Daniel Melnick

Central and Southern Andean Tectonic Evolution Inferred from Arc Magmatism

Patterns of spatial distribution, and geochemical and isotopic evolution from subduction-related igneous rocks provide tools for scaling, balancing and predicting orogenic processes and mechanisms. We discuss patterns from two Andean key arc segments, which developed into fundamentally different types of orogens: (1) A plateau-type orogen with thick crust in the central Andes, and (2) a non-plateau orogen with normal crust in the southern Andes.

Reawakening of large earthquakes in south central Chile: The 2016 Mw 7.6 Chiloé event

On 25 December 2016, the Mw 7.6 Chiloe earthquake broke a plate-boundary asperity in South- Central Chile near the center of the rupture zone of the Mw 9.5 Valdivia earthquake of 1960. To gain insight on decadal-scale deformation trends and their relation with the Chiloe earthquake, we combine geodetic, teleseismic and regional seismological data. GPS velocities increased at continental scale after the 2010 Maule earthquake, probably due to a readjustment in the mantle flow and an apparently abrupt end of the viscoelastic mantle relaxation following the 1960 Valdivia earthquake. It also produced an increase in the degree of plate locking. The Chiloe earthquake occurred within the region of increased locking, breaking a circular patch of ~15 km radius at ~30 km depth, located near the bottom of the seismogenic zone. We propose that the Chiloe earthquake is a first sign of the seismic reawakening of the Valdivia segment, in response to the interaction between postseismic viscoelastic relaxation and changes of interseismic locking between Nazca and South-America.

Quantifying offshore fore‐arc deformation and splay‐fault slip using drowned Pleistocene shorelines, Arauco Bay, Chile

Most of the deformation associated with the seismic cycle in subduction zones occurs offshore, and has been therefore difficult to quantify with direct observations at millennial time scales. Here we study millennial deformation associated with an active splay-fault system in the Arauco Bay area off south-central Chile. We describe hitherto-unrecognized drowned shorelines using high-resolution multibeam bathymetry, geomorphic, sedimentologic, and paleontologic observations, and quantify uplift rates using a landscape evolution model. Along a margin-normal profile, uplift rates are 1.3 m/ka near the edge of the continental shelf, 1.5 m/ka at the emerged Santa Maria Island, -0.1 m/ka at the center of the Arauco Bay, and 0.3 m/ka in the mainland. The bathymetry images a complex pattern of folds and faults representing the surface expression of the crustal-scale Santa Maria splay fault system. We modeled surface deformation using two different structural scenarios: deep-reaching normal faults and deep-reaching reverse faults with shallow extensional structures. Our preferred model comprises a blind reverse fault extending from 3 km depth down to the plate interface at 16 km that slips at a rate between 3.0 and 3.7 m/ka. If all the splay fault slip occurs during every great megathrust earthquake, with a recurrence of ~150-200 years, the fault would slip ~0.5 m per event, equivalent to a magnitude ~6.4 earthquake. However, if the splay fault slips only with a megathrust earthquake every ~1000 years, the fault would slip ~3.7 m per event, equivalent to a magnitude ~7.5 earthquake.

Recovery of black-necked swans, macrophytes and water quality in a Ramsar wetland of southern Chile: Assessing resilience following sudden anthropogenic disturbances

In 2004 migration and mortality for unknown reasons of the herbivorous Black necked swan (Cygnus melancorhyphus (Molina, 1782)) occurred within the Río Cruces wetland (southern Chile), a Ramsar Site and nature sanctuary. Before 2004, this wetland hosted the largest breeding population of this water bird in the Neotropic Realm. The concurrent decrease in the spatial occurrence of the aquatic plant Egeria densa Planch. 1849 - the main food source of swans - was proposed as a cause for swan migration and mortality. Additionally, post-mortem analyses carried out on swans during 2004 showed diminished body weight, high iron loads and histopathological abnormalities in their livers, suggesting iron storage disease. Various hypotheses were postulated to describe those changes; the most plausible related to variations in water quality after a pulp mill located upstream the wetland started to operate in February 2004. Those changes cascaded throughout the stands of E. densa whose remnants had high iron contents in their tissues. Here we present results of a long-term monitoring program of the wetland components, which show that swan population abundance, body weights and histological liver conditions recovered to pre-disturbance levels in 2012. The recovery of E. densa and iron content in plants throughout the wetland, also returned to pre-disturbance levels in the same 8-year time period. These results show the temporal scale over which resilience and natural restoring processes occur in wetland ecosystems of temperate regions such as southern Chile.

Postseismic uplift of the Andes following the 2010 Maule earthquake: Implications for mantle rheology: Postseismic Deformation of Maule Earthquake

Postseismic surface deformation associated with great subduction earthquakes is controlled by asthenosphere rheology, frictional properties of the fault, and structural complexity. Here by modeling GPS displacements in the 6 years following the 2010Mw 8.8 Maule earthquake in Chile, we investigate the impact of heterogeneous viscosity distribution in the South American subcontinental asthenosphere on the 3-D postseismic deformation pattern. The observed postseismic deformation is characterized by flexure of the South America plate with peak uplift in the Andean mountain range and subsidence in the hinterland. We find that, at the time scale of observation, over 2 orders of magnitude gradual increase in asthenosphere viscosity from the arc area toward the cratonic hinterland is needed to jointly explain horizontal and vertical displacements. Our findings present an efficient method to estimate spatial variations of viscosity, which clearly improves the fitting to the vertical signal of deformation. Lateral changes in asthenosphere viscosity can be correlated with the thermomechanical transition from weak subvolcanic arc mantle to strong subcratonic mantle, thus suggesting a stationary heterogeneous viscosity structure. However, we cannot rule out a transient viscosity structure (e.g., power law rheology) with the short time span of observation.