Modelling of crustal anomalies of Lanzarote (Canary Islands) in light of gravity data

The application of a gravity inversion method enables us to obtain a 3-D de nsity contrast model of the upper crustal anomalies of the volcanic island of Lanzarote (Canary Islands). For this, we use a network of 296 gravity st ations distributed over the whole island, and a digital terrain model of ab out 45 000 terrestrial and oceanic data to determine the corresponding terr ain correction. A density value of 2480 kg m(-3) is chosen for this correct ion by means of a new approach. The resulting Bouguer anomaly is analysed b y means of a least-squares prediction which gives us a mean level of uncorr elated observational noise of about 1.2 mgal. This anomaly is considered in order to obtain independent information about the inner anomalous mass den sity distribution by means of a 3-D gravity inversion based on a systematic exploration on a prismatic partition of the subsoil volume, and adopting e t priori values of the density contrast (positive and negative) to determin e the geometry of the anomalous bodies. The problem of non-uniqueness of th e solution is avoided by using a minimization mix condition on the weighted residuals and the weighted whole anomalous mass. The structural solution i s finally presented by means of horizontal sections and vertical profiles. A main intrusive body is located under the central-eastern area and could c orrespond to a dilated volcanic activity of shield formation. It shows a pr ismatic form of more than 15 km depth, subducted with only the ridges remai ning as horst blocks. Moreover, the SW and NE extreme areas of the island s how smaller and shallower positive bodies, interpreted as less-developed ma gmatic intrusions. Conversely, several density lows offer interesting shall ow alignments, 45 degreesN (ENE-WSW) and 125 degreesN (WNW-ESE), which coul d be associated with a fracture system corresponding to structural stress, and also correlate with historic eruptions, such as, for instance, the Tima nfaya eruption. The monitoring of several geophysical parameters at two und erground geodynamic stations, in the NE zone of the island and Timanfaya, s hows characteristic differences between the two zones which confirm crustal anomalies in the second station.