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1. Slab interactions in 3D subduction settings: The Philippine Sea Plate region.

   https://doi.org/doi:10.1016/j.epsl.2018.02.024  

   Holt, Adam F. ( April 2018 , Earth and planetary science letters)

   The importance of slab–slab interactions is manifested in the kinematics and geometry of the Philippine Sea Plate and western Pacific subduction zones, and such interactions offer a dynamic basis for the first-order observations in this complex subduction setting. The westward subduction of the Pacific Sea Plate changes, along-strike, from single slab subduction beneath Japan, to a double-subduction setting where Pacific subduction beneath the Philippine Sea Plate occurs in tandem with westward subduction of the Philippine Sea Plate beneath Eurasia. Our 3-D numerical models show that there are fundamental differences between single slab systems and double slab systems where both subduction systems have the same vergence. We find that the observed kinematics and slab geometry of the Pacific–Philippine subduction can be understood by considering an along-strike transition from single to double subduction, and is largely independent from the detailed geometry of the Philippine Sea Plate. Important first order features include the relatively shallow slab dip, retreating/stationary trenches, and rapid subduction for single slab systems (Pacific Plate subducting under Japan), and front slabs within a double slab system (Philippine Sea Plate subducting at Ryukyu). In contrast, steep to overturned slab dips, advancing trench motion, and slower subduction occurs for rear slabs in a double slab setting (Pacific subducting at the Izu–Bonin–Mariana). This happens because of a relative build-up of pressure in the asthenosphere beneath the Philippine Sea Plate, where the asthenosphere is constrained between the converging Ryukyu and Izu–Bonin–Mariana slabs. When weak back-arc regions are included, slab–slab convergence rates slow and the middle (Philippine) plate extends, which leads to reduced pressure build up and reduced slab–slab coupling. Models without back-arcs, or with back-arc viscosities that are reduced by a factor of five, produce kinematics compatible with present-day observations. 

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2. Earthquake Focal Mechanisms and Stress Field for the Intermediate‐Depth Cauca Cluster, Colombia

   https://doi.org/10.1029/2018JB016804  

   Chang, Y. ; Warren, L. M. ; Zhu, L. ; Prieto, G. A. ( January 2019 , Journal of Geophysical Research: Solid Earth)

   Beneath southwestern Colombia, intermediate‐depth earthquakes in the Cauca cluster locate in the subducting Nazca plate and in two columns extending ~40‐km into the mantle wedge above the slab. To investigate the cluster, we determine focal mechanisms for 69 small‐to‐moderate‐sized (2.3 ≤ Ml ≤ 4.7) earthquakes in the cluster by fitting short‐period body wave waveforms using the cut‐and‐paste method. The focal mechanisms have various faulting types and variably oriented nodal planes. We invert the focal mechanisms of the intraslab earthquakes for the intraslab stress field but cannot fit the region with a homogeneous stress tensor. We find that the principal stress axes rotate with the slab geometry, which has a concave shape and increases in dip angle from north to south. The northern region has slab normal compression and similar‐magnitude maximum and intermediate principal stresses. The minimum stress axis is oriented ~41° counterclockwise from the downdip direction. In the steeper southern region, the intermediate stress axis orients in the downdip direction. Deviation from a typical downdip extensional stress field may result from a buoyant young slab, an eastward mantle flow push, and/or along‐strike compression from the concave shape of the slab. This stress field would allow slip along preexisting faults of various orientations, such as the trench‐perpendicular seafloor features presently observed offshore, and contribute to the apparent heterogeneity of the intraslab stress field. The mantle wedge earthquakes also have various focal mechanisms but tend to have a subvertical nodal plane that aligns with the earthquake locations.

    

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3. Macrofracturing of Oceanic Lithosphere in Complex Large Earthquake Sequences

   https://doi.org/10.1029/2020JB020137  

   Lay, Thorne ; Ye, Lingling ; Wu, Zhenbo ; Kanamori, Hiroo ( October 2020 , Journal of Geophysical Research: Solid Earth)

   Major earthquakes in oceanic lithosphere seaward of the subduction zone outer trench slope are relatively uncommon, but several recent occurrences have involved very complex sequences rupturing multiple nonaligned faults and/or having high aftershock productivity with diffuse distribution. This includes the 21 December 2010M_W7.4 Ogasawara (Bonin), 11 April 2012M_W8.6 Indo‐Australia, 23 January 2018M_W7.9 Off‐Kodiak Island, and 20 December 2018M_W7.3 Nikol'skoye (northwest Pacific) earthquakes. Major oceanic intraplate event sequences farther from plate boundaries do not tend to be as complex in faulting or aftershocks. Outer trench slope extensional faulting can involve complex distributed sequences, particularly when activated by great megathrust ruptures such as 11 March 2011M_W9.1 Tohoku and 15 November 2006M_W8.3 Kuril Islands. Intense faulting sequences also occur near subduction zone corners, with many fault geometries being activated, including some in nearby oceanic lithosphere, as for the 29 September 2009M_W8.1 Samoa, 6 February 2013M_W8.0 Santa Cruz Islands, and 16 November 2000M_W8.0 New Ireland earthquakes. The laterally varying plate boundary stresses from heterogeneous locking, recent earthquakes, or boundary geometry influence the specific faulting geometries activated in nearby major intraplate ruptures in oceanic lithosphere. Preexisting lithospheric structures and fabrics exert secondary influences on the faulting. Intraplate stress release in oceanic lithosphere near subduction zones favors distributed macrofracturing of near‐critical fault systems rather than localized, single‐fault failures, both under transient loading induced by plate boundary ruptures and under slow loading by tectonic motions and slab pull.

    

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4. Cenozoic slip along the southern Sierra Nevada normal fault, California (USA): A long-lived stable western boundary of the Basin and Range

   https://doi.org/10.1130/GES02574.1  

   Lee, Jeffrey ; Stockli, Daniel F. ; Blythe, Ann E. ( April 2023 , Geosphere)

   The uplift history of the Sierra Nevada, California, is a topic of long-standing disagreement with much of it centered on the timing and nature of slip along the range-bounding normal fault along the east flank of the southern Sierra Nevada. The history of normal fault slip is important for characterizing the uplift history of the Sierra Nevada, as well as for characterizing the geologic and geodynamic factors that drove, and continue to drive, normal faulting. To address these issues, we completed new structural studies and extensive apatite (U-Th)/He (AHe) thermochronometry on samples collected from three vertical transects in the footwall to the east-dipping southern Sierra Nevada normal fault (SNNF). Our structural studies on bedrock fault planes show that the SNNF is a steeply (~70°) east-dipping normal fault. The new AHe data reveal two elevation-invariant AHe age arrays, indicative of two distinct periods of cooling and exhumation, which we interpret as initiation of normal faulting along the SNNF at ca. 28–27 Ma with a second phase of normal faulting at ca. 17–13 Ma. We argue that beginning in the late Oligocene, the SNNF marked the now long-standing stable western limit, or break-away zone, of the Basin and Range. Slip along SNNF, and the associated unloading of the footwall, likely resulted in two periods of uplift of Sierra Nevada during the late Cenozoic. Trench retreat, driven by westward motion of the North American plate, along the Farallon–North American subduction zone boundary, as well as the gravitationally unstable northern and southern Basin and Range pushing on the cold Sierra Nevada, likely drove the late Oligocene- aged normal slip along the SNNF and the similar-aged but generally local and minor extension within the Basin and Range. We posit that the thick proto–Basin and Range lithosphere was primed for late Oligocene extension by replacement of the steepening Farallon slab with hot and buoyant asthenosphere. While steepening of the Farallon slab had not yet reached the southern Sierra Nevada by late Oligocene time, we speculate that late Oligocene slip along the SNNF reactivated a late Cretaceous dextral shear zone as the Sierra Nevada block was pulled and pushed westward in response to trench retreat and gravitational potential energy. The dominant middle Miocene normal fault-slip history along the SNNF is contemporaneous with high-magnitude slip recorded along range-bounding normal faults across the Basin and Range, including the east-adjacent Inyo and White mountains, indicating that this period of extension was a major regional tectonic event. We infer that a combination of slab-driven trench retreat along the Juan de Fuca–North America subduction zone boundary and clockwise rotation of the southern ancestral Cascade Range superimposed on continental lithosphere pre-conditioned for extension drove this episode of middle Miocene normal slip along the SNNF and extension to the east across the Basin and Range. Transtensional plate motion along the Pacific–North America plate boundary, and likely a growing slab window, continued to drive extension along the SNNF and the western Basin and Range, but not until ca. 11 Ma when the Mendocino triple junction reached the latitude of our northernmost (U-Th)/He transect. 

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5. Expedition 371 Scientific Prospectus: Tasman Frontier Subduction Initiation and Paleogene Climate

   https://doi.org/10.14379/iodp.sp.371.2016  

   Sutherland, R. ; Dickens, G.R. ; Blum, P. ( December 2016 , Scientific prospectus)

   null (Ed.)

   During International Ocean Discovery Program Expedition 371, we will core and log Paleogene and Neogene sediment sequences within the Tasman Sea. The cores will be analyzed for their sediment composition, microfossil components, mineral and water chemistry, and physical properties. The research will improve our understanding of how convergent plate boundaries form, how greenhouse climate systems work, and how and why global climate has evolved over the last 60 my. The most profound subduction initiation event and global plate-motion change since 80 Ma appears to have occurred in the early Eocene, when Tonga-Kermadec and Izu-Bonin-Mariana subduction initiation corresponded with a change in direction of the Pacific plate (Emperor-Hawaii bend) at ~50 Ma. The primary goal of Expedition 371 is to precisely date and quantify deformation and uplift/subsidence associated with Tonga-Kermadec subduction initiation in order to test predictions of alternate geodynamic models. This tectonic change may coincide with the pinnacle of Cenozoic “greenhouse” climate. However, paleoclimate proxy data from lower Eocene strata in the southwest Pacific show especially warm conditions, presenting a significant discrepancy with climate model simulations. The second goal is to determine if paleogeographic changes caused by subduction initiation may have led to anomalous regional warmth by altering ocean circulation. Late Neogene sediment cores will complement earlier drilling to investigate the third goal: tropical and polar climatic teleconnections. During Expedition 371, we will drill in a significant midlatitude transition zone influenced by both the Antarctic Circumpolar Current and the Eastern Australian Current. The accumulation of relatively thick carbonate-rich Neogene bathyal strata make this a good location for generating detailed paleoceanographic records from the Miocene through the Pleistocene that can be linked to previous ocean drilling expeditions in the region (Deep Sea Drilling Project Legs 21, 29, and 90; Ocean Drilling Program Leg 189) and elsewhere in the Pacific Ocean. 

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