Geophysics

Regional geophysical information has been gathered in the study area from different sources, qualities and resolution over the last 30 years. Relevant to this study are the crustal seismicity, gravity and magnetic data. Crustal seismicity is a key parameter given the fact that deformation inside the inverted Abanico Basin remains active today, representing part of the permanent deformation associated with the Andean subduction cycle. Gravity and magnetic signals are complementary tools that can be used to define major lithological and structural domains.

4.3.1 Seismicity

Within the last 20 Ma, the Andes of central Chile and Argentina have experienced ~ 100 km of shortening (Farías et al., 2010), corresponding to a deformation rate on the order of 5 mm/year. This permanent deformation is about 5-7% of the convergence rate between the Nazca and South American plates, and produces active crustal seismicity and sporadically some large earthquakes, such as the Las Melosas 1958 Mw 6.9 earthquake (Sepulveda et al., 2008; Alvarado et al., 2009; Fig. 4.3A).

Chapter 4 – Arc-oblique fault systems in the Andes of Central Chile last 50 years (National Earthquake Information Centre), with error envelops in the range of 10/20 km in the horizontal and vertical dimension and magnitudes over 3 Mw. In addition, two local networks (CHASE 2005, Pardo et al., 2008; ANILLO 2006-2008, Farías et al., 2010), provide a representative data set that characterizes the active deformation of the crust in the study area. Databases considered in this study provide a good representation for magnitudes over 2.5 Mw with errors in the range of 5-10 km in horizontal coordinates and 8-15 km in depth. On the other hand, the time window of 40 years for the regional network and 4 years for the local networks ensures that enough small seismic events are recorded to characterize crustal deformation, given the fact that the mean deformation rate is about 5 mm/yr and relatively large earthquakes (~ 6.5 Mw) occur every ~ 25 years.

Figure 4.3A shows the total crustal seismicity over the structural network of the study area. A depth filter was applied, so that only the events with depths above 20 km are shown. The seismic events associated with the mining operations (Rio Blanco-Los Bronces and El Teniente) were also eliminated. Seismicity is mostly concentrated in the N-striking eastern border of the former Abanico basin, in particular in the southern segment. However, clustering of seismic events and NE - NW trends are observed in the central and western parts of the Abanico Basin, some of them spatially correlated with the location of the Rio Blanco-Los Bronces and El Teniente deposits. These seismic clusters and alignments are strongly concentrated along regional-scale oblique structures (Fig. 4.3A).

Figure 4.3. Main arc-oblique and arc-parallel fault systems (from Fig. 4.1C) plotted against different regional geophysical data: A. Shallow hypocenters. Red triangle marks the location of the Las Melosas 1958 Mw 6.9 earthquake. B. Shallow hypocenters and gravity field. C. Shallow hypocenters and the reduced to pole magnetic field. The two different surveys mentioned in the text are delimitated by a narrow mask zone in order to clarify which domain belongs to each survey.

4.3.2 Gravimetry

Gravity data have being gathered within the framework of the Ring Project ACT N°18 (Yañez et al., 2008). This is a regional data set with stations at an average sampling distance of 5 km, with differential GPS height control and errors on the order of 0.3 mGal. The residual gravity field for the study area is shown in Figure 4.3B, along with the structural network and crustal seismicity. A simple

interpretation of this data set shows that the strongest deformation of the Tertiary volcanic rocks, observed in the structural geology and seismicity record, is spatially correlated with a gradient between medium and low density domains on the eastern flank of the area. In the southern part, this gradient is N-trending and follows the

Chapter 4 – Arc-oblique fault systems in the Andes of Central Chile eastern margin of the Abanico Basin. Towards the north, this transition zone acquires a NW trend, parallel to the NW-striking structures which traverse the Rio Blanco- Los Bronces district (Fig. 4.3B). In the western part of the area, long wavelength positive anomalies are associated with dense and deep crustal blocks. Previous studies (Yañez and Rivera, 2009; Rivera and Cerda, 2012) have suggested a relationship between these dense blocks and the emplacement of giant copper deposits. More precise correlation with structures is hampered by the low resolution of this regional data set.

4.3.3 Magnetometry

The regional magnetic field of the western part of the study area was surveyed in the early 1980s by the Chilean Geological Survey. The eastern part was surveyed in the early 1990s by Codelco. Even though technical conditions at the time of the two surveys were limited (pre GPS era for the western survey and without the appropriate aircraft for high altitude surveys on the eastern side), data quality is good enough for the purposes of the present study. The western survey was flown with NS lines every 2.5 km and nominal clearance of 600-1000m. The eastern survey specifications were NS lines every 1 km and a clearance of 1000m, however in practice the mean altitude above the topography was 1500m. With cross-checking calibration between both surveys, we estimate errors below 500m in plan view. Figure 4.3C shows the reduced to the pole magnetic field of the study area, overlapped by the structural network and crustal seismicity. The magnetic data highlights the major oblique structural trends much better than the gravity data, due to better resolution and line orientations (N-S) at high angles with respect to the structural pattern.