Jamshid Hassanzadeh

Exhumation of the west-central Alborz Mountains, Iran, Caspian subsidence, and collision-related tectonics

Crystallization and thermal histories of two plutons in the west-central Alborz (also Elburz, Elburs) Mountains, northern Iran, are combined with crosscutting relations and kinematic data from nearby faults to determine the Cenozoic tectonic evolution of this segment of the youthful Euro-Arabian collision zone. U/Pb, ^(40)Ar/^(39)Ar, and (U-Th)/He data were obtained from zircon, biotite, K-feldspar, and apatite. The Akapol pluton intruded at 56 ± 2 Ma, cooled to ∼150 °C by ca. 40 Ma, and stayed near that temperature until at least 25 Ma. The nearby Alam Kuh granite intruded at 6.8 ± 0.1 Ma and cooled rapidly to ∼70 °C by ca. 6 Ma. These results imply tectonic stability of the west-central Alborz from late Eocene to late Miocene time, consistent with Miocene sedimentation patterns in central Iran. Elevation-correlated (U- Th)/He ages from the Akapol suite indicate 0.7 km/m.y. exhumation between 6 and 4 Ma, and imply ∼10 km of Alborz uplift that was nearly synchronous with rapid south Caspian subsidence, suggesting a causal relation. Uplift, south Caspian subsidence and subsequent folding, reversal of Alborz strike-slip (from dextral to sinistral) and(?) eastward extrusion of central Iran, coarse Zagros molasse deposition, Dead Sea transform reorganization, Red Sea oceanic spreading, and(?) North and East Anatolian fault slip all apparently began ca. 5 ± 2 Ma, suggesting a widespread tectonic event that we infer was a response to buoyant Arabian lithosphere choking the Neo-Tethyan subduction zone.

Accommodation of transpressional strain in the Arabia‐Eurasia collision zone: new constraints from (U‐Th)/He thermochronology in the Alborz mountains, north Iran

The Alborz range of N Iran provides key information on the spatiotemporal evolution and characteristics of the Arabia-Eurasia continental collision zone. The southwestern Alborz range constitutes a transpressional duplex, which accommodates oblique shortening between Central Iran and the South Caspian Basin. The duplex comprises NW-striking frontal ramps that are kinematically linked to inherited E-W-striking, right-stepping lateral to obliquely oriented ramps. New zircon and apatite (U-Th)/He data provide a high-resolution framework to unravel the evolution of collisional tectonics in this region. Our data record two pulses of fast cooling associated with SW-directed thrusting across the frontal ramps at ~ 18–14 and 9.5-7.5 Ma, resulting in the tectonic repetition of a fossil zircon partial retention zone and a cooling pattern with a half U-shaped geometry. Uniform cooling ages of ~ 7–6 Ma along the southernmost E-W striking oblique ramp and across its associated NW-striking frontal ramps suggests that the ramp was reactivated as a master throughgoing, N-dipping thrust. We interpret this major change in fault kinematics and deformation style to be related to a change in the shortening direction from NE to N/NNE. The reduction in the obliquity of thrusting may indicate the termination of strike-slip faulting (and possibly thrusting) across the Iranian Plateau, which could have been triggered by an increase in elevation. Furthermore, we suggest that ~ 7-6-m.y.-old S-directed thrusting predated inception of the westward motion of the South Caspian Basin.

A Paleogene extensional arc flare‐up in Iran

Arc volcanism across Iran is dominated by a Paleogene pulse, despite protracted and presumably continuous subduction along the northern margin of the Neotethyan ocean for most of Mesozoic and Cenozoic time. New U-Pb and ^(40)Ar/^(39)Ar data from volcanic arcs in central and northern Iran constrain the duration of the pulse to ~17 Myr, roughly 10% of the total duration of arc magmatism. Late Paleocene-Eocene volcanic rocks erupted during this flare-up have major and trace element characteristics that are typical of continental arc magmatism, whereas the chemical composition of limited Oligocene basalts in the Urumieh-Dokhtar belt and the Alborz Mountains which were erupted after the flare-up ended are more consistent with derivation from the asthenosphere. Together with the recent recognition of Eocene metamorphic core complexes in central and east central Iran, stratigraphic evidence of Eocene subsidence, and descriptions of Paleogene normal faulting, these geochemical and geochronological data suggest that the late Paleocene-Eocene magmatic flare-up was extension related. We propose a tectonic model that attributes the flare-up to decompression melting of lithospheric mantle hydrated by slab-derived fluids, followed by Oligocene upwelling and melting of enriched mantle that was less extensively modified by hydrous fluids. We suggest that Paleogene magmatism and extension was driven by an episode of slab retreat or slab rollback following a Cretaceous period of flat slab subduction, analogous to the Laramide and post-Laramide evolution of the western United States.

Late Cenozoic shortening in the west-central Alborz Mountains, northern Iran, by combined conjugate strike-slip and thin-skinned deformation

The west-central Alborz Mountains of northern Iran have deformed in response to the Arabia-Eurasia collision since ca. 12 Ma and have accommodated 53 ± 3 km of shortening by a combination of range-parallel, conjugate strike-slip faulting and range-normal thrusting. By our interpretation, ∼17 km of shortening across the Alborz is accommodated by westward relative motion of a crustal wedge bounded by conjugate dextral and sinistral strike-slip fault systems. The Nusha, Barir, and Tang-e-Galu fault zones strike west-northwest, constrain the north side of the wedge, and, prior to ca. 5 Ma, accumulated a total of ∼25 km of dextral slip. The south side of the wedge is bounded by the active sinistral reverse Mosha and Taleghan faults, which merge northwest of Tehran and have a total slip estimate of 30–35 km. A restored cross section across the range indicates a minimum of 36 ± 2 km of fold-and-thrust–related, range-normal shortening. Combined, wedge motion, thrusting, and folding yield a net shortening of 53 ± 3 km across the range, which is within the error of the shortening estimate predicted by assuming that the present-day shortening rate (5 ± 2 mm/yr) has been constant since ca. 12 Ma (∼60 km of predicted shortening). A second restored cross section farther west, which includes the wedge, gives a total shortening of 15–18 km and a long-term shortening rate of 1.25–1.5 mm/yr (constant shortening rate since ca. 12 Ma). These strong along-strike finite-strain and long-term strain-rate gradients are important for our understanding of how long-term strain rates compare with instantaneous strain rates derived from global positioning system (GPS) data, and should be considered when planning mountain belt–scale GPS surveys. Finally, a 60-km-long right-hand bend in the Mosha-Taleghan fault system has driven the development of a transpressional duplex south of the fault. The southern boundary of the duplex is the active Farahzad–Karaj–North Tehran thrust system. The kinematic development of this strike-slip duplex has implications for seismic hazard assessment in the heavily populated Karaj and Tehran areas.

Permian–Triassic boundary interval in the Abadeh section of Iran with implications for mass extinction: Part 1 – Sedimentology

The uppermost Permian strata of the Abadeh section in Iran consist of 56 m of skeletal limestone (Abadehian), that grades upward into 18 m of grey, bioturbated lime mudstone (Djulfian), which in turn grades upward to 18 m of red, nodular wackestone (Dorashamian) containing an abundant pelagic fauna. The overlying lowermost Triassic includes two enigmatic layers at its base. Immediately above the boundary is a 1-m-thick layer of inorganically precipitated synsedimentary carbonate cement. The cement is composed of 10–20-cm-long crystals oriented either perpendicular to the bedding or as dome-shaped features resembling botryoids. About 0.5 m of wackestone overlies the cement layer. The second layer is a 1.5-m-thick grainstone overlying the cement horizon and consists of recrystallised spherical grains (ooids or peloids). The remainder of unit ‘A’ is composed of 100+ m of grey, bioturbated to nodular lime mudstone. The lithofacies succession of the uppermost Permian is interpreted to represent deposition under increasing water depth, related to rising of relative sea level, which led to drowning of the carbonate platform to below storm wave base. Sedimentologic characteristics, lithology, wide distribution, and slow sedimentation rates indicate that strata immediately below the Permian–Triassic (P–T) boundary (Dorashamian interval) were deposited in deep, oxygenated waters. The enigmatic lowermost Triassic synsedimentary carbonate cement and grainstone layers are interpreted to have been deposited in shallow waters, indicating a rapid and major drop in relative sea level at the end of Permian time in this area, followed by a relative sea-level rise during the earliest Triassic. Precipitation of synsedimentary cement immediately above the P–T boundary in Iran and elsewhere (South China) suggests a global change in ocean chemistry. One possible explanation is that dissolved calcium and bicarbonate concentrations in seawater increased sufficiently to promote spontaneous carbonate precipitation, a rare event in the Phanerozoic but common during Precambrian time. Alternatively, a massive heating event could have resulted in CO2 degassing and decreases in oceanic carbonate solubility, both leading to inorganic, synsedimentary carbonate precipitation. Inorganic precipitation of synsedimentary carbonate cement may indicate that biochemical production of carbonate was halted due to the P–T boundary events. Our study indicates that shallow and moderately deep waters during the latest Permian were well oxygenated in the open ocean setting of the central Tethys Sea in Iran. Therefore, P–T mass extinction scenarios that invoke upon widespread marine anoxia should be viewed with caution.

Thermal histories from the central Alborz Mountains, northern Iran: Implications for the spatial and temporal distribution of deformation in northern Iran

We integrate new and existing thermochronological, geochronological, and geologic data from the western and central Alborz Mountains of Iran to better constrain the late Cenozoic tectonic evolution of northern Iran in the context of the Arabia-Eurasia collision. New data are presented for two granitic plutons north of the Alborz Range crest. Additional new apatite (U-Th)-He data are also presented for volcanic, intrusive, and detrital apatite grains from two transects south of the range crest. Our most definitive results include zircon and apatite (U-Th)-He and limited K-feldspar ^(40)Ar/^(39)Ar thermal history data from the Cretaceous (ca. 98 Ma) Nusha pluton that reveal that the Alborz basement underwent generally slow denudation (∼0.1 km/m.y.) as late as 12 Ma with more accelerated exhumation (∼0.45 km/m.y.) that likely began shortly after 12 Ma. The Lahijan pluton, a late Neoproterozoic–Cambrian basement exposure near the Caspian shore, records apatite (U-Th)-He closure at 17–13 Ma. Additional (U-Th)-He results from detrital apatites sampled along two separate horizontal transects all consistently yielded latest Miocene to Pliocene apparent ages that imply that even supracrustal cover rocks within the Alborz have undergone significant, regionally extensive exhumation. Overall, our data are consistent with ∼5 km of regionally extensive denudation since ca. 12 Ma. The onset of rapid exhumation in the Alborz at ca. 12 Ma appears to be consistent with other timing estimates that place the onset of the Arabia-Eurasia collision between 14 and 10 Ma.

Geology and thermochronology of Tertiary Cordilleran-style metamorphic core complexes in the Saghand region of central Iran

An ~100-km-long north-south belt of metamorphic core complexes is localized along the boundary between the Yazd and Tabas tectonic blocks of the central Iranian micro-continent, between the towns of Saghand and Posht-e-Badam. Amphibolite facies mylonitic gneisses are structurally overlain by east-tilted supracrustal rocks including thick (>1 km), steeply dipping, nonmarine siliciclastic and volcanic strata. Near the detachment (the Neybaz-Chatak fault), the gneisses are generally overprinted by chlorite brecciation. Crosscutting relationships along with U-Pb zircon and ^(40)Ar/^(39)Ar age data indicate that migmatization, mylonitic deformation, volcanism, and sedimentation all occurred in the middle Eocene, between ca. 49 and 41 Ma. The westernmost portion of the Tabas block immediately east of the complexes is an east-tilted crustal section of Neoproterozoic–Cambrian crystalline rocks and metasedi-mentary strata >10 km thick. The ^(40)Ar/^(39)Ar biotite ages of 150–160 Ma from structurally deep parts of the section contrast with ages of 218–295 Ma from shallower parts, and suggest Late Jurassic tilting of the crustal section. These results define three events: (1) a Late Jurassic period of upper crustal cooling of the western Tabas block that corresponds to regional Jurassic–Cretaceous tectonism and erosion recorded by a strong angular unconformity below mid-Cretaceous strata throughout central Iran; (2) profound, approximately east-west middle Eocene crustal extension, plutonism, and volcanism (ca. 44–40 Ma); and (3) ~2–3 km of early Miocene (ca. 20 Ma) erosional exhumation of both core complex and Tabas block assemblages at uppermost crustal levels, resulting from significant north-south shortening. The discovery of these and other complexes within the mid-Tertiary magmatic arcs of Iran demonstrates that Cordilleran-style core complexes are an important tectonic element in all major segments of the Alpine-Himalayan orogenic system.

The geology of Damavand volcano, Alborz Mountains, northern Iran

Damavand volcano, located in northern Iran, is a large (>400 km^3) composite cone that is currently dormant; it shows fumarolic activity near the summit but no evidence of eruption in the past 1000 yr. The volcano represents an isolated focus of magmatism of uncertain tectonic affinity, although geophysical and geochemical constraints point toward a local hotspot/plume origin, possibly associated with lithospheric delamination, rather than any association with subduction. New (U-Th)/He and ^(40)Ar/^(39)Ar geochronological constraints indicate that the present cone (Young Damavand) has been constructed over ∼600 k.y. on an older, eroded edifice of indistinguishable composition (younger than 1.8 Ma). Damavand activity has been characterized by the eruption of radially directed trachyandesite lava flows, almost exclusively from summit vents. Limited pyroclastic activity has yielded thin fallout pumice lapilli layers and a few pyroclastic flows. Only one significant pyroclastic event is recognized in the remnants of a welded ignimbrite, ponded and preserved along the Haraz River drainage. Relatively short periods of volcanic eruptive activity were interspersed with longer periods of erosion in which volcanic products were transported, particularly as hyperconcentrated flows, into the surrounding drainage systems to be further reworked into epiclastic deposits. Occasional catastrophic events punctuated this interplay between volcanism and erosion. At least one sector collapse is signified by the presence of a large debris avalanche deposit, and the regional drainage systems appear to have been frequently dammed by incursions of volcanic material.

Diagenetic origin of carbon and oxygen isotope compositions of Permian-Triassic boundary strata

Bulk carbonate δ^(13)C and δ^(18)O compositions of profiles across Permian–Triassic (P–T) boundary sections in China, Italy, Austria, and Iran show wide varieties of trends. The δ^(13)C depletions occur in all sections and range from 2 to 8‰ PDB in magnitude. These excursions take place over intervals ranging from less than 0.1 to more than 40 m. The δ^(18)O values may increase or decrease toward the P–T boundary, but decrease sharply by 2–9‰ PDB at or above the boundary. Cross-plots of δ^(13)C and δ^(18)O values from all sections show positive covariance. Wide differences in magnitudes, trends, and position of the excursions relative to the boundary, as well as the covariance patterns suggest that P–T boundary δ^(18)O and δ^(13)C values are partially or entirely diagenetic in origin, formed in association with exposure surfaces. This interpretation implies that P–T boundary sections studied till date were subaerially exposed before, during, and after the mass extinction, resulting in the removal of strata containing key information about the extinction mechanism. This inference is consistent with the paleontological studies that have shown the presence of gaps at the boundary, and further supported by the sharp lithologic changes observed at virtually all P–T boundary sections. Subaerial exposures are documented by detailed sedimentologic and isotopic studies from central Tethyan sections in Abadeh and Shah Reza in Iran. Proposed P–T boundary extinction models are based on isotopic values that are diagenetic in origin and stratigraphic sections that are incomplete, leading to extinction mechanisms with little physical supporting evidence.

Magnetic fabrics of Tertiary sandstones from the Arc of Fars (Eastern Zagros, Iran)

An analysis of magnetic fabric (45 sites, 514 samples) has been performed in folded but apparently not penetratively deformed Mio-Pliocene sedimentary rocks (mainly sandstones) from the Arc of Fars (Eastern Zagros, Iran). Different types of magnetic fabrics and shapes of susceptibility ellipsoids were observed from an oblate form reflecting a sedimentary fabric to a prolate form signifying a magnetic lineation of tectonic origin. Classically in such weakly deformed sedimentary rocks the magnetic lineation is perpendicular to the shortening direction and is interpreted as recording a pre-folding layer-parallel shortening. In the studied area, the trends of the magnetic lineation show two clusters oriented ENE-WSW and WNW-ESE. To a first approximation, the magnetic lineation follows the bending of the fold trend. However, a quite systematic obliquity is observed. According to regional geology and to other considerations arising from the magnetic analysis, we consider that the tectonic transport direction is NNW-SSE. The arc pattern observed in the fold and magnetic lineation trends may result either from an inherited structural framework or from post-folding clockwise rotations or both. In any case, our data rule out any interpretation of the whole Arc of Fars as resulting from the superimposition of two non-coaxial folding events.