Gonzalo Yáñez

Magnetic anomaly interpretation across the southern central Andes (32°–34°S): The role of the Juan Fernández Ridge in the late Tertiary evolution of the margin

Marine and terrestrial magnetic surveys have been integrated to study the tectonic structure of the convergent margin of Chile between 32°–34°S. Three magnetic domains have been identified: oceanic, continental margin, and subaerial. The oceanic domain has seafloor spreading anomalies (16 (∼37 Ma) to 18 (∼39.5 Ma)) disturbed by anomalies of the Juan Fernandez hot spot chain. In the continental margin, the most prominent fabric are E-W anomalies in the upper slope corresponding to onshore E-W anomalies of large intrusive bodies. Onshore, a N-S lineament of short-wavelength anomalies defines the roots of a Cretaceous volcanic arc. A resembling lineament offshore indicates a submerged older volcanic arc and that continental basement extends to ∼50 km landward of the trench. Absolute Cenozoic plate motion for Nazca and South American plates and dating of the Juan Fernandez chain provide a kinematic model of ridge-continent collision. The reconstruction indicates rapid southward migration of the collision point along ∼1400 km of the margin from 20 to 11 Ma (∼20 cm yr−1). From 11 Ma to present the collison point has migrated at a slower rate along ∼375 km of the margin (3.5 cm yr−1). The predicted location of the subducted portion of the Juan Fernandez chain coincides with the south edge of the southward migrating flat slab segment of the subducted lithosphere and with a cluster of deep earthquakes indicating a causal relationship. In the last ∼10 Myr the ridge has separated a sediment starved trench to the north where subduction erosion may dominate from a sediment filled trench to the south where recent sediment accretion dominates. These observations indicate that subduction of the Juan Fernandez chain plays a major role in arc-forearc tectonics.

Revised tectonic implications for the magnetic anomalies of the western Weddell Sea

Abstract Ten years after the USAC ( U.S. – A rgentina– C hile) Project, which was the most comprehensive aeromagnetic effort in the Antarctic Peninsula and surrounding ocean basins, questions remain regarding the kinematics of the early opening history of the Weddell Sea. Key elements in this complex issue are a better resolution of the magnetic sequence in the western part of the Weddell Sea and merging the USAC data set with the other magnetic data sets in the region. For this purpose we reprocessed the USAC data set using a continuation between arbitrary surfaces and equivalent magnetic sources. The equivalent sources are located at a smooth crustal surface derived from the existing bathymetry/topography and depths estimated by magnetic inversions. The most critical area processed was the transition between the high altitude survey over the Antarctic Peninsula and the low altitude survey over the Weddell Sea that required downward continuation to equalize the distance to the magnetic source. This procedure was performed with eigenvalue analysis to stabilize the equivalent magnetic source inversion. The enhancement of the Mesozoic sequence permits refining the interpretation of the seafloor-spreading anomalies. In particular, the change in shape and wavelength of an elongated positive in the central Weddell Sea suggests that it was formed during the Cretaceous Normal Polarity Interval. The older lineations in the southwestern Weddell Sea are tentatively attributed to susceptibility contrasts modeled as fracture zones. Numerical experimentation to adjust synthetic isochrons to seafloor-spreading lineations and flow lines to fracture zones yields stage poles for the opening of the Weddell Sea since 160 Ma to anomaly 34 time. The corresponding reconstructions look reasonable within the known constraints for the motions of the Antarctic and South America plates. However, closure is not attained between 160 and 118 Ma if independent published East Antarctica–Africa–South America rotations are considered. The lack of closure may be overcome by considering relative motion between the Antarctic Peninsula and East Antarctica until 118 Ma time, an important component of convergence.

El Cenozoico del alto río Teno, Cordillera Principal, Chile central: estratigrafía, plutonismo y su relación con estructuras profundas

The Cenozoic of the upper Teno River, Cordillera Principal, Central Chile: stratigraphy, plutonism and their relation with deep structures. The Cenozoic geologic evolution of the central part of the Cordillera Principal at ~35°S, is intimately related to the geodynamic evolution of deep crustal structures, which during different stages con- trolled the deposition of volcanosedimentary sequences, and the ascent and emplacement of epizonal intrusions. Newly defined stratigraphy around these structures confirms the Cenozoic age of a group of pyroclastic and sedimentary rocks, which conformably underlie andesitic lavas of the Abanico Formation (assigned to the Late Eocene-Early to Middle Miocene). Intrusive rocks correspond to four main phases (from oldest to youngest: diorite, granodiorite, rhyo-dacitic and dacitic porphyry), which occurs in a North-South trending belt. The granodiorite was dated at 7.8±0.4 Ma (K-Ar in biotite). Rhyo-dacitic porphyries, considered as a marginal lithodeme of the granodiorite, yielded 7.9±0.4 Ma (K-Ar in plagioclase phenocrysts). Two main structures of regional importance were observed: the El Fierro thrust, and, towards the west, the Infiernillo-Los Cipreses Fault System. In the characterization of the latter, magnetic modeling of cross-sections were analyzed as a complement to the geologic information. The ascent of the different intrusive phases mentioned before, is interpreted as being controlled by the Infiernillo-Los Cipreses Fault System. This structure, as well as the El Fierro thrust, acted as a basin-margin normal fault during the Late Eocene-Middle Miocene, controlling the deposition of the Abanico Formation. These faults were reactivated as reverse faults during an episode of major tectonic contraction and magmatic-induced high fluid pressure in the Late Miocene, focusing the ascent of the intrusive bodies.

Coseismic slip and afterslip of the 2015 Mw 8.3 Illapel (Chile) earthquake determined from continuous GPS data

We analyzed the coseismic and early postseismic deformation of the 2015, Mw 8.3 Illapel earthquake by inverting 13 continuous GPS time series. The seismic rupture concentrated in a shallow (<20 km depth) and 100 km long asperity, which slipped up to 8 m, releasing a seismic moment of 3.6 × 1021 Nm (Mw = 8.3). After 43 days, postseismic afterslip encompassed the coseismic rupture. Afterslip concentrated in two main patches of 0.50 m between 20 and 40 km depth along the northern and southern ends of the rupture, partially overlapping the coseismic slip. Afterslip and aftershocks confined to region of positive Coulomb stress change, promoted by the coseismic slip. The early postseismic afterslip was accommodated ~53% aseismically and ~47% seismically by aftershocks. The Illapel earthquake rupture is confined by two low interseismic coupling zones, which coincide with two major features of the subducting Nazca Plate, the Challenger Fault Zone and Juan Fernandez Ridge.

Age‐dependent three‐dimensional magnetic modeling of the North Pacific and North Atlantic oceanic crust at intermediate wavelengths

Three-dimensional magnetic modeling of the North Atlantic and northeast Pacific is performed at intermediate wavelengths using three models for the acquisition of a natural remanent magnetization. It is shown that a remanent magnetization which is dependent on the crustal age is the dominant source for the intermediate-wavelength pattern in both basins. However, a pure thermoremanent magnetization of layer 2 alone is insufficient to model the intensity and shape of the observed magnetic anomalies at satellite altitude. We conclude that the best fitting magnetization model for both basins is a combination of a chemical remanent magnetization in the altered upper crust and a thermoviscous remanent magnetization of the slowly cooling lower crust and upper mantle. The North Pacific requires a bulk magnetization which is 50% higher than that of the North Atlantic in order to fit the Magsat field. Geological processes associated with a faster spreading rate such as a faster hydrothermal alteration and the growth of a thicker gabbro layer at the expense of a weakly magnetized sheeted dike layer are plausible explanations for the higher North Pacific magnetization. The lineated positive magnetic anomaly observed over the North Atlantic spreading center is not well reproduced by our models. This anomaly is likely due to a highly magnetized body along or in the vicinity of the spreading center. This highly magnetic body could be an unstable serpentinized lens of crustal material younger than 20 m.y. which develops within the zone cooled by hydrothermal circulation.

Splay fault slip during the Mw 8.8 2010 Maule Chile earthquake: COMMENT

Over the past decade, the Isla Santa Maria (south-central Chile) has provided one of the most exquisitely detailed records of deformation and tectonically controlled sea-level change anywhere along the Andean margin ([Bookhagen et al., 2006][1]; [Melnick et al., 2006][2], [2009][3], [2012][4]).

Topographic correction of magnetic data on rugged topography with application to Río Blanco-Los Bronces and El Teniente porphyry copper districts, Southern Andes, Chile

Airborne magnetic surveys over rugged topography generate noticeable magnetic signatures that are likely equivalent in amplitude with the signal of geological interest. Synthetic models demonstrate that this magnetic terrain effect has the same wavelength as the topography and cannot be overcome by drape flying. The magnetic terrain effect amplifies negative and positive magnetic signals over steep valleys and ridges, respectively. These magnetic artefacts may induce incorrect geological interpretation of magnetic features. In order to remove these spurious signals, we develop a semiquantitative methodology based on 3D magnetic modelling of the topographic effect. Observed total magnetic field is then corrected by subtracting the synthetic field related to the topographic effect. The key element in this approach is the appropriate estimate of the magnetisation associated with the topography, which is especially difficult to determine in areas characterised by rugged terrain. We estimate the magnetic signal related to the topographic effect by: (1) filtering the magnetic data based on the wavelength band in which magnetic and topographic data show maximum coherency, (2) inverting the filtered magnetic data in order to obtain a model of the magnetic susceptibility distribution associated with the topography, and finally (3) calculating the magnetic signal response of the topography-related susceptibility model. We successfully tested this approach in the Río Blanco-Los Bronces and El Teniente porphyry copper districts (Andes of Central Chile), which are characterised by rugged topography and the presence of highly magnetised volcanic rocks. Validation is achieved by comparing the magnetic response over zones with a good geological and petrophysical knowledge. In these examples, the topography-corrected magnetic data show the distribution of geological units and susceptibility better than the non-corrected magnetic data. This paper describes a semiquantitative, 3D-modelling based methodology that has been developed to remove topographic effects on magnetic data. Through synthetic and real cases in the Andes, we show that airborne magnetic surveys over rugged topography generate magnetic signatures that could be of similar amplitude with the signal of geological interest.

Relationship between bulk mineralogy and induced polarisation responses in iron oxide-copper-gold and porphyry copper mineralisation, northern Chile

We have studied the correlation between bulk mineralogy and induced polarisation (IP) responses in iron oxide-copper-gold (IOCG) and porphyry copper mineralised systems in northern Chile. Twelve drillholes (> 5000 m) that intersect IP sections were mapped and sampled to obtain geological characteristics and to quantify metallic minerals concentration in ore bodies. Geological parameters and modelled geophysical responses (electrical chargeability and resistivity) were compared using qualitative and quantitative criteria. Data analyses show that bulk sulphide concentration is the major factor that explains variations in the IP effect, even for IOCG ore bodies associated with magnetite mineralisation (up to 5 vol. %). Electrical chargeability exhibits a direct, but non-linear proportionality with sulphides content, while no clear trend is observed when chargeability is compared to magnetite concentration. In drillhole segments where macroscopic sulphides were not mapped, but IP results suggested high chargeabilities, petrographic studies revealed at least 1 vol. % of pyrite occurring as micro crystals (< 0.125 mm). The apparent resistivity data do not show any direct relationship with chargeability or sulphide contents. We have studied the correlation between bulk mineralogy and induced polarisation (IP) responses in iron oxide-copper-gold (IOCG) and porphyry copper concealed mineralised systems in northern Chile. Data analyses show that bulk sulphide concentration is the first order factor that explains variation in the IP effect, even for the case of magnetite-related IOCG mineralisation.

High magma oxidation state and bulk crustal shortening

El segmento andino comprendido entre los 31 y 34oS documenta una evolucion tectono-magmatica que involucra la generacion de tres porfidos cupriferos de clase mundial: Los Pelambres, Rio Blanco-Los Bronces y El Teniente. La genesis de estos tres depositos gigantes habria ocurrido como la culminacion de un ciclo de mineralizacion que actuo progresivamente de norte a sur, en estrecha asociacion con el emplazamiento de granitoides calcoalcalinos, fuertemente oxidados (razon Fe2O3/FeO = entre 1 y 3). Estos granitoides fueron emplazados en un ambiente de acortamiento regional y se fraccionaron a lo largo de zonas de falla sub-verticales activas, oblicuas al margen continental. Se propone que la actividad de estas zonas de cizalle habria jugado un rol clave en la exsolucion de los fluidos mineralizadores. El elevado estado de oxidacion de los magmas, junto con la ausencia de un aumento significativo en las razones iniciales 87Sr/86Sr, respaldados por nuevos datos geoquimicos y por datos previamente publicados, podria ser el resultado de un aumento de componentes de corteza oceanica alterada en la fuente mantifera de los magmas, bajo el arco. Se propone que este incremento de componentes de corteza oceanica en los magmas del Mioceno Superior estaria ralacionado con la progresiva subduccion, de norte a sur, de la dorsal de Juan Fernandez.