The Nyasa/ Malawi rift is characterized by poor magma with relatively large earthquakes. There has been a controversy as to the stress kinematics of the rift, some considering it as part of the transform fault and some considering it as a rift structure characterized by normal faulting. To review this controversy, we collect fault slip data from the central to the southern end of the rift and integrate our results with published focal mechanisms fault slip data on the rift. Results show that the central part of the rift is under radial extension whereas the southern half is under oblique NNE-SSW transtensive tectonic regime with the horizontal axis of minimum extension = 020˚. Further south, the obliquity extension rotates by about 15˚ reaching N-S with Shmin = 175˚. The level of structural penetration and intensity of faulting show that the N-S opening is more important and prominent in the south than towards the north. We also find that the faults that dip to the east and trending NW-SE are characterized by sinistral sense of movement whereas those that dip to the southwestern side are characterized by dextral sense of movements. This implies that regionally, the rift is essentially under normal faulting regime but with a significant strike –slip component – hence the obliquity kinematics. Tectonic regimes obtained from fault-slip data are related to lithospheric scale and involve both the crust and the upper mantle. Thus, the pure NNW-SSE extension related to focal mechanism data are crust deformation related events.
The Great Slave Lake shear zone (GSLsz) is a type example for deeply eroded continental transform boundaries located in the Northwest Territories, Canada. Formed during the oblique convergence of the Archean Rae and Slave cratons, the GSLsz has accommodated up to 700 km of dextral shear. Here we present the results of in situ U-Pb apatite and titanite geochronology from 11 samples that were collected across the strike of the shear zone. Both geochronometers record a near-continuous history of ductile shear during crustal cooling and exhumation that spans ca. 1920–1740 Ma. By integrating the geochronological data with structural and metamorphic observations across the structure, we propose a tectonic model for the shear zone that consists of three stages. The first stage (ca. 1920–1880 Ma) is characterized by strain accommodation along two coeval fault strands. During the second stage (ca. 1880–1800 Ma), ductile shear ceases along the northernmost fault strand and the locus of strain migrates southwards towards the hinterland of the Rae cratonic margin. In the third stage (ca. 1800–1740 Ma), ductile strain localizes back along the southern of the two original fault strands, after which the present-day surface level of the shear zone transitions to brittle shear. Our results highlight both the significance of the lateral migration of the zone of active deformation in major crustal shear zones as well as the localization of strain along existing crustal structures.
Detailed understanding of crustal components and tectonic history of forearcs is important, due to their geological complexity and high seismic hazard. The principal component of the Cascadia forearc is Siletzia, a composite basaltic terrane of oceanic origin. Much is known about the lithology and age of the province. However, glacial sediments blanketing the Puget lowland obscure its lateral extent and internal structure, hindering our ability to fully understand its tectonic history and its influence on modern deformation. In this study, we apply map-view interpretation and two-dimensional modeling of aeromagnetic and gravity data to the magnetically stratified Siletzia terrane revealing its internal structure and characterizing its eastern boundary. These analyses suggest the contact between Siletzia (Crescent Formation) and the Eocene accretionary prism trends northward under Lake Washington. North of Seattle, this boundary dips east where it crosses the Kingston arch, while south of Seattle the contact dips west where it crosses the Seattle uplift. This westward dip is opposite of the dip of the Eocene subduction interface, implying obduction of Siletzia upper crust at this location. Elongate pairs of high and low magnetic anomalies over the Seattle uplift suggest imbrication of steeply-dipping, deeply-rooted slices of Crescent Formation within Siletzia. We hypothesize these features result from duplication of Crescent Formation in an accretionary fold-thrust belt during the Eocene. The active Seattle fault divides this Eocene fold-thrust belt into two zones with different structural trends and opposite frontal ramp dips, suggesting the Seattle fault may have originated as a tear fault during accretion.
The Indian plate underthrusting the Himalaya is considered to be segmented along the collision belt arc and seismic images of the Indian mantle lithosphere (IML) suggest along-arc variations in the angle of underthrusting and its northern limit beneath Tibet. The pre-existing transverse tectonic structures of the Indian plate mapped in the Ganga foreland basin have been related to these segmentation boundaries. These segmentations imply changes in mechanical properties of adjoining blocks which should manifest in the form of spatial variations in topography build-up. We have analysed a geomorphic index, normalized channel steepness (ksn), along the Himalayan arc using the ALOS elevation dataset to test whether there is any correlation between the and these segmentation boundaries. Our results bring out spatial variability in the along the arc. Based on these results, the arc can be segmented into five blocks, similar to the ones delineated based on correlation between the width of the Ganga foreland basin and the disposition of major Himalayan thrusts from the foothills. Thus, the can be used as a proxy to demarcate different tectonic blocks along the Himalayan arc. Further, we have found a good correlation between the basin width and the northern limit of the IML for all block except the Uttarakhand block. We infer that transverse crustal heterogeneities in this block due to the continuation of different litho-units of the Aravalli-Delhi Fold Belt could be a plausible cause for this anti-correlation.
Zagros Orogeny System resulted due to collision of the Arabian with Eurasia. The region has the ocean-continent subduction and continent-continent collision; and convergence velocity shows variation from east to west. Therefore, this region shows the complex tectonic stress and a wide range of diffuse or localized deformation between both plates. The in-situ stress and GPS data are very limited in this region, therefore, we performed a numerical simulation of the stresses causing deformation in the Zagros-Iran region. The deviatoric stresses resulting from the variations in lithospheric density and thickness; and those from shear tractions at the base of lithosphere due to mantle convection were computed using thin-sheet approximation. Surface observations of strain rates, SHmax, plate velocities etc. are explained using the joint models of lithosphere and mantle, suggesting a good coupling between lithosphere and mantle in most parts of Zagros and Iran. However, the deformation in east of Iran is caused primarily by lithospheric stresses. Plate motion of Arabian plate is found to vary along the Zagros belt from north-northeast in south-east of Zagros, north in central Zagros to slight northwest in northwestern Zagros. The output of this study can be used in seismic hazards estimations.