Non-linear Teleseismic Tomography Research Articles

Post-collisional lithospheric delamination in eastern Iran, revealed by non-linear teleseismic tomography and residual topography

The Eastern Iranian Mountain Ranges (EIR) emerged as a consequence of the Late Cretaceous collision between the Afghan and Lut blocks. However, the response of the uppermost mantle to this collision remains enigmatic. Additionally, although petrological evidence suggests that post-collisional delamination is possible, it has not been conclusively identified in prior regional seismic imagery. This observation leads us to further explore this possibility using a dense seismic network. To gain insight into the geodynamic implications for eastern Iran and address knowledge gaps, we extensively investigated the seismic structure of the uppermost mantle beneath the EIR using a dense seismic network of 34 temporary stations, complemented by data from nine additional local permanent stations. By meticulously analyzing 6589 relative arrival time residuals from teleseismic records with favorable signal-to-noise ratios, we applied a non-linear tomography method to map P-wave velocity perturbations in a relative sense. Our tomographic images unveiled distinct instances of rapid high-velocity anomalies beneath low-velocity regions in the shallow mantle, suggesting the potential occurrence of lithospheric dripping, followed by subsequent asthenospheric upwelling. This observation offers a plausible explanation for the observed post-collisional magmatism over the Lut Block. Furthermore, to maintain the approximately 1.5-km positive residual topography across the EIR, beyond the influence of crustal properties, additional support from the hot and buoyant asthenosphere becomes crucial, particularly in the absence of a substantial lithospheric mantle.

Crustal and upper mantle structure of northern Algeria inferred from a 3-D inversion of teleseismic tomography

The northern Algeria forms the southern edge of the Western Mediterranean Sea that formed within the zone of convergence between the African and Eurasian plates. It represents a key area in the Mediterranean formation where controversial kinematic and geodynamical models are proposed. The current structure of northern Algeria is the result of the geodynamic evolution that occurred in the western Mediterranean. Several tomographic studies revealed the presence of velocity anomalies beneath the south of the western Mediterranean region. Namely, these anomalies suggest the existence of abnormally cold material in the asthenosphere, interpreted as the trace of subducted plunging panels (slab). Unfortunately, the lack of suitable data remains the main limit confronted by most tomographic studies in the northern Algeria, which remains to date one of the poorly studied regions in the western Mediterranean. During the recent decades, the seismological network in northern Algeria has significantly developed, giving rise to a significant increase in the amount of information given by the network database. This development motivated us to investigate the crust and the upper mantle structure beneath the northern Algeria using a non-linear teleseismic tomography method. An overall of 29 stations distributed along the northern part of Algeria have been used to collect 11018 P-waves from 2847 earthquakes recorded over ten years. The proposed model suggests the presence of high-velocity anomalies along of the northern Algeria. We interpreted these abnormal anomalies as a slab teared into two panels. A central segment bending along the Atlasic massif of Hodna and plunging under the Lesser Kabylia. The second segment at the North-East, shifted to the north plunging towards the NE. A series of Tellian high-velocity anomalies axes mark the limit between the African and the Eurasian plate. The south eastern region covered by our model (Aurès massif) is characterized by a low-velocity anomaly potentially linked to the remnant of the Mesozoic rifting.

Shear wave velocity structure of the upper-mantle beneath the northern Zagros collision zone revealed by nonlinear teleseismic tomography and Bayesian Monte-Carlo joint inversion of surface wave dispersion and teleseismic P-wave coda

The continental collision between the Arabian and Eurasian plates plays an important role in the geodynamical evolution of the Zagros and Iranian plateau. In order to investigate the upper mantle structure across the collision zone, we calculated the absolute and relative shear-wave velocity structure along a dense and long temporary seismic profile in NW Iran. The probabilistic 1-D absolute shear-wave velocity (Vs) depth profiles beneath 23 seismic stations were calculated using a trans-dimensional joint inversion approach of P-wave coda together with the Rayleigh wave phase and group velocity dispersion data. These 1-D models are then interpolated to obtain a 2-D Vs model along the profile. In addition, we used a nonlinear teleseismic tomography method to determine shear wave velocity variations beneath 44 seismic stations. Both the absolute and relative velocity models, obtained by different data sets and methods, show a thick (>150 km) high-velocity anomaly beneath the northern part of the Zagros, which could be the thickened Zagros lithosphere associated with continental shortening. A relatively thin lithosphere (~80 km) and lower S-wave velocity beneath Central Iran may suggest a weakened Iranian mantle lithosphere by upwelling of the hot asthenosphere following slab break-off. The presence of a thickened lithosphere beneath the Zagros implies that the Arabian-Eurasian convergence is accommodated mainly as shortening in the mantle lithosphere beneath the Zagros, rather than underthrusting beneath Central Iran. A narrow low-velocity corridor between the Zagros lithosphere and overriding plate (Central Iran), south of the Sanandaj-Sirjan Zone (SSZ), probably due to delamination of the subducting Arabian lithosphere from the Zagros lower crust, likely prohibits transferring stress from the Zagros lithosphere to Central Iran, causing less deformation in the SSZ which is the edge of the overriding plate.

Upper-mantle velocity structure beneath the Zagros collision zone, Central Iran and Alborz from nonlinear teleseismic tomography

Upper-mantle velocity structure beneath the Zagros collision zone, Central Iran and Alborz from nonlinear teleseismic tomography

Upper mantle structure around the Trans-European Suture Zone obtained by teleseismic tomography

Abstract. The presented study aims to resolve the upper mantle structure around the Trans-European Suture Zone (TESZ), which is the major tectonic boundary in Europe. The data of 183 temporary and permanent seismic stations operated during the period of the PASsive Seismic Experiment (PASSEQ) 2006–2008 within the study area from Germany to Lithuania was used to compile the data set of manually picked 6008 top-quality arrivals of P waves from teleseismic earthquakes. We used the TELINV nonlinear teleseismic tomography algorithm to perform the inversions. As a result, we obtain a model of P wave velocity variations up to about ±3% with respect to the IASP91 velocity model in the upper mantle around the TESZ. The higher velocities to the east of the TESZ correspond to the older East European Craton (EEC), while the lower velocities to the west of the TESZ correspond to younger western Europe. We find that the seismic lithosphere–asthenosphere boundary (LAB) is more distinct beneath the Phanerozoic part of Europe than beneath the Precambrian part. To the west of the TESZ beneath the eastern part of the Bohemian Massif, the Sudetes Mountains and the Eger Rift, the negative anomalies are observed from a depth of at least 70 km, while under the Variscides the average depth of the seismic LAB is about 100 km. We do not observe the seismic LAB beneath the EEC, but beneath Lithuania we find the thickest lithosphere of about 300 km or more. Beneath the TESZ, the asthenosphere is at a depth of 150–180 km, which is an intermediate value between that of the EEC and western Europe. The results imply that the seismic LAB in the northern part of the TESZ is in the shape of a ramp dipping to the northeasterly direction. In the southern part of the TESZ, the LAB is shallower, most probably due to younger tectonic settings. In the northern part of the TESZ we do not recognize any clear contact between Phanerozoic and Proterozoic Europe, but further to the south we may refer to a sharp and steep contact on the eastern edge of the TESZ. Moreover, beneath Lithuania at depths of 120–150 km, we observe the lower velocity area following the boundary of the proposed paleosubduction zone.

Traces of the crustal units and the upper-mantle structure in the southwestern part of the East European Craton

Abstract. The presented study is a part of the passive seismic experiment PASSEQ 2006–2008, which took place around the Trans-European Suture Zone (TESZ) from May 2006 to June 2008. The data set of 4195 manually picked arrivals of teleseismic P waves of 101 earthquakes (EQs) recorded in the seismic stations deployed to the east of the TESZ was inverted using the non-linear teleseismic tomography algorithm TELINV. Two 3-D crustal models were used to estimate the crustal travel time (TT) corrections. As a result, we obtain a model of P-wave velocity variations in the upper mantle beneath the TESZ and the East European Craton (EEC). In the study area beneath the craton, we observe up to 3% higher and beneath the TESZ about 2–3% lower seismic velocities compared to the IASP91 velocity model. We find the seismic lithosphere–asthenosphere boundary (LAB) beneath the TESZ at a depth of about 180 km, while we observe no seismic LAB beneath the EEC. The inversion results obtained with the real and the synthetic data sets indicate a ramp shape of the LAB in the northern TESZ, where we observe values of seismic velocities close to those of the craton down to about 150 km. The lithosphere thickness in the EEC increases going from the TESZ to the NE from about 180 km beneath Poland to 300 km or more beneath Lithuania. Moreover, in western Lithuania we find an indication of an upper-mantle dome. In our results, the crustal units are not well resolved. There are no clear indications of the features in the upper mantle which could be related to the crustal units in the study area. On the other hand, at a depth of 120–150 km we indicate a trace of a boundary of proposed palaeosubduction zone between the East Lithuanian Domain (EL) and the West Lithuanian Granulite Domain (WLG). Also, in our results, we may have identified two anorogenic granitoid plutons.

Lithospheric structure beneath the Zagros collision zone resolved by non-linear teleseismic tomography

The upper-mantle structure across the Zagros collision zone, in southwest Iran, is investigated using a non-linear weighted damped least-squares teleseismic tomography approach. The resolution of the structures/transitions in the upper mantle is enhanced significantly by correcting the teleseismic relative arrival time residuals for an a priori crustal velocity model and then performing the inversion with fixed crustal blocks. To investigate whether or not the lithospheric blocks and major transitions in the resulting model are required by the data or are artefacts of the inversion, the data were inverted using two different inverse methods (singular value decomposition and a quadratic programming method). New high-quality seismic velocity models show apparent correlation between surface geological features and seismic velocity structures at lithospheric depth across the Zagros collision zone. The image shows a sharp lithospheric boundary at the Main Zagros Thrust between 100 km and 250 km depth with P-wave velocity about 3 per cent faster within the Arabian Shield to the south. A step-like increase in lithospheric thickness across the Zagros collision zone is assumed to separate two different mantle structures namely the Arabian (to the south) and the Eurasian (to the north) domains. The most striking feature resolved is a north-dipping slab-like positive velocity anomaly.

Upper-mantle S-velocity structure across the Zagros collision zone resolved by nonlinear teleseismic tomography

Non-linear teleseismic S-phase tomography across the Zagros collision zone in southwestern Iran is used to determine a high-resolution image of the upper-mantle structure. The inversion was done using 41 high-quality earthquakes recorded by 19 broad-band and medium-band stations along a 620 km long profile across the collision zone. Smearing from strong crustal velocity anomalies into the upper-mantle is suppressed by travel-time corrections calculated based on a 3-D crustal model for the study area. Our results show that the relatively old and cold Arabian shield has a higher velocity (up to 6% faster, at depths between 70 and 300 km) than the younger lithosphere farther north in Central Iran. These two upper-mantle domains are separated by a sharp near-vertical transition whose surface expression coinciding with the Main Zagros Thrust.

High-resolution body wave tomography beneath the SVEKALAPKO array - II. Anomalous upper mantle structure beneath the central Baltic Shield

A number of different geodynamic models have been proposed to explain the early tectonic evolution of the Baltic Shield. To provide additional geophysical constraints on these models, we performed a teleseismic tomography traveltime inversion for the central part of the Baltic Shield. The SVEKALAPKO project is focused on the investigation of the lithosphere—asthenosphere structure down to 400 km depth under central Fennoscandia (Baltic Shield). A total of 143 stations were deployed including 15 permanent stations from the Finnish seismic network. The temporal network was composed of 40 broad-band and 88 short-period instruments distributed in a rectangular array of 1000 km by 900 km from 1998 August to 1999 May. The results are based on a non-linear teleseismic tomography algorithm. They reveal significant P-velocity variations (up to 4 per cent) throughout the SVEKALAPKO array. The most prominent feature is a positive anomaly that can be followed down to 250 km depth beneath the centre of the array. We interpret this anomaly as the signature of the tectosphere (Jordan 1978) beneath the Fennoscandian Shield. It correlates spatially with an anomalous high-velocity lower crust. Other shallow (crustal) anomalies can be correlated with magmatic events surrounding this nucleus of high velocity. Comparison of images before and after correction by crustal structure proves that this methodology yields solid and coherent tomographic results. Further observations of relative P traveltime residuals from six teleseismic events with different azimuths show delay variations of ±2.0 s between stations located in the North German basin and stations on the Svecofennian Shield.

Non-linear body wave teleseismic tomography along the TOR array

Summary The TOR project investigates the lithosphere–asthenosphere structure under the Tornquist Zone between Denmark and Sweden. Around 150 seismic stations (108 short-period, 28 broadband and 14 other permanent stations) were employed in a rectangular array along a 900 km long by 100 km wide strip across the Tornquist Zone from July 1996 to August 1997. The results obtained based on a non-linear teleseismic tomography algorithm reveal significant P-velocity variations (up to 5 per cent) along the TOR array from northern Germany to southern Sweden. Distinct lithospheric blocks are also recognized in the inversion results. Two inverse methods (singular value decomposition and a quadratic programming method) were implemented in order to investigate whether or not the lithospheric blocks and major boundaries in the inversion are required by the data or are artefacts of the inversion. According to the results, the lithosphere under northern Germany is thin but reaches to intermediate thicknesses of about 120 km in the Tornquist Zone area. Farther north in the Baltic Shield, more than 200 km of continental lithosphere is recognized in the model. Abrupt lateral P-velocity changes (maximum 5 per cent) are seen at both the southern and northern sides of the Tornquist Zone.

Nonlinear teleseismic tomography at Long Valley Caldera, using three‐dimensional minimum travel time ray tracing

We explore the impact of three‐dimensional minimum travel time ray tracing on nonlinear teleseismic inversion. This problem has particular significance when trying to image strongly contrasting low‐velocity bodies, such as magma chambers, because strongly refracted and/or diffracted rays may precede the direct P wave arrival traditionally used in straight‐ray seismic tomography. We use a simplex‐based ray tracer to compute the three‐dimensional, minimum travel time ray paths and employ an iterative inversion technique to cope with nonlinearity. Results from synthetic data show that our algorithm results in better model reconstructions compared with traditional straight‐ray inversions. We reexamine the teleseismic data collected at Long Valley caldera by the U.S. Geological Survey. The most prominent feature of our result is a 25–30% low‐velocity zone centered at 11.5 km depth beneath the northwestern quadrant of the caldera. Beneath this at a depth of 24.5 km is a more diffuse 15% low‐velocity zone. In general, the low velocities tend to deepen to the south and east. We interpret the shallow feature to be the residual Long Valley caldera magma chamber, while the deeper feature may represent basaltic magmas ponded in the midcrust. The deeper position of the prominent low‐velocity region in comparison to earlier tomographic images is a result of using three‐dimensional rays rather than straight rays in our ray tracing. The magnitude of our low‐velocity anomaly is a factor of ∼3 times larger than earlier models from linear arrival time inversions and is consistent with models based on observations of ray bending at sites within the caldera. Our results imply the presence of anywhere from 7 to 100% partial melt beneath the caldera.