Abstract
Asthenospheric mantle flow drives lithospheric plate motion and constitutes a relevant feature of Earth gateways. It most likely influences the spatial pattern of seismic velocity and deep electrical anisotropies. The Drake Passage is a main gateway in the global pattern of mantle flow. The separation of the South American and Antarctic plates since the Oligocene produced this oceanic and mantle gateway connecting the Pacific and Atlantic oceans. Here we analyze the deep crustal and upper mantle electrical anisotropy of its northern margin using long period magnetotelluric data from Tierra del Fuego (Argentina). The influence of the surrounding oceans was taken into account to constrain the mantle electrical conductivity features. 3D electrical models were calculated to fit 18 sites responses in this area. The phase tensor pattern for the longest periods reveals the existence of a well-defined NW-SE electrical conductivity anisotropy in the upper mantle. This anisotropy would result from the mantle flow related to the 30 to 6 Ma West Scotia spreading, constricted by the subducted slab orientation of the Pacific plate, rather than the later eastward mantle flow across the Drake Passage. Deep electrical anisotropy proves to be a key tool for a better understanding of mantle flow.
Highlights
Date[7,8,9,10,11,14], but never with a focus on mantle gateways
In 2012, a long period magnetotelluric (LMT) study with 18 stations distributed along two profiles was performed in Tierra del Fuego to investigate the deep crustal and upper mantle conductivity (Fig. 1b)
The main results of MT research in Tierra del Fuego reveal the existence of a main NW-SE oriented anisotropy in the upper mantle
Summary
In 2012, a long period magnetotelluric (LMT) study with 18 stations distributed along two profiles was performed in Tierra del Fuego to investigate the deep crustal and upper mantle conductivity (Fig. 1b). The influence of seawater on MT parameters (phase tensor and tipper vectors) at coastal areas strongly depends on the bathymetry[8,26,27]. It is obvious that toward the end of the long period the conducting isotropic asthenosphere reduces the length of the tipper vectors considerably, while the phase split does not appear, either with or without asthenosphere. The conductive principal axis in N150°E direction) explains both the large observed tipper vectors and the phase split along the two profiles (Fig. 2d) We found that an anisotropic asthenosphere with a resistive principal axis in a N60°E direction (resp. the conductive principal axis in N150°E direction) explains both the large observed tipper vectors and the phase split along the two profiles (Fig. 2d)
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