Reseach Interest

My research interest has focused on understanding the role of hydrous and magmatic fluids in orogenic systems; an explanation to their possible origin(s) in the crust and upper mantle and their implications to partial melting and rheology changes, their role in fault systems, and the relation between fluid flow and lithospheric deformation.

I specialized in the Magnetotelluric (MT) and Geomagnetic Deep Sounding (GDS) methods, which measure the natural time variations of the electric & magnetic fields at the earth's surface originating from ionospheric & magnetospheric currents, inducing telluric currents under the surface. The aim of MT and GDS measurements is to obtain the electrical conductivity values of the underground as a function of depth at each station, and build two-dimensional or three-dimensional conductivity models of the earth by calculation of the Maxwell field equations through numerical forward algorithms (e.g., finite element or finite difference), and through model inversion. I have built 2-D and 3-D electrical conductivity models of the earth's crust and mantle in active continental margins (Andean subduction zone) and in a craton (the Slave, NW Canada; Jones et al., 2003), giving a geo-tectonic interpretation in the context of other geological & geophysical observations such as seismic reflection and attenuation, gravimetry, heat flow measurements and geochemistry.

Hydrous fluids play a key role in the temperature dependent metamorphic reactions of rocks producing state and compositional changes, affecting the bulk electrical conductivity of the material. Salinary fluids flowing in a permeable porous material will enhance ionic conductivity, increasing the total conductivity value with increasing porous interconnection.

My PhD thesis research involved the analysis and 3-D modeling of magnetotelluric data collected in the Southern Central Andean subduction zone. The conductivity structures of the model integrated with geothermal gradients and seismic reflectors bring insights into the properties of lithospheric deformation for the fore- and magmatic-arc region of the subduction zone (Fig.1).

Fluids flowing in partially molten rocks matrix (of enhanced bulk conductivity) and beyond cause a reduction of the melting point temperature that can lead to an expansion of the melting process beyond the existing boundaries. The brittle- ductile transition zone may be affected when molten rocks are displaced into deeper zones of higher temperatures.
A good example of an anomalous ductile rheology of the lithosphere in an active orogenic system is found from geophysical observations in the back-arc of the Andean subduction zone at latitude 24 S, underneath the Puna plateau. An upward lithospheric displacement of the brittle-ductile transition zone beneath the Puna occur, with a ductile -partially melted- lower crust associated to active magmatism and tectonic deformation. The asthenospheric upwelling is supported by both high seismic attenuation values and the temperature dependent electrical conductivity values obtained from magnetotelluric modelling ( Lezaeta et al., 2001).

Conductvity model of the Andean subduction zone at latitude 20-21.5 S

The model on the right shows a W-E cross section at latitude 20.5 S of a 3-D electrical resistivity (the inverse of conductivity) model obtained from magnetotelluric surveys carried along the Southern Central Andes subduction system. Explanations of the main features presented in the model are shown in the figure caption (taken from: PhD Thesis, pp 159-167). Interpetation of the conductivity model is explained below for each along strike geological region forming the fore- and magmatic-arc of the Andean subduction system at latitute 20- 21.5 S:

Coastal Cordillera mountain range (forearc):

The resolution in depth of an upper crustal high conductivity zone (<10 km depth) along the Atacama mega strike-slip fault (AF) of the Coastal Cordillera cannot be resolved from on-shore measurements due to the screen effect of the conductive ocean. Ocean bottom surveying would allow discriminating the presence of a conductive zone in the (coupling) seismogenic zone near the mantle wedge associated to an hydrated oceanic slab (high Vp/Vs ratio and low attenuations), in which case fluids could be released from dehydration and find their pathway upward through the fractured brittle crust. This hypothesis would be proofed by finding high conductivity values along vertical pathways associated to the Atacama fault system reaching down to moho depths (40-50 km). In the current model obtained from land measurements, the Atacama fault system at latitudes between 20 S and 21.5 S appears to be highly conductive in a vertical dike-like fashion at least down to depths of 10 km. These structures are vertical pathways for salinary fluids that may have origined from meteoric (surficial) waters at least for the upper crustal levels. Besides the meteoric water, that is rainwater and fluvial sources, surficial fluids may also have come originally from thermal convections developed at mid-crustal level excited by intrusions.
Porous rocks can also have retained water from dehydration of past methamorphic reactions occured within the mid- crust. The Coastal Cordillera was subject to postgradational metamorphism during the Jura (210-120 Ma), and may have continued with retrograde metamorphism during the emplacement of the volcanic arc to the east. Metamorphic fluid fluxes and crustal porosities could have well be maintained long after the end of dehydration metamorphism (>100 Ma), with fluid production ending after about 70 Ma (e.g., Thompson & Connolly, 1990). Thus, water may have been trapped in the fractured rocks until presently, especially if the vertical pathways extend beyond 10 km depth downto mid-crustal levels.
Mineralizations in the active shear zone or ore fluids can also enhance conductivity. Ore minerals have higher densities than the average crust, leading to enhance the P-wave velocities and resulting in positive isostatic residual field, in accordance with geophysical observations in the Coastal Cordillera. On the other hand, highly conductive minerals such as illmenite, magnetite and pyrhotine can develop aligned in shear zones. The widespread nitrate deposits east of the Atacama fault are probably related to metallogenetic events, where ore fluids may have been channeled in the Atacama fault system, such as that from epithermal gold-silver.
The origin of the fluids in the Atacama fault can be a combination of the mechanisms explained above. However, the hypothesis regarding the upward migration of water into the crustal fractures released from the seismogenic zone, can not be supported by the MT observations on land. The levels of the slab are beyond the penetration depths of the MT data since measurements carried out off-shore are required to resolve this zone as outlined above.

Precordillera Fault system (forearc):

The High Conductivity Zone (HCZ) in the Precordillera (PC) can be separated into two blocks. A shallow structure (<5 Ohm.m) between 2 and 10 km deep in the form of approximately N-S elongated conductors along the Precodillera fault system (PCFS); a deeper block below 10 km depth with lateral N-S gradient of conductivity decreasing from north to south. Its central part (at 20.5°S) extends to the west in the Longitudinal Valley. See overview of the 3-D model at different depths to identify the structure of the Precordillera HCZ.
The upper crustal dike-type conductors can be related to the Precordillera fault system, and hence explain the conductivity enhancement as being due to salinary fluids circulating in the fractures of the brittle upper crust (temperatures <200°C; fig.1).
In the western PC as well as part of the Longitudinal Valley, a N-S thrust-fold system has been inferred from an isostatic model (fig.1; west of 69°W), and is interpreted as the west-flank of the Miocene-Holocene tectonic Altiplano uplift which developed in connection with ignimbrite-magmatism (Victor et al., 2000). The thrust-fold in the western PC as well as the West Fissure (WF) shear zone are geological structures recognizable in the field. The upper crustal dikes at latitude 20.5°S correlate well with the PC thrust-fold system and the WF (fig.1). The depth of the WF ($\sim$25 km) is presumed by accounting for the isostatic model of the thrust-fold system by Victor et al. (2000), the geothermal gradient and the low seismic velocity zone at a 30-40 km depth modeled by Yuan (2000) from P-S converted seismic phases. Thus, the crust in the Precordillera can be considered to have a brittle regime at depths above 30 km, whereas at depths below about 30 to 40 km it undergoes a brittle-ductile transition (with temperatures between 300°C and 500°C; Fig.1). The temperatures in the lower crust and upper mantle do not exceed 650°C, indicating that partial melt is not possible here, supported also by the lack of high seismic attenuation registered in this zone (e.g., Haberland, 1999).
A larger high conductivity zone (HCZ) extends below the conductive dikes at depths between 10 and 35 km, with a N-S gradient of conductivity decreasing from north (<5 Ohm m) to south (>10 Ohm m; PC in "presentation of the 3-D model at different depths"). Although the lower boundary is not well constrained, it could be constrained by inserting another HCZ at the levels of the oceanic crust (70-90 km depth at 69°W; Fig.1; right), which is seen to give an equivalent response under 2-D modelling.
The origin of the fluids in the fractures can be argued in a similar manner for the PC-fault system as was explained for the Atacama fault system. The conductive dikes of the upper crust may be fed by meteoric water trapping from the surface and/or by ore fluids (inferred from the huge porphyry copper in Chuquicamata at latitude 22°S, associated with the West Fissure zone; Reutter, 1995). Water released from fractured hydrated porous rock, remaining from older metamorphic reactions, can also be a possibility for the brittle crust.
In the Precordillera however, the rheology and the tectonic history differ from the crust of the near coast. The last important compressional deformation in the Precordillera occurred about 35 Ma ago (Reutter et al., 1995}), when the crust was thickened subsequently subject to a retrograde metamorphism (Reutter, pers. comm.). Thermotectonic modeling has shown that a lithosphere subject to a strong and sudden thickening by overthrusting will need a long period of time (>100 Ma) to recover its original thermal state (Thompson, 1981). Within a period of about 70 Ma the crust can still undergo metamorphic reactions, adjusting fluid productions Thompson (1990). This model can be suitable for the Precordillera if we relate the pressure-temperature diagrams of metamorphic hydrous rocks which characterize a subduction zone (Peacock et al., 1996) with the geotherms estimated by Springer (1998) for the Precordillera (Fig.1). The mid-crust undergoes intermediate metamorphic reactions (temperatures from 300°C to 550°C) where greenschists transform to epidote amphibolite ( fig.1; EA on left diagram) indicating the first appearance of water, and liberating fluids break down in the absence of partial melting (Wannamaker, 1986). At greater depths (30-45 km) the crust contains wet rocks with epidote blueschist or epidotite amphibolite. At the lowest crustal depths, the P-T diagram indicates a composition of eclogite (dry rock) for the ductile lower crust of the Precordillera (Fig.1; left; 45-60 km depth and T=500-600°C), which does not indicate conductivity enhancement since eclogite can not retain water.

Along strike variations of volcanic activity:

It is proposed that continuing metamorphic reactions in the mid-crust, which liberate fluids from cracks and ascend into the fractures of the Precordillera fault system, are the cause of the conductivity enhancement at mid-crustal levels (10-30 km). In the north the conductivity is clearly higher than to the south, which may be related to the different volcanic history known at these latitudes. North of 21.S magmatism ceased about 25 Ma, whereas to the south volcanism is younger (<10 Ma; de Silva [1989]). If magmatism and tectonic compression would occurred simultaneously (Victor [2000]), then tectonic overthrusting would have started at about 10 Ma in the south whereas to the north it would have began ~30 Ma ago (Victor [2000], pers. comm.). In this manner, according to a thermotectonic model of a thickened crust by overthrusting that induces a thermal perturbation over time (Thompson [1981]), metamorphic reactions may evolve under different P-T conditions from north to south in the Precordillera. This suggests that a distinct production of fluids in the crust might develop north and south of 21.S. A ~30 Ma thickened crust can lead to a higher production of fluids (and hence higher electrical conductivity values in the north) than a recently deformed crust subject to younger metamorphism.
An additional explanation is that in the north, where magmatism is older (>25 Ma) than in the south (<10 Ma), the crust might be more fractured due to a cooler and more brittle regime than in the south, allowing the fluids to be better interconnected and hence the electrical conductivity to be enhanced.

2-D Modelling of the Back-Arc of the Andean Subduction Zone at Latitude 24 S

Figure 2

Fluids flowing in partially molten rocks matrix (of enhanced bulk conductivity) and beyond cause a reduction of the melting point temperature that can lead to an expansion of the melting process beyond the existing boundaries. The brittle- ductile transition zone may be affected when molten rocks are displaced into deeper zones of higher temperatures.
Fig. 2 shows an example of seismic attenuation and MT surveys performed in the Central Andes zone at latitude 24 S. The 2-D resistivity model supports one of the two seismic attenuation models proposed by Whitman et al. (1992) in the back-arc zone, the former showing a western upwelling of the electrical asthenosphere in direction to the Puna volcanism. More recent seismic measurements show the Puna plateau with a zone of low P velocities and high seismic attenuations (not shown here) at lower crustal levels. Partial melts and/or magma at crust /upper mantle levels below the Puna zone is a good candidate to explain the seismic anomalies, confirmed also by high conductivity values in the mid-low crust modelled from magnetotelluric sites located west of the MT profile shown here (Lezaeta & Brasse, 2002). Some authors (as Kay & Kay, 1993) support the well-known hypothesis of upper mantle delamination due to a thickening of crust of the Puna plateau, causing crustal uplift, asthenospheric upwelling, and pressure-release melting. Both MT and seismic models support a thinning of the rheologic lithosphere beneath the Puna, interpreted as an asthenospheric upwelling due to an increase of mantle temperature according to heat flow measurements in the area. An upward lithospheric displacement of the brittle- ductile transition zone beneath the Puna occur, with a ductile -partially melted- lower crust associated to active magmatism and tectonic deformation. The asthenospheric upwelling is supported by both high seismic attenuation values and the temperature dependent electrical conductivity values obtained from magnetotelluric modelling ( Lezaeta et al., 2001).

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