Heinrich Brasse

Seismic imaging of a convergent continental margin and plateau in the central Andes (Andean Continental Research Project 1996 (ANCORP'96))

[1]xa0A 400-km-long seismic reflection profile (Andean Continental Research Project 1996 (ANCORP96)) and integrated geophysical experiments (wide-angle seismology, passive seismology, gravity, and magnetotelluric depth sounding) across the central Andes (21°S) observed subduction of the Nazca plate under the South American continent. An east dipping reflector (Nazca Reflector) is linked to the down going oceanic crust and shows increasing downdip intensity before gradual breakdown below 80 km. We interpret parts of the Nazca Reflector as a fluid trap located at the front of recent hydration and shearing of the mantle, the fluids being supplied by dehydration of the oceanic plate. Patches of bright (Quebrada Blanca Bright Spot) to more diffuse reflectivity underlie the plateau domain at 15–30 km depth. This reflectivity is associated with a low-velocity zone, P to S wave conversions, the upper limits of high conductivity and high Vp/Vs ratios, and to the occurrence of Neogene volcanic rocks at surface. We interpret this feature as evidence of widespread partial melting of the plateau crust causing decoupling of the upper and lower crust during Neogene shortening and plateau growth. The imaging properties of the continental Moho beneath the Andes indicate a broad transitional character of the crust-mantle boundary owing to active processes like hydration of mantle rocks (in the cooler parts of the plate margin system), magmatic underplating and intraplating under and into the lowermost crust, mechanical instability at Moho, etc. Hence all first-order features appear to be related to fluid-assisted processes in a subduction setting.

The Bolivian Altiplano conductivity anomaly

[1]xa0A long-period magnetotelluric study was carried out in the central Andes between latitudes 19.5°S and 21°S along two almost parallel profiles of 220 and 380 km length, respectively. The investigation area extends from the Pacific coast to the southern Altiplano Plateau in the back arc of the South American subduction zone. The main geoelectrical structure resolved is a broad and probably deep-reaching highly conductive zone in the middle and deeper crust beneath the high plateau. Although the data show deviations from two-dimensionality, a two-dimensional approach is justified for large parts of the profiles. Sensitivity studies were carried out in order to constrain the depth extent. Another electrically conductive structure was resolved in the middle crust of the Chilean forearc, thought to be connected with the Precordillera fault system. The Andean Continental Research Program (ANCORP) seismic reflection profile, carried out along the same line at 21°S, revealed highly reflective zones below the Altiplano, in good correlation with the upper boundary of the Altiplano conductor. This highly conductive domain also coincides with low seismic velocities and a zone of an elevated vp/vs ratio and, although not well resolved, with low Qp seismic quality factors. Taking into account the enhanced heat flow and a derived temperature model, the most probable explanation lies in the assumption of granitic partial melts. The good conductor below the volcanic arc which was found in regions farther south at 22°S gradually vanishes toward the north; this is consistent with the results of seismic tomography concerning Qp values and a gap of recent volcanism.

Partial melting below the magmatic arc in the central Andes deduced from geoelectromagnetic field experiments and laboratory data

Abstract Magnetotelluric and geomagnetic deep soudings in northern Chile revealed a pronounced high conductivity zone (HCZ). Below the Western Cordillera, which constitutes the present magmatic arc with active volcanism of the South American continental margin, conductivities in the range of 1 S/m are observed. The anomalously high conductivities in a broad depth range from approximately 20 km to at least 60 km, are interpreted in terms of partial melting. Other geophysical observations, such as a zone of low seismic velocities (LVZ) at similar depths, high heat flow values (> 100 mW/m2) and a pronounced negative anomaly in the residual gravity field, are also considered. Impedance spectroscopic laboratory experiments up to and in the temperature range of partial melting were performed under controlled oxygen fugacities. At sub-solidus temperatures, electrical behavior is described by defect electrons with an activation energy of 1.34 eV and a conductivity of 2.5 mS/m at 900°C. Model calculations using a modified-brick-layer model (MBL) were compared with experimental observations. A good agreement between calculations and experiments is achieved with an electrical resistivity of the melt phase of 7 S/m at 1250°C assuming an activation energy of 1 eV. The same MBL model is used to calculate melt proportions beneath the Western Cordillera. Between 14 and 27 vol.% of interconnected melt are necessary to explain the observed HCZ. The stability of the melt rich crust is explained by a dynamic melting-crystallisation behavior during crustal anatexis and by magma filled dikes.

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.

Geophysical Signatures and Active Tectonics at the South-Central Chilean Margin

The ISSA 2000 (Integrated Seismological experiment in the Southern Andes) and SPOC 2001 (Subduction Processes Off Chile) onshore and offshore projects surveyed the Chilean margin between 36 and 40° S. This area includes the location of the 1960 earthquake (M w = 9.5) that ruptured the margin from ∼38° S southwards for ~1000 km. Together with gravity and magnetotelluric components, the active-passive seismic experiments between 36 and 40° S provide the first, complete, high-resolution coverage of the entire seismogenic plate interface.

Electrical conductivity beneath the Bolivian Orocline and its relation to subduction processes at the South American continental margin

[1]xa0A long-period magnetotelluric data set was obtained during 2002 and 2004 in the central Andes to study the deep electrical conductivity structure in the region of the Bolivian Orocline between latitudes 17°S and 19°S. The profile extends from the Coastal Cordillera in northernmost Chile, crosses the volcanic arc and the Altiplano high plateau in central Bolivia, and ends in the Eastern Cordillera. Two-dimensional inversion revealed several well-defined conductivity anomalies: in upper crustal levels the conductive sedimentary basins of the central Altiplano and the resistive Arequipa block beneath the western Altiplano are imaged. Earlier seismological and magnetotelluric investigations on the southern Altiplano inferred a large, highly conductive (partially molten) body in the mid to deep crust. It was assumed that this structure would be underlying the entire plateau, but this is not the case according to the new models. Instead, the most prominent feature in the new investigation area is a high-conductivity zone at upper mantle depths below the high plateau, which may be interpreted as an image of partial melts and fluids triggered by water supply from the subducting Nazca slab. This conductor would be in accordance with the standard subduction scenario; it is, however, laterally offset by almost 100 km from the volcanic arc. In contrast, the deep crust and upper mantle beneath the arc is moderately resistive. Both observations may hint at an emerging shift of the magmatic/fluid system in the central Andes.

Coincident anomalies of seismic attenuation and electrical resistivity beneath the southern Bolivian Altiplano plateau

[1]xa0Reassessment of local earthquake data from the ANCORP seismological network allowed the calculation of 3D attenuation (Qp) tomographic images of crust and upper mantle beneath the southern Bolivian Altiplano around 21° S. The images reveal a low-Qp middle and lower crust and a moderate-Qp upper mantle beneath the southern Altiplano. Beneath the recent magmatic arc, Qp is not significantly decreased at this latitude. The distribution of crustal Qp coincides with the variation of electrical resistivity, thus limiting the possible mechanisms causing the anomalies. Our findings support the hypothesis that partial melts in middle and lower crust beneath the Altiplano are present on a large scale. We see no evidence for a shallow asthenosphere beneath the southern Altiplano.

ELECTROMAGNETIC STUDY OF THE ACTIVE CONTINENTAL MARGIN IN NORTHERN CHILE

Magnetotelluric and geomagnetic deep sounding measurements were carried out in the magmatic arc and forearc regions of northern Chile between 19.5° and 22°S to study the electrical conductivity structures of this active continental margin. The instruments used covered a very broad period range from 10−4 s to approx. 2 × 104 s and thus enabled a resolution of deep as well as shallow structures. n nIn this paper we focus on the interpretation of data from an east-west profile crossing Chile from the Pacific coast to the Western Cordillera at 20.5°S. A decomposition of the impedance tensors using the Groom-Bailey decomposition scheme shows that a two-dimensional interpretation is possible. The resulting regional strike direction is N9°W. Two-dimensional models were calculated in this coordinate frame and include the significant bathymetry of the trench as well as the topography of the Andes. The final model shows a generally high resistivity in the forearc and a very good conductor below the Precordillera. Unlike earlier models from areas further south, a good conductor is not observed below the magmatic arc itself. This correlates with the so-called Pica gap in the volcanic chain and a higher age of volcanic activity compared with adjacent areas.

A magnetotelluric study in the southern Chilean Andes

Long-period magnetotelluric investigations were carried out along two transects traversing the Southern Chilean Andes from the Pacific Ocean until the Argentinian border near latitude 39°S to study the mid- and deep-crustal distribution of electrical conductivity. Dimensionality analysis showed that the MT data may be regarded as 2-D with the exception of areas in the Longitudinal Valley and the region east of the volcanic chain; a subsequent multi-site decomposition of magnetotelluric impedances yielded approx. N-S regional strike directions for most parts of the two profiles. First 2-D inversion revealed a zone of moderately high conductivities (approx. 0.1 S/m) in the deep crust below the active volcanic arc, extending eastwards to the Argentinian border. Inversion results of both profiles are very similar, except of a good mid-crustal conductor in the Longitudinal Valley, which only appears in the north and may image a fault zone running obliquely to the main South Andean structures.

Deep electrical resistivity structure of northwestern Costa Rica

First long-period magnetotelluric investigations were conducted in early 2008 in northwestern Costa Rica, along a profile that extends from the coast of the Pacific Ocean, traverses the volcanic arc and ends currently at the Nicaraguan border. The aim of this study is to gain insight into the electrical resistivity structure and thus fluid distribution at the continental margin where the Cocos plate subducts beneath the Caribbean plate. Preliminary two-dimensional models map the only moderately resistive mafic/ultramafic complexes of the Nicoya Peninsula (resistivity of a few hundred Ωm), the conductive forearc and the backarc basins (several Ωm). Beneath the backarc basin the data image a poor conductor in the basement with a clear termination in the south, which may tentatively be interpreted as the Santa Elena Suture. The volcanic arc shows no pronounced anomaly at depth, but a moderate conductor underlies the backarc with a possible connection to the upper mantle. A conductor at deep-crustal levels in the forearc may reflect fluid release from the downgoing slab.