Zones Of Crustal Weakness Research Articles

Neogene Hydrothermal Fe‐ and Mn‐Oxide Mineralization of Paleozoic Continental Rocks, Amerasia Basin, Arctic Ocean

AbstractRocks dredged from water depths of 1,605, 2,500, 3,300, and 3,400 m in the Arctic Ocean included Paleozoic continental rocks pervasively mineralized during the Neogene by hydrothermal Fe and Mn oxides. Samples were recovered in three dredge hauls from the Chukchi Borderland and one from Mendeleev Ridge north of Alaska and eastern Siberia, respectively. Many of the rocks were so pervasively altered that the protolith could not be identified, while others had volcanic, plutonic, and metamorphic protoliths. The mineralized rocks were cemented and partly to wholly replaced by the hydrothermal oxides. The Amerasia Basin, where the Chukchi Borderland and Mendeleev Ridge occur, supports a series of faults and fractures that serve as major zones of crustal weakness. We propose that the stratabound hydrothermal deposits formed through the flux of hydrothermal fluids along Paleozoic and Mesozoic faults related to block faulting along a rifted margin during minor episodes of Neogene tectonism and were later exposed at the seafloor through slumping or other gravity processes. Tectonically driven hydrothermal circulation most likely facilitated the pervasive mineralization along fault surfaces via frictional heating, hydrofracturing brecciation, and low‐ to moderate temperature Fe‐ and Mn‐rich hydrothermal fluids, which mineralized the crushed, altered, and brecciated rocks.

Constraints on Growth and Stabilization of the Western Superior Craton From Inversion of Magnetotelluric Data

AbstractA data set consisting of 376 broadband and long‐period MT measurements was used to generate the first ever 3D resistivity model of the Archean western Superior Craton. The modeled resistivity structure is compared to coincident seismic reflection data. The observed geophysical signatures are interpreted within the context of the late stages of crustal growth and cratonization of the region via the progressive accretion of terranes against the initial cratonic core. The northern‐most terranes comprising the cratonic core exhibit a nearly homogenous highly resistive crust. The lower crust of the southern terranes contains largely continuous low resistivity bands which run subparallel to major terrane boundaries and corresponding fault systems. In some cases, low resistivity features are coincident with dense packages of sub‐horizontal to listric reflections within the mid‐ to lower crust. These resistivity structures are inferred to represent preserved geoelectric signatures of late to post‐orogenic magmatic pulses likely related to delamination of locally overthickened crust. Increased mantle heat flow resulted in partial melting of the lower crust and upper mantle and upward migration of CO2‐rich melts and fluids through crustal weak zones corresponding to shear and/or suture zones formed during terrane amalgamation. Thermal softening of the mid‐ to lower crust led to orogenic collapse and reactivation of the crustal shear zones, resulting in formation and interconnection of graphitic films which were preserved within the stable craton. These results have implications for the tectono‐magmatic history of the western Superior Craton, as well toward the understanding of the geodynamic regime of the Archean Earth.

Geophysical characterization of lamproite fields in the Dharwar craton using VLF-EM and advanced filtering techniques: insights from conductivity analysis and analytical signal mapping

Abstract This study presents a geophysical investigation of the lamproite fields located in the Dharwar craton, aiming to map conductivity variations using contemporary techniques. The study employs very low-frequency electromagnetic (VLF-EM) methods, applying Hilbert transformations, and first-order vertical derivatives to the Fraser and Karous–Hjelt filtered contoured of VLF-EM data. The peninsular gneissic complex (PGC) granitic rocks in the study area experienced tectonic forces, resulting in fractures along specific WNW–ESE to NW–SE trends. Within these crustal weak zones, these lamproites are emplaced. The lamproite pipes are volcanic rocks. As a result, the top portions are weathered and tend to conductive, and the conductivity tends to decrease with depth. The volumetric size of lamproites ranges from centimetres to hundreds of metres, unlike kimberlites that are larger. Hence, the exploration of lamproites poses challenges. The contours of in-phase and quadrature components were used to identify the cluster of lamproite zones within the study area. From this study, the boundaries of the lamproite pipes were clearly identified using real component's analytical and first-order vertical derivative signal contour maps. The VLF-EM pseudo-depth current density section was used to identify anomalous lamproite, pipes, and their subsurface extensions, along with the surrounding formations. The current investigation findings specify that the lamproites exhibit weak conductive. These results provide valuable insights for exploration efforts within the Dharwar craton, and can aid in the identification and mapping of the lamproite fields.

Zones of Weakness Within the Juan de Fuca Plate Mapped From the Integration of Multiple Geophysical Data and Their Relation to Observed Seismicity

AbstractThis study aims to explain the nonuniform earthquake pattern along the Cascadia Subduction Zone. In particular, we investigate the relationship between the tectonic features of the subducting oceanic Juan de Fuca slab and the onshore seismicity pattern. We have integrated multiple geophysical data sets toward three general objectives. The first study intends to study variations in physical properties along three 2‐dimensional models through regions of different seismicities that combine public gravity, magnetic, and seismic data sets. These models reveal multiple zones of decreased crustal density that we interpret as regions of weaker oceanic crust. The second objective is to delineate major tectonic features by performing spatial analysis of potential fields. The overall methodology comprises gravity and magnetic data filtering, followed by lineaments mapping and cross‐referencing interpretation with available seismic reflection data. This process allows delineating zones of crustal weakness by extrapolating outside our three 2‐D models. We also map multiple seamounts that appear to cluster along identified zones of weaker crust. Third, we investigate the relationship between the mapped tectonic elements, namely the zones of weak crust with accompanying seamounts, and the observed seismicity trends within the subducted slab. The alignment between those suggests that mapped weak crust zones and associated seamounts may have an influence on the overall subduction process. As more of these structures are heading toward the Washington portion of the margin than to the Oregon portion, more earthquakes are observed in the north than in the south.

A validated geomechanical model for the strike-slip restraining bend in Lebanon

Most of the methodologies used to validate complex strike-slip structures mainly rely on comparison with other well-known geological features or analogue laboratory models. This study adopts an approach based on the boundary element method at the regional scale to test the structural interpretation of a complex transpressional mountain range. Lebanon restraining bend represents the most prominent topographic transpressional feature along the Dead Sea Transform (DST). It consists of two mountain ranges: the Mount Lebanon and the Anti-Lebanon ranges. We built a 3D geometrical model of the fault surfaces based on previously studied natural examples, structural maps, satellite images, DEM interpretation and experimental analogue models of restraining bend or transpressional structures. Using a boundary element method, we modelled fault deformation response to the regional stress field. The simulation accurately predicts the shape and magnitude of positive and negative topographic changes and fault slip directions throughout the study area. We propose an original approach, which uses implementation of well-known fault geometries, surface and subsurface data, for structural validation in the complex strike-slip domain. Our results, validated by structural evidences, highlight that various structural styles lead to formation of Mt. Lebanon, Anti-Lebanon and Palmyrides structures. Furthermore, this simulation supports the hypothesis that the restraining bend of the DST formed in the widespread crustal weakness zone developed in the Late Jurassic to Early Createceous. We also propose recent Neogene tectonic evolution of the region based on our modelling and integrated with published U/Pb dating of fault zones and tectonostratigraphic evidence.

Continental-scale structural heritage from rift extension to postrift inversion: Implications for the central Brazilian Equatorial Margin evolution

The link between major basement anisotropies and basin evolution is crucial for understanding the role of tectonic inheritance on continental margins. However, it is still a matter of debate how and why it happens, especially in the Equatorial Atlantic, where the interplay between Precambrian fabric and the shearing of major oceanic fractures has been studied. Here, we investigate the offshore Ceará Basin and its basement in the central part of the Brazilian Equatorial Margin to (1) map basement anisotropies underneath marginal basins and show how Precambrian crustal domains influenced basin evolution; (2) determine the deformation style of reactivated basement faults; and (3) assess the interplay between reactivated faults and the oceanic Romanche Fracture Zone. The combined geological and geophysical data show that reactivation of NE-SW to ENE-WSW continental-scale ductile shear zones that acted as crustal weakness zones started in the Early Cretaceous under a normal stress field. Stress field inversion occurred in the Barremian during the continental breakup when the South American plate started to became an independent intraplate area under a strike-slip stress regimes. The shear zones arrest at the E-W-striking Romanche Fracture Zone, which marks the continental-oceanic boundary. The major fault offsets occurred along the Transbrasiliano Shear Zone, which represented a collisional boundary between paleocontinents in the Neoproterozoic. Fault reactivation also occurred in the eastern boundary of the Ceará Terrace marginal ridge along the Transbrasiliano Shear Zone, uplifted during tectonic inversion. We conclude that fault reactivation mainly occurred between contrasting Precambrian terrains separated by ductile shear zones. Additionally, the folds associated with the Romanche Fracture Zone were mainly generated along reactivated faults that affected syn- to postrift units on the Equatorial Margin.

Seismicity around Southern Lake Erie during 2013–2020 in Relation to Lake Water Level

Abstract The water level change in the Great Lake region can modulate stresses in the nearby fault system and potentially induce earthquakes. We perform a systematic analysis of local seismicity around southern Lake Erie during 2013–2020 to investigate the relation between seismicity and lake water level change. We obtain a newly detected catalog of tectonic earthquakes, which reveals 20–40 M > 0 earthquakes/yr before 2019. The peak seismicity rate in 2019 is dominated by active aftershocks following the 2019 ML 4.0 Ohio earthquake. The clustering analysis reveals both clusters with multiple events and stand-alone events that might represent isolated zones of crustal weakness in this region. The absolute location analysis identifies a group of earthquakes around the epicenter of the 1986 M 5 earthquake, which might be linked to the nearby injection activities. To evaluate the potential interaction with changing water loading of Lake Erie, we then model the coulomb stress change (ΔCSC) caused by elastic lake-water loading and find that ΔCSC strongly depends on effective friction coefficient (μ′). Considering the geometry of the receiver fault, higher lake water level results in positive ΔCSC and would promote failure when μ′≤0.3, and leads to negative ΔCSC and would discourage rupture when μ′>0.3. Further analysis indicates a lack of temporal correlation between long-term seismicity and water level variations before the 2019 Ohio sequence, suggesting that water level fluctuations do not modulate long-term seismicity. However, we cannot fully rule out the impact of increasing water level on reactivating the fault that hosted the 2019 Ohio earthquake sequence because of the uncertainty of μ′. Our results highlight the necessity of denser and closer monitoring of lake seismicity to further investigate the impact of changing water loading on reactivating shallow faults in this region.

Electrical structures of the Yinchuan Basin and adjacent area, western North China Craton, inferred from magnetotelluric imaging

• The Alxa and Ordos blocks are characterized by simple, laterally uniform, layered electrical structures. • The East Helanshan and Yellow River faults correspond to major electrical boundaries beneath the Yinchuan Basin. • Electrically conductive crustal weak zone is the main driver of earthquakes in the Yinchuan Basin. The Yinchuan Basin is located in the contact zone between the Alxa block and the Ordos block, North China Craton . The Yinchuan Basin also lies within the northern segment of the North-South seismic zone in China, which has experienced frequent earthquakes, including the M 8.0 Pingluo earthquake in 1739. In this study, two magnetotelluric profiles that cross the Yinchuan Basin and the adjacent Helanshan Uplift were used to obtain a 3-D image of the crustal electrical structure in this region. The results show that the electrical structure of the Yinchuan Basin and its surrounding areas differ not only in the east–west direction but also from north to south. The Yinchuan Basin and Helanshan Uplift comprise a typical basin-range tectonic feature , with a high-resistivity body in the mountain range and a low-resistivity body in the basin. Within this region, the East Helanshan and Yellow River faults appear as significant large-scale electrical boundaries along the east and west edges of a seismically imaged “flower” structure within the deep part of the Yinchuan Basin. The Yinchuan Fault is believed to extend northward as a buried fault, and here we speculate that it may be the master fault of the “flower” structure. The epicenter of the 1739 Pingluo earthquake falls along the boundary between high and low resistivity bodies. The low-resistivity body, within the Yinchuan Basin, extends to a depth of more than 50 km and is regarded as a weak zone; we speculate that the Pingluo earthquake was triggered by the interaction of multiple seismogenic faults within that weak zone. In addition, the alternating high and low-resistivity bodies in Yinchuan Basin and Helanshan Uplift, along which deformation is accommodated between the stable Alxa and Ordos blocks, likely defines the geodynamic framework of this earthquake. Comparing the electrical structure from the southwestern and northeastern margins of the Ordos block, we can conclude that the different tectonic frameworks can explain different kinematic properties within these active fault zones.

МОДЕЛЬ ГЛУБИННОГО СТРОЕНИЯ ЮЖНОЙ КАМЧАТКИ ПО РЕЗУЛЬТАТАМ ПЛОТНОСТНОГО 3D-МОДЕЛИРОВАНИЯ И КОМПЛЕКСУ ГЕОЛОГО-ГЕОФИЗИЧЕСКИХ ДАННЫХ

In the south of Kamchatka, a number of deep geophysical studies have been conducted along the profile lines. The aim of the research was to study the lithosphere in the zone of present-day volcanism and active seismicity. Geological and geophysical models of the Earth's crust and upper mantle were constructed along the profiles. The results were obtained as part of two-dimensional modeling of geophysical fields. But the analysis of materials shows that the territory is characterized by a complex geological structure, which is reflected in three-dimensional distribution of gravitating masses. For the first time, the article presents the results of volumetric density modeling covering the territory of southern Kamchatka, including areas covered by the Sea of Okhotsk and the Pacific Ocean. The model is based on the technology of three-dimensional imaging of 2D modeling results obtained along the grid of intersecting profiles. The 3D modeling generated isodensity surfaces that enclose regions with layers of high density (≥ 3.33 g/cm3). Thus, the surface identified beneath the ocean is interpreted as a fragment of the top of the subducting plate and the surface under the peninsula is identified as the top of the paleosubduction zone. A subhorizontal high-gradient zone (3.0–3.3 g/cm3) is recognized in the density structures that intersect the 3D model, which is identified with the Moho boundary. A model of subduction interaction between oceanic and continental lithospheric plates is proposed. The two-dimensional model shows the formation of a transitional layer between the Moho boundary of the overhanging lithospheric plate and the top of the paleosubduction zone. In the transition layer, a low-density zone is distinguished, where individual areas of maximum low-density are associated with melting chambers. Conditions are shown for the formation of the crust block with abundant basic-ultrabasic intrusions and the diorite-granodiorite intrusive massif. All ore occurrences and gold deposits of the Karymshinsky ore cluster are located within the contours of projection onto the ground surface of the deep high-gradient zone that encloses the low-density zone. Ore occurrences are genetically related to the zones of crustal weakness where epithermal deposits are formed in closed hydrothermal systems. Based on the analogy, it is possible to prognosticate gold occurrences in other areas of the projection of the high-gradient zone.

Influence of Zones of Pre‐Existing Crustal Weakness on Strain Localization and Partitioning During Rifting: Insights From Analog Modeling Using High‐Resolution 3D Digital Image Correlation

AbstractPre‐existing crustal structures are known to influence rifting, but the factors controlling their influence remain poorly understood. We present results of digital image correlation that allows for the surface strain analysis of a series of analog rifting experiments designed to test the influence of the size, orientation, depth, and geometry of pre‐existing crustal weak zones on strain localization and partitioning. We apply distributed basal extension to crustal‐scale models consisting of a silicone weak zone embedded in a quartz sand layer. We vary the size and orientation (α‐angle) of the weak zone with respect to the extension direction, reduce the thickness of the sand layer to simulate a shallow weak zone, and vary the geometry of the weak zone. Our results show that at higher α‐angle (≥60°) both small‐scale and large‐scale weak zones localize strain into graben‐bounding (oblique‐) normal faults. At lower α‐angle (≤45°), small‐scale weak zones do not localize strain effectively, unless they are shallow. In most models, we observe diffuse, second‐order strike‐slip intra‐graben structures, which are conjugate and antithetic under orthogonal and oblique extension, respectively. Generally, the observed spectrum of rift faulting styles (from discrete fault planes to diffuse fault zones, from normal to oblique and strike‐slip) highlights the sensitivity of rift architecture to the orientation, size, depth, and geometry of pre‐existing weak zones. Our generic models are comparable to observations from many natural rift systems like the North Sea and East Africa, and thus have implications for understanding the role of structural inheritance in rift basins.

Geology and conceptual model of the Domuyo geothermal area, northern Patagonia, Argentina

The western slope of Cerro Domuyo in northern Patagonia is characterized by thermal springs with boiling waters, Quaternary silicic domes, and pyroclastic deposits that suggest the existence of a geothermal reservoir. According to geochemical studies, the reservoir may have a temperature of 220°C and one of the largest advective heat fluxes reported for a continental volcanic center. In this paper, we propose a more refined conceptual model for the Domuyo geothermal area, based on a geological survey supported by UPb, UTh, and ArAr geochronology and by magnetotelluric and gravity surveys. Our study indicates that the Domuyo Volcanic Complex (DVC) is a Middle Pleistocene dome complex overlying middle Miocene to Pliocene volcanic sequences, which in turn cover: 1) the Jurassic-Early Cretaceous Neuquén marine sedimentary succession, 2) silicic ignimbrites dated at ~186.7Ma, and 3) the Paleozoic metamorphic basement intruded by ~288Ma granite bodies. The volcanic cycle in the DVC is distinctly bimodal, characterized by the emplacement of massive silicic domes and less voluminous olivine basalts on its southern slope. A major collapse of the central dome at ~600ka produced a voluminous (19.4km3 and 133km2) block-and-ash flow, and associated pyroclastic flows, that filled a valley to the southwest at distances up to ~30km from Cerro Domuyo summit. This was followed by a period of intense effusive activity that formed the Cerro Guitarra, Cerro Las Pampas, Cerro Domo, and Cerro Covunco silicic domes. The last two domes are the youngest and largest edifices, dated at 0.50Ma (ArAr age) and 0.25Ma (UTh age). Pre-Cenozoic successions were affected by N-S reverse and thrust faults that were later displaced by an ENE-WSW-trending transtensional belt. The basement rocks at the northern termination of the Cordillera del Viento anticlinorium were also displaced towards the east-northeast by this belt, which is observed NNW of Cerro Domuyo. The DVC was emplaced within this zone of crustal weakness. The integration of geologic observations with magnetotelluric and gravity data, allowed us to develop an updated conceptual model of the geothermal system. The geothermal reservoir is inferred at a depth of less than 2km within pre-Pliocene fractured rocks, bounded by ~WSW-ENE trending faults and sealed by the pyroclastic deposits and rhyolitic lavas of the DVC. The location of most thermal springs is not directly controlled by faults. Instead, flows emerge at the contact between the fractured and faulted basement and the caprock.

The Role of Pre‐existing Crustal Weaknesses in the Uplift of the Eastern Tibetan Plateau: 2D Thermo‐Mechanical Modeling

AbstractThe crust of the eastern Tibetan Plateau is cut through by several ancient fracture zones, which existed prior to the Cenozoic uplift of eastern Tibet. However, the role of these pre‐existing crustal weak zones (CWZs) in the Cenozoic evolution of eastern Tibet is still poorly understood. Here, we conduct a series of 2D thermo‐mechanical simulations to investigate the role of CWZs in the uplift of eastern Tibet. Our results show that CWZs embedded in the continental lithosphere facilitate crustal thickening via folding and thrusting deformation and uplift of a rugged surface at the early stage of convergence. As crustal thickening continues, the lowermost crust begins to partially melt due to strain and radioactive heating. With continued thickening and heating, melt volume gradually increases, leading to the lower crust melt preferentially accumulating where the CWZs are situated. At a later stage, the partially molten layer becomes thick enough and starts to flow in a channel with respect to its upper and lower solid layers. CWZs behave as windows to steer the ascension of the partial melts, leading to melt doming through the CWZs. The melt doming contributes to a second local surface uplift. Parameter tests demonstrate that increasing the number of CWZs can accelerate such geodynamical processes, and that the CWZ orientation controls the polarity of lithospheric underthrusting. We suggest that the Cenozoic episodic uplift of the eastern Tibetan Plateau, revealed by thermochronological studies, was likely caused by a combination of pure shear crustal thickening and melt doming along the pre‐existing CWZs.

Tectonic inheritances in rifts: The meaning of NNE lineaments in the continental rift of SE-Brazil

The effect of previous structures inheritance is known to be important in the development of tectonic rifts. A series of overlapping structures generally can be represented by lineaments marking the successive tectonic events. We studied the NNE structural lineaments corridor in the central region of the Ribeira Belt. We used a digital elevation model (DEM) and new and previous fieldwork data to investigate the structural control of such lineaments and their relevance for the Brazilian continental margin. Our results suggest that the NNE direction is a crustal weakness zone characterising corridors of intense ductile and brittle deformation which was recurrently reactivated. Aligned NNE Neoproterozoic-Ordovician ductile and brittle structures as foliations, shear zones, lithological boundaries, and fractures filled by pegmatitic veins coincide with the lineaments. During the Cretaceous rift, a transtensional sinistral regime generated NNE T-fractures filled by mafic dykes. In the Cenozoic, the NNE direction is represented by transfer and domino faults developed within a mega accommodation zone in an intracontinental rift system. Our results suggest that the NNE direction was active in this region throughout the Phanerozoic and has high relevance for the structural development of the continental margin of southeastern Brazil.

Miocene to Recent collapse calderas of the southern and central volcanic zones of the Andes and their tectonic constraints

Central (18–28° S) and Southern (33–46° S) Andean Volcanic Zones (CVZ and SVZ, respectively) have several Miocene to Recent collapse calderas. We compare their characteristics based on published data to identify how the tectonic variables have influenced their features. Typical calderas of the CVZ are large, have a masked pre-caldera stage, have multiple collapses and a large lifespan, have emitted high volumes of crystal-rich dacitic to rhyodacitic magma, and show a resurgent stage; other calderas with smaller sizes are crystal-poor, rhyolitic and younger, and they do not display resurgence stages. SVZ calderas are typically smaller than those of the CVZ, with generally a long-lived pre-caldera stage, a single collapse history, a wide spectrum of compositions in the same eruptive cycle and rare resurgence. Resurgence is always related to calderas with major axis > 14 km and to magmas with > 26 vol% of crystals. The observed main differences are highly dependent on tectonic variables, such as slab anchoring in the lower mantle and maturity of the subduction zone. Large calderas are related to thick crust, horizontal extension of the upper crust and zones of heat flow peaks. In the CVZ, calderas are also related with a high magmatic flux at the base of the crust and melted magma bodies within the crust, in addition to crustal weakness zones. The SVZ, with lesser crustal thickness, lacks the presence of large magma bodies and thus, also lacks giant caldera complexes.

Post-Variscan tectonic and landscape evolution of the Elbe Fault Zone and the Lusatian Block based on apatite fission-track data and geomorphologic constraints

The Elbe Fault Zone is a continental scale NW striking structure that contains faults of different kinematics. Some faults were active at different times since at least the Variscan orogenesis in the Late Carboniferous. The Lusatian Fault (LF) was one of the major structures of the Elbe Fault Zone in the Late Cretaceous, when NW striking faults of Central Europe experienced basin inversion and thrusting during a short lived pulse of contraction. The LF displays a curved appearance on earth surface, what is not compatible with its activity at only one well defined event. Possibly, different faults where active along strike of the LF at different times.Apatite fission-track thermochronology can distinguish differential exhumation or burial of neighbouring fault blocks, if the vertical difference comprised at least 1–2 km. This study presents apatite fission-track data from the footwall and the hangingwall block of the LF and some other faults in its vicinity. Our new data show a clear difference between the hangingwall block of the LF (the Lusatian Block) with uniform 70–85 Ma apparent ages and relatively long track lengths and the footwall-block of the LF with 66–173 Ma apparent ages and shorter and more variable track lengths. For one section of the LF, we can show, that the Cretaceous fault is not visible geomorphologically, but was substituted by a neighbouring Cenozoic reverse fault with a distinct fault scarp. We used thermal history modelling to determine the timing and magnitude of exhumation and burial events from apparent apatite fission track ages and track length distributions. We infer, that the present morphological expression of the LF consist of smaller faults of different kinematics and age and present an evolution model for this section of the LF.This study establishes the existence of similar striking reverse faults in direct vicinity and partly intersecting each other in the Elbe Fault Zone. This observation emphasizes the significance of the Elbe Fault Zone as a continental scale zone of crustal weakness. It has not only been reactivated several times since the Variscan orogenesis, but represents a cluster of similar striking faults of different age and kinematics.

Upper Crustal Structure and Magmatism in Southwest Washington: Vp, Vs, and Vp/Vs Results From the iMUSH Active‐Source Seismic Experiment

AbstractStructural details of the crust play an important role in controlling the distribution of volcanic activity in arc systems. In southwest Washington, several different regional structures associated with accretion and magmatism have been invoked to explain the broad distribution of Cascade volcanism in this region. In order to image these regional structures in the upper crust, Pg and Sg travel times from the imaging Magma Under St. Helens (iMUSH) active‐source seismic experiment are inverted for Vp, Vs, and Vp/Vs models in the region surrounding Mount St. Helens. Several features of these models provide new insights into the regional structure of the upper crust. A large section of the Southern Washington Cascades Conductor is imaged as a low Vp/Vs anomaly that is inferred to represent a broad sedimentary/metasedimentary sequence that composes the upper crust in this region. The accreted terrane Siletzia is imaged west of Mount St. Helens as north/south trending high Vp and Vp/Vs bodies. The Vp/Vs model shows relatively high Vp/Vs regions near Mount St. Helens and the Indian Heaven Volcanic Field, which could be related to the presence of magmatic fluids. Separating these two volcanic regions below 6‐km depth is a northeast trending series of high Vp and Vs bodies. These bodies have the same orientation as several volcanic/magmatic features at the surface, including Mount St. Helens and Mount Rainier, and it is argued that these high‐velocity features are a regional‐scale group of intrusive bodies associated with a crustal weak zone that focuses magma ascent.

High-frequency sequences, paleogeography, and syn-depositional tectonism on a shallow clastic ramp: Doe Creek and Pouce Coupe members of the Late Cenomanian Kaskapau Formation, Western Canada Foreland Basin

AbstractThe great Late Cenomanian transgression of the Greenhorn Sea is recorded in western Alberta and adjacacent British Columbia by shallow-marine deposits of the lower Kaskapau Formation that blanket underlying deltaic deposits of the Dunvegan Formation. Overlying the basal ‘A-X unit’ of the Kaskapau are the allostratigraphically-defined Doe Creek and Pouce Coupe units, each of which forms a SW-thickening wedge up to about 100 m thick. The Doe Creek and Pouce Coupe units comprise, respectively, 10 and 8 allomembers, each defined by a marine flooding surface. Doe Creek facies are organized in upward-shoaling successions less than about 10 m thick. Shallow-water marginal-marine facies prevail in the west and north, with transgressive mudstones overlain by thin, sharp-based shoreface sandstones that, in places, contain roots and dinosaur tracks. In some instances, marine mudstone grades directly upward into a rooted, muddy paleosol. Mapped regionally, these facies define a broad, shallow, SE-facing embayment approximately 200 x 200 km. Trace and molluscan fossils suggest that salinity was below normal marine level, and benthic fauna were, at times, stressed by high turbidity and sedimentation rate. The central part of the embayment is dominated by offshore mudstone that encloses isolated, sharp-based lenticular bodies of clean sandstone up to 9 m thick. Sandstones are ovoid to elongate in plan view, and commonly extend many tens of km N-S or NE-SW. These sandstones are interpreted as top-truncated lowstand delta and strandplain deposits that became isolated on the outer ramp following marine transgression. The sharp base of most shoreface sandstones implies that deposition took place during relative sea-level fall. All Doe Creek allomembers thicken into the foredeep, with the tectonically-flexed surface inclined to the SW. Palaeogeographic maps show, however, that lowstand shorelines were oriented approximately NE-SW, and that the depositional surface sloped to the SE. This geometric relationship implies that, despite tectonic subsidence increasing to the SW, the rate of sediment supply and efficiency of redistribution were able to maintain a wave-graded surface sloping to the SE. Because shorelines migrated across, rather than up and down tectonic dip, allomembers are interpreted to have been controlled primarily by high-rate, high-frequency eustatic cycles, rather than by tectonic movements. The Pouce Coupe unit represents a complete reversal of basin paleogeography relative to the Doe Creek. Pouce Coupe shoreface sandstones are stacked in a linear belt just west of 120°W and grade westward into offshore muddy facies that thicken to the WSW. The entire unit is truncated eastward by the K1 unconformity and is absent over most of Alberta. It is postulated that the Proterozoic Kiskatinaw domain acted as a crustal weak zone, forming a hinge that defined the eastern margin of the Pouce Coupe flexural depocentre.

Upper Crustal Weak Zone in Central Tibet: An Implication From Three‐Dimensional Seismic Velocity and Attenuation Tomography Results

AbstractA series of conjugate strike‐slip faults is the most prominent geologic feature in central Tibet and is considered to accommodate east‐west extension and coeval north‐south contraction. The development mechanism of the conjugate strike‐slip fault system is under debate because of unclear crustal physical properties and compositional variations. P and S wave arrivals from 414 local earthquakes recorded by the temporary Seismic Array Integrated Detection for a Window of Indian Continental Head array and the permanent China National Seismic Network were used for the velocity tomography, with additional P and S wave arrivals from 12 shots of the International Deep Profiling of Tibet and the Himalaya III reflection/refraction profile. The local earthquakes were simultaneously relocated with the updated velocity models. We also inverted for a three‐dimensional upper crustal Qp model with the same earthquake data set. The Vp structure near the surface shows that low‐Vp anomalies generally correspond to sedimentary basins and high‐Vp anomalies are related to exhumed metamorphic blocks in the study area. Relatively low Vp/Vs ratios in the upper crust indicate widely distributed quartz‐rich rocks. The low‐Vp zone from 0‐ to 10‐km depth (resolving depth limit) is spatially correlated with the Bangong‐Nujiang suture, possibly reflecting the compositional difference along the ophiolitic mélange belt accompanied by twin volcanic arcs from a double‐sided subduction. This interpretation is supported by relatively heterogeneous Qp values. This low‐velocity zone also implies relatively uniform stress and continuous deformation in the upper crust of central Tibet. The relatively weak materials in at least the upper crust would result in strain concentration and help the development of the conjugate strike‐slip fault system along the Bangong‐Nujiang suture.

Upper crustal fluids in a large fault system: microstructural, trace element and oxygen isotope study on multi-phase vein quartz at the Bavarian Pfahl, SE Germany

Fault systems are zones of crustal weakness and are used as pathways for ascending hydrothermal fluids. One impressive example is the more than 150 km long Bavarian Pfahl system at the SW boundary of the Bohemian Massif. Dextral ductile shearing of Moldanubian basement has shaped a several hundred meter wide zone of mylonites and ultramylonites. This setting was reactivated under brittle conditions and infiltrated by fluids resulting in a multi-phase quartz lode. Structural analysis using scanning electron microscopy cathodoluminescence (SEM–CL) images has been carried out in combination with trace element and oxygen isotope analysis on quartz along the system. Three successive quartz phases have been distinguished due to their structural details and trace element contents. Cryptocrystalline quartz fabric in the first quartz phase results from crystallization from a silica gel, whereas in phases II and III, quartz precipitation is characterized by repeated fragmentation and sealing, visible due to strongly varying CL intensities. The δ18O of Bavarian Pfahl quartz decreases from 13.8‰ in the NW to 8.5‰ in the SE, whereas quartz from the close-by-situated Wölsendorf fluorite deposit shows δ18O between 15.9 and 18.5‰ (vs. SMOW). Flat shale-normalized rare earth element patterns point towards an upper crustal origin of the hydrothermal fluids. The southeastern part of the Bavarian Pfahl is interpreted to represent a deeper crustal level with higher mineralization temperatures (about 350 °C) compared to the rest of the Pfahl (< 250 °C).

The Yanshan orogeny and late Mesozoic multi-plate convergence in East Asia—Commemorating 90th years of the “Yanshan Orogeny”

The Yanshan movement/orogeny has been proposed for 90 years, which is of special significance in the history of geological research in China. This study conducted a review by synthesizing major achievements regarding episodic deformation features, sedimentary and magmatic records of the Yanshan orogeny in China, and clarified the episodic tectono-magmatism and its geodynamic origins. The tectonic implications of the Yanshan orogeny are discussed in the context of global plate tectonics and supercontinent reconstruction. Lines of evidence from structural, sedimentary and magmatic data suggest that the Yanshan orogeny represents a regional-scale tectonic event that affected the entire China continent in late Mesozoic period. Numerous age and structural constraints consistently indicate that the Yanshan orogeny was initiated in the Jurassic (at ∼170±5 Ma). and was characterized by alternating stages of crustal shortening at ∼170–136 Ma, crustal extension at ∼135–90 Ma, and weak shortening at ∼80 Ma. The 170–136 Ma crustal shortening was reflected in the generation of two regional stratigraphic unconformities (the Tiaojishan and Zhangjiakou unconformities), which were initially named the A and B episodes of “the Yanshan Orogeny” by Mr. Wong Wenhao in 1928. Geodynamically, the Yanshan orogeny in East Asia was associated with nearly coeval oceanic subduction and continental convergence in the Paleo-Pacific, Neo-Tethys, and Mongol-Okhotsk tectonic domains. As a consequence, three giant accretionary-collisional tectonic systems were formed along the continental margins of East Asia, i.e., the Mongol-Okhotsk, Bangonghu-Nujiang, and SE China subduction- and collision-related accretionary systems. The Yanshan orogeny induced widespread crustal-scale folding and thrusting, tectonic reactivation of long-lived zones of crustal weakness, and extensive magmatism and mineralization in intraplate regions. Based on the time principle of supercontinent assembly and break-up, we propose that the mid-Late Jurassic multi-plate convergence in East Asia might represent the initiation of the assembly of the Amasia supercontinent, and the Yanshan orogeny might be the first “stirrings” that is a prerequisite for the birth of the Amasia supercontinent.