Identification of and Favorable Conditions for a New Long-Offset Seismic Phase from Offshore–Onshore Seismic Surveys

Offshore–onshore seismic survey is one of the main methods to study crustal structures in offshore–onshore transitional zones. At present, the seismic waves commonly used in imaging are the crustal refraction (Pg), the crustal reflection from the Moho (PmP), and the upper-mantle refraction (Pn) waves. The propagation distances of Pg and PmP are commonly less than 210 km, and Pn propagates with an apparent velocity of ∼8 km/s. In 2015, two offshore–onshore wide-angle seismic lines with a length of ∼350 km were acquired in the Pearl River Estuary. In addition to Pg, PmP, and Pn, a new seismic phase was observed, which has a long propagation distance (offset of ∼170–290 km), low apparent velocity (∼5.85 km/s), and low frequency (∼4–7 Hz). Similar seismic phases have been widely reported in previous offshore–onshore and reservoir seismic surveys, but the understanding of these phases is still limited. Herein, we used both raytracing and waveform modeling methods to identify the new seismic phase as the secondary Pg phase, which reflects from the surface (named Pg2Pg). We also discuss favorable conditions for Pg2Pg, including (1) a thin sedimentary layer with low velocity at the surface in which the reflection of Pg occurs, which can reduce the incidence angles and hence increase the energy of the reflected waves; (2) a sedimentary basement dipping toward the sea at the positions of the air gun shots, which focuses seismic waves; (3) relatively smooth interfaces of the medium, which can reduce the scattering of Pg2Pg; and (4) air guns that can excite low-frequency signals, which can reduce the attenuation of seismic waves. Checkerboard tests and practical applications show that Pg2Pg can significantly improve upper-crustal resolution, especially for onshore areas. Our research promotes the data mining of offshore–onshore seismic surveys, which is important for obtaining more detailed crustal structures.

Evidence of salt rich Jurassic extension in the Tethyan-Pyrenean linkage zone. Pre-orogenic inheritance in the Corbières-Languedoc Transfer Zone (Corbières Arc, France)

A detailed field study of Jurassic tectono-stratigraphic architecture of the southwestern part of the Corbières-Languedoc Transfer Zone (CTZT) in the French Pyrenees demonstrates for the first time 3D variations in thickness and stratigraphic geometries on near-orthogonal syn-sedimentary structures linked to Jurassic extension with salt. Some of these structures were previously interpreted as compressional and Pyrenean in origin (Late Santonian-Eocene). Our study instead shows that these are extensional salt- related structures that were reactivated during Pyrenean compression and again during Oligo-Miocene extension. We propose that the structures evolved through a strong interaction between inherited crustal structures of the same orientation, and salt tectonics. Strong segmentation of the CLTZ supra-salt cover by oblique structures, is inherited from Jurassic and is linked to interaction between basement EW structures of the Pyrenean rift domain and NE-SW structures of the European Tethyan margin. We distinguish NE-SW trending stuctures (Cévenoles) as extensional forced folds and NE-SW trending salt ridges that developed above basement cutting faults NE-SW oriented. Salt ridges delimited the future NE-SW trending orogenic domains (retro-foreland, Frontal, Main Nappe). N110 trending Pyrenean structures are represented by the Treilles Fault, a major syn sedimentary fault that roots into Keuper evaporites. This study shows that the Corbières is a key area to better understand Pyreneo-provençal evolution along its whole Wilson cycle (rift to rift) and to better understand the processes that govern the formation of a salt-rich rift transfer zone in a strongly pre-structured crust, its positive inversion and the role of salt tectonics in different deformation phases.

Climatic control on the location of the Southern Andes volcanic arc

Volcanic arcs at convergent plate margins are primary surface expressions of plate tectonics. Although climate affects many of the manifestations of plate tectonics via erosion, the upwelling of magmas and location of volcanic arcs are considered insensitive to climate. In the Southern Andes, subduction of the Nazca oceanic plate below the South American continent generates the Southern Andes Volcanic zone. Orographic interactions with Pacific westerlies lead to high precipitation and erosion on the western slopes of the belt between 42-46°S. At these latitudes, the topographic water divide and the volcanic arc are respectively farther and closer to the subduction trench than at lower latitudes, despite a constant subduction dip angle along strike. Here, we use thermomechanical numerical modeling to investigate how magma upwelling is affected by topographic changes due to orography. We show that a leeward topographic shift may entail a windward asymmetry of crustal structures accommodating the magma upwelling, consistent with the observed trench-ward migration of the Southern Andes Volcanic Zone. A climatic control on the location of volcanic arcs via orography and erosion is thus revealed.

Tectonic Influence on Bed-Character Variability under Thwaites Glacier, West Antarctica

<p>Subglacial topography and bed character are important controls on glacier and ice-sheet flow. Previous studies using reflection-seismic data from the upper half of Thwaites Glacier, West Antarctica, have shown variations in the bed character in the along-flow direction with continuous soft bed in the flatter “lowland” areas and a mix of soft and hard bed over more elevated, rugged “highland” areas. Here we use long-offset reflection/refraction seismic and aerogravity data over a ~40-km section 230-km inland of the current grounding line to model the upper-crustal structures and relate them to the previously identified bed-character variability. We identified a sedimentary basin ~11-km long and up to ~400-m deep beneath the lowland area with continuous soft bed. The downstream end of this sedimentary basin aligns with the transition from the lowland to highland area which indicates its existence could be related to the formation of the subglacial topography. The sedimentary basin is a graben or half-graben potentially formed due to rifting associated with the development of the West Antarctic Rift System, suggesting tectonic influence on the bed character variability and, in turn, on the glacier flow. We will further analyze the seismic reflection data and also add aeromagnetic data to model the crustal structures more accurately and clarify the potential tectonic control on bed-character variability.</p>