Melt migration modeling in partially molten upper mantle

The objective of this thesis is to investigate the importance of melt migration in shaping major characteristics of geological features associated with the partial melting of the upper mantle, such as sea-floor spreading, continental flood basalts and rifting. The partial melting produces permeable partially molten rocks and a buoyant low viscosity melt. Melt migrates through the partially molten rocks, and transfers mass and heat. Due to its much faster velocity and appreciable buoyancy, melt migration has the potential to modify dynamics of the upwelling partially molten plumes. I develop a 2-D, two-phase flow model and apply it to investigate effects of melt migration on the dynamics and melt generation of upwelling mantle plumes and focusing of melt migration beneath mid-ocean ridges. Melt migration changes distribution of the melt-retention buoyancy force and therefore affects the dynamics of the upwelling plume. This is investigated by modeling a plume with a constant initial melt of 10% where no further melting is considered. Melt migration polarizes melt-retention buoyancy force into high and low melt fraction regions at the top and bottom portions of the plume and therefore results in formation of a more slender and faster upwelling plume. Allowing the plume to melt as it ascends through the upper mantle also produces a slender and faster plume. It is shown that melt produced by decompressional melting of the plume migrates to the upper horizons of the plume, increases the upwelling velocity and thus, the volume of melt generated by the plume. Melt migration produces a plume which lacks the mushroom shape observed for the plume models without melt migration.Melt migration forms a high melt fraction layer beneath the sloping base of the impermeable oceanic lithosphere. Using realistic conditions of melting, freezing and melt extraction, I examine whether the high melt fraction layer is able to focus melt from a wide partial melting zone to a narrow region beneath the observed neo-volcanic zone. My models consist of three parts; lithosphere, asthenosphere and a melt extraction region. It is shown that melt migrates vertically within the asthenosphere, and forms a high melt fraction layer beneath the sloping base of the impermeable lithosphere. Within the sloping high melt fraction layer, melt migrates laterally towards the ridge. In order to simulate melt migration via crustal fractures and cracks, melt is extracted from a melt extraction region extending to the base of the crust. Performance of the melt focusing mechanism is not significantly sensitive to the size of melt extraction region, melt extraction threshold and spreading rate. In all of the models, about half of the total melt production freezes beneath the cooling base of the lithosphere, and the rest is effectively focused towards the ridge and forms the crust.To meet the computational demand for a precise tracing of the deforming upwelling plume and including the chemical buoyancy of the partially molten zone in my models, a new numerical method is developed to solve the related pure advection equations. The numerical method is based on Second Moment numerical method of Egan and Mahoney [1972] which is improved to maintain a high numerical accuracy in shear and rotational flow fields. In comparison with previous numerical methods, my numerical method is a cost-effective, non-diffusive and shape preserving method, and it can also be used to trace a deforming body in compressible flow fields

Negative magnetic anomalies at satellite altitude over passive marginal basins

Among the most significant features of satellite magnetic anomaly maps is the association of negative anomalies with deep sedimentary basins, as observed in the Nova Scotia basin. A possible explanation is partial demagnetization of the oceanic lithosphere by thermal blanketing of sediments and subsidence of the lower crust and uppermost mantle. I test this hypothesis by computing magnetization of oceanic lithosphere beneath the Nova Scotia basin based on a detailed thermal evolution model which includes rifting, subsidence and sea-floor spreading. The results confirm the possibility of demagnetization, however it is not substantial to explain the observed negative magnetic anomaly. As an alternative explanation, I propose a model in which the oceanic crust and uppermost mantle have age-varying magnetization and the continent has an average total magnetization contrast of 20000 A greater than the oceanic. This model produces a magnetic anomaly map having first order features which are in good agreement with satellite magnetic anomaly maps

Sensor Orientation of Iranian Broadband Seismic Stations from P‐Wave Particle Motion

Knowing the orientation of horizontal components of seismic sensors is important for many seismological applications such as waveform modeling, receiver function analysis, and shear‐wave splitting. We determined the sensor orientations for broadband seismic stations belonging to the Iranian National Seismic Network (INSN) and the Iranian Seismological Center (IRSC) to enable such studies. For both networks, we have catalogs of event‐based seismic waveforms of Iranian earthquakes. Sensor orientations were found by P‐wave energy minimization on the transverse component and validated by long‐period waveform modeling of events with well‐constrained source parameters. We obtained stable sensor orientations for 28 (of 29) INSN sites and for 66 (of 92) IRSC sites. About 75% and 59% of all INSN and IRSC orientation estimates, respectively, are oriented within 15° of true north leaving many sites with largely misoriented sensors. We found temporally changing sensor orientations for 36 (of 121) sites

A new tectonic map of the Iranian plateau based on aeromagnetic identification of magmatic arcs and ophiolite belts

The Iranian plateau is one of the most complex geodynamic settings within the Alpine-Himalayan belt. The Paleo-Tethys and Neo-Tethys ocean subduction is responsible for the formation of several magmatic arcs and sedimentary basins within the plateau. These zones mostly are separated by thrust faults related to paleo-suture zones, which are highlighted by ophiolites. Sediment cover and overprint of a different magmatic phase from late Triassic to the Quaternary impede identification of some magmatic arcs and ophiolite belts. We track the known magmatic arcs, such as the Urmia-Dokhtar Magmatic Arc (UDMA), and unknown, sediment covered magmatic arcs by aeromagnetic data. We present a new map of average susceptibility calculated by the radially averaged power spectrum method. High average susceptibility values indicate the presence of a number of lineaments that correlate with known occurrences of Magmatic-Ophiolite Arcs (MOA), and low average susceptibility coincides with known sedimentary basins like Zagros, Makran, Kopeh-Dagh, and Tabas. In analogy to Zagros, low average susceptibility values indicate sedimentary basins to the south of the Darouneh fault and in the northern part of the Lut, Tabas and Yazd blocks. We interpret the Tabas basin as a pull-apart or back-arc basin. We identify hitherto unknown parallel MOAs in eastern Iran and the SE part of UDMA which both indicate steeply dipping (>60° dip) paleo-subduction zones. In contrast, we interpret shallow subduction (<20° dip) of Neo-Tethys in the NW part of UDMA as well as in the Sabzevar-Kavir MOA

Characterizing Near‐Field Surface Deformation in the 1990 Rudbar Earthquake (Iran) Using Optical Image Correlation

International audienc

The Ahar-Varzeghan earthquake doublet (M<inf>w</inf> 6.4 and 6.2) of 11 August 2012: Regional seismic moment tensors and a seismotectonic interpretation

On 11 August 2012 an earthquake doublet (Mw 6.4 and 6.2) occurred near the city of Ahar, northwest Iran. Both events were only 6 km and 11 minutes apart, producing a surface rupture of about 12 km in length. Historical and modern seismicity has so far been sparse in this area. Spatially, the region represents a transitional zone between different tectonic domains, including compression in Iran, westward extrusion of the Anatolian plate, and thrusting beneath the Caucasus. In this study, we inverted the surface waveforms of the two mainshocks and 11 aftershocks (Mw ≥4:3) to obtain regional seismic moment tensors. The earthquakes analyzed can be grouped into pure strike slip (including the first mainshock) and oblique reverse mechanisms (including the second mainshock). The sequence provides information about faulting mechanisms at the spatial scale of the entire rock volume affected by the earthquake doublet, including coinciding deformation on minor faults (sub)parallel to the main fault and Riedel shears. It occurred on a so far unknown fault structure, which we call the Ahar fault. Alongside the seismological data, we used geological maps, satellite images, and digital elevation data to analyze the geomorphology of the region. Our analysis suggests that the adjacent North Tabriz fault, which accomodates up to 7 mm=yr of rightlateral strike-slip faulting, does not compensate the entire lateral shear strain, and that part of it is compensated farther north. Combined, our results suggest a temporally and spatially complex style of deformation (reverse and strike slip) overprinting older reverse deformation

Seismotectonic Modeling of the 2017 Hojedk (Kerman) Earthquake Sequence from Joint Inversion of InSAR and Offset Tracking Techniques

We Investigate the Geometric/kinematic Characteristics of Faulting a Triplet of Mw ∼ 6 Earthquakes (E1, E2, and E3) in Kerman Province, Central Iran, over Twelve Days in December 2017. a Geodetic Framework is Implemented and Combined with Seismic and Geological Methods to Provide an Integrated Model of Faulting within the Sequence. to Retrieve Coseismic Displacements in the Complex and High-Gradient Near-Field Area, an Adaptive Method is Proposed based on Image-Alignment Techniques Applied to Both SAR (Sentinel-1) and Optical Data (PlanetScope). a Continuous Deformation Map is Created by Combining Interferometric and Pixel Offset Tracking Results through Principal Component Analysis. We Jointly Invert the Geodetic Data, Including Along- and Across-Track Displacement Maps, through a Constraint-Based Bayesian Inversion Process to Optimize the Source Parameters of the Causative Faults. the Results Reveal that the NE-Dipping Nodal Planes, with Variable Slip Peaks of 63 and 17.5 Cm at Depths of 5.5 and 9.1 Km, Best Fit the Data for the E1 and E2 Events, Respectively, Whereas the E3 Event Was on a SW-Dipping Fault Plane Including ∼E- and NW-Striking Segments. the E3 Plane Was Shallow with Maximum Slip of 1.2 M at a Depth of 2.2 Km. We Relocate 93 Events and Use the InSAR Slip Distributions as Constraints to Indirectly Calibrate the Epicenter of the Earthquake Cluster, and Obtain the Local Stress State from the Inversion of the Focal Mechanism Solutions of the Ten Largest Events of the Hojedk Cluster. the Resultant Compressional Stress Explains the Dextral and Sinistral Components Found on the Western and Eastern Segments of the E3 Rupture, Respectively. All Together Suggest a NW-Striking Pop-Up Structure in a Restraining Stepover between the ∼N-Striking Dextral Lakarkuh and Golbaf-Sirch-Gowk Fault Systems. This Structural Model Highlights the Interaction between Thick-Skinned and Thin-Skinned Seismic Deformation within Iran

Seismicity in the western coast of the South Caspian Basin and the Talesh Mountains

We have studied the seismicity of the western margin of the South Caspian Basin (SCB) and the neighbouring Talesh fold and thrust belt. We have used the hypocentroidal decomposition multiple-event location technique to obtain accurate location of events recorded during 2 yr of observation. Data from a temporary seismic network in northwest Iran and other national and regional networks were combined to make an accurate assessment of seismicity in the region. Significant offshore seismicity is observed in a 50-km wide margin of the SCB. East of the Talesh Fault along the Caspian coastline, the depth of seismicity varies from 20 to 47 km. This pattern extends inland about 20–25 km west of the North Talesh Fault. This pattern of seismicity indicates that the basement slab of the South Caspian is undergoing intense seismic deformation as it is underthrusting beneath the northern Talesh, whereas the sedimentary cover deforms aseismically. The seismicity, depths, and previous focal mechanisms of the larger offshore events are consistent with low-angle underthrusting of the South Caspian floor. Within the Talesh, seismicity is mostly concentrated in the northern and southern structural arcs of the range, where deformation is more intense and complicated. Shallow crustal seismicity in the eastern flank of the Talesh is much less intense than in the western flank, where it signifies the deformation of the upper continental crust. One major observation is the lack of any significant N–S alignment of shallow epicentres inside the central Talesh to match the observed right-lateral shear deformation there. This suggests that shear deformation inside the Talesh may have a distributed nature, rather than being concentrated on a single thorough-going fault zone, as the Talesh moves northward relative to the South Caspian. We have determined a new moment tensor solution in the southwestern Talesh, with a dominant N–S trending right-lateral motion, the only solution so far confirming along-strike shear deformation in the Talesh