How seismicity relates to lithospheric heterogeneity in the Alps

2021 ◽  
Author(s):  
Ajay Kumar ◽  
Cameron Spooner ◽  
Magdalena Scheck-Wenderoth ◽  
Mauro Cacace
Keyword(s):  
Lower Crust ◽  
Convergence Rates ◽  
Upper Rhine Graben ◽  
Topographic Effects ◽  
Relative Role ◽  
Upper Crust ◽  
Radiogenic Heat ◽  
First Order ◽  
Tectonic Inheritance ◽  
The Alps

<p>The Alps mountains and its forelands consist of a heterogeneous lithosphere, comprised of a multitude of tectonic blocks from different tectonic provinces with different thermo-physical properties. Patterns of seismicity distribution are also observed to vary significantly throughout the region. However, the relationship between seismicity and lithospheric heterogeneity has been often overlooked in previous studies. We present an overview of recent results that have attempted to address these questions through the use of integrated 3D modelling techniques, thereby including: (i) a gravity and seismic data constrained, 3D, density structural model of the lithosphere; (ii) a 3D thermal model constrained against available wellbore temperature data; and,  (iii) a 3D rheological model of the long-term lithospheric strength and effective viscosities. Our models support the existence of a first-order correlation between the distribution of seismicity (laterally and with depth) and the strength of the lithosphere, with the former being clustered mainly within weaker domains. Beneath the Alps, observed upper-crustal level (i.e., unimodal) seismicity correlates with a weaker lithosphere where plate strength is controlled by the thick crustal root. Whereas in the southern foreland, weaker zones are found preferentially around the stronger Adriatic indenter while in the northern foreland they are located in the crust beneath the the Upper Rhine Graben (URG). We found that this correlation is primarily controlled by resolved thermal gradients and is a function of the tectonic inheritance setting (e.g., UGR), crustal architecture (e.g., thickness of sediments, upper and lower crust) and LAB depth. Sediment thickness and topographic effects controls the shallow thermal filed (0 – 10 km) whereas the deeper thermal field is controlled by the thickness of felsic upper crust (higher radiogenic heat contribution), the mafic lower crust (less radiogenic heat contribution) and basal thermal boundary condition from LAB depth. Seismicity is bounded by specific isotherms, 450 <sup>o</sup>C in the crust and < 600 <sup>o</sup>C in the mantle, except in regions where slabs are imaged by seismic tomography models. This is in contrast to the recent proposition that convergence velocity is a first-order factor controlling seismicity in an orogen rather than its architecture. Fast convergence rates (e.g., Himalayas) have been related to the subduction of the cold crust to deeper crustal depths thereby leading to a deepening of the brittle  domain and to a bimodal (i.e., upper and lower crust) seismicity character. In contrast, slow convergence (e.g., Alps) is thought to lead to a hotter ductile lower crust thus limiting brittle deformation within the upper crust. We therefore end our contribution by opening a discussion on the relative role of convergence rates and lithospheric heterogeneities, inherited and/or developed during orogenesis, in controlling the seismicity. In doing so we carry out a comparison between observed seismicity and lithospheric architecture in the other mountain ranges of the western Alpine-Himalayan collision zone where  convergence velocities are of a similar order of magnitudes as Alps, i.e., the Betics, the Pyrenees and the Apennines but where seismicity is observed to occur both at upper and lower crustal levels.</p>

scholarly journals Verification of Convergence Rates of Numerical Solutions for Parabolic Equations

2019 ◽  
Vol 2019 ◽  
pp. 1-10
Author(s):  
Darae Jeong ◽  
Yibao Li ◽  
Chaeyoung Lee ◽  
Junxiang Yang ◽  
Yongho Choi ◽  
...  
Keyword(s):  
Parabolic Equations ◽  
Convergence Rates ◽  
Numerical Solutions ◽  
Second Order ◽  
Order Convergence ◽  
Numerical Convergence ◽  
Verification Method ◽  
First Order ◽  
Convergence Test ◽  
Second Order Convergence

In this paper, we propose a verification method for the convergence rates of the numerical solutions for parabolic equations. Specifically, we consider the numerical convergence rates of the heat equation, the Allen–Cahn equation, and the Cahn–Hilliard equation. Convergence test results show that if we refine the spatial and temporal steps at the same time, then we have the second-order convergence rate for the second-order scheme. However, in the case of the first-order in time and the second-order in space scheme, we may have the first-order or the second-order convergence rates depending on starting spatial and temporal step sizes. Therefore, for a rigorous numerical convergence test, we need to perform the spatial and the temporal convergence tests separately.

Results of a seismic reflection survey across the fault zone between the Thompson nickel belt and the Churchill Tectonic Province, northern Manitoba

1981 ◽  
Vol 18 (1) ◽  
pp. 13-25 ◽  
Author(s):  
A. G. Green
Keyword(s):  
Fault Zone ◽  
Lower Crust ◽  
Seismic Reflection ◽  
High Amplitude ◽  
Small Scale ◽  
Upper Crust ◽  
Travel Times ◽  
Reflection Point ◽  
Point Data ◽  
Amplitude Reflection

Approximately 11 km of four-fold common reflection point data have been recorded across a region that spans the contact fault zone between the Thompson nickel belt and the Churchill Tectonic Province. From these data it is shown that the upper crust in this region and, to a lesser extent, the lower crust are characterized by numerous scattered events that originate from relatively small-scale features. Within the Thompson nickel belt two extensive and particularly high-amplitude reflection zones, at two-way travel times of t = 5.0–5.5 s and t = 6.0–6.5 s, are recorded with apparent northwesterly dips of 0–20 °C. These reflection zones, which have a laminated character, are truncated close to the faulted contact with the Churchill Province. Both the contact fault zone and the Churchill Province in this region have crustal sections that are relatively devoid of significant reflectors. The evidence presented here confirms that the crustal section of the Thompson nickel belt is fundamentally different from that of the Churchill Tectonic Province.

Dynamics of the extension in the Fonualei Rift in the northern Lau Basin at 16 °S

2021 ◽  
Author(s):  
Anna Jegen ◽  
Anke Dannowski ◽  
Heidrun Kopp ◽  
Udo Barckhausen ◽  
Ingo Heyde ◽  
...  
Keyword(s):  
Velocity Gradient ◽  
Lower Crust ◽  
Pacific Plate ◽  
P Wave ◽  
Upper Crust ◽  
Lau Basin ◽  
Australian Plate ◽  
The Pacific ◽  
Refraction Seismic ◽  
Roll Back

<p>The Lau Basin is a young back-arc basin steadily forming at the Indo-Australian-Pacific plate boundary, where the Pacific plate is subducting underneath the Australian plate along the Tonga-Kermadec island arc. Roughly 25 Ma ago, roll-back of the Kermadec-Tonga subduction zone commenced, which lead to break up of the overriding plate and thus the formation of the western Lau Ridge and the eastern Tonga Ridge separated by the emerging Lau Basin.</p><p>As an analogue to the asymmetric roll back of the Pacific plate, the divergence rates decline southwards hence dictating an asymmetric, V-shaped basin opening. Further, the decentralisation of the extensional motion over 11 distinct spreading centres and zones of active rifting has led to the formation of a composite crust formed of a microplate mosaic. A simplified three plate model of the Lau Basin comprises the Tonga plate, the Australian plate and the Niuafo'ou microplate. The northeastern boundary of the Niuafo'ou microplate is given by two overlapping spreading centres (OLSC), the southern tip of the eastern axis of the Mangatolu Triple Junction (MTJ-S) and the northern tip of the Fonualei Rift spreading centre (FRSC) on the eastern side. Slow to ultraslow divergence rates were identified along the FRSC (8-32 mm/a) and slow divergence at the MTJ (27-32 mm/a), both decreasing southwards. However, the manner of divergence has not yet been identified. Additional regional geophysical data are necessary to overcome this gap of knowledge.</p><p>Research vessel RV Sonne (cruise SO267) set out to conduct seismic refraction and wide-angle reflection data along a 185 km long transect crossing the Lau Basin at ~16 °S from the Tonga arc in the east, the overlapping spreading centres, FRSC1 and MTJ-S2, and extending as far as a volcanic ridge in the west. The refraction seismic profile consisted of 30 ocean bottom seismometers. Additionally, 2D MCS reflection seismic data as well as magnetic and gravimetric data were acquired.</p><p>The results of our P-wave traveltime tomography show a crust that varies between 4.5-6 km in thickness. Underneath the OLSC the upper crust is 2-2.5 km thick and the lower crust 2-2.5 km thick. The velocity gradients of the upper and lower crust differ significantly from tomographic models of magmatically dominated oceanic ridges. Compared to such magmatically dominated ridges, our final P-wave velocity model displays a decreased velocity gradient in the upper crust and an increased velocity gradient in the lower crust more comparable to tectonically dominated rifts with a sparse magmatic budget.</p><p>The dominance of crustal stretching in the regional rifting process leads to a tectonical stretching, thus thinning of the crust under the OLSC and therefore increasing the lower crust’s velocity gradient. Due to the limited magmatic budget of the area, neither the magnetic anomaly nor the gravity data indicate a magmatically dominated spreading centre. We conclude that extension in the Lau Basin at the OLSC at 16 °S is dominated by extensional processes with little magmatism, which is supported by the distribution of seismic events concentrated at the northern tip of the FRSC.</p>

Thermodynamic constraints on assimilation of silicic crust by primitive magmas

2021 ◽  
Author(s):  
Jussi S Heinonen ◽  
Frank J Spera ◽  
Wendy A Bohrson
Keyword(s):  
Phase Equilibria ◽  
Lower Crust ◽  
Primitive Mantle ◽  
Upper Crust ◽  
Basaltic Composition ◽  
Primitive Magma ◽  
Partial Melts ◽  
Thermodynamic Limits ◽  
Melt Composition ◽  
Lower Crustal

<p>Some studies on basaltic and more primitive rocks suggest that their parental magmas have assimilated more than 50 wt.% (relative to the initial uncontaminated magma) of crustal silicate wallrock. But what are the thermodynamic limits for assimilation by primitive magmas? This question has been considered for over a century, first by N.L. Bowen and many others since then. Here we pursue this question quantitatively using a freely available thermodynamic tool for phase equilibria modeling of open magmatic systems — the Magma Chamber Simulator (MCS; https://mcs.geol.ucsb.edu).</p><p>In the models, komatiitic, picritic, and basaltic magmas of various ages and from different tectonic settings assimilate progressive partial melts of average lower, middle, and upper crust. In order to pursue the maximum limits of assimilation constrained by phase equilibria and energetics, the mass of wallrock in the simulations was set at twice that of the initially pristine primitive magmas. In addition, the initial temperature of wallrock was set close to its solidus at a given pressure. Such conditions would approximate a rift setting with tabular chambers and high magma input causing concomitant crustal heating and steep geotherms.</p><p>Our results indicate that it is difficult for any primitive magma to assimilate more than 20−30 wt.% of upper crust before evolving to intermediate/felsic compositions. However, if assimilant is lower crust, typical komatiitic magmas can assimilate more than their own weight (range of 59−102 wt.%) and retain a basaltic composition. Even picritic magmas, more relevant to modern intraplate settings, have a thermodynamic potential to assimilate 28−49 wt.% of lower crust before evolving into intermediate/felsic compositions.</p><p>These findings have important implications for petrogenesis of magmas. The parental melt composition and the assimilant heavily influence both how much assimilation is energetically possible in primitive magmas and the final magma composition. The fact that primitive mantle melts have potential to partially melt and assimilate significant fractions of (lower) crust may have fundamental importance for how trans-Moho magmatic systems evolve and how crustal growth is accomplished. Examples include generation of siliceous high-magnesium basalts in the Precambrian and anorogenic anorthosite-mangerite-charnockite-granite complexes with geochemical evidence of considerable geochemical overprint from (lower) crustal sources.</p>

scholarly journals Evaluation of crustal inhomogeneity parameters in the southern Longmenshan fault zone and adjacent regions

2020 ◽  
Vol 24 (6) ◽  
pp. 1175-1188
Author(s):  
Xiao-Ping Fan ◽  
Yi-Cheng He ◽  
Cong-Jie Yang ◽  
Jun-Fei Wang
Keyword(s):  
Fault Zone ◽  
Lower Crust ◽  
Tectonic Deformation ◽  
Upper Crust ◽  
Crust And Upper Mantle ◽  
Study Region ◽  
Waveform Data ◽  
Longmenshan Fault Zone ◽  
Deformation Features ◽  
Longmenshan Fault

AbstractBroadband teleseismic waveform data from 13 earthquakes recorded by 70 digital seismic stations were selected to evaluate the inhomogeneity parameters of the crustal medium in the southern Longmenshan fault zone and its adjacent regions using the teleseismic fluctuation wavefield method. Results show that a strong inhomogeneity exists beneath the study region, which can be divided into three blocks according to its structure and tectonic deformation features. These are known as the Sichuan-Qinghai Block, the Sichuan-Yunnan Block, and the Mid-Sichuan Block. The velocity fluctuation ratios of the three blocks are approximately 5.1%, 3.6%, and 5.1% in the upper crust and 5.1%, 3.8%, and 4.9% in the lower crust. The inhomogeneity correlation lengths of the three blocks are about 10.1 km, 14.0 km, and 10.7 km in the upper crust and 11.8 km, 17.0 km, and 11.8 km in the lower crust. The differences in the crustal medium inhomogeneity beneath the Sichuan-Yunnan Block, the Sichuan-Qinghai Block, and the Mid-Sichuan Block may be related to intensive tectonic movement and material flow in the crust and upper mantle.

Inelastic strain rate and stress fields in and around an aseismic zone of Kyushu Island, Japan, inferred from seismic and GNSS data

2020 ◽  
Vol 221 (1) ◽  
pp. 289-304
Author(s):  
Y Yuasa ◽  
S Matsumoto ◽  
S Nakao ◽  
T Matsushima ◽  
T Ohkura
Keyword(s):  
Stress Concentration ◽  
Strain Rate ◽  
Crustal Deformation ◽  
Lower Crust ◽  
Seismic Activity ◽  
Deformation Zone ◽  
Inelastic Deformation ◽  
Upper Crust ◽  
Localized Deformation ◽  
Concentration Process

SUMMARY Understanding earthquake processes and crustal deformation requires knowledge of the stress concentration process in the crust. With the enhancement of observation networks, it has become possible to consider in detail the relationships between localized deformation and seismic activity in island arcs and the process of stress concentration. According to previous studies, inelastic deformation in localized weak zones in the crust is considered to play an important role in the stress concentration process. Kyushu, located in southwest Japan, has a 20–30 km band-like active seismic activity and an enclosed aseismic zone. In particular, a part of the seismic active region called the Beppu-Simahara Graben, which is dominated by north–south extensional deformation, is characterized by high seismic activity and a remarkable aseismic zone. We identified the relationship between inelastic deformation and stress concentration processes in this area by using analyses of geodetic and seismic data. The results inverted from both the strain rate field obtained by the geodetic observations and the deviatoric stress field estimated from focal mechanism data reveal a large inelastic deformation zone ($\sim {10^{ - 7}} \,\mathrm{ yr}^{-1}$) beneath the area of active seismicity. From comparison with previous works, the inelastic deformation zone in the lower crust may correspond to an area with high temperature and/or fluid. This may suggest that inelastic deformation is in progress in the area where the strength of lower crustal rocks has reduced due to the presence of geothermics and/or fluids. Furthermore, we confirmed that this inelastic deformation causes stress concentrations of up to $10\,\,{\rm{kPa}}\,\,{\rm{yr}}^{-1}$ in the upper crust. These results show that stress concentration occurs locally in the upper crust, above the inelastic deformation zone in the weakened lower crust, owing to the presence of geothermal and/or fluid; this stress concentration induces seismic activity and crustal deformation.

scholarly journals Determination of rock densities in the Carpathian-Pannonian Basin lithosphere: based on the CELEBRATION 2000 experiment

2016 ◽  
Vol 46 (4) ◽  
pp. 269-287 ◽  
Author(s):  
Barbora Šimonová ◽  
Miroslav Bielik
Keyword(s):  
Lower Crust ◽  
Pannonian Basin ◽  
Average Density ◽  
East European Craton ◽  
P Wave ◽  
Seismic Velocities ◽  
Upper Crust ◽  
East European ◽  
P Wave Velocity

Abstract The international seismic project CELEBRATION 2000 brought very good information about the P-wave velocity distribution in the Carpathian-Pannonian Basin litosphere. In this paper seismic data were used for transformations of in situ P-wave velocities to in situ densities along all profiles running across the Western Carpathians and the Pannonian Basin: CEL01, CEL04, CEL05, CEL06, CEL09, CEL11 and CEL12. The calculation of rock densities in the crust and lower lithosphere was done by the transformation of seismic velocities to densities using the formulae of Sobolev-Babeyko, Christensen-Mooney and in the lower lithosphere also by Lachenbruch-Morgan’s formula. The density of the upper crust changes significantly in the vertical and horizontal directions, while the interval ranges of the calculated lower crust densities narrow down prominently. The lower lithosphere is the most homogeneous - the intervals of the calculated densities for this layer are already very narrow. The average density of the upper crust (ρ̅ = 2.60 g · cm−3) is the lowest in the Carpathian Foredeep region. On the contrary, the highest density of this layer (ρ̅ = 2.77 g · cm−3) is located in the Bohemian Massif. The average densities ρ̅ of the lower crust vary between 2.90 and 2.98 g · cm−3. The Palaeozoic Platform and the East European Craton have the highest density (ρ̅ = 2.98 g · cm−3 and ρ̅ = 2.97 g · cm−3, respectively). The lower crust density is the lowest (ρ̅ = 2.90 g · cm−3) in the Pannonian Basin. The range of calculated average densities ρ̅ for the lower lithosphere is changed in the interval from 3.35 to 3.40 g · cm−3. The heaviest lower lithosphere can be observed in the East European Craton (ρ̅ = 3.40 g · cm−3). The lower lithosphere of the Transdanubian Range and the Palaeozoic Platform is characterized by the lowest density ρ̅ = 3.35 g · cm−3.

Some results of COCORP seismic reflection profiling in the Grenville-age Adirondack Mountains, New York State

1985 ◽  
Vol 22 (2) ◽  
pp. 141-153 ◽  
Author(s):  
S. L. Klemperer ◽  
L. D. Brown ◽  
J. E. Oliver ◽  
C. J. Ando ◽  
B. L. Czuchra ◽  
...  
Keyword(s):  
New York ◽  
Lower Crust ◽  
Seismic Reflection ◽  
New York State ◽  
Three Dimensional ◽  
Upper Crust ◽  
Adirondack Mountains ◽  
East Central ◽  
York State ◽  
Seismic Reflection Profiling

COCORP deep seismic reflection profiling in the Adirondack Mountains of northern New York State has revealed a prominent zone of layered reflectors in the lower crust of the east-central Adirondacks. The strong, layered reflectors (here termed the Tahawus complex) occur between 18 and 26 km depth, beneath the sparsely reflective, granulite-grade, surface terrane, which has been uplifted from depths greater than 20 km. The Tahawus complex apparently represents layered rocks of some type in the lower crust of the Adirondacks. Possibilities include gneissic layering, cumulate igneous layering, a layered sill complex, and underthrust sedimentary strata, The Tahawus complex may be spatially coincident with a previously detected, high-conductivity zone in the lower crust, suggesting that either unusual mineralogies or interstitial electrolytes are present in the Tahawus complex. In contrast to layered reflections discovered in the lower crust of the east-central Adirondacks and southeast of the Adirondacks, cross-cutting and discontinuous reflections are recorded from the upper crust on all the COCORP Adirondack lines, including lines in both the Adirondack Highlands and Lowlands. Available three-dimensional control suggests that reflections in the upper crust of the central Adirondacks are parallel to, and hence may be related to, the folded gneisses mapped at the surface. Shallow events are also observed on a COCORP profile close to the epicenter of the 7 October 1983 magnitude 5.2 earthquake in the central Adirondacks, but their relation to the earthquake is uncertain.

Sign in / Sign up

Export Citation Format

Share Document