Francis Lucazeau

Large uplift of rift flanks : a genetic link with lithospheric rigidity?

Abstract Large and permanent uplifts may develop along the flanks of passive rifts, and the most likely mechanism for explaining them is flexural bending of the lithosphere due to an isostatic load resulting from necking of the lithosphere. A numerical model that accounts for the elastic and viscoplastic behaviour of the lithosphere is used to determine the important parameters of this mechanism, in order to explain large uplifts such as those observed in the Red Sea and the Transantarctic Mountains (more than 2000 m). Large uplifts of the flanks and large subsidence in the rift are associated with a thick mechanical lithosphere.

Heat flow density data from France and surrounding margins

A new compilation of heat flow density data was made in the framework of the “Geothermal Atlas of Europe”. This work uses oil exploration data and classical measurements. The consistency between the two sets of data is briefly discussed and a heat flow density map of France is compiled using all available data (oil exploration data, classical measurements and data from neighbouring countries). The main trends of heat flow density are discussed in relation to the geological and geodynamic setting of the various areas. The high heat flow density values (100–110 mW m−2) observed over the Rhine graben in the northeastern part of France, the Massif Central and in the Provencal Basin in the southern part of France seem to be directly connected with the post-Oligocene rifting activity. The mantle heat flow ranges between 20–25 mW m−2 in stable Hercynian provinces (Brittany and Maures Esterel) and 50–60 mW m−2 in the rift zones.

The problem of heat flow density determination from inaccurate data

Abstract We present an inverse method for processing rough data from oil wells in order to evaluate the heat flow density. Assuming that the evaluations of temperatures and thermal resistivities present gaussian uncertainties, we give formulas for computing the optimal value of the heat flow density and its standard deviation. A synthetic example, using likely assumptions on various uncertainties, leads to the prediction that a standard deviation of about 10 mW m−2 could apply to heat flow estimates from oil well data for typical situations.

Interpretation of heat flow data in the french massif central

Abstract We investigate in this paper the possible origin of the high heat flow anomaly observed in the Massif Central (100–110 mW m−2). A detailed study of crustal heat production shows that the radioactive component cannot account for the entire surface anomaly. However, when we remove the integrated radioactive contribution from the surface heat flow, it appears that the anomaly is reduced to a smaller area close to the Cenozoic graben and the volcanic area, with an amplitude of 25–30 mW m−2. This mantle heat flow anomaly is interpreted as a transient component related to a mantle diapir ascent in the Massif Central since the Oligocene, also outlined by several other geological and geophysical studies. A kinematic model of thermal diapir in the mantle can explain this transient anomaly as well as other independent sets of data (gravity anomaly, topography, lithospheric thickness, partial melting) provided the vertical velocity of the ascent is 1 cm yr−1, the width of the diapir 40 km and the depth where it initiates 150 km.

Heat flow over Reunion hot spot track: Additional evidence for thermal rejuvenation of oceanic lithosphere

We present 99 new heat flow determinations obtained with a regular sampling (2–3 km) along two seismic profiles across the Mascarene Ridge, Indian Ocean. This aseismic ridge is volcanic relief due to the activity of a hot spot, now responsible of the active volcanism of Reunion Island. Our sampling technique allows us to identify short-wavelength processes such as slope-driven water flows that affect heat flow on the ridge. With the exception of these perturbed sites, heat flow appears to be very uniform across the ridge and significantly higher (6–8 mW m−2) than the reference heat flow predicted for the seafloor age. Although this anomaly is low, it confirms other evidence for thermal rejuvenation of the lithosphere based on gravity and bathymetry. The set of information now available for the Mascarene Ridge seems to indicate a mixed mode of compensation with dynamical support below the front of the swell track and thermal support behind for the older part of the swell.

Subsidence, extension and thermal history of the West African margin in Senegal

Abstract The subsidence of the Atlantic margin in Senegal clearly shows two rapid stages related to the formation of (1) the Central Atlantic during the early Jurassic (around 200 Ma), and (2) the Equatorial Atlantic during the Cretaceous (100 Ma). A simple model of extension is used to interpret the subsidence history and to derive the thermal evolution of this basin. The present-day gravity, bathymetry, bottom hole temperatures (BHT) in oil exploration boreholes and heat flow density are used to control the validity of the model. Two cross sections from the outcropping basement to oceanic crust are used, one in Casamance and the other one at the south to latitude of Dakar. The model can fully explain the first-order subsidence history as well as the present-day observations, and therefore can provide valuable information about the thermal evolution of sediments and about the structure of the continental crust along the margin. Comparisons with the opposite margin in North America (Blake Plateau and Carolina trough) indicate a rather different evolution (the North American margin did not undergo the second stage of rifting) and a different crustal structure (crustal thinning is less important on the African margin).

Heat flow variations on a slowly accreting ridge: Constraints on the hydrothermal and conductive cooling for the Lucky Strike segment (Mid-Atlantic Ridge, 37°N)

We report 157 closely spaced heat flow measurements along the Lucky Strike segment in the Mid-Atlantic Ridge (MAR) for ages of the ocean floor between 0 and 11 Ma. On the eastern flank of a volcanic plateau delimiting off-axis and axial domains, the magnitude of heat flow either conforms to the predictions of conductive lithospheric cooling models or is affected by localized anomalies. On the western flank it is uniformly lower than conductive model predictions. We interpret the observed patterns of heat flow by lateral fluid circulation in a highly permeable oceanic basement. The circulation geometries are probably 3-D rather than 2-D and are determined by the configuration of the basement/sediment interface and the distribution of effectively unsedimented seamounts where water recharge can occur. Two major hydrothermal circulation systems can possibly explain the observations off-axis: the first would involve lateral pore water flow from west to east, and the second would have a reverse flow direction. The wavelengths and magnitudes of heat flow anomalies require Darcy velocities of the order of 1–4 m/year, which are similar to those proposed for fast-accreted crust elsewhere. However, a large proportion of this MAR domain remains unaffected by hydrothermal cooling, which is a relatively unusual observation but confirms the validity of conductive thermal models for seafloor ages between 5 and 10 Ma. Closer to the ridge axis (<5 Myr old crust), water circulation affects the overall axial domain, as larger proportions of basement are exposed. As much as 80–90% of the heat flux from the axial domain may be transferred to the Lucky Strike vent field, in agreement with the estimated discharge.

Exploring Structural Controls on Sumatran Earthquakes

A series of linked marine and land studies have recently targeted the Sumatra subduction zone, focusing on the 2004 and 2005 plate boundary earthquake ruptures in Indonesia. A collaborative research effort by scientists from the United Kingdom (UK Sumatra Consortium), Indonesia, United States, France, and Germany is focusing on imaging the crustal structure of the margin to examine controls on along-strike and updip earthquake rupture propagation. The fundamental science objective is to examine how margin architecture and properties control earthquake rupture location and propagation.

Thermal Regime and Evolution of the Congo Basin as an Intracratonic Basin

The Congo Basin (CB) lies over a thick (200–250 km) and cold lithosphere: we estimate the present-day surface heat-flow to 40 ± 5 mW m−2, from the BHT temperatures, lithology and porosity recorded in two oil wells and in agreement with the only measurement in this area (44 mW m−2). This value is consistent with the thickness of the lithosphere inferred from seismic tomography, assuming stationary conditions. The paleo-thermal regimes can be constrained by additional information, such as the pressures and temperatures derived from kimberlites studies, the variations of vitrinite reflectance with burial or the reconstructed subsidence history. The pressures and the temperatures derived from kimberlites xenocrysts suggest that the conditions were similar for at least 120 Myr. The long-term subsidence can be interpreted by the thermal relaxation of a thick lithosphere after a Neo-Proterozoic rifting stage (ca. 700–635 Ma) with a thinning factor β = 1.4 and a possible reactivation during the Karoo period (ca. 320 Ma). Because the magnitude of the crustal thinning is small, the past thermal conditions throughout the Phanerozoic were probably not very different from the present-day. Additional short term variations (~20–40 Myr) of the subsidence are interpreted by dynamic subsidence or uplift caused by sublithospheric mantle instabilities at the transition between litho-spheres of different thicknesses. These short term variations should not be associated with significant thermal changes. In order to explain the observed maturation of the vitrinite as well as angular uncomformities on seismic lines, one should assume one or two stages of erosion in the basin, representing at least 4 km of removed material. Heat advection by hydrothermal or volcanic fluids can conversely reduce the magnitude of this erosion.

Neoproterozoic to Lower Paleozoic Sequences of the Congo Shield: Comparisons Between the Congo and Its Peripheral Basins

The Congo Basin (CB) was initiated during a Neoproterozoic intracratonic rifting process across the Congo Shield, possibly related to the break-up of the Rodinia Supercontinent. The Neoproterozoic history of the CB remains poorly constrained because it is largely concealed by Phanerozoic sequences and is thus accessible only by drilling and geophysical investigations. Neoproterozoic basins peripheral to the CB, however, share first-order similarities in their stratigraphic successions: an initial coarse clastic sequence followed by a thick carbonate sequences and terminating by a dominantly siliciclastic sequence. The subsequent transition to the Lower Paleozoic is influenced by regional Pan-African deformation and molasses-like foreland basin sediment-fill. The Neoproterozoic stratigraphy and structure beneath the CB can be tested also using data from two fully cored stratigraphic wells (Samba: 2,038 m and Dekese: 1,836 m), and two exploration wells (Gilson-1: 4,563 m and Mbandaka-1: 4,350 m), seismic refraction and reflection profiles, gravimetric and magnetic data. Synthesizing this data, we show that the CB shares a similar early evolution as its flanking basins in that they all initiated in extensional tectonic environments and were subsequently affected by compressional deformation during late Neoproterozoic to Early Cambrian Pan-African orogenesis.