Deep lithospheric structures along the southern central Chile Margin from wide-angle P-wave modellilng

Crustal- and upper-mantle structures of the subduction zone in south central Chile, between 42 degrees S and 46 degrees S, are determined from seismic wide-angle reflection and refraction data, using the seismic ray tracing method to calculate minimum parameter models. Three profiles along differently aged segments of the subducting Nazca Plate were analysed in order to study subduction zone structure dependencies related to the age, that is, thermal state, of the incoming plate. The age of the oceanic crust at the trench ranges from 3 Ma on the southernmost profile, immediately north of the Chile triple junction, to 6.5 Ma old about 100 km to the north, and to 14.5 Ma old another 200 km further north, off the Island of Chiloe. Remarkable similarities appear in the structures of both the incoming as well as the overriding plate. The oceanic Nazca Plate is around 5 km thick, with a slightly increasing thickness northward, reflecting temperature changes at the time of crustal generation. The trench basin is about 2 km thick except in the south where the Chile Ridge is close to the deformation front and only a small, 800-m-thick trench infill could develop. In south central Chile, typically three quarters (1.5 km) of the trench sediments subduct below the decollement in the subduction channel. To the north and south of the study area, only about one quarter to one third of the sediments subducts, the rest is accreted above. Similarities in the overriding plate are the width of the active accretionary prism, 35-50 km, and a strong lateral crustal velocity gradient zone about 75-80 km landward from the deformation front, where landward upper-crustal velocities of over 5.0-5.4 km s<SU-1</SU decrease seaward to around 4.5 km s<SU-1</SU within about 10 km, which possibly represents a palaeo-backstop. This zone is also accompanied by strong intraplate seismicity. Differences in the subduction zone structures exist in the outer rise region, where the northern profile exhibits a clear bulge of uplifted oceanic lithosphere prior to subduction whereas the younger structures have a less developed outer rise. This plate bending is accompanied by strongly reduced rock velocities on the northern profile due to fracturing and possible hydration of the crust and upper mantle. The southern profiles do not exhibit such a strong alteration of the lithosphere, although this effect may be counteracted by plate cooling effects, which are reflected in increasing rock velocities away from the spreading centre. Overall there appears little influence of incoming plate age on the subduction zone structure which may explain why the M-w = 9.5 great Chile earthquake from 1960 ruptured through all these differing age segments. The rupture area, however, appears to coincide with a relatively thick subduction channel

Across-arc geochemical variations in the Southern Volcanic Zone, Chile (34.5- 38.0°S): Constraints on Mantle Wedge and Input Compositions

Crustal assimilation (e.g. Hildreth and Moorbath, 1988) and/or subduction erosion (e.g. Stern, 1991; Kay et al., 2005) are believed to control the geochemical variations along the northern portion of the Chilean Southern Volcanic Zone. In order to evaluate these hypotheses, we present a comprehensive geochemical data set (major and trace elements and O-Sr-Nd-Hf-Pb isotopes) from Holocene primarily olivine-bearing volcanic rocks across the arc between 34.5-38.0°S, including volcanic front centers from Tinguiririca to Callaqui, the rear arc centers of Infernillo Volcanic Field, Laguna del Maule and Copahue, and extending 300 km into the backarc. We also present an equivalent data set for Chile Trench sediments outboard of this profile. The volcanic arc (including volcanic front and rear arc) samples primarily range from basalt to andesite/trachyandesite, whereas the backarc rocks are low-silica alkali basalts and trachybasalts. All samples show some characteristic subduction zone trace element enrichments and depletions, but the backarc samples show the least. Backarc basalts have higher Ce/Pb, Nb/U, Nb/Zr, and Ta/Hf, and lower Ba/Nb and Ba/La, consistent with less of a slab-derived component in the backarc and, consequently, lower degrees of mantle melting. The mantle-like δ18O in olivine and plagioclase phenocrysts (volcanic arc = 4.9-5.6 and backarc = 5.0-5.4 per mil) and lack of correlation between δ18O and indices of differentiation and other isotope ratios, argue against significant crustal assimilation. Volcanic arc and backarc samples almost completely overlap in Sr and Nd isotopic composition. High precision (double-spike) Pb isotope ratios are tightly correlated, precluding significant assimilation of older sialic crust but indicating mixing between a South Atlantic Mid Ocean-Ridge Basalt (MORB) source and a slab component derived from subducted sediments and altered oceanic crust. Hf-Nd isotope ratios define separate linear arrays for the volcanic arc and backarc, neither of which trend toward subducting sediment, possibly reflecting a primarily asthenospheric mantle array for the volcanic arc and involvement of enriched Proterozoic lithospheric mantle in the backarc. We propose a quantitative mixing model between a mixed-source, slab-derived melt and a heterogeneous mantle beneath the volcanic arc. The model is consistent with local geodynamic parameters, assuming water-saturated conditions within the slab

Hydrogeological system of erosional convergent margins and its influence on tectonics and interplate seismogenesis

[1] Fluid distribution in convergent margins is by most accounts closely related to tectonics. This association has been widely studied at accretionary prisms, but at half of the Earth's convergent margins, tectonic erosion grinds down overriding plates, and here fluid distribution and its relation to tectonics remain speculative. Here we present a new conceptual model for the hydrological system of erosional convergent margins. The model is based largely on new data and recently published observations from along the Middle America Trench offshore Nicaragua and Costa Rica, and it is consistent with observations from other erosional margins. The observations indicate that erosional margins possess previously unrecognized distinct hydrogeological systems: Most fluid contained in the sediment pores and liberated by early dehydration reactions drains from the plate boundary through a fractured upper plate to seep at the seafloor across the slope, rather than migrating along the décollement toward the deformation front as described for accretionary prisms. The observations indicate that the relative fluid abundance across the plate-boundary fault zone and fluid migration influence long-term tectonics and the transition from aseismic to seismogenic behavior. The segment of the plate boundary where fluid appears to be more abundant corresponds to the locus of long-term tectonic erosion, where tectonic thinning of the overriding plate causes subsidence and the formation of the continental slope. This correspondence between observations indicates that tectonic erosion is possibly linked to the migration of overpressured fluids into the overriding plate. The presence of overpressured fluids at the plate boundary is compatible with the highest flow rates estimated at slope seeps. The change from aseismic to seismogenic behavior along the plate boundary of the erosional margin begins where the amount of fluid at the fault declines with depth, indicating a control on interplate earthquakes. A previously described similar observation along accreting plate boundaries strongly indicates that fluid abundance exerts a first-order control on interplate seismogenesis at all types of subduction zones. We hypothesize that fluid depletion with depth increases grain-to-grain contact, increasing effective stress on the fault, and modifies fault zone architecture from a thick fault zone to a narrower zone of localized slip

Early-stage rifting of the northern Tyrrhenian Sea Basin: Results from a combined wide-angle and multichannel seismic study

Extension of the continental lithosphere leads to the formation of rift basins and ultimately may create passive continental margins. The mechanisms that operate during the early stage of crustal extension are still intensely debated. We present the results from coincident multichannel seismic and wide-angle seismic profiles that transect across the northern Tyrrhenian Sea Basin. The profiles cross the Corsica Basin (France) to the Latium Margin (Italy) where the early-rift stage of the basin is well preserved. We found two domains, each with a distinct tectonic style, heat flow and crustal thickness. One domain is the Corsica Basin in the west that formed before the main rift phase of the northern Tyrrhenian Sea opening (∼8–4 Ma). The second domain is rifted continental crust characterized by tilted blocks and half-graben structures in the central region and at the Latium Margin. These two domains are separated by a deep (∼10 km) sedimentary complex of the eastern portion of the Corsica Basin. Travel-time tomography of wide-angle seismic data reveals the crustal architecture and a subhorizontal 15–17 ± 1 km deep Moho discontinuity under the basin. To estimate the amount of horizontal extension we have identified the pre-, syn-, and post-tectonic sedimentary units and calculated the relative displacement of faults. We found that major faults initiated at angles of 45°–50° and that the rifted domain is horizontally stretched by a factor of β ∼ 1.3 (∼8–10 mm/a). The crust has been thinned from ∼24 to ∼17 km indicating a similar amount of extension (∼30%). The transect represents one of the best imaged early rifts and implies that the formation of crustal-scale detachments, or long-lived low-angle normal faults, is not a general feature that controls the rift initiation of continental crust. Other young rift basins, like the Gulf of Corinth, the Suez Rift or Lake Baikal, display features resembling the northern Tyrrhenian Basin, suggesting that half-graben formations and distributed homogeneous crustal thinning are a common feature during rift initiation

Crustal structure of the propagating TAMMAR ridge segment on the Mid-Atlantic Ridge, 21.5°N

Active ridge propagation frequently occurs along spreading ridges and profoundly affects ridge crest segmentation over time. The mechanisms controlling ridge propagation, however, are poorly understood. At the slow spreading Mid-Atlantic Ridge at 21.5°N a seismic refraction and wide-angle reflection profile surveyed the crustal structure along a segment controlled by rapid ridge propagation. Tomographic traveltime inversion of seismic data suggests that the crustal structure along the ridge axis is controlled by melt supply; thus, crust is thickest, 8 km, at the domed segment center and decreases in thickness toward both segment ends. However, thicker crust is formed in the direction of ridge propagation, suggesting that melt is preferentially transferred toward the propagating ridge tip. Further, while seismic layer 2 remains constant along axis, seismic layer 3 shows profound changes in thickness, governing variations in total crustal thickness. This feature supports mantle upwelling at the segment center. Thus, fluid basaltic melt is redistributed easily laterally, while more viscose gabbroic melt tends to crystallize and accrete nearer to the locus of melt supply. The onset of propagation seems to have coincided with the formation of thicker crust, suggesting that propagation initiation might be due to changes in the melt supply. After a rapid initiation a continuous process of propagation was established. The propagation rate seems to be controlled by the amount of magma that reaches the segment ends. The strength of upwelling may govern the evolution of ridge segments and hence ultimately controls the propagation length

Seamounts

Definition: Seamounts are literally mountains rising from the seafloor. More specifically, they are “any geographically isolated topographic feature on the seafloor taller than 100 m, including ones whose summit regions may temporarily emerge above sea level, but not including features that are located on continental shelves or that are part of other major landmasses” (Staudigel et al., 2010). The term “guyot” can be used for seamounts having a truncated cone shape with a flat summit produced by erosion at sea level (Hess, 1946), development of carbonate reefs (e.g., Flood, 1999), or partial collapse due to caldera formation (e.g., Batiza et al., 1984). Seamounts <1,000 m tall are sometimes referred to as “knolls” (e.g., Hirano et al., 2008). “Petit spots” are a newly discovered subset of sea knolls confined to the bulge of subducting oceanic plates of oceanic plates seaward of deep-sea trenches (Hirano et al., 2006)

PickBlue: Seismic Phase Picking for Ocean Bottom Seismometers With Deep Learning

Detecting phase arrivals and pinpointing the arrival times of seismic phases in seismograms is crucial for many seismological analysis workflows. For land station data, machine learning methods have already found widespread adoption. However, deep learning approaches are not yet commonly applied to ocean bottom data due to a lack of appropriate training data and models. Here, we compiled an extensive and labeled ocean bottom seismometer (OBS) data set from 15 deployments in different tectonic settings, comprising ∼90,000 P and ∼63,000 S manual picks from 13,190 events and 355 stations. We propose PickBlue, an adaptation of the two popular deep learning networks EQTransformer and PhaseNet. PickBlue joint processes three seismometer recordings in conjunction with a hydrophone component and is trained with the waveforms in the new database. The performance is enhanced by employing transfer learning, where initial weights are derived from models trained with land earthquake data. PickBlue significantly outperforms neural networks trained with land stations and models trained without hydrophone data. The model achieves a mean absolute deviation of 0.05 s for P-waves and 0.12 s for S-waves, and we apply the picker on the Hikurangi Ocean Bottom Tremor and Slow Slip OBS deployment offshore New Zealand. We integrate our data set and trained models into SeisBench to enable an easy and direct application in future deployments

PickBlue: Seismic Phase Picking for Ocean Bottom Seismometers With Deep Learning

Detecting phase arrivals and pinpointing the arrival times of seismic phases in seismograms is crucial for many seismological analysis workflows. For land station data, machine learning methods have already found widespread adoption. However, deep learning approaches are not yet commonly applied to ocean bottom data due to a lack of appropriate training data and models. Here, we compiled an extensive and labeled ocean bottom seismometer (OBS) data set from 15 deployments in different tectonic settings, comprising ∼90,000 P and ∼63,000 S manual picks from 13,190 events and 355 stations. We propose PickBlue, an adaptation of the two popular deep learning networks EQTransformer and PhaseNet. PickBlue joint processes three seismometer recordings in conjunction with a hydrophone component and is trained with the waveforms in the new database. The performance is enhanced by employing transfer learning, where initial weights are derived from models trained with land earthquake data. PickBlue significantly outperforms neural networks trained with land stations and models trained without hydrophone data. The model achieves a mean absolute deviation of 0.05 s for P-waves and 0.12 s for S-waves, and we apply the picker on the Hikurangi Ocean Bottom Tremor and Slow Slip OBS deployment offshore New Zealand. We integrate our data set and trained models into SeisBench to enable an easy and direct application in future deployments

Investigations of the Oligocene-Miocene opening of the Ligurian Basin using amphibious refraction seismic data

The Ligurian Basin is located north-west of Corsica at the transition from the western Alpine orogen to the Apennine system. The Back-arc basin was generated by the southeast trench retreat of the Apennines-Calabrian-Maghrebides subduction zone. The opening took place from late Oligocene to Miocene. While the extension led to extreme continental thinning and un-roofing of mantle material little is known about the style of back-arc rifting. To shed light on the present day crustal and lithospheric architecture of the Ligurian Basin, active seismic data have been recorded on short period ocean bottom seismometers in the framework of SPP2017 4D-MB, the German component of AlpArray. Two refraction seismic profiles were shot across and along the centre of the Ligurian Basin. P01 was shot in an E-W direction from the Gulf of Lion to Corsica. The profile extends onshore Corsica to image the necking zone of continental thinning. P02 is a transect along the basin in NE-SW direction extending a previous shot seismic profile reaching to the Italian cost near Genua. The majority of the ocean bottom seismometer data show sedimentary and crustal phases of good quality and weaker in amplitude mantle phases to offsets up to 70 km. The arrivals of seismic phases were picked and inverted in a travel time tomography. The results for p01 show a crust-mantle boundary in the central basin at ~12 km depth below sea surface. The crust-mantle boundary deepens from ~12 km to ~18 km within 25 - 30 km towards Corsica. The results do not map an axial valley as expected for oceanic spreading. However, an extremely thinned continental crust indicates a long-lasting rifting process that possibly did not initiate oceanic spreading before the opening of the Ligurian Basin stopped. This is in good agreement with recent kinematic modelling performed in the second phase of the SPP2017 4D-MB. The modelling results of p01 indicate that continental crust can be stretched over several million years when the opening rate is low, i.e. <2 mm/year, and syn-rift sedimentation rate is high. Subduction initiation could occur in ultra-thinned continental crust as basin inversion has been observed at the northern Ligurian margin as a result of the African-European convergence. Additionally, the observations from the Ligurian Basin might be transferred to the evolution of the Piemont-Liguro Ocean. So far oceanic crust was assumed as initial conditions for the subduction of the Piemont-Liguro Ocean. An ultra-thin continental crust as initial condition would explain the observed thin subducted Piemont-Liguro plate which seemed to be thinner than 6-7 km oceanic crust. Further, a dry continental crust could explain why no back-arc volcanism was observed. The along-basin profile p02 shows a deepening crust-mantle boundary from 11 to 13 km. Based on the retrieved velocity model, gravity modelling and further results from surrounding studies we conclude that the continental crust is thinning from the northeast to the southwest which is related to the increase of extension away from the rotation pole of the anticlockwise rotation of the Corsica-Sardinia block. It remains unclear if at the southern end of the profile the mantle is overlain directly by sediments or by extremely thinned continental crust of up to 2.5 km thickness. The results however document, that seafloor spreading and the formation of mantle-derived oceanic crust was not initiated during the extension of the Ligurian Basin