Seismotectonic Analysis of the 2019–2020 Puerto Rico Sequence: The Value of Absolute Earthquake Relocations in Improved Interpretations of Active Tectonics

We present a new catalog of calibrated earthquake relocations from the 2019–2020 Puerto Rico earthquake sequence related to the 7 January 2020 Mw 6.4 earthquake that occurred offshore of southwest Puerto Rico at a depth of 15.9 km. Utilizing these relocated earthquakes and associated moment tensor solutions, we can delineate several distinct fault systems that were activated during the sequence and show that the Mw 6.4 mainshock may have resulted from positive changes in Coulomb stress from earlier events. Seismicity and mechanisms define (1) a west–southwest (∼260°) zone of seismicity comprised of largely sinistral strike-slip and oblique-slip earthquakes that mostly occurs later in the sequence and to the west of the mainshock, (2) an area of extensional faulting that includes the mainshock and occurs largely within the mainshock’s rupture area, and (3) an north–northeast (∼30°)-striking zone of seismicity, consisting primarily of dextral strike-slip events that occurs before and following the mainshock and generally above (shallower than) the normal-faulting events. These linear features intersect within the Mw 6.4 mainshock’s fault plane in southwest Puerto Rico. In addition, we show that earthquake relocations for M 4+ normal-faulting events, when traced along their fault planes, daylight along east–west-trending bathymetric features offshore of southwest Puerto Rico. Correlation of these normal-faulting events with bathymetric features suggests an active fault system that may be a contributor to previously uncharacterized seismic hazards in southwest Puerto Rico.

Regional Seismic Attribute Analysis and Tectono-Stratigraphy of Offshore South-Western Taranaki Basin, New Zealand

<p>This study investigates the nature, origin, and distribution of Cretaceous to Recent sediment fill in the offshore Taranaki Basin, western New Zealand. Seismic attributes and horizon interpretations on 30,000 km of 2D seismic reflection profiles and three 3D seismic surveys (3,000 km²) are used to image depositional systems and reconstruct paleogeography in detail and regionally, across a total area of ~100,000 km² from the basin's present-day inner shelf to deep water. These data are used to infer the influence of crustal tectonics and mantle dynamics on the development of depocentres and depositional pathways. During the Cretaceous to Eocene period the basin evolved from two separate rifts into a single broad passive margin. Extensional faulting ceased before 85 Ma in the present-day deep-water area of the southern New Caledonia Trough, but stretching of the lithosphere was higher (β=1.5-2) than in the proximal basin (β<1.5), where faulting continued into the Paleocene (~60 Ma). The resulting differential thermal subsidence caused northward tilting of the basin and influenced the distribution of sedimentary facies in the proximal basin. Attribute maps delineate the distribution of the basin's main petroleum source and reservoir facies, from a ~20,000 km²-wide, Late Cretaceous coastal plain across the present-day deep-water area, to transgressive shoreline belts and coastal plains in the proximal basin. Rapid subsidence began in the Oligocene and the development of a foredeep wedge through flexural loading of the eastern boundary of Taranaki Basin is tracked through the Middle Miocene. Total shortening within the basin was minor (5-8%) and slip was mostly accommodated on the basin-bounding Taranaki Fault Zone, which detached the basin from much greater Miocene plate boundary deformation further east. The imaging of turbidite facies and channels associated with the rapidly outbuilding shelf margin wedge illustrates the development of large axial drainage systems that transported sediment over hundreds of kilometres from the shelf to the deep-water basin since the Middle Miocene. Since the latest Miocene, south-eastern Taranaki Basin evolved from a compressional foreland to an extensional (proto-back-arc) basin. This structural evolution is characterised by: 1) cessation of intra-basinal thrusting by 7-5 Ma, 2) up to 700 m of rapid (>1000 m/my) tectonic subsidence in 100-200 km-wide, sub-circular depocentres between 6-4 Ma (without significant upper-crustal faulting), and 3) extensional faulting since 3.5-3 Ma. The rapid subsidence in the east caused the drastic modification of shelf margin geometry and sediment dispersal directions. Time and space scales of this subsidence point to lithospheric or asthenospheric mantle modification, which may be a characteristic process during back-arc basin development. Unusual downward vertical crustal movements of >1 km, as inferred from seismic facies, paleobathymetry and tectonic subsidence analysis, have created the present-day Deepwater Taranaki Basin physiography, but are not adequately explained by simple rift models. It is proposed that the distal basin, and perhaps even the more proximal Taranaki Paleogene passive margin, were substantially modified by mantle processes related to the initiation of subduction on the fledgling Australia-Pacific plate boundary north of New Zealand in the Eocene.</p>

Regional Seismic Attribute Analysis and Tectono-Stratigraphy of Offshore South-Western Taranaki Basin, New Zealand

<p>This study investigates the nature, origin, and distribution of Cretaceous to Recent sediment fill in the offshore Taranaki Basin, western New Zealand. Seismic attributes and horizon interpretations on 30,000 km of 2D seismic reflection profiles and three 3D seismic surveys (3,000 km²) are used to image depositional systems and reconstruct paleogeography in detail and regionally, across a total area of ~100,000 km² from the basin's present-day inner shelf to deep water. These data are used to infer the influence of crustal tectonics and mantle dynamics on the development of depocentres and depositional pathways. During the Cretaceous to Eocene period the basin evolved from two separate rifts into a single broad passive margin. Extensional faulting ceased before 85 Ma in the present-day deep-water area of the southern New Caledonia Trough, but stretching of the lithosphere was higher (β=1.5-2) than in the proximal basin (β<1.5), where faulting continued into the Paleocene (~60 Ma). The resulting differential thermal subsidence caused northward tilting of the basin and influenced the distribution of sedimentary facies in the proximal basin. Attribute maps delineate the distribution of the basin's main petroleum source and reservoir facies, from a ~20,000 km²-wide, Late Cretaceous coastal plain across the present-day deep-water area, to transgressive shoreline belts and coastal plains in the proximal basin. Rapid subsidence began in the Oligocene and the development of a foredeep wedge through flexural loading of the eastern boundary of Taranaki Basin is tracked through the Middle Miocene. Total shortening within the basin was minor (5-8%) and slip was mostly accommodated on the basin-bounding Taranaki Fault Zone, which detached the basin from much greater Miocene plate boundary deformation further east. The imaging of turbidite facies and channels associated with the rapidly outbuilding shelf margin wedge illustrates the development of large axial drainage systems that transported sediment over hundreds of kilometres from the shelf to the deep-water basin since the Middle Miocene. Since the latest Miocene, south-eastern Taranaki Basin evolved from a compressional foreland to an extensional (proto-back-arc) basin. This structural evolution is characterised by: 1) cessation of intra-basinal thrusting by 7-5 Ma, 2) up to 700 m of rapid (>1000 m/my) tectonic subsidence in 100-200 km-wide, sub-circular depocentres between 6-4 Ma (without significant upper-crustal faulting), and 3) extensional faulting since 3.5-3 Ma. The rapid subsidence in the east caused the drastic modification of shelf margin geometry and sediment dispersal directions. Time and space scales of this subsidence point to lithospheric or asthenospheric mantle modification, which may be a characteristic process during back-arc basin development. Unusual downward vertical crustal movements of >1 km, as inferred from seismic facies, paleobathymetry and tectonic subsidence analysis, have created the present-day Deepwater Taranaki Basin physiography, but are not adequately explained by simple rift models. It is proposed that the distal basin, and perhaps even the more proximal Taranaki Paleogene passive margin, were substantially modified by mantle processes related to the initiation of subduction on the fledgling Australia-Pacific plate boundary north of New Zealand in the Eocene.</p>

Review of Recent Drilling Projects in Unconventional Geothermal Resources at Campi Flegrei Caldera, Cornubian Batholith and Williston Sedimentary Basin

Unconventional geothermal resource development can contribute to increase power generation from renewable energy sources in countries without conventional hydrothermal reservoirs, which are usually associated with magmatic activity and extensional faulting, as well as to expand the generation in those regions where conventional resources are already used. Three recent drilling experiences focused on the characterization of unconventional resources are described and compared: the Campi Flegrei Deep Drilling Project (CFDDP) in Italy, the United Downs Deep Geothermal Power (UDDGP) project in the United Kingdom, and the DEEP Earth Energy Production in Canada. The main aspects of each project are described (geology, drilling, data collection, communication strategies) and compared to discuss challenges encountered at the tree sites considered, including a scientific drilling project (CFDDP) and two industrial ones (UDDGP and DEEP). The first project, at the first stage of pilot hole, although not reaching deep supercritical targets, showed extremely high, very rare thermal gradients even at shallow depths. Although each project has its own history, as well as social and economic context, the lessons learned at each drilling site can be used to further facilitate geothermal energy development.

Middle Jurassic arc reversal, Victoria–Katha Block and Sibumasu Terrane collision, jadeite formation and Western Tin Belt generation, Myanmar

Myanmar is occupied by the N-wards continuation of the Sunda arc and by the Shan Plateau and its continuation through Yunnan into Tibet. Our new tectonic interpretation of the ophiolite–flysch belts, world-famous jadeite and tin deposits in Myanmar west of the Salween adopts previous proposals that, before 450-km post-early Oligocene dextral displacement along the Sagaing Fault, the ophiolite belt in NE Myanmar continued through the topography that is now located west of the fault in the Indo-Burman Ranges. Differences in cross-section through Mogok and the Shan Scarps are reconciled by the recently proposed emplacement, in our view during Permian time, of the Mogok Metamorphic Group onto the Slate Belt to form Sibumasu. We argue that during Early Jurassic time a Neo-Tethys ophiolite nappe was obducted over turbidites on Sibumasu’s passive western margin. Following reversal in tectonic polarity, the remaining Neo-Tethys subducted E-wards generating the 113–128 Ma Mondaung Arc. During ocean closure the Victoria–Katha Block and its Triassic flysch subducted beneath Sibumasu, resulting in jadeite veins in overlying serpentinite that ascended in the subduction zone and were exhumed at Hpakant and Nat Hmaw, bordering the Jade Mines Uplift. Subduction of the Indian Ocean since Albian time generated the Popa–Loimye arc, while extensional faulting led to uplift of the Indo-Burman Ranges and to the formation of the Western Tin Belt granites. Tectonic effects in Myanmar of the India–Asia collision may be confined to the Disang thrust belt in the Naga Hills.

Post-magmatic fracturing, fluid flow, and vein mineralization in supra-subduction zones: a comparative study on vein calcites from the Troodos ophiolite and the Izu–Bonin forearc and rear arc

Based on the published data of pillow lava-hosted mineralized veins, this study compares post-magmatic fracturing, fluid flow, and secondary mineralization processes in the Troodos and Izu–Bonin supra-subduction zone (SSZ) and discusses the crucial factors for the development of distinct vein types. Thin section and cathodoluminescence petrography, Raman spectroscopy, fluid inclusion microthermometry, and trace element and isotope (87Sr/86Sr, δ18O, δ13C, Δ47) geochemistry indicate that most veins consist of calcite that precipitated from pristine to slightly modified seawater at temperatures < 50 °C. In response to the mode of fracturing, fluid supply, and mineral growth dynamics, calcites developed distinct blocky (precipitation into fluid-filled fractures), syntaxial (crack and sealing), and antitaxial (diffusion-fed displacive growth) vein microtextures with vein type-specific geochemical signatures. Blocky veins predominate in all study areas, whereas syntaxial veins represent subordinate structures. Antitaxial veins occur in all study areas but are particularly abundant in the Izu–Bonin rear arc where the local geological setting was conducive of antitaxial veining. The temporal framework of major calcite veining coincides with the onset of extensional faulting in the respective areas and points to a tectonic control on veining. Thus, major calcite veining in the Troodos SSZ began contemporaneously with volcanic activity and extensional faulting and completed within ~ 10–20 Myr. This enabled deep seawater downflow and hydrothermal fluid upflow. In the Izu–Bonin forearc, reliable ages of vein calcites point to vein formation > 15 Myr after subduction initiation. Therefore, high-T mineralization (calcite, quartz, analcime) up to 230 °C is restricted to the Troodos SSZ.

Ammonite occurrences in North Sea cores: implications for Jurassic Arctic–Mediterranean marine seaway connectivity

The paucity of ammonite recovery from North Sea wells has meant that offshore correlations are largely dependent upon microfossil assemblages. While rare, ammonites have been found in a few boreholes during the course of oil exploration activities. The occurrence of ammonites in ten wells in the UK sector of the Viking Graben and the Moray Firth rift arms provides a new basis by which to demonstrate that there was a distinct separation between Arctic and sub-Mediterranean species that lasted from Bajocian to Early Callovian times. Five wells contain ‘Boreal Bathonian' ammonites from the Arctic Realm. Arctocephalites from the Boreal Arcticus Zone (uppermost Bajocian) correlates basinal partly anoxic mudstones in the Beryl Embayment (9/13b) with both bioturbated siltstones in the southern Viking Graben (9/10b), and calcareous mudstones in the East Shetland Basin (211/21). Upper Bajocian Pompeckji Zone Cranocephalites and younger Arcticoceras from Lower to Middle Bathonian Greenlandicus, Ishmae and Cranocephaloide zones are confined to 211/21 demonstrating that the marine transgression began earlier and lasted longer. A Cadoceras from well 3/3-8 dates to the Lower Callovian Koenigi and Calloviense zones during which renewed extensional faulting re-established ammonite migration routes between the Boreal and sub-Mediterranean realms. A Middle Oxfordian (Densiplicatum Zone) Perisphinctes from well 22/5b-8 confirms an episode of northward migration from the sub-Mediterranean into the Boreal Realm. Upper Oxfordian (Regulare to Rosenkantzi zones) Amoeboceras in wells 211/21-1 and 9/13b-19 are close to Upper Bajocian/Lower Bathonian faunas, suggesting an absence of Upper Bathonian to Middle Oxfordian strata as a result of rift-related footwall uplift and erosion. In four wells from Block 15/21 (-4, -11, -12A and -25) Lower Kimmeridgian ammonites have been documented, including Rasenia, Amoebites, Aulacostephanoides and Zenostephanoides, from the Baylei (?), Cymodoce, Mutabilis and Eudoxus zones, the latter (confirmed at well 13/28b-8) dating a widespread regional marine flooding surface in the Inner Moray Firth.Supplementary material: The detailed measurements of dimensions of the ammonites described are available at: https://doi.org/10.6084/m9.figshare.c.5087313

Active faulting in western Turkey: the challenge of earthquake hazard estimation in a complex extensional regime based on cosmogenic isotope analyses

&lt;p&gt;In zones of distributed continental faulting, it is critical to understand how slip is partitioned onto brittle structures over both long-term millennial time scales and shorter-term earthquake cycles. Measuring earthquake slip histories on different timescales is challenging due to earthquake repeat-times being longer or similar to historical earthquake records, and a paucity of data on fault activity covering millennial to Quaternary scales in detail. Cosmogenic isotope analyses from bedrock fault scarps have the potential to bridge the gap, as these datasets track the exposure of fault planes due to earthquakes with millennial resolution. In this presentation, we show new &lt;sup&gt;36&lt;/sup&gt;Cl data combined with active fault maps to document the spatial and temporal complexity of extensional faulting in western Turkey.&lt;/p&gt;&lt;p&gt;Extensional faulting covers an area ~460 x 460 km in western Turkey. The dynamics controlling extension are debated, but there is an overall pattern of anticlockwise rotation superimposed on N-S directed upper-plate extension related to eastern Mediterranean subduction. This has resulted in several major east-west trending extensional grabens along the western coast of Turkey and NE-SW to NW-SE trending conjugate grabens towards the southern coast and central Anatolia. The active fault map of Turkey is well characterised by the MTA (General Directorate of Mineral Research and Exploration), but recent mid-magnitude earthquakes have occurred on some un-characterised fault zones, suggesting that there is further complexity in the trace and locations of active faults. This complexity may indicate recent reactivation of pre-existing structures.&lt;/p&gt;&lt;p&gt;Most of the major bedrock normal fault scarps are well preserved in carbonate and marble successions distributed across the region. These scarps preserve an excellent record of Late Pleistocene to Holocene earthquake activity, which can be quantified using cosmogenic isotopes that track the exposure of the bedrock fault scarps. &lt;sup&gt;36&lt;/sup&gt;Cl accumulates in the fault scarps as the footwall is progressively exhumed by earthquakes and the concentration of &lt;sup&gt;36&lt;/sup&gt;Cl measured up the fault plane reflects the rate and patterns of slip. In this presentation, we utilise Bayesian modelling techniques to estimate slip histories based on new cosmogenic data from several faults across western Turkey. Each sampling site is carefully characterised using field mapping and LiDAR to ensure that fault plane exposure is due to slip during earthquakes and not sediment transport processes. We will compare several neighbouring fault zones with variable slip rates to investigate how they interact over multiple earthquake cycles, and put this temporal complexity into the context of spatial complexity, and the resultant challenges for hazard forecasts in western Turkey.&lt;/p&gt;

Multiphase tectonic interaction of Tyrrhenian - Tunisia Margin - Ionian systems: Implications for regional seismogenesis

&lt;p&gt;The region at the transition from the west to the east Mediterranean is a complex puzzle of terrains spanning in age from the Mesozoic Ionian lithosphere to the Pleistocene arc and back arc domains of the Tyrrhenian system. Although the region has had a complicated evolutionary history, the current configuration of terrains fundamentally denotes Miocene to recent kinematics.&lt;/p&gt;&lt;p&gt;In this contribution we present new data from Tunisia Margin showing the evolution from its formation in early Miocene to recent, the tectonic interaction with the opening of the Tyrrhenian system and its current inversion, and discuss the implications for the regional kinematics evolution. &amp;#160;&lt;/p&gt;&lt;p&gt;The Tyrrhenian is no longer extending, but all basin borders indicate currently active large-scale thrusting &amp;#160;to strike slip tectonics. Tunisia margins formed by a well-know contractional tectonic phase in early Miocene expressed in large-scale tectonics with a clearly imaged thrust &amp;#160;and fold belt, cut by Messinian to Pliocene extensional faulting. However, high resolution multibeam bathymetry and images of the shallowest layers indicates ongoing inversion tectonics.&lt;/p&gt;&lt;p&gt;We compare the tectonic evolution of north Tunisia and Tyrrhenian with the patterns of deformation of the Ionian tectonic wedge observed in new and reprocessed seismic images. We interpret the current deformation of the Ionian tectonic wedge based on the integration of evolution of the kinematics from the data sets of observations from the three systems.&lt;/p&gt;&lt;p&gt;We conclude that the entire region is currently under collision of the Africa Plate with the Adria Plate and the Neogene terrains of the Tyrrhenian Domain. &amp;#160;The corollary is the subduction of the Ionian lithosphere is fundamentally stalled so that the megathrust fault is possibly not any longer accumulating significant shortening and most deformation is currently occurring in steeper faults re-activation or cutting the previous structural framework.&lt;/p&gt;