Subducted Plate Research Articles

Extension of Aseismic Slip Propagation Theory to Slow Earthquake Migration

AbstractNatural faults host various types of migrating slow earthquake phenomena, with migration speeds much lower than seismic wave speeds and different moment‐duration scaling from regular earthquakes. To advance the obtained quantitative understanding of the migration process and long duration of slow earthquakes, I study a chain reaction model in a population of brittle asperities based on a rate‐ and state‐dependent friction on a 3‐D subduction plate boundary. Simulation results show that the migration speed is quantitatively related to frictional properties by an analytical relation derived here. By assuming that local pore water in front of the migration drives rapid tremor reversal and is so local as to hold a constant stress drop, the application of the analytical solution to observational results suggests that (a) the temporal changes of observed migration speeds for the rapid tremor reversal could be explained by about 70% reduction of the effective normal stress; (b) effective normal stress for the deeper extension of seismogenic segment in the western part of Shikoku is about 1.5 times greater than that in the central part. Applying rupture time delay between slow earthquake asperities for the duration longer than regular earthquake, I also conclude that (c) the characteristic slip distance of rate‐and‐state friction for low‐frequency earthquakes is roughly between 30 µm and 30 mm; (d) the stress and strength drops of very low‐frequency earthquakes is much smaller than 1 MPa.

Aftershock Regions of Aleutian‐Alaska Megathrust Earthquakes, 1938–2021

AbstractWith five earthquakes (1938, 1946, 1957, 1964, and 1965), the Aleutian‐Alaska subduction plate boundary ruptured a length of 3,548 km. We revisit these five earthquakes—first studied in detail by Sykes (1971, https://doi.org/10.1029/JB076i032p08021)—through probabilistic relocation of carefully selected mainshocks and aftershocks. Our final catalog of 324 events is established from a set of 12 mainshocks that includes all Mw ≥ 7.7 megathrust earthquakes. Using the relocated catalog, we create revised aftershock regions delimited both parallel and normal to the trench. These aftershock regions exhibit significant differences from previous studies, with the following basic findings: the 10 November 1938 Mw 8.3 earthquake extended further west, to the Shumagin Islands, and further east, into the Kodiak region, relative to the prevailing aftershock region established by McCann et al. (1979, https://doi.org/10.1007/978-3-0348-6430-5_2). The 01 April 1946 Mw 8.6 sequence was anomalously concentrated near the trench, which implies near‐trench coseismic slip that contributed to the exceptionally large tsunami. The 09 March 1957 Mw 8.6 aftershocks spanned a 1,230 km length with numerous aftershocks within the outer‐rise region of the incoming Pacific plate. The 28 March 1964 Mw 9.2 aftershocks extended east into the Pamplona thrust system (south of Icy Bay, Alaska), suggesting coseismic rupture into this region; this is consistent with coseismic static displacements, as well as current estimates of interseismic locking. The post‐1965 events we examine are all smaller than the earlier events, but since they occurred in an era of improved data collection (seismic, geodetic, and tsunami), they provide a better opportunity for assessing the link between the distribution of aftershocks and the occurrence of coseismic, postseismic, and interseismic slip.

Outer trench slope extension to frontal wedge compression in a subducting plate

The occurrence of faulting in subducting plates is a major process that changes the mechanical properties of the subducting lithosphere and carries surface materials into mantle wedges. Two ocean-bottom seismometer networks deployed on the frontal accretionary wedge of the northern Manila trench in 2005 and on the outer slope of the trench in 2006 were used to detect earthquakes in the subducting plate. All available P and S manually picked phases and the waveforms of 16 short-period, three-component stations were used. Relocation was performed using the double-difference method with differential times derived from the phase-picked data. Two intraplate earthquake sequences of small-to-moderate magnitudes in the northern Manila subduction system were investigated in this study. The results revealed distinct fault planes, but a contrasting seismogeny over the northern Manila Trench. The seismicity in the frontal wedge (as measured in 2005) was mainly contributed by a fluid overpressure sequence, whereas that in the incoming plate (as measured in 2006) was contributed by the aftershocks of an extensional faulting sequence. The obtained seismic velocity models and Vp/Vs ratios revealed that the overpressure was likely caused by high pore-fluid pressure within the shallow subduction zone. By using the near-field waveform inversion algorithm, we determined focal mechanism solutions for a few relatively large earthquakes. Through the use of data obtained from global seismic observations, we determined that stress transfer may be responsible for the seismic activity in the study area during the period of 2005–2006. In late 2005, the plate interface in the frontal wedge area was unlocked by the overpressure effect due to a thrusting-dominant sequence. This event changed the stress regime across the Manila Trench and triggered a normal fault extension at the outer trench slope in mid-2006. However, in the present study, a hybrid focal mechanism solution indicating reverse and strike–slip mechanisms was implemented, and it revealed that the plate interface locked again in late 2006.Graphical

Types and Eruption Patterns of the Carboniferous Volcanic Edifices in the Shixi Area, Junggar Basin

The types of volcanic edifices and volcanic eruption patterns control the accumulation and distribution of oil and gas. By means of drillings, seismic data, and geochemical analysis, the types and distribution of the Carboniferous volcanic edifices in the Shixi area of Junggar Basin were studied, the formation mechanism of magma was clarified, and the eruption patterns of volcanoes were investigated. The results show that the types of the Carboniferous volcanic edifices in the Shixi area mainly include stratovolcanic edifices, shield basic volcanic edifices, and mound superimposed volcanic edifices. Stratovolcanic edifices and mound superimposed volcanic edifices are developed around the Shixi fault, while single stratovolcanic edifices are developed around the Mobei fault. Shield basic volcanic edifices are only developed in the south of the Shixi area. Vertically, volcanic edifices are mainly developed on the hanging walls of the faults. When the faults develop, the volcanic vents are dendritic, forming mound superimposed volcanic edifices in space. On the plane, there are more volcanic edifices in the Shixi salient than in the Mobei salient. Intermediate-acid volcanic edifices are distributed in beaded shapes along one side of the Shixi fault and the Mobei fault, while basic volcanic edifices are distributed in sheet shapes on both sides of the Dinan fault. In the early Carboniferous, the Shixi area was in the island arc environment under the subduction of the oceanic crust. The basic magma came from the melting of the subduction plate, and the intermediate-acid magma came from the partial melting of the crust. In the late Carboniferous, the extensional environment formed by the retraction of the subduction plate led to the formation of large-scale volcanic eruptions. The extensional environment formed by the retreat of the subduction plate in the later stage resulted in large-scale volcanic eruptions.

Rapid Exhumation Processes of the Gaoligong Mountain Range in the Southeastern Margin of the Qinghai–Tibet Plateau Since the Late Cenozoic

Three continent-scale ductile shear zones trending N-S are distributed in the southeast margin of the Qinghai–Tibet Plateau. Their tectonic deformation and exhumation histories are of great significance to understanding the orogenic processes within the continent and the growth as well as expansion mechanism of the plateau. The Gaoligong shear zone (GLGSZ) is the westernmost zone near the boundary of the Indian subduction plate and is less well studied than the Ailaoshan–Red River shear zone (ASRRSZ) in the east. In this study, low-temperature thermochronological methods including apatite (U-Th)/He (AHe), zircon (U-Th)/He (ZHe), and apatite fission track (AFT) were used to date the vertical profile samples from the Gaoligong Mountain to understand the exhumation processes that occurred in the late Cenozoic. Our results show that the GLGSZ has experienced two stages of rapid exhumation events since the late Cenozoic: in the middle Miocene (∼14.5 Ma) and early Pleistocene (∼2.9 Ma). Based on our data, we divided the late Cenozoic tectonic deformation and exhumation processes in the Gaoligong Mountain area into two stages: 1) From the middle to the late Miocene, large-scale regional dextral strike-slip movements and lateral compressions controlled the ductile shear zone and continuously denuded the mountain surface; 2) in the Pleistocene, rapid river erosion and undercutting caused by fluctuations of the monsoon system, together with the continuous activity of brittle faults, drove the latest rapid exhumations. A comparison of the thermochronological data of the different areas along the Gaoligong Mountain shows that the exhumation rate in its northern transect is significantly higher and the time of the onset of exhumation is earlier than that in the southern transect. These results indicate that the deformation processes began in the north and continued southwards and controlled the geomorphological characteristics of the Gaoligong Mountain, whose elevation is higher in the northern part than in the southern part.

An 8‐Year‐Long Low‐Frequency Earthquake Catalog for Southern Cascadia

AbstractLow‐frequency earthquakes (LFEs) are small magnitude earthquakes with typical magnitude less than 2 and reduced amplitudes at frequencies greater than 10 Hz relative to ordinary small earthquakes. Their occurrence is often associated with tectonic tremor and slow‐slip events along the plate boundary in subduction zones and occasionally transform fault zones. They are usually grouped into families with all the earthquakes of a given family originating from the same small patch on the plate interface and recurring more or less episodically in a bursty manner. In this study, we extend the LFE catalog obtained by Plourde et al. (2015), https://doi.org/10.1002/2015gl064363 during an episode of high tremor activity in April 2008 to the 8‐year period 2004–2011. All of the tremors in the Boyarko et al. (2015), https://doi.org/10.1016/j.epsl.2015.06.026 catalog south of 42° North have associated LFE activity, but we have identified several smaller episodes of LFEs without associated tremor activity, and extend their catalog forward and backward by a total of about 3 years. As in northern Cascadia, the downdip LFE families have recurrence intervals several times smaller than the updip families. For the April 2008 Episodic Tremor and Slip event, the best recorded LFE families exhibit a strong tidal Coulomb stress sensitivity starting 1.5 days after the rupture front passes by each LFE family. The southernmost LFE family, which has been interpreted to be on the subduction plate boundary, near the updip limit of tremor, has a very short recurrence time. Also, these LFEs tend to occur during times when predicted tidal Coulomb stress is discouraging slip on the plate boundary. Both observations suggest that this LFE family may be on a different fault.

3D constrained gravity inversion to model Moho geometry and stagnant slabs of the Northwestern Pacific plate at the Japan Islands

The motivation for this work was to model the crustal thickness, and the geometry of the Mohorovičić (Moho) discontinuity, the crust-upper mantle boundary around the Japanese archipelago. The western Pacific Ocean is one of the most eminent subduction zones in the world. The subduction zone complexity is due to alteration within the geometry of the stagnant slab. We compiled a three-dimensional density distribution model for subduction plates around the Japan Islands, and estimated the depth to Moho using the constrained non-linear inversion of gravity data derived from the latest Gravity field and Ocean Circulation Explorer, or GOCE, model known as GOCO06s. The study had three steps, starting with the data corrections followed by the inversion, and ending with the modeling and comparing of the estimated results with previous models and studies. In this study, we used available receiver function results, seismic tomography models, global crustal thickness, and geological and tectonic information as constraints. The model estimated the crustal thickness, surface geometry of the Moho, and geometry of the stagnant slab in the Kuril Trench, Japan Trench, Nankai Trough, and Izu-Bonin Trough. In gravity inversion, we used 34 km and 600 kg m−3 as the Moho depth reference and density contrast, respectively. Results showed that the crustal thickness ranged from 14 to 43 km with a − 0.78 mean misfit for Moho depths derived from seismic P-wave analysis. The calculated gravity data had a 0.16 mGal misfit with the observed data. The normalized analytical signal helped to sharply delineate the different plates and their edges. Three cross-sections were conducted to model the stagnant slab along the subduction zone of the Eastern Eurasian plate, western Pacific plate, Okhotsk plate, and Philippine Sea plate. The eastern area of the entire region (Pacific plate) had a low crust thickness and high density compared to the western area (Eurasian plate), which had a high crust thickness and low density.

An Experimental Study of Chlorite Stability in Varied Subduction Zone Lithologies with Implications for Fluid Production, Melting, and Diapirism in Chlorite-Rich Mélange Rocks

AbstractFour ultramafic bulk compositions comprising only natural minerals were used to constrain the stability field of chlorite in a variety of subducted, chlorite-rich rocks through an examination of key chlorite dehydration reactions relevant to the sub-arc. Seventy-four piston cylinder experiments were conducted at a range of pressures (1.0–5.0 GPa) and temperatures (500°C–1150°C).Bulk 1 represents a chlorite mélange (Mg# = 0.94) typically formed in the subduction channel. This composition was used to examine the terminal chlorite reactions to olivine, orthopyroxene, and spinel at low pressure and to olivine, garnet, and spinel at high pressure. Chlorite achieves a thermal maximum stability at 2.0 GPa, 850°C; at 3.0 GPa, 850°C; and at 5.0 GPa, 760°C. The terminal chlorite breakdown reaction rises at a much steeper Clapeyron slope than shown in previous studies. Bulk 2 contains additionally antigorite and tremolite, to constrain phase relations in more fertile compositions. Chlorite reacts with clinopyroxene at ~100°C lower temperatures and with orthopyroxene at ~20°C–60°C lower temperatures than the terminal chlorite breakdown. The reactions have a subparallel Clapeyron slope and none of the three chlorite dehydration reactions crosses the antigorite breakdown reaction up to 5 GPa. This demonstrates that chlorite is the most stable carrier of H2O to high temperatures in subducted ultramafic rocks. Chlorite mélanges that form at the subduction plate interface will dehydrate at 850°C–800°C, 80–120 km depth for intermediate to hot subduction geotherms and liberate 10–12 wt.% of H2O, triggering wet melting in associated sediments. For cold subduction geotherms, chlorite dehydration occurs at 780°C–740°C, 120–170 km depth. Interaction of such fluids with sediments will likely produce a supercritical fluid phase. No melting in the ultramafic rocks has been observed at the chlorite breakdown reactions. Wet melting of the chlorite mélange at 3 GPa occurred between 1100°C and 1150°C.The stability of chlorite in more Fe-rich mélanges (bulk Mg# = 0.50 and 0.68, respectively) were conducted at 3.0 GPa and revealed thermal maxima at 650°C and 765°C, respectively. Collectively, the thermal stability of chlorite is dependent upon the Mg# of the bulk composition and spans over 200°C at sub-arc depths. The density of run products was calculated to test the validity of the chlorite mélange diapir model. With the progressive breakdown of chlorite, ultramafic chlorite mélanges transform into garnet peridotite, thereby losing any buoyancy they initially possessed. This makes the likelihood of mélange diapirs as a major transport mechanism through the sub-arc unfeasible.

Discovery of late Early Cretaceous diorite porphyrite from the Shamuluo Formation in the Gaize area, Tibet: Response to the northward subduction plate rollback event of Bangongco-Nujiang Tethys Ocean

改则地区闪长玢岩位于藏西北班公湖-怒江成矿带西段北缘，呈岩脉状侵位在成岩于浅海环境的沙木罗组地层中，是班公湖-怒江特提斯洋向北俯冲造山消减过程的产物，而其岩石成因、地质意义和深部动力学过程等尚欠缺详细的研究。本文研究了发育于改则地区的闪长玢岩，对其开展了岩相学、锆石LA-ICP-MS U-Pb年代学以及元素和同位素地球化学的研究，对其岩石成因、成岩构造地质背景、深部地球动力学过程等进行了分析和讨论，并对其含矿性进行了评价，为该地区下一步找矿方向提供了初步的设想。锆石U-Pb同位素年代测试显示闪长玢岩具有较多锆石结晶年龄段，相较于研究区出露的沙木罗组碎屑锆石年代学特征，其最新年龄段（117~126Ma）加权平均年龄为120.4Ma，可代表其结晶年龄。元素地球化学结果显示闪长玢岩具有弧岩浆的典型特征，属于高钾钙碱性和准铝质系列岩石，结合锆石微量元素等指标综合判断其属于I型花岗岩类岩石。本文综合全岩Sr-Nd-Pb同位素组成特征等分析认为，闪长玢岩形成于新生玄武质下地壳的部分熔融，岩浆源区残留相以角闪岩相为主，源区物质有少量沉积物组份的参与，侵位过程中分离结晶和地壳混染作用不明显。通过多种方法判别并结合区域构造背景，我们认为闪长玢岩成岩于班公湖-怒江特提斯洋北向俯冲板片折返的地球动力学背景。由于地处俯冲板片后端根部，改则地区闪长玢岩俯冲角度小于成岩于俯冲板片前端同时代的多龙矿集区中酸性岩浆岩，亦小于成岩于相同板片构造位置的103.5Ma的革吉闪长玢岩，暗示120.4~103.5Ma时班公湖-怒江特提斯洋北向俯冲板片仍处于持续折返阶段。由于其俯冲角度较小以及岩浆源区低氧逸度等因素导致其无法成矿。因此建议在多龙矿集区和改则县中间位置寻找可能的与俯冲背景有关的成矿靶区，但是否见矿还应考虑构造、围岩、后期保存等条件的影响。;The diorite porphyrite in Gaize area, located in the northern margin of the western section of the Bangongco-Nujiang metallogenic belt, emplaced into the Shamuluo Formation formed from shallow sea environment. It was the product of the northward subduction of Bangongco-Nujiang Tethys Ocean (BNTO). Taking diorite porphyrite in Gaize area as the research object, this paper studied petrography, zircon U-Pb chronology, elements, and isotope geochemistry, analyzed its petrogenesis and source characteristics, discussed its diagenetic tectonic background, described its deep dynamic process in detail, explained the possible reasons for its non-mineralization, and provided a preliminary idea for the next prospecting direction in this area. Zircon U-Pb isotopic dating shows that diorite porphyrite has many stages of age. Compared with the detrital zircon chronological characteristics of Shamulo Formation in the study area, the weighted average age of the latest age group (117~126Ma) is 120.4Ma, which can represent its crystallizing age. The element geochemical results show that diorite porphyrite is a typical island arc magma with high potassium calc-alkaline characteristics, belonging to quasi-aluminous series. Combined with the characteristics of zircon trace elements, the diorite porphyrite has the characteristics of I-type granitoids. Based on the analysis of the Sr-Nd-Pb isotopic composition of the whole rock, it is considered that the diorite porphyrite was formed in the partial melting of the juvenile basaltic lower crust, the residual facies in the magmatic source are mainly amphibolite facies, with the addition of a small amount of sediment components, and the separation, crystallization and crustal contamination in the emplacement process is not obvious. Through a variety of methods and combined with the regional tectonic background, we present that the diorite porphyrite was formed in the geodynamic background of rollback within the process of northward subduction of the BNTO. Because it is located at the root of the back end of the subducted plate, the subduction angle of the diorite porphyrite in Gaize area is smaller than that of the intermediate acid magmatic rock in the Duolong ore district, which formed from the front of the subducted plate at same time, and is also smaller than that of Geji diorite porphyrite, which is located at the same tectonic location of the subducted plate and formed at 103.5Ma. It is suggested that the northward subduction plate of the BNTO is still in the stage of continuous rollback during 120.4~103.5Ma. It cannot be mineralized because of its small subduction angle and low oxygen fugacity in the magmatic source area. Therefore, it is suggested to look for possible ore-forming targets related to the subduction background between the Duolong ore concentration area and Gaize County, but the influence of structure, surrounding rocks and late preservation should also be considered.

Petrogenesis and tectonic setting of the late Early Cretaceous Kong Co A-type granite in the northern margin of Central Lhasa Subterrane, Tibet

在中拉萨地块北部尼玛控错地区发育着一套碱性长石花岗岩，对该花岗岩体开展成因和形成背景的研究，能为探索班公湖-怒江洋的构造演化提供有价值的信息。用LA-ICP-MS方法测得该花岗岩的锆石²⁰⁶Pb/²³⁸U年龄加权平均值为104.9±1.4Ma （MSWD=1.5）和104.6±1.3Ma （MSWD=1.3），表明该岩体形成于早白垩世。花岗岩具有高硅（SiO₂=76.75%~77.51%，平均77.27%）、高钾（K₂O=4.61%~4.85%，平均4.77%）、高碱（K₂O+Na₂O=8.24%~8.57%，平均8.44%）、低钙（CaO=0.28%~0.48%，平均0.35%）、低镁（MgO=0.11%~0.16%，平均0.13%）和低铝（Al₂O₃=11.79%~12.22%，平均12.09%）等特征，里特曼指数（σ）为1.96~2.15（平均2.08），A/NK值为1.06~1.09，A/CNK值为1.01~1.04。这些特征表明控错花岗岩为弱过铝质的高钾钙碱性-钾玄岩系列岩石。控错花岗岩相对富集Zr、Nb、Ce、Y和Hf等微量元素，相对亏损Ti、Ba、Sr和P等微量元素，分异系数（DI）为95.5~96.9（平均：96.3），还具有较高的FeO^T/MgO值（5.61~10.22，平均7.26）、10000Ga/Al值（2.78~2.56，平均2.84）、Y/Nb值（2.29~4.97，平均3.57）、Rb/Nb值（11.6~18.2，平均15.2）；此外，该岩体还具有较高的全岩Zr饱和温度（875~910℃，平均890℃）和锆石Ti饱和温度（848~919℃，平均890℃），明显的Eu负异常（δEu=0.04~0.09，平均0.06），以及向右缓倾的Ⅴ型稀土元素配分曲线，这些特征表明控错花岗岩为产于碰撞后环境的A2型花岗岩。正的锆石ε_Hf（t）值（4.26~6.38，平均5.16）、相对年轻的锆石Hf地壳模式年龄（t_DM2=757~889Ma，平均833Ma）、下地壳与地幔混合特征的（⁸⁷Sr/⁸⁶Sr）_t（0.7194~0.7407，平均0.7313）、ε_Nd（t）（-3.39~-3.00，平均-3.24））和Pb同位素特征（（²⁰⁶Pb/²⁰⁴Pb）_t=18.792~18.845，（²⁰⁷Pb/²⁰⁴Pb）_t=15.708~15.718，（²⁰⁸Pb/²⁰⁴Pb）_t=38.870~38.037），指示控错花岗岩熔融于幔源物质加入的新生地壳。研究结果揭示，控错花岗岩形成于羌塘-拉萨地块碰撞作用下，俯冲板片的断离后，软流圈上涌诱发的地壳部分熔融，并经历了显著的以钾长石和角闪石为主的分离结晶作用。;A set of alkali feldspar granite develops well in the Kong Co of Nyima County in the northern margin of Central Lhasa Subterrane, Tibet, which can provide invaluable information to explore the geological evolution of the Bangong Co-Nujiang Ocean. The weighted averaged zircon ²⁰⁶Pb/²³⁸U ages of the granite measured by LA-ICP-MS are 104.9±1.4Ma (MSWD=1.5) and 104.6±1.3Ma (MSWD=1.3), respectively, indicating that it was formed in the late phase of Early Cretaceous magmatism. The rocks in Kong Co granite are grayish-white, with fine-grained equigranular structure and medium coarse-grained porphyritic-like texture. The rock-forming minerals mainly include quartz, potassium feldspar, plagioclase, and a small amount of transparent accessory minerals such as biotite and amphibole, as well as a few melanocratic mineral like magnetite. The granite is characterized by high contents of SiO₂ (6.75%~77.51%), K₂O (4.61%~4.85%), and K₂O+Na₂O (8.24%~8.57%), with low contents of CaO (0.28%~0.48%), MgO (0.11%~0.16%) and Al₂O₃ (11.79%~12.22%), and a Rittman index (σ₄₃) of 1.96~2.15 and A/CNK ratios of 1.01~1.04. These characteristics reveal that the Kong Co granite is weakly peraluminous high potassium calc-alkaline-potassium basalt series rock. The granite is relatively enriched in Zr, Nb, Ce, Y and Hf, and deficient in Ti, Ba, Sr and P. It shows a highly variation of index (DI:95.50~96.90), and also has high values of FeO^T/MgO (5.61~10.22), 10000Ga/Al (2.78~2.56), Y/Nb (2.29~4.97), Y/Nb (2.29~4.97), Rb/Nb (11.6~18.2), which is consistent with A2-type granites produced in post collision environment. In addition, the rock has high whole-rock Zr saturation temperatures (875~910℃) and zircon Ti saturation temperatures (848~919℃), with a significant negative Eu anomaly (δEu=0.04~0.09), and a Ⅴ-shaped rare earth element curve gently inclined to the right. These characteristics also prove the Kong Co granite is A2 type granite. They have relatively positive ε_Hf(t) values (4.26~6.38), and two-stage Hf model ages (t_DM2) ranging from 757Ma to 889Ma, with (⁸⁷Sr/⁸⁶Sr)_t=0.7194~0.7407 and ε_Nd(t)=-3.39~-3.00, and Pb isotopic ((²⁰⁶Pb/²⁰⁴Pb)_t=18.792~18.845, (²⁰⁷Pb/²⁰⁴Pb)_t=15.708~15.718, (²⁰⁸Pb/²⁰⁴Pb)_t=38.870~38.037) characteristics of lower crust-mantle mixture. These data indicate that the granite was sourced from interaction between mantle-derived and juvenile lower crust-derived melts. This occurred when the post collision extension of Qiangtang and Lhasa terranes the subduction plate split to create a slab window, and rising asthenosphere triggered re-melting of the lower crust basalt, resulting in the formation of the late Early Cretaceous A-type granite around Kong Co.

Decadal and Yearly Forerunning Earthquakes to Cape Mendocino, Northern California, Mainshock of 1992 Magnitude 6.9

The area near the Cape Mendocino earthquake of 1992, magnitude 6.9, was the site of many moderate to large shocks during the previous decades. It and the Honeydew event of 1991, however, are distinguished from most earthquakes in the region by their thrust-fault mechanisms. The magnitude of the 1991 shock was also unusually large for the preceding decades, Mw 6.1. The mechanisms of most other large events involved strike-slip faulting. The 1992 mainshock occurred in a volume of space characterized by few decadal forerunning earthquakes of moderate to large size. Most of those forerunners took place on the periphery of that volume. The presence of that zone suggests that it broke previously in a large to great earthquake. Precise locations indicate that slip in the 1991 and 1992 earthquakes occurred on faults dipping shallowly to the NE and ENE. They likely took place within the North American plate above the subduction plate boundary. Their implications for earthquake forecasting using sparse precursors are discussed.

Eclogite with unusual atoll garnet from the southern Armorican Massif, France: Pressure-temperature path and geodynamic implications

Eclogites form in different geotectonic environments such as subduction zones or roots of overthickened continental crust. The specific environment can be identified by the construction of pressure-temperature (P-T) trajectories. We determined the P-T path of eclogite from the Les Essarts high-pressure unit, Southern Armorican zone in western France. The eclogite contains unusual atoll garnet characterized by a younger generation (Grt2) that replaced the core of an older generation (Grt1). Large-area (cm2) X-ray mapping with the electron microprobe was applied to determine the quantities of Grt1 and Grt2 in the studied eclogites. Detailed characterization of the mineral chemistry was undertaken followed by thermodynamic modelling. The assemblage of amphibole, quartz, rutile, and ilmenite enclosed in Grt1 formed in the amphibolite facies during a first metamorphic event. Grt1 grew at P-T conditions around 20 kbar and 540 °C probably after nearly isothermal burial during a second metamorphic cycle. An isobaric heating to ca. 650 °C followed at which Grt2 and, thus, atoll garnet formed. A time period of ca. 15 million years for exhumation in the temperature range 620–655 °C was estimated by diffusional modelling of the contact between Grt1 and Grt2. We correlate the Southern Armorican zone with the Malpica-Tuy zone (NW Spain) that contains eclogite and related rocks with a similar metamorphic evolution. We outline a geodynamic scenario in which the eclogite-facies rocks of both zones were tectonically eroded from the upper continental plate by a subducting plate to reach maximum depths of 70–90 km in the Late Devonian. Exhumation to depths of ca. 45–50 km occurred in a subduction channel just before continent-continent collision started.

Terrane geodynamics: Evolution on the subduction conveyor from pre-collision to post-collision and implications on Tethyan orogeny

Terranes are passengers within drifting oceanic plate, and ride with the plate to subduction plate boundaries. Although oceanic lithosphere readily subducts into the mantle at the plate boundary, terranes may resist sinking or not, depending on a variety of controlling factors that are not very well understood. Further, the tectonic development of terranes prior to their arrival at a subduction zone is not well documented. We performed numerical experiments to explore these unknowns of terrane geodynamics during the whole pre- to post-collisional period. Our analyses reveal that terranes can undergo considerable extension prior to their arrival to subduction plate boundaries owing to the pull force exerted by sinking oceanic slabs. Increasing terrane crustal thickness, decreasing terrane width or imposed convergence velocity, and an intra-oceanic subduction setting aggrandizes the pre-collisional terrane extension. The numerical models identify increasing terrane crustal thickness and width, and decreasing imposed convergence velocity as factors that promote terrane accretion. Additionally, having a continental overriding plate at the subduction boundary increases the propensity for terrane accretion. Some intra-oceanic subduction experiments demonstrate ablative subduction and subduction polarity reversal events in connection with terrane collisions/subduction. We compare our models to the evolution of a controversial Tethyan terrane, the Nilufer oceanic plateau. Our models suggest that the Nilufer terrane may have undergone pre-collisional extension owing to the pull of the sinking Tethys Ocean plate. Further, uninterrupted subduction of the Tethys Ocean, despite the accretion of the Nilufer terrane, is illustrated by our results which have implications on other regions along the Tethyan orogenic belt.

Three-stage modification of lithospheric mantle: Evidence from petrology, in-situ trace elements, and Sr isotopes of mantle xenoliths in the Cenozoic basalts, northeastern North China Craton

Abstract We present mineralogical and geochemical compositions of mantle xenoliths from two Cenozoic basalt localities of the northeastern North China Craton. These xenoliths include lherzolite, harzburgite, and websterite. They are generally fertile in major elements and different from the typical cratonic lithosphere, which is consistent with previous hypotheses regarding craton destruction. The ratios of 87Sr/86Sr and (La/Yb)N of clinopyroxenes (Cpx) in one lherzolite are relatively low in the core but high in the rim. The center of the Cpx grain has a high U concentration. Changes in trace elements and Sr isotopes indicate that later stage high 87Sr/86Sr melt metasomatism superimposed on the early hydrous melt/fluid. The Cpxs in some xenoliths are low in Ti/Eu but high in Ca/Al and light rare earth elements, which indicates carbonate melt metasomatism. 87Sr/86Sr is increased in the core and decreased in the rim of most Cpx grains, which reflects the superposition of two-stage metasomatism. The early agent should be high in 87Sr/86Sr, and the recent agent should be low in 87Sr/86Sr. The Cpxs in olivine websterite are low in 87Sr/86Sr (0.70220–0.70320), which reflects the recent metasomatism of asthenosphere-derived melt. Collectively, these observations reflect a three-stage modification of the lithospheric mantle. First-stage hydrous melt/fluid could come from the dehydration of young subducted plates. Second-stage melt/fluid of high 87Sr/86Sr could derive from the partial melting of the subducted altered oceanic crust, and the recent melt/fluid of low 87Sr/86Sr should be from the asthenosphere.

Toward Waveform‐Based Characterization of Slab & Mantle Wedge (SAM) Earthquakes

AbstractEarthquakes in subduction zones occur in the slab mantle, in the subducting crust, on the subduction plate interface, and, in some cases, in the mantle wedge–regions that are separated by strong seismic discontinuities. These discontinuities are typically imaged with techniques using teleseismic waves, while local earthquakes are located based on arrival times. While this combination of imaging and earthquake location provides a good initial overview of where the earthquakes are located, the uncertainties associated with the two approaches are too large (i.e., few kilometers) to robustly identify on which side of a discontinuity (with thickness 100 m) the earthquakes occurred. Here we investigate how the waveforms of local earthquakes, which contain secondary phases arising from wave scattering at discontinuities, can be exploited to determine the source region of subduction zone earthquakes more robustly. Our investigation involves a three‐step approach and includes an application to data from western Greece. First, to identify characteristic secondary phases, we analyzed synthetic seismograms from a generic 2‐D subduction zone. Second, to enhance the visibility of secondary phases in field data, we implemented a workflow to process three‐component seismograms. Third, to identify individual secondary phases in the data, we matched their timing to arrivals computed in a 3‐D velocity model. We identified on average two to three secondary arrivals per station. These include P‐ and S‐reflections from the plate interface which indicate hypocenters in the mantle wedge, and P‐reflections from the slab Moho which indicate hypocenters on the plate interface and in the subducting crust.

Constraints on Element Mobility During Deformation Within the Seismogenic Zone, Shimanto Belt, Japan

AbstractSubduction interfaces exhibit a variety of slip behaviors, including megathrust and slow earthquakes. Field observations are consistent with crack‐seal deformation, in which tensile cracks are sealed by fluid‐transported solute. However, there are few constraints on the mass fluxes and length and time scales of such deformation and attendant increases in cohesion within the seismogenic zone. Here, we present a systematic geochemical investigation of mass transport associated with development of crack‐seal veins in the Shimanto Belt—an accretionary complex that preserves a record of plate boundary slip behavior at temperatures relevant to the seismogenic zone: 150–350°C. These mélanges show evidence for shear across decameter‐scale zones of deformation dominated by anastomosing scaly fabrics and pervasive veins. We use meso‐ and microstructural observations with geochemical observations of scaly fabrics and crack‐seal veins to evaluate the role of silica redistribution in healing fracture porosity along the plate interface and modulating slip behavior. Crack‐seal veins contain primarily quartz with albite and calcite, and vein textures provide evidence for partial sealing. Bulk rock analyses determined that the amount of phyllosilicates, specifically illite and chlorite, increases with temperature. A simple mass‐balance model based on the immobile chemical component TiO2 shows a systematic trend in mobility for all three mélanges and increased element mobility as a function of temperature. Scaly fabrics and veins show compositional evidence for locally sourced mineral redistribution. This study supports a model where development of scaly slip surfaces and fracture healing through temperature‐dependent mineral redistribution can impact slip behavior and fluid flow along the subduction plate interface.

PILLOWED METABASALTS AND METABRECCIAS FROM THE VALAISAN OCEAN-CONTINENT TRANSITION PRESERVED IN THE BREUIL VALLEY, AOSTA (ITALIAN WESTERN ALPS)

Subduction plate margins have an intrinsically poor preservation potential for pre-subduction settings because most rocks sink and disappear into Earth’s mantle. However, a preserved pre-subduction setting has been reported in the Lower Penninic Units (Valaisan Basin) of the Italian Western Alps (Beltrando et al., 2012). Here, we review two main aspects of the Valaisan Basin cropping out in Italy: the nature of the syn-rift detrital sediments and the characteristics of the metabasalts in proximity to the continental crust metagranitoid. New fossil findings are reported, which confirm a Mesozoic age for the Valaisan rifting. Field and petrographic data from metabreccias and pillowed metabasalts near the fossil-rich high-pressure metasediments support the interpretation of the Valaisan Domain in the Breuil valley (Aosta) as a fossil ocean-continent transition zone.

Insights from elastic thermobarometry into exhumation of high-pressure metamorphic rocks from Syros, Greece

Abstract. Retrograde metamorphic rocks provide key insights into the pressure–temperature (P–T) evolution of exhumed material, and resultant P–T constraints have direct implications for the mechanical and thermal conditions of subduction interfaces. However, constraining P–T conditions of retrograde metamorphic rocks has historically been challenging and has resulted in debate about the conditions experienced by these rocks. In this work, we combine elastic thermobarometry with oxygen isotope thermometry to quantify the P–T evolution of retrograde metamorphic rocks of the Cycladic Blueschist Unit (CBU), an exhumed subduction complex exposed on Syros, Greece. We employ quartz-in-garnet and quartz-in-epidote barometry to constrain pressures of garnet and epidote growth near peak subduction conditions and during exhumation, respectively. Oxygen isotope thermometry of quartz and calcite within boudin necks was used to estimate temperatures during exhumation and to refine pressure estimates. Three distinct pressure groups are related to different metamorphic events and fabrics: high-pressure garnet growth at ∼1.4–1.7 GPa between 500–550 ∘C, retrograde epidote growth at ∼1.3–1.5 GPa between 400–500 ∘C, and a second stage of retrograde epidote growth at ∼1.0 GPa and 400 ∘C. These results are consistent with different stages of deformation inferred from field and microstructural observations, recording prograde subduction to blueschist–eclogite facies and subsequent retrogression under blueschist–greenschist facies conditions. Our new results indicate that the CBU experienced cooling during decompression after reaching maximum high-pressure–low-temperature conditions. These P–T conditions and structural observations are consistent with exhumation and cooling within the subduction channel in proximity to the refrigerating subducting plate, prior to Miocene core-complex formation. This study also illustrates the potential of using elastic thermobarometry in combination with structural and microstructural constraints, to better understand the P–T-deformation conditions of retrograde mineral growth in high-pressure–low-temperature (HP/LT) metamorphic terranes.

Yield failure of the subducting plate at the Mariana Trench

Two tectonic plates converge at subduction zones where the subducting plate bends. Flexural bending of a subducting plate at a trench results in pervasive normal faulting, providing conduits for plate hydration and influencing the water budget and seismic behavior of the plate interface. 2-D plate bending simulations have demonstrated high strains due to strong bending may result in loss of strength of the lithosphere and intraplate earthquakes near the trench axis. However 3-D plate bending deformation may affect the along-strike slab pull force, especially when the deflection changes along the trench. Here we simulated the 3-D plate bending deformation and calculated bending stresses, and brittle failure of the Pacific plate at the Mariana Trench. We find that both the plate deflection and the yield zone depth (~16–20 km) increases from the northern to southern Mariana Trench. The water flux in the plate at the southern Mariana Trench is estimated to be about 15% greater than that of the northern Mariana Trench. By comparing with 2-D models, we further find that the 2-D approaches may undereastimate the yield zone depth as they ignored the along-trench effects of plate deflection. The new results provide a self-consistent framework for interpretation of the observed surface normal faults, extensional earthquakes, and the inferred hydration of the subducting plate as constrained by seismic velocity anomalies.

Tectonic and eustatic control of Mesaverde Group (Campanian–Maastrichtian) architecture, Wyoming-Utah-Colorado region, USA

AbstractWe describe and analyze the depositional history and stratigraphic architecture of the Campanian and Maastrichtian succession of the southern greater Green River basin of Wyoming, USA, and surrounding areas to better understand the interplay between tectonic and eustatic drivers that built the stratigraphy. By integrating new measured sections with published outcrop, well-log, and paleogeographic data, two new stratigraphic correlation diagrams, 35 new paleogeographic reconstructions, and six new tectonic diagrams were created for this part of the Western Interior Seaway. From this work, two time-scales of organization are evident: (1) 100–300 k.y.-scale, mainly eustatically driven regressive-transgressive shoreline oscillations that generated repeated sequences of alluvial-coastal plain-shoreline deposits, passing basinward to subaqueous deltas, then capped by transgressive estuarine/barrier lagoon deposits, and (2) 3.0–4.0 m.y.-scale, tectonically driven groups of 10 to 15 of these eustatically driven units stacked in an offset arrangement to form larger clastic units, which are herein referred to as clastic wedges. Four regional clastic wedges are recognized, based on the architectures of these clastic packages. These are the: (1) Adaville, (2) Rock Springs, (3) Iles, and (4) Williams Fork clastic wedges. Pre-Mesaverde deposition in the Wyoming-Utah-Colorado (USA) region during the Middle Cretaceous was characterized by thickening of the clastic wedge close to the thrust-front, driven primarily by retroarc foreland basin (flexural) tectonics. However, a basinward shift in deposition during the Santonian into the early Campanian (Adaville clastic wedge) signaled a change in the dominant stratigraphic drivers in the region. Shoreline advance accelerated in the early to middle Campanian (Rock Springs clastic wedge), as the end of activity in the thrust belt, growing importance of flat-slab subduction, and steady eastward migration of the zone of dynamic subsidence led to loss of the foredeep and forebulge, with the attendant formation of a low-accommodation shelf environment. This “flat-shelf” environment promoted large shoreline advances and retreats during sea-level rise and fall. During the middle to late Campanian (Iles clastic wedge), deep erosion on the crest of the Moxa Arch, thinning on the crests of the Rock Springs and Rawlins uplifts, and subsequent Laramide-driven basin formation occurred as the Laramide blocks began to partition the region. The next clastic package (Williams Fork clastic wedge) pushed the shoreline over 400 km away from the thrust belt during the late Campanian. This was followed by a very large and persistent marine transgression across the region, with the formation of a Laramide-driven deepwater turbidite basin with toe-of-slope fans into the early Maastrichtian. The Mesaverde Group in the Wyoming-Utah-Colorado region is thus characterized by: (1) a succession of four tectonically driven classic wedges, each comprised of a dozen or so eustatically driven packages that preserve large basinward and landward shoreline shifts, (2) broad regional sand and silt dispersal on a low-accommodation marine shelf setting, (3) a progressive, tectonically driven, basinward shift of deposition with offset, basinward stacking of successive clastic wedges, and (4) the gradual formation of various uplifts and sub-basins, the timing and sizes of which were controlled by the movement of deep-seated Laramide blocks. The Mesaverde Group in the Wyoming-Utah-Colorado region provides an outstanding opportunity to study the dynamic interaction among the tectonic control elements of a subducting plate (crustal loading-flexure, dynamic subsidence/uplift, and regional flat-slab basin partitioning), as well as the dynamic interaction of tectonic and eustatic controls.