Sequential Fragmentation / Transport Theory, Pyroclast Size-Density Relationships, and the Emplacement Dynamics of Pyroclastic Density Currents – A Case Study on the Mt. St. Helens (USA) 1980 Eruption

Pyroclastic density currents (PDCs) are the most dangerous hazard associated with explosive volcanic eruptions. Despite recent advancements in the general understanding of PDC dynamics, limited direct observation and/or outcrop scarcity often hinder the interpretation of specific transport and depositional processes at many volcanoes. This study explores the potential of sequential fragmentation / transport theory (SFT; cf. Wohletz et al. 1989), a modeling method capable of predicting particle mass distributions based on the physical principles of fragmentation and transport, to retrieve the transport and depositional dynamics of well-characterized PDCs from the size and density distributions of individual components within the deposits. The extensive vertical and lateral exposures through the May 18th, 1980 PDC deposits at Mt. St. Helens (MSH) provide constraints on PDC regimes and flow boundary conditions at specific locations across the depositional area. Application to MSH deposits suggests that SFT parameter distributions can be effectively used to characterize flow boundary conditions and emplacement processes for a variety of PDC lithofacies and deposit locations. Results demonstrate that (1) the SFT approach reflects particle fragmentation and transport mechanisms regardless of variations in initial component distributions, consistent with results from previous studies; (2) SFT analysis reveals changes in particle characteristics that are not directly observable in grain size and fabric data; (3) SFT parameters are more sensitive to regional transport conditions than local (outcrop-scale) depositional processes. The particle processing trends produced using SFT analysis are consistent with the degree of particle processing inferred from lithofacies architectures: for all lithofacies examined in this study, suspension sedimentation products exhibit much better processing than concentrated current deposits. Integrated field observations and SFT results provide evidence for increasing density segregation within the depositional region of the currents away from source, as well as for comparable density-segregation processes acting on lithic concentrations and pumice lenses within the current. These findings further define and reinforce the capability of SFT analysis to complement more conventional PDC study methods, significantly expanding the information gained regarding flow dynamics. Finally, this case study demonstrates that the SFT methodology has the potential to constrain regional flow conditions at volcanoes where outcrop exposures are limited

Dynamics of Pyroclastic Density Currents: Conditions That Promote Substrate Erosion and Self-Channelization - Mount St Helens, Washington (USA)

The May 18th, 1980 eruption of Mount St. Helens (MSH) produced multiple pyroclastic density currents (PDCs), burying the area north of the volcano under 10s of meters of deposits. Detailed measurements of recently exposed strata from these PDCs provide substantial insight into the dynamics of concentrated currents including inferences on particle-particle interactions, current mobility due to sedimentation fluidization and internal pore pressure, particle support mechanisms, the influence of surface roughness and the conditions that promote substrate erosion and self-channelization. Four primary flow units are identified along the extensive drainage system north of the volcano. Each flow unit has intricate vertical and lateral facies changes and complex cross-cutting relationships away from source. Each flow unit is an accumulation from an unsteady but locally sustained PDC or an amalgamation of several PDC pulses. The PDCs associated with Units I and II likely occurred during the pre-climactic, waxing phase of the eruption. These currents flowed around and filled in the hummocky topography, leaving the massive to diffusely-stratified deposits of Units I and II. The deposits of both Units I and II are generally more massive in low lying areas and more stratified in areas of high surface roughness, suggesting that surface roughness enhanced basal shear stress within the flow boundary. Units III and IV are associated with the climactic phase of the eruption, which produced the most voluminous and wide-spread PDCs. Both flow units are characteristically massive and enriched in vent-derived lithic blocks. These currents flowed over and around the debris avalanche deposits, as evidenced by the erosion of blocks from the hummocks. Unit III is massive, poorly sorted, and shows little to no evidence of elutriation or segregation of lithics and pumice, suggesting a highly concentrated current where size-density segregation was suppressed. Unit IV shows similar depositional features but typically has a basal lithic-rich region, is variably fines-depleted and contains pumice lobes, suggesting density segregation in a less concentrated current relative to Unit III. Deep, erosive channels cut by the Unit III current and thick lithic levee deposits within Unit IV occur in an area where debris avalanche relief is limited, suggesting self-channelization developed as a function of internal flow dynamics. An increase in the proportion and size of lithic blocks is found (1) downstream of debris avalanche hummocks, suggesting the PDCs were energetic enough to locally entrain accidental lithics from the hummocks and transport them tens of meters downstream, and (2) within large channels cut by later PDCs into earlier PDC deposits, suggesting self-channelization of the flows increased the carrying capacity of the subsequent channelized currents. Finally, the combination of thick, massive deposits with a high percentage of fine ash within Unit III and in the medial-distal depositional regions of Units II-IV suggests the PDCs developed and maintained a high internal pore pressure during transport and deposition. The most important include our ability to understand the role of internal pore pressure on current mobility, the influence of self-channelization on carrying capacity of the currents and the influence of surface roughness on substrate erosion. These observations have critical consequences for understanding the flow dynamics and hazard potential of PDCs

Andean deformation and basin evolution during changes in subduction zone geometry (29–33°S)

Understanding the surface, crustal, and tectonic processes controlling deformation and crustal evolution along subduction margins remains an outstanding challenge in Earth science, with implications for characterizing Earth systems, resource potential, and natural hazard assessment. The southern Central Andes of Chile and Argentina define type examples of deformation, arc magmatism, and basin evolution during long-lived subduction, and provide unique opportunities to study the overriding plate response to changing plate margin conditions and tectonic regimes. This dissertation presents new geo- and thermochronological assessments of sediment provenance, basin accumulation, and deformational timing at ~29–33°S that provide critical insight into retroarc deformational patterns and subsidence mechanisms over the past ~200 Myr. Mesozoic to mid-Cenozoic deposits recorded the unroofing of basin margins and sediment contributions from the Andean magmatic arc during Late Triassic to Early Cretaceous extension, thermal subsidence, and possible slab rollback, followed by initial shortening and sediment derivation from western sources during the Late Cretaceous. Newly dated volcanic and sedimentary deposits in the retroarc hinterland provide evidence for a late Paleogene episode of intra-arc and proximal retroarc extension preceding Negoene growth of the modern Andes. Geochronological and thermochronological data for thrust sheets and Neogene foreland basin fill indicate Andean shortening initiated with the inversion of the intra-arc basin system, followed by sequential eastward migration of the fold-thrust belt–foreland basin system along a regionally connected, hybrid thin- and thick-skinned decollement. Finally, new structural interpretations and flexural thermo-kinematic model results quantify the influence of inherited structures, precursor stratigraphic architectures, thrust belt critical taper, and non-flexural geodynamic processes during Neogene construction of the modern AndesEarth and Planetary Science

Stress transmission along mid-crustal faults highlighted by the 2021 Mw 6.5 San Juan (Argentina) earthquake

International audienceAbstract Understanding the mechanisms of crustal deformation along convergent margins is critical to identifying seismogenic structures and assessing earthquake hazards for nearby urban centers. In the southern central Andes (28–33°S), differences in the style of middle to upper-crustal deformation and associated seismicity are highlighted by the January 19th, 2021 (Mw 6.5) San Juan earthquake. We integrate waveforms recorded at regional and teleseismic distances with co-seismic displacements calculated from local Global Navigation Satellite System time series, to re-estimate the source parameters of the 2021 San Juan earthquake, confirming a mid-crustal nucleation depth (21 ± 2 km) and right-lateral transpressional mechanism. Considered alongside decades of seismic observations and geological data, this event provides evidence for retroarc deformation partitioning among inherited basement faults and upper-crustal structures in response to oblique convergence of the Nazca and South American plates. As they may transfer shortening to active upper-crustal faults associated with historically devastating shallower earthquakes, a better understanding of seismogenic basement faults such as the mid-crustal structure activated during the 2021 San Juan earthquake could help future re-assessment of the seismic risk in western Argentina

Neogene retroarc foreland basin evolution, sediment provenance, and magmatism in response to flat slab subduction, western Argentina

Understanding the effects of flat slab subduction on mountain building, arc magmatism, and basin evolution is fundamental to convergent-margin tectonics, with implications for potential feedbacks among geodynamic, magmatic, and surface processes. New stratigraphic and geochronological constraints on Cenozoic sedimentation and magmatism in the southernCentral Andes of Argentina (31°S) reveal shifts in volcanism, foreland/hinterland basin development, sediment accumulation, and provenance as the retroarc region was structurally partitioned during slab flattening. Detrital zircon U-Pb age distributions from the western(Calingasta basin), central (Talacasto and Albarracín basins), and eastern (Bermejo foreland basin) segments of the retroarc basin system preserve syndepositional volcanism and orogenic unroofing of multiple tectonic provinces. Initial shortening-related exhumation of the Principal Cordillera at 24-17 Ma was recorded by the accumulation of distal eoliandeposits bearing Oligocene-Eocene zircons from the Andean magmatic arc. The Calingasta basin chronicled volcanism and basement shortening in the Frontal Cordillera at ~17-11 Ma, as marked by an upward coarsening succession of fluvial to alluvial-fan deposits with a sustained zircon U-Pb age component that matches pervasive Permian-Triassic bedrock in thehinterland. A ~450 km eastward inboard sweep of volcanism at 11 Ma coincided with the inception of flat slab subduction, and subsequent thin-skinned shortening in the Precordillera fold-thrust belt that exhumed wedge-top deposits and induced cratonward (eastward) advance of flexural subsidence into the Bermejo foreland basin. This foreland basin was structurally partitioned as basement uplifts of the Sierras Pampeanas transformed a fluvial megafan sediment routing network into smaller isolated alluvial-fan systems fed by adjacent basement blocks.Fil: Capaldi, Tomas N.. University of Texas at Austin; Estados UnidosFil: Horton, Brian K.. University of Texas at Austin; Estados UnidosFil: McKenzie, N. Ryan. University Of Hong Kong; ChinaFil: Mackaman Lofland, Chelsea. University of Texas at Austin; Estados UnidosFil: Stockli, Daniel F.. University of Texas at Austin; Estados UnidosFil: Ortiz, Gustavo Federico. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - San Juan. Centro de Investigaciones de la Geosfera y Biosfera. Universidad Nacional de San Juan. Facultad de Ciencias Exactas Físicas y Naturales. Centro de Investigaciones de la Geosfera y Biosfera; ArgentinaFil: Alvarado, Patricia Monica. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - San Juan. Centro de Investigaciones de la Geosfera y Biosfera. Universidad Nacional de San Juan. Facultad de Ciencias Exactas Físicas y Naturales. Centro de Investigaciones de la Geosfera y Biosfera; Argentin

Andean Mountain Building and Foreland Basin Evolution During Thin‐ and Thick‐Skinned Neogene Deformation (32–33°S)

The southern Central Andes recorded retroarc shortening, basin evolution, and magmatic arcmigration during Neogene changes in subduction. At 31?33°S, above the modern flat‐slab segment, spatialand temporal linkages between thin‐ and thick‐skinned foreland shortening, basement‐involvedexhumation of the main Cordillera, and lower‐crustal hinterland thickening remain poorly resolved. Weintegrate new geochronological and thermochronological data for thrust sheets and Neogene foreland basinfill with structural, sedimentological, and passive seismic results to reconstruct the exhumation historyand evaluate potential geometric linkages across structural domains. 40Ar/39Ar ages for volcanic horizonsand zircon U‐Pb ages for synorogenic clastic deposits in the Manantiales Basin constrain the minimumduration of synorogenic sedimentation to ~22?14 Ma. Detrital zircon age distributions record sequentialunroofing of hinterland thrust sheets until ~15 Ma, followed by eastward (cratonward) advance of thedeformation front, shutoff of western sediment sources, and a shift from fluvial to alluvial fan deposition at~14 Ma. Apatite (U‐Th)/He cooling ages confirm rapid exhumation of basement‐involved structural blocksand basin partitioning by ~14?5 Ma, consistent with the timing of the Manantiales facies and provenanceshifts and a coeval (~12?9 Ma) pulse of thin‐skinned shortening and exhumation previously identified in theeastern foreland. Late Miocene?Pliocene (~8?2 Ma) cooling ages along the Chile‐Argentina border point tohinterland uplift during the latest stage of Andean orogenesis. Finally, geophysical constraints on crustalarchitecture and low‐temperature thermochronometry results are compatible with a hybrid thin‐ andthick‐skinned décollement spanning retroarc domains.Fil: Mackaman Lofland, Chelsea. Jackson School Of Geosciences; Estados Unidos. Department Of Geological Sciences; Estados UnidosFil: Horton, Brian K.. Jackson School Of Geosciences; Estados Unidos. Institute For Geophysics; Estados UnidosFil: Fuentes, Facundo. YPF; ArgentinaFil: Constenius, Kurt N.. University of Arizona; Estados UnidosFil: Ketcham, Richard A.. Jackson School Of Geosciences; Estados UnidosFil: Capaldi, Tomas N.. Jackson School Of Geosciences; Estados Unidos. Department Of Geological Sciences; Estados UnidosFil: Stockli, Daniel F.. Jackon School Of Geosciences; Estados Unidos. Department Of Geological Sciences; Estados UnidosFil: Ammirati, Jean Baptiste. Universidad de Chile. Facultad de Ciencias Físicas y Matemáticas. Departamento de Geología; Chile. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - San Juan. Centro de Investigaciones de la Geosfera y Biosfera. Universidad Nacional de San Juan. Facultad de Ciencias Exactas Físicas y Naturales. Centro de Investigaciones de la Geosfera y Biosfera; ArgentinaFil: Alvarado, Patricia Monica. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - San Juan. Centro de Investigaciones de la Geosfera y Biosfera. Universidad Nacional de San Juan. Facultad de Ciencias Exactas Físicas y Naturales. Centro de Investigaciones de la Geosfera y Biosfera; Argentina. Universidad Nacional de San Juan. Facultad de Ciencias Exactas, Físicas y Naturales. Departamento de Geofísica y Astronomía; ArgentinaFil: Orozco Chirino, Paola Gabriela. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - San Juan. Centro de Investigaciones de la Geosfera y Biosfera. Universidad Nacional de San Juan. Facultad de Ciencias Exactas Físicas y Naturales. Centro de Investigaciones de la Geosfera y Biosfera; Argentin