Bertrand Meyer

Zagros orogeny: a subduction-dominated process

This paper presents a synthetic view of the geodynamic evolution of the Zagros orogen within the frame of the Arabia–Eurasia collision. The Zagros orogen and the Iranian plateau preserve a record of the long-standing convergence history between Eurasia and Arabia across the Neo-Tethys, from subduction/obduction processes to present-day collision (from ~ 150 to 0 Ma). We herein combine the results obtained on several geodynamic issues, namely the location of the oceanic suture zone, the age of oceanic closure and collision, the magmatic and geochemical evolution of the Eurasian upper plate during convergence (as testified by the successive Sanandaj–Sirjan, Kermanshah and Urumieh–Dokhtar magmatic arcs), the P–T–t history of the few Zagros blueschists, the convergence characteristics across the Neo-Tethys (kinematic velocities, tomographic constraints, subduction zones and obduction processes), together with a survey of recent results gathered by others. We provide lithospheric-scale reconstructions of the Zagros orogen from ~ 150 to 0 Ma across two SW–NE transects. The evolution of the Zagros orogen is also compared to those of the nearby Turkish and Himalayan orogens. In our geotectonic scenario for the Zagros convergence, we outline three main periods/regimes: (1) the Mid to Late Cretaceous (115–85 Ma) corresponds to a distinctive period of perturbation of subduction processes and interplate mechanical coupling marked by blueschist exhumation and upper-plate fragmentation, (2) the Paleocene–Eocene (60–40 Ma) witnesses slab break-off, major shifts in arc magmatism and distributed extension within the upper plate, and (3) from the Oligocene onwards (~ 30–0 Ma), collision develops with a progressive SW migration of deformation and topographic build-up (Sanandaj–Sirjan Zone: 20–15 Ma, High Zagros: ~12–8 Ma; Simply Folded Belt: 5–0 Ma) and with partial slab tear at depths (~10 Ma to present). Our reconstructions underline the key role played by subduction throughout the whole convergence history. We finally stress that such a long-lasting subduction system with changing boundary conditions also makes the Zagros orogen an ideal natural laboratory for subduction processes.

Late Cenozoic and modern stress fields in the western Fars (Iran): Implications for the tectonic and kinematic evolution of central Zagros

[1]xa0The Zagros (Iran) developed during Mio-Pliocene times in response to Arabia-Eurasia convergence. The western Fars highlights a major bend of the deformation front and displays a remarkable set of nearly N-S right-lateral strike-slip faults (the Kazerun-Borazjan/Karebass/Sabz-Pushan/Sarvestan faults) oblique at high angle to the belt. The region likely plays a major kinematic role by accommodating the change in shortening modes from partitioned in the western central Zagros to nonpartitioned in the eastern Zagros. The inversion of focal mechanisms from small and moderate earthquakes shows a consistent N020°–030° compression with a low ratio between differential stresses. This regime accounts for the combination of strike-slip and thrust-type mechanisms through likely σ2/σ3 permutations. Fault slip analysis reveals two successive late Cenozoic regional compressional trends, NE-SW then N020°. The latter is in good agreement with the present-day stress. The significance of the NE-SW compression is discussed alternatively in terms of stress deviations or block rotations in relation to the strike-slip fault system. Fieldwork and satellite imagery suggest that these faults behave first as transfer faults during folding of the cover and later as strike-slip faults, in agreement with the succession of stress regimes and the evolution of the dominant deformation style from thin-skinned to thick-skinned. The first-order stability of the collision-related state of stress since ∼5 Ma supports that the Arabia-Eurasia convergence did not give rise to partitioning in the western Fars but rather was (and is still) accommodated by distributed deformation involving both shortening and strike-slip motion throughout the cover and the basement.

Transient, synobduction exhumation of Zagros blueschists inferred from P‐T, deformation, time, and kinematic constraints: Implications for Neotethyan wedge dynamics

[1]xa0We present the first P-T, deformation time, and kinematic constraints on the only known blueschist facies rocks (BS) present in the Zagros (Hajiabad area). The BS were underplated below the Sanandaj-Sirjan zone and crop out as kilometer-scale bodies within extensive colored melange units marking discontinuously the Neotethyan suture zone. P-T estimates point to high-pressure/low-temperature (HP-LT) conditions around 11 kbar and 520–530°C for the majority of BS, along a ∼15°C km−1 gradient. Some exotic blocks in matrix serpentinite reached 17–18 kbar at ∼500°C. In situ laser probe 40Ar-39Ar radiometric age constraints on phengite cluster between 85 and 95 Ma and suggest that (1) synconvergence exhumation of Zagros BS from 35–50 km to depths <15–20 km was accomplished before 80 Ma, (2) BS exhumation corresponded to a transient process with respect to the long-lived subduction beneath Iran (∼150–35 Ma), and (3) age constraints for Zagros BS are 5–10 Myr older than for the nearby Oman HP-LT rocks and broadly coincide with obduction processes in the region (circa 95–70 Ma). We propose that the mechanical coupling across the Neotethyan subduction zone (NSZ) beneath Iran was modified by the large-scale plate rearrangement accompanying obduction, allowing for a short-lived exhumation of Zagros BS. Exhumation ceased at the end of obduction, when subduction of the Arabian continental margin stopped. Kinematic calculations suggest that convergence velocities across the NSZ likely doubled (to ∼5–6 cm yr−1) during the period 118–85 Ma, so that BS exhumation may have been promoted by a combination of obduction movements and increased convergence velocities.

Rapid active thrusting along the northwestern range front of the Tanghe Nan Shan (western Gansu, China)

The western part of the Tanghe Nan Shan range southwest of Subei (western Gansu, China) is presently growing on thrust ramps splaying from the left-lateral Altyn Tagh Fault. Late Cenozoic thrusting has folded and sliced Oligocene-Miocence red beds into an imbricate wedge, capped by warped and uplifted Quaternary terraces that form a 2- to 5-km-wide ledge, north of the steeply faceted range front. Seismic scarps 1.5 to 4.5 m high cut young fans along the outer thrusts. Carbon 14 dating of organic remains collected on strath terraces constrains the chronology of deposition and incision by the streams. Most of the fans and terraces in the southern part of the Subei basin appear to have been emplaced after the last glacial maximum, particularly during the early Holocene optimum (9–5 ka). Measurements of the shapes of the warped terraces constrain minimum and maximum throws on the outer thrusts. The minimum vertical throw of 34 ± 2 m of a surface dated at 8411 ± 530 years B.P. at one site provides a minimum vertical uplift rate of 4.1±0.5 mm/yr. The maximum possible uplift (115±15 m) of the oldest terrace surface, whose probable age is 15 to 18 ka, places an upper bound on the uplift rate of 7±2 mm/yr. The thrust geometry at depth and the cumulative shortening (10–20 km) deduced from balancing sections logged across the imbricate thrust wedge are consistent with a shortening rate of about 5 mm/yr and an onset of thrusting at about 4±2 Ma. Such a shortening rate implies a significant northward decrease in slip rate along the Altyn Tagh Fault. The recent growth of the western Qilian mountain ranges thus appears to be intimately coupled with sinistral motion on the Altyn Tagh Fault and the extrusion of Tibet.

Strike‐slip kinematics in Central and Eastern Iran: Estimating fault slip‐rates averaged over the Holocene

[1]xa0According to GPS measurements, the right-lateral shear between Central Iran and Afghan blocks amounts to 16 mm/yr. A model based on very long-term estimates of fault-rates suggests the current shear originated about 5 Ma ago and has been accommodated by strike-slip faulting limited to the western (∼2 mm/yr, Gowk-Nayband fault) and eastern (∼14 mm/yr, Sistan system fault) edges of the Lut block. We have used high-resolution SPOT5 (pixel size 2.5 m) images to measure recent cumulative offsets and estimate slip-rates over shorter time periods that average several seismic cycles only. Recent offsets, a few tens of meters, have been found along the Anar fault inside the Central Iran plateau and along the Sistan faults east of the Lut. The offset-morphologies postdate the last incision of the network and are most probably of Holocene age (12 ± 2 ka). The corresponding slip-rates range between ∼0.5–0.75 mm/yr, ∼1.75–2.5 mm/yr, ∼1–5 mm/yr, ∼1–2.5 mm/yr for the Anar, East Neh, West Neh, and Asagie fault, respectively. These estimates suggest the GPS shear-rate across the Lut may not extrapolate over the Holocene. They also indicate strike-slip faulting is not confined to the Lut edges, but also occurs in Central Iran, suggesting the ongoing strike-slip tectonics might have originated between 8 and 22 Ma ago, earlier than considered previously and consistent with observations in NE Iran.

Buildup of a dynamically supported orogenic plateau: Numerical modeling of the Zagros/Central Iran case study

The Iranian plateau is a vast inland region with a smooth average elevation of c. 1.5 km formed nat the rear of the Zagros orogen as a result of the Arabia-Eurasia collision (i.e., over the last 30–35 Myr). This ncollision zone is of particular interest due to its disputed resemblance to the faster Himalayan collision, nwhich gave birth to the Tibetan plateau around 50 Myr ago. Recent studies have suggested that a recent n(10–5 Ma) slab break-off event below Central Iran caused the formation of the Iranian plateau. Here, we test nseveral hypotheses through large-scale (3082 n3 n590 km) numerical models of continental subduction mod- nels that incorporate a free upper surface erosion, rheological stratification, brittle-elastic-ductile rheologies, nand metamorphic phase changes (density and physical properties) and account for the specific crustal and nthermal structure of the Arabian and Iranian continental lithospheres. We test the impact of the transition nfrom oceanic to continental subduction and the topographic consequences of the progressive slowdown of nthe convergence rate during continental subduction. Our results demonstrate the role of mantle flow nbeneath the overriding plate, initiated as an indirect consequence of slab break-off. This flow creates a ndynamic topography support during continental subduction and results in delamination of the overriding nplate lithospheric mantle followed by isostatic readjustment, hence of further uplift and maintenance of a nplateau-like topography without significant crustal thickening. The slowdown of the convergence rate dur- ning the development of the continental subduction/collision phase largely contributes to this process by ncontrolling the timing and depth of slab break-off

Height of faceted spurs, a proxy for determining long-term throw rates on normal faults: Evidence from the North Baikal Rift System, Siberia

[1]xa0We present new results on the long-term throw rates of active normal faults in the North Baikal Rift (NBR), eastern Siberia, based on a statistical analysis of triangular faceted scarps. Fault-bounded ridges in the NBR display typical morphologies with several contiguous facets separated by fault-perpendicular catchments. Over a range of 20 fault segments analyzed, triangular facet heights vary from ∼200 to >900 m. As fault scarps have been developing under similar long-term climatic conditions, we infer that the scatter in mean facet height arises from long-term differences in fault throw rate. We compare the morphology of NBR facets with results obtained in a previously published numerical model of facet growth. Using facet height as an input, model results provide estimates of the long-term fault throw rate. NBR throw rates vary between 0.2 and 1.2 mm yr−1. The throw rates are then compared with the cumulated throw, which has been constrained by geophysical and stratigraphic data in the basins. This provides an estimate of the age of fault and basin initiation. We show that the modern stage of basin development started circa 3 Myr ago, except for the North Baikal basin (∼8 Ma). Our results also suggest that a proportion of the observed throw is inherited from an earlier tectonic stage.

Late Pleistocene‐Holocene right slip rate and paleoseismology of the Nayband fault, western margin of the Lut block, Iran

The 290-km-long, Nayband strike-slip fault bounds the western margin of the Lut block and cuts across a region thought to have been quiescent during the last few millennia. Cl-36 cosmic ray exposure (CRE) and optically stimulated luminescence (OSL) dating of cumulative geomorphic offsets are used to derive the long-term slip rate. The measured offsets at two sites along the fault range between 9 ± 1 m and 195 ± 15 m with ages from 6.8 ± 0.6 ka to ∼100 ka, yielding minimum and maximum bounds of late Pleistocene and Holocene slip rates of 1.08 and 2.45 mm yr-1, respectively. This moderate slip rate of 1.8 ± 0.7 mm yr-1, averaged over several earthquake cycles, is compared to the paleoseismic record retrieved from the first trench excavated across the fault. Combining the paleoseismic evidence with 18 OSL ages obtained from this trench site demonstrates the occurrence of at least four large (M ∼7) earthquakes during the last 17.4 ± 1.3 ka and of two older wearthquakes, one before ∼23 ka and another before 70 ± 5 ka. The exposed sediment succession also indicates a significant gap at the end of MIS-2 and the beginning of MIS-1. The age of the most recent regional incision is accurately bracketed between 6.1 ka and 7.4 ka. Sediments from the last ∼7 ka contain evidence of the three younger earthquakes. Interestingly, the penultimate and antepenultimate events occurred between 6.5 ± 0.4 ka and 6.7 ± 0.4 ka within a time interval lasting at most 1 ka whereas the most recent earthquake occurred within the last millennium. Such an irregular earthquake occurrence suggests the seismic behavior of the Nayband fault is not strictly time dependent but possibly related to clustering. From this and taking into account the occurrence of the most recent earthquake within the last 800 years, the imminence of an earthquake along the Nayband fault cannot be discarded. Although the most recent surface-rupturing event seems to have occurred after AD 1200, this event went unnoticed in the historical records. This provides a marked illustration of the incompleteness of the historical seismic catalogs in Central Iran, challenging any assessment of regional seismic hazard without appropriate geologic and geochronological information. Large and infrequent earthquakes are characteristic of the seismic behavior of the slow-slipping strike-slip faults slicing Central and Eastern Iran. Also, the slip rates summed across Central and Eastern Iran from the Iran Plateau up to the Afghan lowlands appear in agreement with the most recent GPS data.

Normal Faulting during the August 1989 Earthquakes in Central Afar: Sequential Triggering and Propagation of Rupture along the Dôbi Graben

In August 1989, an earthquake sequence including ten events with 6:3 ! M ! 5:5 in the first two days produced widespread ground deformation in the Dobi graben of central Afar. Numerous surface breaks with complex geometry, including fresh scarplets with vertical throws up to 30 cm high and open fissures up to 30 cm wide, were observed. Coseismic slip incremented the deformation (normal faulting, block tilting, and counterclockwise rotation of basaltic slices) accumulated in the last 2 m.y. in the transfer zone between the Dobi and Hanle grabens. By combining maps of surface ruptures, relative event relocations with the local Djibouti network, published focal mechanisms, and source sizes, we tentatively relate most of the main- shocks of the sequence to slip on individual faults. The largest shocks at 11h16 on 20 August 1989 (MS 6:2) and at 1h09 on 21 August 1989 (MS 6:3) ruptured southern segments of the southwestern bounding fault of the graben. A dozen other faults also slipped along the edges of, and inside, the graben. On average, triggered seismic fault- ing propagated about 35 km northwestward along the graben in about 20 hr. Slip on the main faults was coupled with slip on secondary antithetic faults branching from them at depth. Although the Dobi earthquakes ruptured part of the fault array between the Asal rift (1978 sequence) and the Serdo region (1969 sequence), an approximately 50-km-long gap subsists along the Derela half-graben. We infer the patterns of sur- face faulting in the Dobi sequence, which coinvolved bookshelf-faulting about both horizontal and vertical axes, to typify the complexity of coseismic stress release in central Afar and in other active zones of distributed extension (e.g., Iceland, Abruzzi, Basin and Range). Online Material: Additional photos and descriptions of surface effects of the Dobi earthquake sequence.