Numerical Simulation of the Effects of Wedge Subduction on the Lithospheric Thermal Structure and the Seismogenic Zone South of Chile Triple Junction

In contrast to common subduction, the young and thin part of the Antarctic Plate subducts first to the south of the Chile Triple Junction (CTJ), followed by the old and thick part, corresponding to wedge subduction. A finite element model was used to simulate the wedge subduction of the Antarctic Plate and to compare it with the slab subduction of the Nazca Plate. The results show that the CTJ is not only a wedge subduction boundary but also an important factor controlling the lithospheric thermal structure of the overriding plate. The computed heat flow curves are consistent with the data observed near the trench of the two selected profiles. The different slab dips to the north and south of the CTJ are considered to be caused by wedge subduction. When the slabs are young and at the same age, the deep dip of the Antarctic slab is 22° smaller than the Nazca slab. Southward from the CTJ, the slab age of the wedge subduction increases, which leads to a larger slab dip, a colder slab, and a wider seismogenic zone. The effect of the slab age of wedge subduction on the focal depth is smaller than that of the convergence rate. A 4.8-cm/year difference in convergence rate of the wedge subduction results in an 11-km difference in the width of the seismogenic zone and a 10-km difference in the depth of the downdip limit. Among these controlling factors, the convergence rate plays a major role in the different focal depths south and north of the CTJ.

Cenozoic plutonism geochemistry in the extra-andean southern Patagonia as evidences of arc migration process

In this work, we present the results of the first study which involves most of several cenozoic satellite plutons located in a 'back-arc' position in west Patagonia, south of the present Chile Triple-Junction. 4 plutons were studied in the northern tip, and 3 in the southern one. The petrographic and geochemical character of these plutons is different enough between them to propose a further classification for them: alkaline (sensu stricto): only the Monte Balmaceda Igneous complex, in the southern extremity; "intra-plate transitional" calc-alkaline: Río de Las Nieves and San Lorenzo plutons in the northern area, and the Torres del Paine Igneous Complex, in the southern area; and "arc transitional" calc-alkaline: the Paso de Las Llaves and Cerro Negro del Ghío plutons in the northern area, and the Cerro Donoso Pluton in the southern area. The "transitional" character, together with the plutons location, and regional context, can be explained by a Mio-Pliocene eastward arc migration coeval with the beginning of the Chile Rise subduction . Slab flattening hypothesis is favoured to explain the arc-migration, and together with the different length and time of arrival of the Chile Rise segments, contributed to the plutons heterogeneous geochemistry.

Thermal regime around the Chile Triple Junction based on JAMSTEC MR18-06 cruise 'EPIC'

<p><span>The Chile triple junction (CTJ) is a unique place where a spreading center of mid-ocean ridge is subducting near the Taitao peninsula. Around CTJ, presence of high heat flow on the continental slope and near-trench young granitic rocks on the Taitao peninsula suggests the thermal and petrological impact of subducting ridge on the continental side. The tectonic history of the southeast Pacific since early Cenozoic to the present suggests that ridge subduction continuously occurred along the Chile trench, which migrated northward.</span></p><p><span>In January 2019, the MR18-06 cruise Leg 2 was conducted at CTJ, as a part of 'EPIC' expedition by using R.V Mirai of JAMSTEC. During the leg, we completed 4 SCS lines, 6 piston coring with heat flow measurements, 2 dredges, and underway geophysics observations, as well as deployment of 13 OBSs. Coring/heatflow sites were located across the ridge axis, HP5 on the seaward plateau of axial graben, HP1/HP2/HP6 on the axis, and HP3/HP7 on the forearc slope near the trench axis. The primary object of heat flow measurement at CTJ is to better constrain the thermal regime around CTJ by adding new data right above CTJ. The key question is whether CTJ is thermally dominated by ridge activity (magmatic, tectonic, and/or hydrothermal) or by subduction initiation (tectonic thickening, accretion, and/or erosion). The ultimate goal is to model the temperature at the plate interface from the heat flow and other data, and to infer how the thermal regime at CTJ contributes the seismogenic behavior at the M~9 megathrust zone. </span></p><p><span>Onboard and post-cruise measurements include; bulk density, porosity, Vp, resistivity, CT imags, iTracks element scan, age dating, etc. Core saples seaward of ridge axis (HP5) has few turbidites with higher density (~2 g/cc) and low sedimentation rate (SR; 0.2 m/ky), whereas cores on the axis the density are turbidite dominant with lower (1.6~1.8 g/cc) and very high SR (1~3 m/ky). The accretionary prism (landward of trench) cores have the density of 1.6~1.7 g/cc and SR=0.5~1 m/ky. They suggest that the turbidite covers only the axial graben. </span></p><p><span>Heat flow in the axial graben range 140-210 mW/m^2, which is lower than on the seaward plateau (370 mW/m^2). This apparent controversy may be due to lower magmatic activity and/or high sedimentation rate on the axis. The lower spreading rate (2.6 cm/yr one side) and the rapid convergent rate at the trench (7.2 cm/yr) may suppress sufficient magma supply or hydrothermal circulation. Heat flow on the accretionary prism (230 mW/m^2) is higher than borehole or BSR-derived heat flow (~<100 mW/m^2). It is suggestive of fluid upwelling along the decollement as proposed in the previous study. Some numerical thermal models will be presented to show the effect of ridge subduction. </span></p>

Southern Chile crustal structure from teleseismic receiver functions: Responses to ridge subduction and terrane assembly of Patagonia

Continental crustal structure is the product of those processes that operate typically during a long tectonic history. For the Patagonia composite terrane, these tectonic processes include its early Paleozoic accretion to the South America portion of Gondwana, Triassic rifting of Gondwana, and overriding of Pacific Basin oceanic lithosphere since the Mesozoic. To assess the crustal structure and glean insight into how these tectonic processes affected Patagonia, we combined data from two temporary seismic networks situated inboard of the Chile triple junction, with a combined total of 80 broadband seismic stations. Events suitable for analysis yielded 995 teleseismic receiver functions. We estimated crustal thicknesses using two methods, the H-k stacking method and common conversion point stacking. Crustal thicknesses vary between 30 and 55 km. The South American Moho lies at 28–35 km depth in forearc regions that have experienced ridge subduction, in contrast to crustal thicknesses ranging from 34 to 55 km beneath regions north of the Chile triple junction. Inboard, the prevailing Moho depth of ∼35 km shallows to ∼30 km along an E-W trend between 46.5°S and 47°S; we relate this structure to Paleozoic thrust emplacement of the Proterozoic Deseado Massif terrane above the thicker crust of the North Patagonian/Somún Cura terrane along a major south-dipping fault.

Isolated Triggered Tremor Spots in South America and Implications for Global Tremor Activity

ABSTRACT We report new observations of triggered tectonic tremor in three regions in South America along the plate boundary between the Nazca and South America plates: southern Chile, Ecuador, and central Colombia. In these regions, tremor was observed during the passage of large‐amplitude surface waves of recent large earthquakes, which occurred in South America and around the world. In southern Chile, triggered tremor was observed around an ambient tremor active zone in the Chile triple junction region. In Ecuador and central Colombia, only one seismic station in each region recorded triggered tremor. With a single‐station approach, we are able to estimate potential tremor sources in these regions. Triggered tremor in Ecuador is likely associated with an inland fault near the volcanic region. In central Colombia, triggered tremor may be associated with the Romeral fault system rather than the subduction zone interface. In addition, we summarize global observations of tremor‐triggering stress and background ambient tremor activity in 24 tremor‐active regions. Based on the global summary of triggered and ambient tremor activity, the relative lack of triggered tremor in central and northern Chile and Peru is consistent with the lack of background tremor activity in these regions, suggesting tectonic tremor occurs only in isolated regions along major faults.

Shallow Nonvolcanic Tremor Activity and Potential Repeating Earthquakes in the Chile Triple Junction: Seismic Evidence of the Subduction of the Active Nazca–Antarctic Spreading Center

ABSTRACT In southern Chile, at ∼46.2°S and ∼75.2°W, the active spreading center between the Nazca and Antarctic plates is colliding with the South American plate, forming the Chile triple junction (CTJ). For 1 yr, from March 2009 to February 2010, five ocean‐bottom seismometers (OBSs) were deployed over the CTJ. We used a portion of the OBS data to study the seismic signatures of the subduction of the active Nazca–Antarctic spreading center. Using the envelope technique, we detected long episodes of shallow nonvolcanic tremor (NVT) activity. To improve the identified location of the NVT activity, we cross‐correlated the vertical and horizontal components of all located NVTs. In different months, we measured the local maximum of the lag‐time correlation near 2 s, which is associated with the lag between the S and P waves (S−Ptime). Furthermore, we observed that in the days with intense tremor activity, the maxima corresponding to S−Ptime emerged in windows without observable NVTs. We suggest that days with intense tremor activity correspond to an almost continuous slow slip, which may accelerate and decelerates nearly randomly, with spatial and temporal heterogeneity. In addition, we detected some potential repeating earthquakes with an S−Ptime near 2 s, as well as NVTs. The detected NVT activity and potential repeating earthquakes suggest the existence of a shallow region close to the CTJ that is able to generate brittle (earthquakes) and brittle–ductile (potential repeating earthquakes and NVTs) ruptures.

Gas Hydrate Estimate in an Area of Deformation and High Heat Flow at the Chile Triple Junction

Large amounts of gas hydrate are present in marine sediments offshore Taitao Peninsula, near the Chile Triple Junction. Here, marine sediments on the forearc contain carbon that is converted to methane in a regime of very high heat flow and intense rock deformation above the downgoing oceanic spreading ridge separating the Nazca and Antarctic plates. This regime enables vigorous fluid migration. Here, we present an analysis of the spatial distribution, concentration, estimate of gas-phases (gas hydrate and free gas) and geothermal gradients in the accretionary prism, and forearc sediments offshore Taitao (45.5°–47° S). Velocity analysis of Seismic Profile RC2901-751 indicates gas hydrate concentration values <10% of the total rock volume and extremely high geothermal gradients (<190 °C·km−1). Gas hydrates are located in shallow sediments (90–280 m below the seafloor). The large amount of hydrate and free gas estimated (7.21 × 1011 m3 and 4.1 × 1010 m3; respectively), the high seismicity, the mechanically unstable nature of the sediments, and the anomalous conditions of the geothermal gradient set the stage for potentially massive releases of methane to the ocean, mainly through hydrate dissociation and/or migration directly to the seabed through faults. We conclude that the Chile Triple Junction is an important methane seepage area and should be the focus of novel geological, oceanographic, and ecological research.

Gas Hydrate Estimate in an Area of Deformation and High Heat Flow at the Chile Triple Junction

Large amounts of gas hydrate are present in marine sediments offshore Taitao Peninsula, near the Chile Triple Junction. Here, marine sediments on the forearc contain carbon that is converted to methane in a zone of very high heat flow and intense rock deformation above the downgoing oceanic spreading ridge separating the Nazca and Antarctic plates. This regime enables vigorous fluid migration. Here we present an analysis of the spatial distribution, concentration, estimate of gas phases (gas hydrate and free gas) and geothermal gradients in the accretionary prism and forearc sediments offshore Taitao (45.5&deg; - 47&deg; S). Velocity analysis of Seismic Profile RC2901-751 indicates gas hydrate concentration values &lt;10% of the total rock volume, and extremely high geothermal gradients (&lt;190 &deg;Ckm-1). Gas hydrates are located in shallow sediments (90-280 meters below the seafloor). The large amount of hydrate and free gas estimated (7.21x1011 m3 and 4.1x1010 m3, respectively), the high seismicity, the mechanically unstable nature of the sediments, and the anomalous geothermal conditions, set the stage for potential massive releases of methane to the ocean mainly through hydrate dissociation and/or migration directly to the seabed through faults. We conclude that the Chile Triple Junction is an important methane seepage area and should be the focus of novel geological and ecological research.