G. Asch

Subduction and collision processes in the Central Andes constrained by converted seismic phases

The Central Andes are the Earths highest mountain belt formed by ocean–continent collision. Most of this uplift is thought to have occurred in the past 20 Myr, owing mainly to thickening of the continental crust, dominated by tectonic shortening. Here we use P-to-S (compressional-to-shear) converted teleseismic waves observed on several temporary networks in the Central Andes to image the deep structure associated with these tectonic processes. We find that the Moho (the Mohorovičić discontinuity—generally thought to separate crust from mantle) ranges from a depth of 75 km under the Altiplano plateau to 50 km beneath the 4-km-high Puna plateau. This relatively thin crust below such a high-elevation region indicates that thinning of the lithospheric mantle may have contributed to the uplift of the Puna plateau. We have also imaged the subducted crust of the Nazca oceanic plate down to 120 km depth, where it becomes invisible to converted teleseismic waves, probably owing to completion of the gabbro–eclogite transformation; this is direct evidence for the presence of kinetically delayed metamorphic reactions in subducting plates. Most of the intermediate-depth seismicity in the subducting plate stops at 120 km depth as well, suggesting a relation with this transformation. We see an intracrustal low-velocity zone, 10–20 km thick, below the entire Altiplano and Puna plateaux, which we interpret as a zone of continuing metamorphism and partial melting that decouples upper-crustal imbrication from lower-crustal thickening.

Complex patterns of fluid and melt transport in the central Andean subduction zone revealed by attenuation tomography

Abstract We present a high resolution 3-D model of P -wave attenuation ( Q p −1 ) for the central Andean subduction zone. Data from 1500, mostly intermediate depth (60–250 km) earthquakes recorded at three temporary seismic networks covering the forearc, arc, and backarc around 23°S were used for tomographic inversion. The forearc is characterised by uniformly high Q p values, indicating low temperature rocks, in accordance with low surface heat flow values. Prominent low Q p anomalies are found beneath the magmatic arc and the backarc in the crust and mantle. Continuous regions of low Q p connect earthquake clusters at 100 km and 200 km depth with zones of active volcanism in the arc and backarc. Fluids fluxed from the subducted oceanic lithosphere into the overlying mantle wedge, where they induce melting, explain our observations. We propose that low Q p regions indicate source and ascent pathways of metamorphic fluids and partial melts. Ascent of fluids and melts, as imaged by seismic Q p , are not vertical, as is often implicitely assumed. Instead, sources of fluids are located at different depth levels, and ascent paths are complex and exhibit significant variation within the study area. The largest Quaternary backarc volcano Cerro Tuzgle is fed by mantle melts which are imaged as a plume of low Q p material that reaches to the strong earthquake cluster at 200 km depth.

The Southern Andes between 36° and 40°S latitude: seismicity and average seismic velocities

Abstract The project ISSA 2000 (Integrated Seismological experiment in the Southern Andes) consists of a temporary seismological network and a seismic refraction profile. A network of 62 seismological stations was deployed across the Southern Andes at ∼38°S. Three hundred thirty-three local seismic events were observed in a 3-month period. P and S arrival times of a subset of high quality data were inverted simultaneously for 1-D velocity structure, hypocentral coordinates and station delays. Seismic refraction data along a transect at 39°S provide further constraints on the crustal structure. Low crustal velocities beneath the forearc may be either due to subducted trench sediments or serpentinized mantle material of the continental lithosphere. The continental Moho is not clearly observed in this region. Average velocities of the crust beneath the arc are higher than those beneath the forearc. Crustal thickness is about 40 km. Crustal seismicity concentrates in the forearc region along the Bio-Bio and Gastre fault zones. The area between these two prominent fault zones seems to be nearly devoid of crustal seismicity but shows highest uplift and topography in the forearc region. Benioff seismicity is observed down to 150 km depth resulting in the first accurate image of the Benioff zone in the Southern Andes. A maximum of seismicity at 60 km depth may be caused by dehydration embrittlement.

P and S velocity structure of the crust and the upper mantle beneath central Java from local tomography inversion

Here we present the results of local source tomographic inversion beneath central Java. The data set was collected by a temporary seismic network. More than 100 stations were operated for almost half a year. About 13,000 P and S arrival times from 292 events were used to obtain three-dimensional (3-D) Vp, Vs, and Vp/Vs models of the crust and the mantle wedge beneath central Java. Source location and determination of the 3-D velocity models were performed simultaneously based on a new iterative tomographic algorithm, LOTOS-06. Final event locations clearly image the shape of the subduction zone beneath central Java. The dipping angle of the slab increases gradually from almost horizontal to about 70°. A double seismic zone is observed in the slab between 80 and 150 km depth. The most striking feature of the resulting P and S models is a pronounced low-velocity anomaly in the crust, just north of the volcanic arc (Merapi-Lawu anomaly (MLA)). An algorithm for estimation of the amplitude value, which is presented in the paper, shows that the difference between the fore arc and MLA velocities at a depth of 10 km reaches 30% and 36% in P and S models, respectively. The value of the Vp/Vs ratio inside the MLA is more than 1.9. This shows a probable high content of fluids and partial melts within the crust. In the upper mantle we observe an inclined low-velocity anomaly which links the cluster of seismicity at 100 km depth with MLA. This anomaly might reflect ascending paths of fluids released from the slab. The reliability of all these patterns was tested thoroughly.

Partial Melting in the Central Andean Crust: a Review of Geophysical, Petrophysical, and Petrologic Evidence

The thickened crust of the Central Andes is characterized by several first-order geophysical anomalies that seem to reflect the presence of partial melts. Magnetotelluric and geomagnetic deep-sounding studies in Northern Chile have revealed a high conductivity zone (HCZ) beneath the Altiplano Plateau and the Western Cordillera, which is extreme both in terms of its size and integrated conductivity of > 20000 Siemens. Furthermore, this region is characterized by an extremely high seismic attenuation and reduced seismic velocity. The interrelation between the different petrophysical observations, in combination with petrological and heat-flow density studies, strongly indicates a huge area of partially molten rocks that is possibly topped with a thin, saline fluid film. The average melt fraction is deduced to be ∼20 vol.%, which agrees with typical values deduced from eroded migmatites. Based on the distribution and geochemical composition of Pliocene to Quaternary silicic ignimbrites in this area, this zone is thought to be dominated by crustally-derived rhyodacite melts with minor andesitic contribution. An interconnected melt distribution — typical for migmatites - would satisfy both the magnetotelluric and seismic observations. The high melt fraction in this mid-crustal zone should lead to strong weakening, which may be a main cause for the development of the flat topography of the Altiplano Plateau.

High-resolution image of the oceanic moho in the subducting Nazca Plate from P-S converted waves

Short-period P-S converted waves outline the oceanic Moho of the descending Nazca plate in northern Chile at 24°S. Comparison with hypocentral locations obtained with temporary local seismic networks show that earthquakes inside the subducting oceanic lithosphere of the Nazca plate are located predominantly in the oceanic crust at depths between 70 and 120 km, and probably in the upper mantle at depths below 120 km. The results suggest that dehydration embrittlement as mechanism for intermediate-depth earthquakes is active both in the former oceanic crust and upper mantle of subducting lithosphere.

Seismicity and average velocities beneath the Argentine Puna Plateau

A network of 60 seismographs was deployed across the Andes at ∼23.5°S. The array was centered in the backarc, atop the Puna high plateau in NW Argentina. P and S arrival times of 426 intermediate depth earthquakes were inverted for 1-D velocity structure and hypocentral coordinates. Average velocities and v p /v s in the crust are low Average mantle velocities are high but difficult to interpret because of the presence of a fast velocity slab at depth. Although the hypocenters sharply define a 35° dipping Benioff zone, seismicity in the slab is not continuous. The spatial clustering of earthquakes is thought to reflect inherited heterogeneties of the subducted oceanic lithosphere. Additionally, 57 crustal earthquakes were located. Seismicity concentrates in the fold and thrust belt of the foreland and Eastern Cordillera, and along and south of the El Toro-Olacapato-Calama Lineament (TOCL). Focal mechanisms of two earthquakes at this structure exhibit left lateral strike-slip mechanisms similar to the suggested kinematics of the TOCL. We believe that the Puna north of the TOCL behaves like a rigid block with little internal deformation, whereas the area south of the TOCL is weaker and currently deforming.

Shear wave anisotropy in the upper mantle beneath the Nazca Plate in northern Chile

Data from the Projecto de Investigacion Sismologica de la Cordillera Occidental (PISCO) seismic network and from six broadband seismographs that were operating in northern Chile were used to investigate the mantle in the convergent boundary zone between Nazca plate and the South American continent for the presence of anisotropy. Broadband data as well as long-period filtered data of teleseismic SKS and PKS phases were analyzed for the presence of shear wave splitting as a possible indicator for seismic anisotropy in the mantle beneath the PISCO network. Measurable shear wave splitting was observed with maximum delay times between the slow and fast split wave of the order of 1 s. Splitting of S waves from intermediate-depth events located directly beneath the PISCO network in the descending Nazca plate is generally associated with small delay times of the order of 0.1 s, a value typical for the continental crust. Near-vertical ScS reflections from two deep earthquakes in Argentina and one nearby intermediate-depth earthquake have similar splitting parameters as the SKS phases. This means that the anisotropic zone causing the splitting of the core phases can be constrained to the Pacific mantle underlying the subducting Nazca plate. It probably does not extend deeper than about 260 km. The majority of the anisotropy directions inferred from the core phases are parallel to the absolute plate motion (APM) direction of the Nazca plate, which is about N80°E. At some stations, however, the fast polarization direction is pointing N160°E, nearly parallel to the strike of the trench and the Andes which would be compatible with the trench-parallel flow model for South America proposed by Russo and Silver [1994]. This direction is observed over an approximately 100-km-wide band to the west of the active volcanic zone. It may represent either a second anisotropy regime in the mantle, a small-scale diversion of slab-entrained mantle flow, or a relatively small area where slab entrainment of mantle flow is reduced or ceases to exist. The large number of observed APM-parallel fast directions suggests, however, that the mantle beneath the descending Nazca plate in northern Chile deforms mainly as the result of slab-entrained mantle flow. The large variations of anisotropy directions in the Andean subduction zone indicate that asthenospheric flow in the Pacific mantle has a complex pattern which may vary over scale lengths of a few hundred kilometers and which may be governed by slab morphology.

Comprehensive observation and modeling of earthquake and temperature‐related seismic velocity changes in northern Chile with passive image interferometry

We report on earthquake and temperature-related velocity changes in high-frequency autocorrelations of ambient noise data from seismic stations of the Integrated Plate Boundary Observatory Chile project in northern Chile. Daily autocorrelation functions are analyzed over a period of 5 years with passive image interferometry. A short-term velocity drop recovering after several days to weeks is observed for the Mw 7.7 Tocopilla earthquake at most stations. At the two stations PB05 and PATCX, we observe a long-term velocity decrease recovering over the course of around 2 years. While station PB05 is located in the rupture area of the Tocopilla earthquake, this is not the case for station PATCX. Station PATCX is situated in an area influenced by salt sediment in the vicinity of Salar Grande and presents a superior sensitivity to ground acceleration and periodic surface-induced changes. Due to this high sensitivity, we observe a velocity response of several regional earthquakes at PATCX, and we can show for the first time a linear relationship between the amplitude of velocity drops and peak ground acceleration for data from a single station. This relationship does not hold true when comparing different stations due to the different sensitivity of the station environments. Furthermore, we observe periodic annual velocity changes at PATCX. Analyzing data at a temporal resolution below 1 day, we are able to identify changes with a period of 24 h, too. The characteristics of the seismic velocity with annual and daily periods indicate an atmospheric origin of the velocity changes that we confirm with a model based on thermally induced stress. This comprehensive model explains the lag time dependence of the temperature-related seismic velocity changes involving the distribution of temperature fluctuations, the relationship between temperature, stress and velocity change, plus autocorrelation sensitivity kernels.

Seismological Studies of the Central and Southern Andes

The central Andes have formed by the complex interaction of subduction-related and tectonic processes on a lithospheric scale. The deep structure of the entire mountain range and underlying subduction zone has been investigated by passive and active seismological experiments. Detailed tomographic features are interpreted to represent the ascent paths of fluid and melts in the subduction zone and provide new insights about the mechanisms of lithospheric deformation. Receiver functions from teleseismic events have been used to observe the upper-plate continental Moho and subducted oceanic Moho, as well as the interaction of subducted oceanic lithosphere and mantle discontinuities. A second working area was established in the southern Andes to compare two different types of Andean subduction and to identify the principal controlling parameters. Besides the first accurate definition of the Wadati-Benioff zone in south-central Chile, a three-dimensional, tomographic velocity model based on local earthquakes in the southern Andes is presented.