Crustal Heterogeneity Research Articles

VOLUMETRIC AND SPATIAL SEGMENTATION OF THE TIEN SHAN LITHOSPHERE ACCORDING TO GEOPHYSICAL DATA

This article consolidates the results of studying the deep structure of the lithosphere of the Central Tien Shan, which aimed to identify the main tectonic elements in its geophysical models. We have compared the structural and geological data with the information on the deep structure obtained by geophysical methods and from the positions of earthquake hypocenters in the study area. According to geological concepts, the Tien Shan orogenic belt is characterized by longitudinal and transverse segmentation. The boundaries of the Northern, Middle, Southern Western and Eastern segments of the Tien Shan are deep-seated fault structures. In deep faults and channels of heat and mass transfer, endogenous processes are localized. High-velocity, geoelectrical and thermal models consider such faults and channels as contrasting objects that can be referred to as indicators of these processes.Our analysis of the locations of earthquake hypocenters from NNC, KNET, CAIIG, KRNET, SOME catalogues shows that seismic events are strongly confined to the fault zones and the boundaries of large blocks. A correlation between the anomalies of geophysical fields suggests the degree of inheritance of tectonic structures and the boundaries of the main tectonic segments of the Tien Shan. To compare the crustal and upper mantle heterogeneities reflected in different geophysical fields, we have analyzed seismic tomographic sections based on volumetric seismotomographic models geoelectric and velocity sections along profiles across the main tectonic elements of the study area. The sections are used to identify the zones with relatively low (i.e. reduced) seismic wave velocities and detect the deep-seated longitudinal segmentation of the folded belt. Objects showing anomalous seismic wave velocities are found in the seismotomographic sections at all the depths under consideration. The most contrasting differences in the velocities of P- and S-waves are typical of the depths of 0-5 km and 50-65 km, showing the most clearly observed Northern, Southern and Western segments of the Tien Shan. In general, the velocities of P- and S-waves at the Northern Tien Shan are higher than those at the Middle and Southern segments. We have analyzed the distribution of geoelectric heterogeneities identified from magnetotelluric sounding data in order to determine the boundaries of the main tectonic elements that are considered as the zones of increased electrical conductivity confined to the boundaries of the fault structures. The distribution of earthquake epicenters clearly reflects the segmentation of the Tien Shan into the Northern, Middle and Southern segments and shows the Western and Eastern Tien Shan relative to the Talas-Fergana fault. Ourstudies of the crust and the upper mantle of the Tien Shan have confirmed that the abovementioned tectonic segments have differences in their deep structures Based on a comprehensive analysis of the study results, we can qualitatively identify a relationship between the distribution of the velocity and geoelectric heterogeneities in the crust and upper mantle, seismicity and the stress-strain state of the crust.

Detrital zircon U–Pb ages and Hf-isotopes from Eocene intermontane basin deposits of the southern Canadian Cordillera

Lower Eocene strata exposed in the hinterland of the southern Canadian Cordillera were deposited in extensional–transtensional basins and provide insights into the evolution of the orogen. We measured stratigraphic sections and collected 22 sandstone samples for detrital zircon uranium–lead (U–Pb) geochronology (n = 2995) and hafnium isotope (εHf) values (n = 67) in order to reconstruct the Eocene physiography and sedimentary history of the region. Early Eocene sedimentation in the interior of the southern Canadian Cordillera occurred in alluvial fan, fluvial, lacustrine and paludal settings in multiple compartmentalized basins. In most locations, detrital zircons have ages of ca. 50 Ma, which are interpreted to be derived from the syndepositional Challis–Kamloops volcanics. Several locations have dominant detrital zircon populations with ages between ca. 145 and 200 Ma, suggesting distinct sediment sources and paleodrainages within the southern Canadian Cordillera. Maximum depositional ages record coeval sedimentation across the southern Canadian Cordillera hinterland, supporting the hypothesis that upper crustal deformation and basin subsidence occurred at the same time across the orogen interior. εHf(t) values of ca. 50 Ma and 145–200 Ma detrital zircons vary between −16 and +12, with more evolved εHf(t) values in the eastern portion of the study area and more juvenile εHf(t) values in the west. Differences in εHf(t) are interpreted to reflect crustal changes across the region, likely related to underlying Laurentian basement rock. Detrital zircons from intermontane basins may provide an additional tool for examining crustal heterogeneity in cordilleran orogens.

Tracing the Sveconorwegian orogen into the Caledonides of West Norway: Geochronological and isotopic studies on magmatism and migmatization

The Sveconorwegian orogen represents a branch of Grenville-age (~1250–950 Ma) orogenic belts that formed during the construction of the supercontinent Rodinia. This study traces the Sveconorwegian records from its type-area in the Baltic Shield of South Norway into basement windows underneath Caledonian nappes, by combining zircon U–Pb geochronology and Hf–O isotopes. Samples along a N-S trending transect reveal multiple magmatic episodes during Gothian (ca. 1650 Ma), Telemarkian (ca. 1500 Ma) and Sveconorwegian (1050–1020 Ma vs. 980–930 Ma) orogenesis as well as Sveconorwegian migmatization (1050–950 Ma). Newly documented 1050–1020 Ma magmatism and migmatization extend the Sirdal Magmatic Belt to a 300 km-long, NNW-SSE trending crustal domain, with the northern boundary corresponding to the gradual transition from Telemarkian to Gothian crust. These Precambrian crustal heterogeneities largely controlled the development of Caledonian shear zones. The ca. 1050–1040 Ma granitic and mafic magmas show similar isotopic signatures with slightly negative or positive εHf(t) and moderate δ18O values (6–7‰), which indicates that crustal reworking was more dominant than juvenile inputs during their genesis. The generation of leucosomes and leucogranites at ca. 1030–1020 Ma, which have a more evolved Hf isotopic composition, probably reflects an even higher degree of remelting of older crust. The Hf–O isotopic patterns show that Sveconorwegian magmas differ from typical arc magmas by lower involvement of sedimentary components and juvenile material. This makes the 1050–930 Ma magmatism incompatible with a long-term subduction setting. The ca. 1650–1500 Ma samples, in contrast, generally have juvenile Hf isotopic compositions associated with varying δ18O values of 4.5–9‰, consistent with subduction-accretion processes involving significant sedimentary recycling. This accretionary margin was most likely transformed into the Sveconorwegian orogen through collisional interactions of Baltica, Laurentia and Amazonia in the context of Rodinia amalgamation.

The characterization and distribution of polygonal impact craters on Ceres and their implications for the cerean crust

Polygonal impact craters (PICs) are supposed to form as a result of impacts into structure systems within the target. The structure systems are an umbrella term which encompass faults, joints, fractures, lithographic boundaries, and other planes of weakness. These impact craters are defined as having at least two linear rim segments with a recognizable angle between the segments. PICs have been reported on both rocky and icy bodies including Ceres. Using a near-global Cerean crater database, PICs were categorized into four different categories: (1) polygonal craters not related to visible structure systems (PICNVS), (2) polygonal craters related to visible structure systems outside of the crater (PICVSO), (3) polygonal craters related to visible structure systems inside of the crater (PICVSI), and (4) polygonal craters related to visible structure systems inside and outside of the crater (PICVSIO). Measurements of the number of linear rim segments, the angle between linear rim segments, mean angle between linear rim segments for the crater, length of linear rim segments, and the mean length of the linear rim segments for the crater were reported for each category. Additionally, the angle between nearby structure systems and the linear rim segments are determined, along with the crater's cardinal direction with relation to the structure system. The most frequent type of PIC is those not related to structure systems. Hexagonal craters are the most prominent polygon shape observed, which is consistent with the observed shape of PICs throughout the solar system. The visible structure systems which displayed the highest number of PICs related to them were the Junina and Samhain Catenae. The average angle between linear rim segments for PICs on Ceres is 121.99° ± 0.25° which is similar to the average angle between linear rim segments reported for PICs on Rhea and Dione. Polygonal craters are found in different degradation states, indicating structure systems have influenced the formation of PICs throughout much of Ceres' history. The observation of greater concentrations of PICs in the northern hemisphere may indicate target strength differences and support the idea of crustal heterogeneity across Ceres; however, the lack of PICs in the southern hemisphere may be due to the younger and larger impact basins (Urvara and Yalode) erasing these craters creating an observational bias.

Eikonal equation-basedP-wave seismic azimuthal anisotropy tomography of the crustal structure beneath northern California

SUMMARYDelineating spatial variations of seismic anisotropy in the crust is of great importance for the understanding of structural heterogeneities, regional stress regime and ongoing crustal dynamics. In this study, we present a 3-D anisotropic P-wave velocity model of the crust beneath northern California by using the eikonal equation-based seismic azimuthal anisotropy tomography method. The velocity heterogeneities under different geological units are well resolved. The thickness of the low-velocity sediment at the Great Valley Sequence is estimated to be about 10 km. The high-velocity anomaly underlying Great Valley probably indicates the existence of ophiolite bodies. Strong velocity contrasts are revealed across the Hayward Fault (2–9 km) and San Andreas Fault (2–12 km). In the upper crust (2–9 km), the fast velocity directions (FVDs) are generally fault-parallel in the northern Coast Range, which may be caused by geological structure; while the FVDs are mainly NE–SW in Great Valley and the northern Sierra Nevada possibly due to the regional maximum horizontal compressive stress. In contrast, seismic anisotropy in the mid-lower crust (12–22 km) may be attributed to the alignment of mica schists. The anisotropy contrast across the San Andreas Fault may imply different mechanisms of crustal deformation on the two sides of the fault. Both the strong velocity contrasts and the high angle (∼45° or above) between the FVDs and the strikes of faults suggest that the faults are mechanically weak in the San Francisco bay area (2–6 km). This study suggests that the eikonal equation-based seismic azimuthal anisotropy tomography is a valuable tool to investigate crustal heterogeneities and tectonic deformation.

Basement Structure and Styles of Active Tectonic Deformation in Central Interior Alaska

Central Interior Alaska is one of the most seismically active regions in North America, exhibiting a high concentration of intraplate earthquakes approximately 700 km away from the southern Alaska subduction zone. Seismological evidence suggests that intraplate seismicity in the region is not uniformly distributed, but concentrated in several discrete seismic zones, including the Nenana basin and the adjacent Tanana basin. Although the location and magnitude of the seismic activity in both basins are well defined by a network of seismic stations in the region, the tectonic controls on these intraplate earthquakes and the heterogeneous nature of Alaska’s continental interior remain poorly understood. We investigated the crustal structure of the Nenana and Tanana basins using available seismic reflection, aeromagnetic and gravity anomaly data, supplemented by geophysical well logs and outcrop data. We developed nine new two-dimensional forward models to delineate internal geometries and the crustal structure of Alaska’s interior. The results of our study demonstrates a strong crustal heterogeneity beneath both basins. The Tanana basin is a relatively shallow (up to 2 km) asymmetrical foreland basin with its southern, deeper side controlled by the northern foothills of the Central Alaska Range. Northeast-trending left lateral strike-slip faults within the Tanana basin are interpreted as a zone of clockwise crustal block rotation. The Nenana basin has a fundamentally different geometry. It is a deep (up to 8 km), narrow transtensional pull-apart basin that is deforming along the left-lateral Minto Fault. This study identifies two distinct modes of current tectonic deformation in Central Interior Alaska and provides a basis for modeling the interplay between intraplate stress fields and major structural features that potentially influence the generation of intraplate earthquakes in the region.

Potential seismogenic asperities in the Garhwal–Kumaun region, NW Himalaya: seismotectonic implications

The Garhwal–Kumaun region (29°–31.5°N; 77°–81°E) in NW Himalaya lies in the Central Seismic Gap where the accumulated strain energy is sufficient for a future great earthquake. A broadband seismic network is continuously monitoring the seismic activity in this region and recorded a total of 3634 well located seismic events justified within the acceptable error bars (ERZ, ERH < 5.0) from 2007 to 2018 with hypocentral distances ranging from 0 to 350 km. In the present work, frequency-magnitude distribution (b-value) and fractal dimension (Dc-value) are investigated along (NW–SE) and across (SW–NE) the strike of Garhwal–Kumaun Himalaya. The study area has been divided into three different seismogenic asperities. Our results revealed that the proximity of Rudraprayag–Chamoli region exhibits significant low b-value (0.702 ± 0.03) and Dc-value (1.38 ± 0.03) and indicates high stress accumulation. The region has witnessed 1999 Chamoli earthquake Mb 6.3 and 2017 Rudraprayag earthquake M 5.7. A detailed study of b-value and Dc-value variation as a function of depth is also examined. The composite trend in abrupt escalation of b-value is observed at depth ≈ 12 km for Garhwal region and ≈ 7 km for Kumaun region. A well demarcation of crustal heterogeneity, high stress regime and associated seismogenic asperity is plotted. The low b-value and Dc-value depth ranges are coinciding with mid-crustal ramp and exhibiting a potential zone (12–15 km) for future great earthquake in the Garhwal region.

Characterization of injection-induced seismicity at north central Oklahoma, USA

The characteristics (foreshock-aftershock patterns, main shock to the largest aftershock magnitude ratio, and difference in magnitude) of two moderate injection-induced seismicity (IIS) earthquake sequences from Oklahoma, USA, namely, Prague (M 5.7, 2011) and Pawnee (M 5.8, 2016) earthquake sequences, are investigated to comprehend the shallow crustal heterogeneity. The analysis of temporal variation of “b” value from 2002 to 2018 suggests an increase in b-value after 2009. A reduction in b-value after 2016, in response to the reduced injection volumes is noted. A sharp fall in b-value usually precedes the main shocks of magnitude M ≥ 3.5. The foreshock b-values are lower than the regional b-value and aftershock b-values are higher than the regional b-value within the error limits. The investigated earthquake sequences fall under Type 2 of Mogi’s model. The largest aftershock to main shock magnitude difference of 2, observed for the Pawnee earthquake, could be a response to drastic reduction in injection rates in May 2016. The characteristics of ISS observed at Oklahoma are similar to the observations from the reservoir-triggered seismicity (RTS). However, with multiple injection wells operating from time to time in the region with varying amount of fluids injected, the entire IIS at Oklahoma has an appearance of a swarm.

Cracks and fluids in the Northeast Tibetan crust: New insight into seismotectonics

We determine high-resolution 3-D images of seismic velocity (Vp and Vs) and Poisson's ratio (σ) of the crust and uppermost mantle beneath NE Tibet by jointly inverting first and later phase (PmP and SmS) arrival times from 6076 local earthquakes recorded at 872 seismic stations of the China Seismic Network and the portable ChinArray and SACHY projects. We then estimate 3-D images of crack density (ε) and saturation rate (ξ) using the obtained Vp, Vs and σ values. Our results reveal significant crustal heterogeneities and anomalous low-σ anomalies in the middle crust beneath the northeastern Bayan Har block, the western Qinling fold zone and the transitional tectonic belt to the east of the Tibetan Plateau. The low-σ features reflect partially saturated cracks due to dilatation or vaporization in the upper to middle crust, rather than lithologic variations alone. Moreover, high-ξ anomalies exhibit a wider distribution than that of high-ε anomalies, which may suggest that micro-cracks exist in more localized areas, such as in and around active fault zones, whereas fluids are permeating through wider areas in the crust. The discrepancy between ε and ξ in the middle crust may be associated with dehydration of materials from regional crustal flow and asthenospheric flow. Our results provide new information on the fault zone rheology and seismogenesis of the study region, such as the generation of the 1920 Haiyuan earthquake (M 8.5), which might be related to fluid-filled, fractured rock matrices in the crust.

Impacts of Topographic Relief and Crustal Heterogeneity on Coseismic Deformation and Inversions for Fault Geometry and Slip: A Case Study of the 2015 Gorkha Earthquake in the Central Himalayan Arc

AbstractFault source models inverted from surface displacement measurements are a common tool to contextualize the seismotectonic significance of earthquakes. However, these models commonly invoke a simplified homogeneous elastic half‐space (flat Earth) description of Earth, which may bias the resulting fault geometries and slip distributions in the presence of spatially varying Earth's elastic structure and topography. Here, we developed both forward and inverse three‐dimensional finite‐element models with planar faults to systematically assess the impacts of topography, crustal elastic structure, and fault geometry on surface deformation and earthquake source inversions, with a focus on the 2015 Gorkha, Nepal earthquake. Our forward models with fixed fault slip confirm the impacts of topography and crustal heterogeneity on surface deformation. These impacts, however, contribute only maximally ∼10% of the total surface displacement, and the fault depth and dip dominantly control the coseismic deformation for the Gorkha case. When using homogeneous half‐space models to invert the synthetic displacement fields generated by forward finite‐element models, we find that ignoring crustal heterogeneity primarily biases inferred dip angle, while ignoring relief primarily biases inferred fault depth. Our inversions based on finite‐element models suggest that topography and crustal heterogeneity introduce negligible bias on recovering the Gorkha line of sight observations. Fault dip angle and depth can systematically trade off with slip area and slip magnitude, regardless of the presence of relief and crustal heterogeneities. Taken together, our findings highlight the importance of fault geometry on inverted fault kinematics with geodetic observations in the central Himalayan arc.

Accounting for uncertain 3-D elastic structure in fault slip estimates

SUMMARY Earthquake source estimates are affected by many types of uncertainties, deriving from observational errors, modelling choices and our simplified description of the Earth’s interior. While observational errors are often accounted for, epistemic uncertainties, which stem from our imperfect description of the forward model, are usually neglected. In particular, 3-D variations in crustal properties are rarely considered. 3-D crustal heterogeneity is known to largely affect estimates of the seismic source, using either geodetic or seismic data. Here, we use a perturbation approach to investigate, and account for, the impact of epistemic uncertainties related to 3-D variations of the mechanical properties of the crust. We validate our approach using a Bayesian sampling procedure applied to synthetic geodetic data generated from 2-D and 3-D finite-fault models. We show that accounting for uncertainties in crustal structure systematically increases the reliability of source estimates.

3D gravity modelling of Colorado and Claromecó basins: new evidences for the evolution of the southwestern margin of Gondwana

Although cratons have long been recognized as an important part of continental tectonic processes, we still have much to learn about their structural features and their relation to the genesis of basins, sedimentary thickness distributions, fold and thrust belts and surface processes. Contributing to a better knowledge of the crustal state and configuration of the southernmost part of the Rio de la Plata Craton, could shed light on the still controversial tectonic processes, which were responsible for the deformation of the southwestern margin of Gondwana during the Late Paleozoic. In particular, the deformation and uplift of the Sierras Australes, which are part of the Claromeco Basin (Buenos Aires, Argentina), would be closely related to the crustal structure of the southern limit of the Rio de la Plata Craton. Therefore, it is of crucial importance to investigate the possible existence of crustal heterogeneities underneath the Claromeco Basin, the Sierras Australes and the Colorado Basin, which can be related to structural features and weakness zones that could have played a major role in the tectonic evolution of the study area. For this purpose, we developed a 3D lithospheric-scale density model integrating various data, such as geological information, global gravity models, well data (thicknesses and lithologies), seismic tomography data (Moho depth), and pre-existing 3D density models of the Colorado Basin. Our model includes layers of sediments, crystalline crust and lithospheric mantle and therefore predicts the thickness variation of the upper and lower crust in the study area and of the main sedimentary sequences infilling the basins. Moreover, by analysing the model results, we propose a tentative location of the southern limit of the Rio de la Plata Craton and its possible tectonic relationship with transfer zones identified in the Atlantic platform southwards. Our results suggest that the southern boundary of the craton is located along the northernmost limit of the Colorado Basin, which is in contrast with the boundary proposed in previous works. Therefore, we propose that the Colorado Basin rifting process could have occurred along this weakness domain.

Quasi-waveform seismic tomography of crustal structures in the capital circle region of China

Seismic tomography is one of the main tools to explore the interior of the earth. In this study, the quasi-waveform seismic tomographic method is used for the first time to reveal the crustal structures in the capital region of China. 3-D high-resolution VP, VS and the Poisson’s ratio models are generated by inverting 29839 direct P- and 29972 direct S-wave traveltimes selected from 3231 local earthquakes. The results reveal strong crustal heterogeneities. The velocity anomalies at shallow depths are well consistent with surface geologic structures. The relatively low-velocity anomaly layer in the middle crust may be the result of multiple phases of tectonic activity. Earthquakes generally occurred on the boundaries of high- and low-velocity and Poisson’s ratio anomalies. There are obvious low-velocity anomalies below the hypocenters of the Tangshan earthquake and the historical Sanhe-Pinggu earthquake, implying the existence of fluids. The similar velocity structures around the hypocenters of the two earthquakes indicate that the occurrences of the two earthquakes may be related to the same mechanism. The highresolution velocity models provide important observational constraints on the small-scale heterogeneities and dynamic mechanism of the crust in the capital region of China.

Seismic study and spatial observations of a & b – values for the different earthquake hazard zones of India

This paper study the recent seismicity in Earthquake hazard zones in India. A large historical earthquake event catalog to cover the period of 1900-2018, the parameters date, time, latitude, longitude, depth and magnitude has been used to calculating frequency-magnitude distribution (b-value) of seismic hazard zones in India. To convert different magnitude scales into a single moment magnitude scale, the general orthogonal regression relation is used. Gamma distribution used for variable corrections also de-clustering method has used for removal of any non-Poisson distribution. The Indian seismic hazard zones are divided into five major seismic sources zones. The seismicity is characterized by Gutenberg-Richter relation. The parameter ‘b’ of FMD and relationship have been determined for these five seismic zones having different vulnerability environment. The ‘b’ values ranges between 0.43 to 1.16. The difference between the b parameters and seismic hazard level from seismic zones II to V considered for the study of high seismo-tectonic complexity and crustal heterogeneity, the parameter ‘a’ value changes accordingly the seismicity of the regions. The lowest b-values found in seismic zone II. The highest FMD b-value has been found in the seismic zone IV. Such high seismicity b-values may be associated with high heterogeneity. In this high b-value predict the low strength in the crust as well as seismic instabilities of that zone. These observations recommend not suggesting the location of important projects like atomic power stations, hydroelectric power stations, neutrino observatory projects, satellite town projects.

Probabilistic assessment of earthquake hazard in the Andaman–Nicobar–Sumatra region

The Andaman–Nicobar–Sumatra (ANS) region is a very hazardous area on the globe, which has witnessed a megathrust earthquake of Mw 9.2 on 26 December 2004 and several dozen large earthquakes in the past. We estimate earthquake hazard parameters (i.e. seismic a- and b-values, maximum expected earthquake magnitudes, mean return periods and probabilities of earthquakes) in 11 shallow and 4 intermediate to deep depth seismogenic zones of the ANS region using a uniform and comprehensive earthquake data for the duration 1906–2018. The earthquake hazard scenarios for all seismogenic zones are calculated using the Gutenberg–Richter frequency–magnitude relation and the Gumbel’s extreme value theory. The low b-values ( 0.90) for the earthquake of Mw 7.0 in the next 50 years and 100 years are observed in shallow zones 4–11 (the Sumatra and Nicobar Islands), while only intermediate to deep zone 4 (Sumatra) shows high probabilities. The low return periods ( 0.90) for the earthquake of Mw 7.0 are observed in shallow zones 5–11 (the Nicobar Islands and Sumatra regions), which suggest high earthquake hazard in these zones. The spatial variations of earthquake hazard parameters from one zone to another suggest a large grade of crustal heterogeneity and seismotectonic complexity present in this area.

Oxygen isotope ratios in zircon and garnet: A record of assimilation and fractional crystallization in the Dinkey Dome peraluminous granite, Sierra Nevada, California

Abstract The 119 Ma Dinkey Dome pluton in the central Sierra Nevada Batholith is a peraluminous granite and contains magmatic garnet and zircon that are complexly zoned with respect to oxygen isotope ratios. Intracrystalline SIMS analysis tests the relative importance of magmatic differentiation processes vs. partial melting of metasedimentary rocks. Whereas δ18O values of bulk zircon concentrates are uniform across the entire pluton (7.7‰ VSMOW), zircon crystals are zoned in δ18O by up to 1.8‰, and when compared to late garnet, show evidence of changing magma chemistry during multiple interactions of the magma with wall rock during crustal transit. The evolution from an early high-δ18O magma [δ18O(WR) = 9.8‰] toward lower values is shown by high-δ18O zircon cores (7.8‰) and lower δ 18O rims (6.8‰). Garnets from the northwest side of the pluton show a final increase in δ18O with rims reaching 8.1‰. In situ REE measurements show zircon is magmatic and grew before garnets. Additionally, δ18O in garnets from the western side of the pluton are consistently higher (avg = 7.3‰) relative to the west (avg = 5.9‰). These δ18O variations in zircon and garnet record different stages of assimilation and fractional crystallization whereby an initially high-δ18O magma partially melted low-δ18O wallrock and was subsequently contaminated near the current level of emplacement by higher δ18O melts. Collectively, the comparison of δ18O zoning in garnet and zircon shows how a peraluminous pluton can be constructed from multiple batches of variably contaminated melts, especially in early stages of arc magmatism where magmas encounter significant heterogeneity of wall-rock assemblages. Collectively, peraluminous magmas in the Sierran arc are limited to small &amp;lt;100 km2 plutons that are intimately associated with metasedimentary wall rocks and often surrounded by later and larger metaluminous tonalite and granodiorite plutons. The general associations suggest that early-stage arc magmas sample crustal heterogeneities in small melt batches, but that with progressive invigoration of the arc, such compositions are more effectively blended with mantle melts in source regions. Thus, peraluminous magmas provide important details of the nascent Sierran arc and pre-batholithic crustal structure.

Crustal origin of the West Florida Terrane, and detrital zircon provenance and development of accommodation during initial rifting of the southeastern Gulf of Mexico and western Bahamas

Abstract Well and seismic data were combined with existing radiometric data and 1328 new laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U–Pb zircon ages from igneous, sedimentary and volcaniclastic rocks in three wells to determine the origin and evolution of key crustal terranes, and the timing of initial rifting and synrift deposition in the southeastern Gulf of Mexico and western Bahamas. These data reveal that several southeastern Gulf of Mexico crustal terranes, distinguished by their zircon age distributions and geochemistries, were amalgamated into a single terrane of mixed Pan-African and Amazonian crustal affinity (the newly named West Florida Terrane) during the formation of Pangaea. Sinistral offset along the Florida Transfer Zone (FTZ) during the Late Triassic–Middle Jurassic was likely to have been facilitated by crustal heterogeneity at the boundary between the West Florida and Suwannee terranes, a prime area for reactivation during the break-up of Pangaea. Initial extension, igneous activity and synrift deposition in the region began during the Middle Triassic in the western Bahamas and offshore North Florida, and rapidly expanded into the South Florida Basin by the Early Jurassic. Peak igneous activity and accommodation in the region was diachronous from north to south, beginning in the north during the Early Jurassic (Sinemurian–Pliensbachian) before ending in the south in the Early–Middle Jurassic (Sinemurian–Aalenian/early Bajocian). Detrital zircons in sedimentary and volcaniclastic rocks were primarily derived from local erosion of pre-existing crustal terranes. Long-distance transport was likely for pre-Neoproterozoic and post-Cambrian Paleozoic zircons. Middle Jurassic detrital zircons in volcaniclastic rocks of the western Bahamas may have been deposited during ash-fall events from hotspot activity in the southeastern Bahamas.

Field observations and numerical models of a Pleistocene-Holocene feeder dyke swarm associated with a fissure complex to the east of the Tatara-San Pedro-Pellado complex, Southern Volcanic Zone, Chile

Magma is transported through the lithosphere as dykes which, during periods of unrest, may feed eruptions at the surface. The propagation path of dykes is influenced by the crustal stress field and can be disturbed by crustal heterogeneities such as contrasting rock units or faults. Moreover, as dykes propagate, they themselves influence the surrounding stress field through processes of stress transfer, crustal deformation and seismic failure. The result is the formation of arrested dykes, as well as contrasting strike and dip angles and dyke segmentation. Here, we study the mechanisms of dyke injection and the role played in modifying the stress field and potential propagation paths of later dyke injections. To do this we combine field data from an eroded and well-exposed shallow feeder dyke swarm with a suite of two-dimensional FEM numerical models. We mapped 35 dyke segments over a ~1 km long dyke swarm exposed ~5 km to the East of Pellado Volcano, in the Tatara-San Pedro-Pellado (TSPP) volcanic complex, Southern Volcanic Zone of the Andes. Detailed mapping of the swarm elucidates two preferential strike orientations, one ~N80°E and the other ~N60°E. Our numerical models simulate both the TSPP volcanic complex and the studied dyke swarm as zones of either magmatic excess pressure or as a rigid inclusion. The crustal segment hosting the volcanic complex and dykes is modelled using an elastic domain subjected to regional compression in select model cases. Model outputs provide the stress and strain fields resulting from the different geometries and applied boundary loads. The model results indicate that individual dyke injections can locally rotate the principal stresses such as to influence the range of orientations over which later dykes will form. The orientation of σ1 at the dyke tip ranges over 60° (±30° either side of the dyke tip) indicating that the strike orientation of later dykes will fall within this range. The effect of adding a bulk regional compression is to locally increase the magnitude of favorably oriented tensile stresses in the bedrock but to reduce the range of σ1 orientations to 40° (±20°). This implies that under a far-field transpressive stress regime, as is common in Andean settings, regional dyke swarms will tend to maintain their strike orientation parallel to the regional bulk stress. These results should be accounted for when studying periods of volcanic unrest in order to discern the location and orientation of potential fissure eruptions in active volcanic areas such as the Southern Volcanic Zone of the Andes.

Strain localization during burial and exhumation of the continental upper crust: A case study from the Northern Sporades (Pelagonian thrust sheet, Greece)

Abstract Extension is a key process controlling the post-orogenic exhumation of metamorphic rocks in subduction and collision zones. Previous studies have largely focused on the mechanics of localized core-complex style post-orogenic extension and the large-scale effects of various internal (e.g. rheology) and external (e.g. plate motions) parameters on the mode of extension. However, many regions on earth underwent rock exhumation during post-orogenic extension, which is characterized by distributed rather than localized deformation. We explore conditions of distributed deformation as illustrated by the Pelagonian unit in the Aegean subduction system. In particular, we explore the influence of structural inheritance related to the pre-extension shortening and mechanical stratigraphy on the localization of extension on the scale of the upper crust through detailed structural analysis on the islands of Skiathos and Skopelos. Additionally, the time frame of deformation has been established by 40Ar/39Ar dating of key shear zones. Shortening on the islands predominantly took place by ductile top-SW thrusting under low-grade metamorphic conditions, localized in weak calcite marble layers within the Upper Cretaceous and Upper Triassic carbonates at ~55 Ma. We show that the presence of shallow decoupling levels in the upper crust resulted in the formation of thin (several 100 m thick) thrust sheets that are defined for the first time on Skiathos. The Early Paleogene accretion of the Pelagonian upper crust to the upper plate (Eurasia/Rhodopia) was followed by the extensional inversion of the nappe stack. Extension was accommodated by opposite-sense, generally top-NE, ductile to brittle shearing, which localized at inherited heterogeneities such as reverse-sense shear zones and stratigraphic contacts at around 35 Ma, as suggested by our 40Ar/39Ar age spectra. The dense network of such northerly-dipping, inherited weakness zones resulted in a highly distributed pattern of extensional deformation dominated by layer-parallel shearing. We argue that the distribution of crustal heterogeneities substantially influences the style of post-orogenic extension.

Effects of Near‐Surface Complexities on Differential Travel Times and Amplitude Ratios Between PP and Its Precursors

AbstractThe characteristics of mantle discontinuities at depths of 410 and 660 km are important to study the mantle dynamics. With the advantage of global samplings, the differential travel times and the amplitude ratios of PP and its precursors (PdP) have been extensively used to estimate the topography and the impedance contrast of mantle discontinuities. According to previous studies, effects of large‐scale structures on the PP phase cannot be neglected. Theoretically, the complex near‐surface structure, including ocean layer and crustal heterogeneity, may complicate the PP phase. It is necessary to assess the uncertainties of waveform by taking into account the near‐surface complexities. Through the usage of an efficient algorithm for constructing synthetic seismograms, the effects of near‐surface structure on the differential travel times and the amplitude ratios between the PP and PdP are explored. We demonstrate that the crustal structure mainly affects PP‐PdP differential travel time with minor effects on the amplitude, while the ocean depth governs PP/PdP amplitude ratio rather than the travel time. Through incorporating the crust model, the anomalies of PP‐PdP travel time and PdP/PP amplitude ratio are globally mapped. A clear continent‐ocean pattern was observed in the measurements of differential times and amplitude ratios. Moreover, time shift caused by the near‐surface complexities may be wrongly attributed to the topography of mantle discontinuity and our tests show that the offset could be up to 15 km, which should be taken into account in future PP precursor studies.