmari Formation (one of the main hydrocarbon reservoir rocks) in Iran, several anticlines have been investigated in the central part of the Zagros folded belt. Combining observations of relative chronology between veins based on calcite-filling phases and crosscutting/abutting relationships, as well as aerial/satellite image interpretation on several anticlines allowed proposing a tectonic model highlighting the widespread development of veins and other extensional micro/meso-structures in the Central Zagros folded belt. Our data suggest that most of the veins affecting the Asmari formation predated the main Miocene-Pliocene folding episode. An early regional vein set striking N50° marked the onset of collisional stress build-up in the region. Then, N150° and N20° trending vein sets were initiated in response to local extension caused by large-scale flexure/drape folds above N-S and N140° basement faults reactivated under the regional NE compression. At the onset and during Miocene-Pliocene folding of the sedimentary cover, the early formed veins were reactivated (reopened and/or sheared) while duplexes, low angle reverse faults and thrusts formed. Beyond regional implications, this study puts emphasis on the need of carefully considering regional/local vein development predating folding as well as influence of underlying basement faults in models of folded-fractured reservoirs in fold-thrust belts

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

Early Silurian palaeolatitude of the Springdale Group redbeds of central Newfoundland: a palaeomagnetic determination with a remanence anisotropy test for inclination error

We studied the palaeomagnetism of red fine-grained sandstones and coarse siltstones of the early Silurian Springdale Group of central Newfoundland. At 10 sites, a high blocking temperature characteristic remanence carried by haematite was isolated. This remanence is shown to predate probable early Devonian folding. Anti-parallel north- and south-directed remanences through a 100 m section of redbeds and a positive conglomerate test on haematite-bearing volcanic clasts suggest absence of remagnetization. Inverting the south-directed sites and unfolding yields a characteristic remanence with a mean declination of 23.6" and a mean inclination of - 14.2" (myg5= 7.3", k = 45.4). The inclination corresponds to a probable early Silurian palaeolatitude of 7"s f 4". We find no significant difference between early Silurian palaeolatitudes for central Newfoundland north and south of the Red Indian Line suture, and conclude that the part of the Iapetus Ocean across the suture had narrowed to less than about 5" by the early Silurian. This is consistent with palaeomagnetic results from Britain and Ireland that suggest no more than a narrow Iapetus at low palaeolatitude by the early Silurian. We also tested whether we have underestimated palaeolatitude because of sediment compaction reducing remanence inclination from that of the early Silurian field. We measured anisotropy of the isothermal remanence (IRM) acquisition for one specimen from each stable site, finding that a field of 200 to 800 rnT applied at 45" to bedding produced an isothermal remanence oriented on average at 42" to bedding. Theory then predicts that sediment compaction caused less than 2" average inclination shallowing in the Springdale Group redbeds, and less than a 1" underestimation of palaeolatitude