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Neighbouring faults can interact, potentially link up and grow, and consequently increase the seismic and related natural hazards in their vicinity. Despite evidence of Quaternary faulting, the kinematic relationships between the neighbouring Mosha Fasham Fault (MFF) and the North Tehran Thrust (NTT) and their temporal evolution in the Alborz mountains are not well understood. The ENE-striking NTT is a frontal thrust that delimits the Alborz mountains to the south with a 2000 m topographic front with respect to the proximal Tehran plain. However, no large instrumentally recorded earthquakes have been attributed to that fault. In contrast, the sigmoidally shaped MFF is a major strike-slip fault, located within the Alborz Mountains. Sinistral motion along the eastern part of the MFF is corroborated by microseismicity and fault kinematic analysis, which documents recent transtensional deformation associated with NNE-SSW oriented shortening. To better understand the activity of these faults on different timescales, we combined fault- kinematic analysis and geomorphic observations, to infer the kinematic history of these structures. Our fault kinematic study reveals an early dextral shear for the NTT and the central MFF, responsible for dextral strike-slip and oblique reverse faulting during NW-oriented shortening. This deformation regime was superseded by NE-oriented shortening, associated with sinistral-oblique thrusting along the NTT and the central-western MFF, sinistral strike-slip motion along subsidiary faults in the central MFF segment, and folding and tilting of Eocene to Miocene units in the MFF footwall. Continued thrusting along the NTT took place during the Quaternary. However, folding in the hanging wall and sinistral stream-offsets indicate a left-oblique component and Quaternary strike-slip reactivation of the eastern NTT- segment, close to its termination. This complex history of faulting under different stress directions has resulted in a composite landscape with inherited topographic signatures. Our study shows that the topography of the hanging wall of the NTT reflects a segmentation into sectors with semi-independent uplift histories. Areas of high topographic residuals and apparent high uplift underscore the fault kinematics. Combined, our data suggest an early mechanical linkage of the NTT and MFF fault systems during a former dextral transpressional stage, caused by NW-compression. During NE-oriented shortening, the NTT and MFF were reactivated and incorporated into a nascent transpressional duplex. The youngest manifestation of motion in this system is sinistral transtension. However, this deformation is not observed everywhere and has not yet resulted in topographic inversion.
Erweiterte Momententensorinversion und ihre seismotektonische Anwendung : Elbursgebirge, Nordiran
(2009)
Der Elburs im Norden Irans ist ein durch die Konvergenz der Arabischen und Eurasischen Platte verursachtes doppelt konvergentes Gebirge. Das komplexe System von Blattverschiebungen und Überschiebungen sowie die Aufnahme der Deformation im Elburs ist noch nicht sehr gut verstanden. Eine neu zu entwicklende Methode zur Inversion von seismischen Momententensoren, die unterschiedliche Beobachtungen verschiedener Stationstypen kombiniert invertiert, soll die bisher hauptsächlich strukturelle/geomorphologische Datengrundlage um Momententensoren auch kleinerer Magnituden (M < 4.5) erweitern. Dies ist die notwendige Grundlage für detaillierte seismotektonische Studien, die wiederum die Basis für seismische Gefährdungsanalysen bilden.
The complex system of strike-slip and thrust faults in the Alborz Mountains, Northern Iran, are not well understood yet. Mainly structural and geomorphic data are available so far. As a more extensive base for seismotectonic studies and seismic hazard analysis we plan to do a comprehensive seismic moment tensor study also from smaller magnitudes (M < 4.5) by developing a new algorithm. Here, we present first preliminary results.
The seismically active Alborz mountains of northern Iran are an integral part of the Arabia-Eurasia collision. Linked strike-slip and thrust/reverse-fault systems in this mountain belt are characterized by slow loading rates, and large earthquakes are highly disparate in space and time. Similar to other intracontinental deformation zones such a pattern of tectonic activity is still insufficiently understood, because recurrence intervals between seismic events may be on the order of thousands of years, and are thus beyond the resolution of short term measurements based on GPS or instrumentally recorded seismicity. This study bridges the gap of deformation processes on different time scales. In particular, my investigation focuses on deformation on the Quaternary time scale, beyond present-day deformation rates, and it uses present-day and paleotectonic characteristics to model fault behavior. The study includes data based on structural and geomorphic mapping, faultkinematic analysis, DEM-based morphometry, and numerical fault-interaction modeling. In order to better understand the long- to short term behavior of such complex fault systems, I used geomorphic surfaces as strain markers and dated fluvial and alluvial surfaces using terrestrial cosmogenic nuclides (TCN, 10Be, 26Al, 36Cl) and optically stimulated luminescence (OSL). My investigation focuses on the seismically active Mosha-Fasham fault (MFF) and the seismically virtually inactive North Tehran Thrust (NTT), adjacent to the Tehran metropolitan area. Fault-kinematic data reveal an early mechanical linkage of the NTT and MFF during an earlier dextral transpressional stage, when the shortening direction was oriented northwest. This regime was superseded by Pliocene to Recent NE-oriented shortening, which caused thrusting and sinistral strike-slip faulting. In the course of this kinematic changeover, the NTT and MFF were reactivated and incorporated into a nascent transpressional duplex, which has significantly affected landscape evolution in this part of the range. Two of three distinctive features which characterize topography and relief in the study area can be directly related to their location inside the duplex array and are thus linked to interaction between eastern MFF and NTT, and between western MFF and Taleghan fault, respectively. To account for inferred inherited topography from the previous dextral-transpression regime, a new concept of tectonic landscape characterization has been used. Accordingly, I define simple landscapes as those environments, which have developed during the influence of a sustained tectonic regime. In contrast, composite landscapes contain topographic elements inherited from previous tectonic conditions that are inconsistent with the regional present-day stress field and kinematic style. Using numerical fault-interaction modeling with different tectonic boundary conditions, I calculated synoptic snapshots of artificial topography to compare it with the real topographic metrics. However, in the Alborz mountains, E-W faults are favorably oriented to accommodate the entire range of NW- to NE-directed compression. These faults show the highest total displacement which might indicate sustained faulting under changing boundary conditions. In contrast to the fault system within and at the flanks of the Alborz mountains, Quaternary deformation in the adjacent Tehran plain is characterized by oblique motion and thrust and strike-slip fault systems. In this morphotectonic province fault-propagation folding along major faults, limited strike-slip motion, and en-échelon arrays of second-order upper plate thrusts are typical. While the Tehran plain is characterized by young deformation phenomena, the majority of faulting took place in the early stages of the Quaternary and during late Pliocene time. TCN-dating, which was performed for the first time on geomorphic surfaces in the Tehran plain, revealed that the oldest two phases of alluviation (units A and B) must be older than late Pleistocene. While urban development in Tehran increasingly covers and obliterates the active fault traces, the present-day kinematic style, the vestiges of formerly undeformed Quaternary landforms, and paleo earthquake indicators from the last millennia attest to the threat that these faults and their related structures pose for the megacity.
Investigation of a right-laterally offset channel at the Miller's Field paleoseismic site yields a late Holocene slip rate of 26.2 +6.4/-4.3 mm/yr (1 sigma) for the main trace of the San Andreas fault at Parkfield, California. This is the first well-documented geologic slip rate between the Carrizo and creeping sections of the San Andreas fault. This rate is lower than Holocene measurements along the Carrizo Plain and rates implied by far-field geodetic measurements (similar to 35 mm/yr). However, the rate is consistent with historical slip rates, measured to the northwest, along the creeping section of the San Andreas fault (<30 mm/yr). The paleoseismic exposures at the Miller's Field site reveal a pervasive fabric of clay shear bands, oriented clockwise oblique to the San Andreas fault strike and extending into the uppermost stratigraphy. This fabric is consistent with dextral aseismic creep and observations of surface slip from the 28 September 2004 M6 Parkfield earthquake. Together, this slip rate and deformation fabric suggest that the historically observed San Andreas fault slip behavior along the Parkfield section has persisted for at least a millennium, and that significant slip is accommodated by structures in a zone beyond the main San Andreas fault trace.
A poorly understood lag time of 15-20 m.y. exists between the initial Arabia-Eurasia continental collision in late Eocene to early Oligocene time and the acceleration of tectonic and sedimentary processes across the collision zone in the early to late Miocene. The late Eocene to Miocene-Pliocene clastic and shallow-marine sedimentary rocks of the Kond, Eyvanekey, and Semnan Basins in the Alborz Mountains (northern Iran) offer the possibility to track the evolution of this orogen in the framework of collision processes. A transition from volcaniclastic submarine deposits to shallow-marine evaporites and terrestrial sediments occurred shortly after 36 Ma in association with reversals in sediment provenance, strata tilting, and erosional unroofing. These events followed the termination of subduction arc magmatism and marked a changeover from an extensional to a contractional regime in response to initiation of continental collision with the subduction of stretched Arabian lithosphere. This early stage of collision produced topographic relief associated with shallow foreland basins, suggesting that shortening and tectonic loading occurred at low rates. Starting from the early Miocene (17.5 Ma), flexural subsidence in response to foreland basin initiation occurred. Fast sediment accumulation rates and erosional unroofing trends point to acceleration of shortening by the early Miocene. We suggest that the lag time between the initiation of continental collision (36 Ma) and the acceleration of regional deformation (20-17.5 Ma) reflects a two-stage collision process, involving the "soft" collision of stretched lithosphere at first and "hard" collision following the arrival of unstretched Arabian continental litho sphere in the subduction zone.
The need to accurately document the spatiotemporal distribution of earthquake-generated strong ground motions is essential for evaluating the seismic vulnerability of sites of critical infrastructure. Understanding the threshold for maximum earthquake-induced ground motions at such sites provides valuable information to seismologists, earthquake engineers, local agencies, and policymakers when determining ground motion hazards of seismically sensitive infrastructures. In this context, fragile geologic features such as precariously balanced rocks (PBRs) serve as negative evidence for earthquake-induced ground motions and provide important physical constraints on the upper limits of ground motions. The three-dimensional (3D) shape of a PBR is a critical factor in determining its static stability and thus susceptibility to toppling during strong ground shaking events. Furthermore, the geomorphic settings of PBRs provide important controls on PBR exhumation histories that are interpreted from surface exposure dating methods. In this paper, we present PBRslenderness, a MATLAB-based program that evaluates the two-dimensional (2D) static stabilities of PBRs from unconstrained digital photographs. The program's graphical user interface allows users to interactively digitize a PBR and calculates the 2D geometric parameters that define its static stability. A reproducibility study showed that our 2D calculations compare well against their counterparts that were computed in 3D (R-2 = 0.77-0.98 for 22 samples). A sensitivity study for single-user and multiuser digitization routines further confirmed the reproducibility of PBRslenderness estimates (coefficients of variation c(v) = 4.3%-6.5% for 100 runs; R-2 = 0.87-0.99 for 20 PBRs). We used PBRslenderness to analyze 261 PBRs in a low-seismicity setting to investigate the local geomorphic controls on PBR stability and preservation. PBRslenderness showed that a PBR's shape strongly controls its static stability and that there is no relationship between a PBR's stability and its geomorphic location in a drainage basin. However, the geomorphic settings of PBRs control their preservation potential by restricting their formation to hillslope gradients <40 degrees and the upper reaches of drainage basins. Such examples of our program's utility have led to its use in archival efforts of PBRs in southern California and Nevada, USA.
Basement-cored ranges formed by reverse faulting within intracontinental mountain belts are often composed of poly-deformed lithologies. Geological data capable of constraining the timing, magnitude, and distribution of the most recent deformational phase are usually missing in such ranges. In this paper, we present new low temperature thermochronological and geological data from a transect through the basement-cored Terskey Range, located in the Kyrgyz Tien Shan. Using these data, we are able to investigate the range's late Cenozoic deformation for the first time. Displacements on reactivated faults are constrained and deformation of thermochronologically derived structural markers is assessed. These structural markers postdate the earlier deformational phases, providing the only record of Cenozoic deformation and of the reactivation of structures within the Terskey Range. Overall, these structural markers have a southern inclination, interpreted to reflect the decreasing inclination of the reverse fault bounding the Terskey Range. Our thermochronological data are also used to investigate spatial and temporal variations in the exhumation of the Terskey Range, identifying a three-stage Cenozoic exhumation history: (1) virtually no exhumation in the Paleogene, (2) increase to slightly higher exhumation rates at similar to 26-20Ma, and (3) significant increase in exhumation starting at similar to 10Ma.
The tectonically driven growth of mountains reflects the characteristics of the underlying fault systems and the applied tectonic forces. Over time, fault networks might be relatively static, but stress conditions could change and result in variations in fault slip orientation. Such a tectonic landscape would transition from a simple to a composite state: the topography of simple landscapes is correlated with a single set of tectonic boundary conditions, while composite landscapes contain inherited topography due to earlier deformation under different boundary conditions. We use fault interaction modeling to compare vertical displacement fields with topographic metrics to differentiate the two types of landscapes. By successively rotating the axis of maximum horizontal stress, we produce a suite of vertical displacement fields for comparison with real landscapes. We apply this model to a transpressional duplex in the south central Alborz Mountains of Iran, where NW oriented compression was superseded by neotectonic NE compression. The consistency between the modeled displacement field and real landforms indicates that the duplex topography is mostly compatible with the modern boundary conditions, but might include a small remnant from the earlier deformation phase. Our approach is applicable for various tectonic settings and represents an approach to identify the changing boundary conditions that produce composite landscapes. It may be particularly useful for identifying changes that occurred in regions where river profiles may no longer record a signal of the change or where the spatial pattern of uplift is complex.
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 similar to 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 similar to 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 similar to 7-6-m.y.-old S-directed thrusting predated inception of the westward motion of the South Caspian Basin. Citation: Ballato, P., D. F. Stockli, M. R. Ghassemi, A. Landgraf, M. R. Strecker, J. Hassanzadeh, A. Friedrich, and S. H. Tabatabaei (2012), Accommodation of transpressional strain in the Arabia-Eurasia collision zone: new constraints from (U-Th)/He thermochronology in the Alborz mountains.