Carlos A. Vargas

Receiver functions and crustal structure of the northwestern Andean region, Colombia

We used the receiver function technique to deduce crustal thickness beneath the northwestern Andean system, using data from the permanent seismic network of Colombia, combined with some of the IRIS and CTBTO stations in Colombia and Ecuador. The estimation of crustal thickness was made using the primary P to s conversion and crustal reverberations. The bulk crustal VP/VS ratio was constrained using a crustal thickness versus VP/VS stacking method, in addition to estimations using a time to depth conversion technique based on results of a modified Wadati diagram analysis. We observed a wide range of crustal thicknesses, including values around 17 km beneath the Malpelo Island on the Pacific Ocean, 20 to 30 km at the coastal Pacific and Caribbean plains of Colombia, 25 to 40 km beneath the eastern plains and foothills, 35 km beneath the Western Cordillera, 45 km at the Magdalena River intermountain valley, 52 to 58 km under the northern Central Cordillera, and reaching almost 60 km beneath some of the volcanoes of the Southern Cordilleran system of Colombia; crustal thickness can be slightly greater than 60 km beneath the plateau of the Eastern Cordillera. The values of VP/VS are particularly high for some of the stations on the volcanic centers, reaching values above 1.79, probably related to the addition of mafic materials to the lower crust, and in the plateau of the Eastern Cordillera near Bogota, where we speculate about the possibility of crustal seismic anisotropy associated with shear zones.

Coda-Wave Attenuation Imaging of Galeras Volcano, Colombia

Abstract The spatial variation of S -wave coda attenuation () in the Galeras volcano region is analyzed. The study region is at present an active magmatic system situated in the southwestern Colombian Andes. is estimated using seismograms from 435 volcano-tectonic earthquakes recorded at 31 stations of the Galeras seismograph network. is then imaged using a 3D spatial stacking procedure. The technique is based on the assumption of uniform distribution of over a spheroidal shell with its volume determined by the associated source–receiver distance. The resultant tomograms also show frequency dependence, which is interpreted in terms of the scale of the heterogeneities producing the scattering. High-attenuation anomalies are detected at high frequencies. Synthetic tests indicate the validity of the inversion technique.

Breathing of the Nevado del Ruiz volcano reservoir, Colombia, inferred from repeated seismic tomography

Nevado del Ruiz volcano (NRV), Columbia, is one of the most dangerous volcanoes in the world and caused the death of 25,000 people in 1985. Using a new algorithm for repeated tomography, we have found a prominent seismic anomaly with high values of the Vp/Vs ratio at depths of 2–5 km below the surface, which is associated with a shallow magma reservoir. The amplitude and shape of this anomaly changed during the current phase of unrest which began in 2010. We interpret these changes as due to the ascent of gas bubbles through magma and to degassing of the reservoir. In 2011–2014, most of this gas escaped through permeable roof rocks, feeding surface fumarole activity and leading to a gradual decrease of the Vp/Vs ratio in the reservoir. This trend was reversed in 2015–2016 due to replenishment of the reservoir by a new batch of volatile-rich magma likely to sustain further volcanic activity. It is argued that the recurring “breathing” of the shallow reservoir is the main cause of current eruptions at NRV.

Shear wave anisotropy in northwestern South America and its link to the Caribbean and Nazca subduction geodynamics

To investigate the subduction dynamics in northwestern South America, we measured SKS and slab-related local S splitting at 38 seismic stations. Comparison between the delay times of both phases shows that most of the SKS splitting is due to entrained mantle flow beneath the subducting Nazca and Caribbean slabs. On the other hand, the fast polarizations of local S-waves are consistently aligned with regional faults, which implies the existence of a lithosphere-confined anisotropy in the overriding plate, and that the mantle wedge is not contributing significantly to the splitting. Also, we identified a clear change in SKS fast directions at the trace of the Caldas Tear (∼5°N), which represents a variation in the subduction style. To the north of ∼5°N, fast directions are consistently parallel to the flat subduction of the Caribbean plate-Panama arc beneath South America, while to the south fast polarizations are subparallel to the Nazca-South America subduction direction. A new change in the SKS splitting pattern is detected at ∼2.8°N, which is related to another variation in the subduction geometry marked by the presence of a lithosphere-scale tearing structure, named here as Malpelo Tear; in this region, NE-SW-oriented SKS fast directions are consistent with the general dip direction of the underthrusting of the Carnegie Ridge beneath South America. Further inland, this NE-SW-trending mantle flow continues beneath the Eastern Cordillera of Colombia and Merida Andes of Venezuela. Finally, our results suggest that the subslab mantle flow in northwestern South America is strongly controlled by the presence of lithospheric tearing structures.

Morphologic Expression of Accretionary Processes and Recent Submarine Landslides Along the Southwestern Pacific Margin of Colombia

We analyzed 9,750 km2 of high-resolution, bathymetric data recorded by a multi-beam echo-sounder along the southwestern Pacific margin of Colombia (1°25′N–2°00′N and 78°44′W–80°14′W). This margin has experienced several great subduction zone earthquakes during the twentieth century (1906, Mw = 8.8; 1942, Mw = 7.8; 1958, Mw = 7.7, 1979, Mw = 8.2). The most prominent morphological features imaged by the survey include the Tumaco high, the Emerald canyon and the Mira canyon, limited on its western edge by the Manglares high. Our results suggest that the Mira canyon was likely formed by a 50-km-long, earthquake-generated landslide that displaced marine sediments to depths of 1,000 m depth. Geomorphology dating using the fault-scarp diffusion hypothesis suggests that the 3200-2800 BP earthquake event could have triggered its configuration.

Lithospheric thickness estimation beneath Northwestern South America from an S‐wave receiver function analysis

We make use of the S-to-P receiver function technique beneath Colombia and neighboring regions to make a first-order approximation of the depth of the Lithosphere-Asthenosphere Boundary (LAB) and, therefore, of lithospheric thickness. A deconvolution technique was used to calculate the receiver functions, and after a moveout correction and a time-depth conversion, LAB depths for different tectonic regions of northwestern South America were obtained. Results are typically between 65 and 110 km, consistent with previous estimates around the world and other regions in South America. Lithospheric thickness beneath an oceanic island in the Caribbean is ∼80 km, whereas for the Ecuador-Colombia Trench it is ∼65 km, and around 100 km for the Panama Arc. The transition to the continent is associated with an increase in LAB depth, where it can reach ∼110 km, with no significant differences among terranes and/or tectonic blocks.

Curie Point Depths in Northwestern South America and the Southwestern Caribbean Sea

Abstract We have estimated the Curie point depth (CPD) in the northwestern corner of South America and the southwestern Caribbean Sea from spectral analysis of magnetic anomalies extracted from the World Digital Magnetic Anomaly Map. To do this, we performed three different spectral methods and chose the model that best fits the geologic and geophysical characteristics of the study area. Then, we calculated the geothermal gradient from these CPD values to assess the likelihood of the hydrocarbon “Golden zone” being present in some of Colombia’s sedimentary basins. Similarly, we tried to establish empirical relationships between CPD, geothermal gradient, and heat flux. Our results show that the CPD lies between 12.6 km (7.8 mi) and 74 km (45.9 mi). The shallowest depths ( 50 km [>31 mi]) occur in parts of the western and central Cordilleras, Santander massif, and middle Magdalena, Catatumbo, Barinas-Apure, and Vaupes-Amazonas Basins. Based on the results, we found a relationship between an unexpected zone of deep CPD values (40–47 km [24.8–29.2 mi]) in the Colombia Basin and the presumable presence of an abnormal thick Caribbean Plateau with a continental inheritance. On the other hand, the contrasting deep and shallow CPD values in the Caribbean support the interpretation of flat subduction of the Caribbean plate beneath South America with a flexural topographic bulge toward the Sinu−San Jacinto and lower Magdalena Basins. Partial erosion of this bulge could have resulted in shallowing of the CPD with a consequent increase in geothermal gradient and heat flux. Also, we found a CPD shallowing beneath Caguan–Putumayo and eastern Llanos Basins. Finally, based on the calculated geothermal gradient values in Colombia, we consider that the Golden zone of hydrocarbon occurrence most likely exists in the Choco–Uraba, eastern Cordillera, Guajira−Los Cayos, eastern Llanos, and lower Magdalena Basins, while the Golden zone would be absent only in the Vaupes–Amazonas Basin.

Morphological Expression of Submarine Landslides in the Accretionary Prism of the Caribbean Continental Margin of Colombia

Morphological analyses of ∼14,500 km2 of high-resolution-bathymetric data in the Caribbean continental margin of Colombia has allowed us to identify submarine landslides in the Sinu Accretionary Prism (SAP) and Magdalena submarine fan (MSF) areas.

Age Estimation of Submarine Landslides in the La Aguja Submarine Canyon, Northwestern Colombia

La Aguja Submarine Canyon (LASC) is a remarkable bathymetric feature that cuts the continental margin of the Colombian Caribbean Coast offshore of the Santa Marta massif. This section of the margin is strongly influenced by two active strike-slip fault systems: the NNW-trending Santa Marta – Bucaramanga Fault (SMBF) and the EW-trending Oca- Fault (OF). Based on 6,500 km2 of high resolution multibeam bathymetry and one 2D seismic reflection profile (∼60 km), we identified submarine landslides associated with the LASC. Along the canyon we observe channel morphologies high sinuosity, variable widths (between 2 and 8 km), and both U- and V-shaped asymmetric cross sections. The occurrence of mud volcanoes in the surrounding area, some of which stand above the flanks of the canyon, is also common. The available data allowed us to identify 31 submarine landslides on the LASC flanks, between water depths of 1,200 and 3,285 m, with slopes ranging between 6.1° and 36.8°. Using numerical solutions of the diffusion equation of these scarps we estimate ages of between <1.0 to ∼631.6 ka. According to observations, the triggering mechanisms of landslides could be closely related to earthquakes originated in the convergence zone of SMBF and OF, and/or to the flanks instabilities driven by mud diapirism.

A New Species of Lycianthes (Solanaceae) from the Pacific Region of Colombia

ABSTRACT A new species of Lycianthes (Dunal) Hassler (Solanaceae, Capsiceae) is described and illustrated. Lycianthes bullata C. I. Orozco, C. A. Vargas & Serralde is found at low altitudes in the Pacific region of Colombia. This species belongs to the subgenus Polymeris (Dunal) Bitter; it is a vine and exhibits stellate and stalked ferruginous hairs and unifoliate sympodia. It is related to L. ferruginea Bitter but can be distinguished from this species by the intense ferruginous color of its indument; its larger foliar lamina (6.5–19 × 5–9 cm), elliptic to ovate, with adaxial surface glossy and bullate, and abaxial surface extremely reticulate; its larger flowers (15–17 mm long); the presence of five or six stamens; and its larger fruits (16–30 mm in diam.).