CoLiBrEA is a multidisciplinary project which aim is to understand the processes involved into the continental breakup of the lithosphere.

We focus our study on the only continental place where the earliest stage of rifting takes place, and where we can look at the magma - tectonic interaction in action.

Our fundings for this project come from ANR, CNES and NSF.

A multidisciplinary project

The project started in 2012, with a collaboration between French, American and Tanzanian institutions and focuses in the North Tanzanian Divergence, a region near the Natron and Manyara Lakes.

We propose here to combine geophysical and geochemical tools to best address the questions that raised in this region. Geophysical tools will help to identify the crustal and upper mantle structures, whereas the geochemical and petrologic analyses will provide useful tool to constrain the dynamic of magma intrusion. Mixing seismology, geodesy, magneto-telluric and gravimetry together with petrologic analysis and volatiles study will provide us powerful tools to understand why in less than 200 km the style of deformation, magmatism and seismicity is so contrasted.

In january 2013, we first installed 35 seismological broadband stations for 2 years in the area of Natron and Arusha region, that were completed by a MT profile perpendicular to the rift axis. GPS measurements on a transverse profile and more than 200 new gravimetric points were added in 2013 and 2014. More than 40 places were sampled for rocks and lavas analyses. In August 2014, 10 short period seismic stations were installed around Manyara Lake to study the deep seismicity that permanently occurs in this region (e.g. Albaric et al., 2014). They stayed until June 2015. A second magneto-telluric profile that runs south Manyara and through the seismic network was added in January 2015.

The first results show Moho variations of ~10 km overall the area, with minimum crustal thickness in the central part of the rift. Low velocity zones are identified in the upper and lower crust near Natron and Kitumbeine, possibly related with partial melt and CO2 presence (Roecker et al., 2017). Normal to low Vp/Vs ratio for this magmatic area is indicative of gaz presence in the crust beneath some of the area. The upper mantle part beneath the rift shoulders (~60-80 km depth) is characterized by a drastic drop of seismic velocity resulting from either partial melt accumulation, a change in the mantle composition or boundary grain sliding processes (Plasman et al., 2017). Local seismicity is present in the lower part of the crust, where border faults are associated with CO2 degazing (Weinstein et al., 2017).

This project is strongly associated with the CNES GoSSIP project (Goce Seismological Simultaneous Inversion Process). The latter aims to develop methodological procedure to jointly associated gradiometry and seismology in a single inversion scheme. This methodological part is a collaborative work between Montpellier, IPGP and the IGN. Matthieu Plasman has started a 2 years CNES post-doc in November 2017 to fulfill this scientific objective.

Scientific rationale

The initiation of rifting in thick continental lithosphere still raises a number of issues that needs to be addressed. Particularly, the role of inherited fabrics, rheologic anisotropy, interaction between magmatic and tectonic processes have to be defined and their interaction determined.

We target the first 5 My of a magmatic rift initiating in thick (>150 km) continental lithosphere, where we can directly image and detect fault and magma interactions, as well as the role of inherited and rheological heterogeneities of the lithosphere on rift localisation. In this context, the North Tanzania system provides an exceptional laboratory, as it is the location of the least evolved stage of magma-assisted continental rift in the East African rift system

The educational dimension

This project includes a lot of field work, and it has allowed us to meet with a lot of local people and helpers. The seismic stations were for the most of them installed in school areas and all this brings us to develop a parallel project with pupils.

Since 2013, we have proposed introduction to geosciences days in Montpellier University for one or two primary schools in Montpellier. Parallel to this, each time we visited the stations in Tanzania, we managed to distribute books on earthquake (Sismo à l’école) and posters to explain the aim of the project. The posters were designed by 3 Montpellier university students (L3) during internship in 2014.

End of May 2015, a professional photograph joined us on the field to report on human relationships that are developing during such a project, and how the scientific goal is perceived by local pupils and non academic. The pictures taken during this field trip will be presented to the French pupils in September-December 2015.