Tectonic Boundary Research Articles

A study of the symbiotic relationship between tectonic fissures and faults in the Fenwei Graben System, China

The Fenwei Graben System is an important extensional fault-depression zone, which is the tectonic boundary between the Ordos Block and the South China Block and the North China Block. The graben system presents an “S” shape, with a total length of 1200 km and a width of 30–60 km and an overall strike of NNE. Due to complex tectonic background and active faults, nearly 612 ground fissures develop in these basins, and more than 90% of these ground fissures are tectonic fissures, which are closely associated with fault activities. To reveal the plane symbiotic phenomena, profile symbiotic modes and symbiotic mechanism between tectonic fissures and faults in the Fenwei Graben System, the developmental characteristics were investigated in detail by surveying, mapping, trenching and drilling in the study area. The main conclusions are as follows. (1) The tectonic fissures of the Fenwei Graben System are mainly distributed along the margin faults of basins, the boundary faults between blocks and the hidden faults within blocks, causing serious damages to farmlands, houses and roads. The symbiosis between these fissures and faults is obvious on the surface. (2) The profile symbiotic morphology includes vertical extension type, syncline and anticline “y” type, and stepped type. (3) The symbiotic mechanisms include vertical growth, lateral rupture and clustery derivation. (4) The activities of underlying faults give impetus to the formation and expansion of tectonic fissures. The exposure of the fissures makes underlying faults continue to develop in the form of surface fractures.

Seismic Characteristics of the Eastern North American Crust With Ps Converted Waves: Terrane Accretion and Modification of Continental Crust

AbstractThe impact of past tectonic events on the formation and modification of continental lithosphere remains as an open question of fundamental importance. Eastern North America provides a complete record of supercontinent assembly and breakup over the past 1.3 Ga, serving as a natural laboratory for our understanding of continental crust and mantle lithosphere and for integrating geologic and geophysical observations. In this study, we used teleseismic Ps receiver functions to image the detailed distribution of crustal thickness beneath eastern North America. The radial‐component receiver functions were calculated from seismic waveforms recorded by a total of 659 broadband stations during 2010–2017, yielding a high‐resolution image of Moho depth distribution. The depths of the Moho and intracrustal layers vary within and across the major tectonic units. Specifically, there are distinct differences in crustal thickness between the northern and southern Grenville Province. A dipping intracrustal feature can be seen within the central Grenville Province, with the depth increasing eastward from 5 to 27 km. The Moho depth decreases southeastward across the Grenville‐Appalachian boundary, with a sharp Moho offset of up to 12–15 km in the central segment and a more gradual variation to the north and south. The thickness difference between the southern and northern Grenville‐aged crusts suggests different tectonic and/or exhumation histories during and after the Grenville Orogeny. The low‐angle eastward dipping crustal feature is interpreted to be a Grenville‐aged collisional structure. Differences in the steepness of the Moho offset along the strike of Appalachians probably reflect variation of the steepness of the subsurface boundary between Laurentia and accreted terranes with different intensities of postorogenic modification. The observed spatial relation between the geologically defined tectonic boundaries and crustal thickness variations provides new constraints on the depth extent of the tectonic units within the crust.

Location of the tectonic boundary between the Paleoproterozoic Jiao–Liao–Ji Belt and the Archean Longgang Block in the Lianshanguan area, northeastern China

The tectonic evolution of the Paleoproterozoic Jiao–Liao–Ji Belt, which is located in the North China Craton in northeastern China, is the subject of ongoing debate. Most previous studies on this belt have been based on traditional geological methods, with most of the data being obtained from the surface, leading to uncertainties regarding the deep geological structure of the belt. This has restricted our understanding of the tectonic character and development of the belt; however, this limitation could be overcome by geophysical studies. Here, we analyze and interpret newly collected rock physical property data (density, magnetic susceptibility, and resistivity) and non-seismic geophysical data (gravity, magnetic, and magnetotelluric sounding data) for the Lianshanguan area to infer the location, configuration, and nature of the tectonic boundary between the Paleoproterozoic Jiao–Liao–Ji Belt and the Archean Longgang Block. The Lianshanguan rock mass, which belongs to the Archean Longgang Block, shows a weak gravity anomaly (0–6 mGal) and high resistivity (103.6–104 Ω·m). The area south of the Lianshanguan rock mass consists of the Paleoproterozoic Liaohe Group and displays an intermediate magnetic anomaly (0–200 nT) and low resistivity (10–102.5 Ω·m). We therefore infer that the tectonic boundary of the Jiao–Liao–Ji Belt is located along the southern margin of the Lianshanguan intrusion. We also show that the tectonic boundary in the Eastern Liaoning–Southern Jilin area trends E–W and extends from Shuangtaling through Lianshanguan, Caohecheng, Balidianzi, and to Guaimozi. The boundary between the Longgang Block and the Jiao–Liao–Ji Belt dips predominantly to the south, suggesting an extensional tectonic regime during the Paleoproterozoic time.

New sedimentological and palynological data from the Yarkand-Fergana Basin (Kyrgyz Tian Shan): Insights on its Mesozoic paleogeographic and tectonic evolution

The Talas Fergana/Karatau Fault, is a major tectonic boundary separating the Kazakh-Turan domain to the west from the Tian Shan domain to the east. During the Jurassic, movements along the fault led to the opening of several basins. Still, the Mesozoic kinematics of the fault and the geodynamic mechanism that led to the opening of these basins are largely unconstrained. Located at its southwestern termination, the Yarkand-Fergana Basin is certainly the best exposed and however still poorly understood. In this study, we provide new sedimentological description of the Jurassic series from the northern part of the Yarkand-Fergana Basin as well as new palynological data. Following a Middle–Late Triassic period dominated by regional erosion, the onset of sedimentation in the Yarkand-Fergana Basin occurred during the Sinemurian(?)–Pliensbachian. The basin opened as a half graben controlled by the Talas Fergana/Karatau Fault and separated from the Fergana Basin by basement highs. Extension persisted during the late Pliensbachian–Middle Jurassic, leading to a general widening of the Yarkand-Fergana Basin. Finally, Late Jurassic–Early Cretaceous renewed tectonic activity in the area led to the inversion of the north Yarkand-Fergana Basin. The Early to Middle Jurassic timing of development of the Yarkand-Fergana Basin suggests that the coeval movements along the Talas Fergana/Karatau Fault are not associated to the collision of the Qiangtang block along the southern margin of Eurasia. We favor the hypothesis of an opening controlled by transtension related to far field effects of back-arc extension along the Neo-Tethys subduction zone to the west.

Geology, U‐Pb Geochronology, and Hf Isotope Geochemistry Across the Mesozoic Alaska Range Suture Zone (South‐Central Alaska): Implications for Cordilleran Collisional Processes and Tectonic Growth of North America

AbstractThe Mesozoic Alaska Range suture zone is defined by a transition from oceanic to continental terranes and is part of a 2000‐km‐long tectonic boundary throughout the northern Cordillera. Surface geologic mapping of the rock types across this suture zone provides critical information about the upper crustal configuration but provides little insight into the sedimentary, igneous, and metamorphic processes that occurred in deeper levels of the collisional zone. To better constrain the timing and mantle‐crust sources of collision‐related magmatism, we combine U‐Pb ages and Hf isotope compositions of detrital zircons from the three main components of the suture zone. U‐Pb/Hf data sets from inboard, continental margin samples have Precambrian‐Paleozoic ages with epsilon Hf(t) values ranging between +10 and −20. U‐Pb/Hf data sets from the outboard, oceanic terrane samples have Pennsylvanian‐Permian detrital zircon ages with epsilon Hf(t) values between +16 and +10. The U‐Pb/Hf data set of detrital zircons from Mesozoic strata that represent intervening sedimentary basin(s) that formed between the continental margin and oceanic terranes record Precambrian‐Mesozoic detrital zircon ages with epsilon Hf(t) values between +14 and −20. Results of the study document four Archean and Proterozoic global crustal magmatic events that are correlated to the tectonic growth of Laurentia. Also, three Phanerozoic magmatic events have been identified that represent more regional tectonic events. This study demonstrates that the combination of U‐Pb geochronology and Hf isotope geochemistry applied to detrital zircons is a powerful tool to define sediment provenance in collisional zones and delineate episodes of global and regional magmatism along convergent plate boundaries.

Fold-Axis Parallel Extension along the Southern Ending of the Quito (Ecuadorian Andes) Fault System: Implications in River Network and Aquifer Geometry

Extension parallel to fold axis represents a rarely documented tectonic process that occur in different tectonic settings such as large-scale orogenic belts, mountain ranges, and thrust and fold belts. Along the Ecuadorian Andes, extensional systems have only been reported as tectonic boundaries of local intermontane basins, pull-apart depressions and outer-arc stretching faults along hinge surfaces in compressional antiforms. The Quito fault system shows in planform view a right-stepping en-echelon arrangement which southernmost anticlinal ridge terminates as an arcuate structure locally dissected by the Saguanchi Gorge. Valley hillslopes of this transverse drainage display evidence of tectonic extension featured by several swarms of normal faults commonly dipping towards the valley bottom. Magnetotellurics sounding obtained across a N‒S line parallel to the crestline axis reveals three contiguous and distinguishable domains of which the central one shows a wedge-shaped geometry pointing down. This tectonic evidence for ongoing extensional deformation suggests that the process of fold-axis-parallel extension controlled the tectonic development and dissection of the Saguanchi Gorge across the anticlinal ridge. This episode marks a significant change in the groundwater recharge and aquifer storage conditions between the Quito basin and the inter-Andean Depression aquifer systems, currently in production to provide the potable water supply for the Quito city.

Evaluating CO2 flux and recharge source in geothermal springs, Garhwal Himalaya, India: stable isotope systematics and geochemical proxies.

Evaluation of geogenic carbon fluxes between solid Earth and its atmosphere is essential to understand the global geological carbon cycle. Some of the key geogenic CO2 suppliers are the magmatic mantle and metamorphic degassing from active and quiescent volcanoes, fault zones, geothermal systems and CO2 rich groundwater. Indian Himalayan geothermal field hosts about 340 geothermal springs in natural as well as artesian condition that eject hot waters and volatiles with varied temperature and chemical composition. These sites provide an opportunity to analyse tectonically driven gas emissions and their impact on regional and global climate. Here we adopt a method for direct measurement of Dissolved Inorganic Carbon (DIC ≈ HCO3) concentration in the geothermal springs to estimate geogenic CO2 flux from an active region based on water discharge and area of geothermal system between the tectonic boundaries of the Main Central Thrust (MCT) and Main Boundary Thrust (MBT) of the Garhwal (Northwest) Himalaya. In the study area, geothermal spring water contain high δ13CDIC ratio (- 8.5‰ to + 4.0‰ VPDB), and among the major ions, bicarbonate (HCO3-) varies by an order of magnitude from 1697 to 21,553μEq/L; chloride and sodium vary from 90 to 19,171μEq/L and 436 to 23181μEq/L. The elevated concentration of Cl- and Na+ in geothermal spring waters suggests affinity towards their deeper origin. These geothermal springs cover a large area of nearly 10,000km2 of the Garhwal region showing a significant discharge of CO2 rich water with an estimated carbon dioxide degassing flux of ~7.2 × 106mol/year to the atmosphere. Considering widespread occurrences of geothermal springs in tectonically active areas worldwide, the proposed direct measurement of DIC may be used as a reliable tool to estimate CO2 fluxes in different active orogenic settings within the Earth system. Results of stable isotopes of δ18O (VSMOW) and δD (VSMOW) in these geothermal spring waters follow the Global Meteoric Water Line (GMWL), suggesting affinity of their recharge through the meteoric origin.

Complex Rupture of an Immature Fault Zone: A Simultaneous Kinematic Model of the 2019 Ridgecrest, CA Earthquakes

AbstractThe 4 July 2019 Mw6.4 and subsequent 6 July 2019 Mw7.1 Ridgecrest sequence earthquakes (CA, USA) ruptured orthogonal fault planes in a low slip rate (1 mm/year) dextral fault zone in the area linking the Eastern California Shear Zone and Walker Lane. This region accommodates nearly one fourth of plate boundary motion and has been proposed to be an incipient transform fault system that could eventually become the main tectonic boundary, replacing the San Andreas Fault. We investigate the rupture process of these events using a novel simultaneous kinematic slip method with joint inversion of high‐rate GNSS, strong motion, GNSS static offset, and Interferometric Synthetic Aperture Radar data. We model the Coulomb stress change to evaluate how the Mw6.4 earthquake may have affected the subsequent Mw7.1 event. Our findings suggest complex interactions between several fault structures, including dynamic and static triggering, and provide important context for regional seismic source characterization and hazard models.

Identification of a nascent tectonic boundary in the San-in area, southwest Japan, using a 3D S-wave velocity structure obtained by ambient noise surface wave tomography

We derived a three-dimensional S-wave velocity model for the San-in area of southwest Japan to examine heterogeneous structures such as tectonic faults. Many earthquakes occur in this area, but much of the activity has been relatively recent, so the fault distribution has yet to be fully clarified. Here, we used continuous ambient noise data from a dense seismic network, deployed from November 2009 to extract Rayleigh and Love wave dispersion data between station pairs, and then applied a direct surface wave inversion to the phase velocities of each station pair to determine a three-dimensional S-wave velocity model. In the resulting model, faults and a previously unrecognized tectonic boundary appeared as low-velocity anomalies or velocity boundaries, and the velocity anomalies were also associated with many past earthquake hypocenters. These results contribute to our understanding of heterogeneous structures caused by recent tectonic motion and of possible future tectonic activity, such as intraplate earthquakes. Surface wave tomography using ambient noise recorded in dense seismic networks could also be applied in other parts of the world to reveal new heterogeneous geological structures (i.e., unrevealed tectonic faults) and could contribute to disaster mitigation.

Late Devonian paleogeography in the framework of global plate tectonics

This paper presents Late Devonian (Frasnian–Famennian) global and regional paleogeographic maps displaying present day coastlines, tectonic elements' boundaries, subductions, rifts, spreading centers, transform faults, paleogeographic configuration and volcanism 370 million years ago. The regional maps illustrate the paleoenvironment and paleolithofacies distribution. The Late Devonian was a time of the onset and development of a major collisional event, the Variscan orogeny. The trench-pulling (or slab-pull) effect of the north dipping subduction, which developed along the Laurussia margin, caused the creation of the back-arc Rheno-Hercynian basin, as well as the transfer of tectonic elements. These tectonic elements included Saxothuringian, Southern Proto-Carpathian and Balkan terranes. The Antler and Ellesmerian orogenies constituted major collisional events in North America. The spreading of the Paleotethys Ocean constituted the main extensional event. This spreading is associated with the movement of tectonic elements towards Laurussia, Siberia and Northern Kazakhstan. In addition, a branch of the Paleotethys Ocean was opened between South China and Gondwana, during the Late Devonian times. The spreading was displayed along the proto-Andean margin of western Gondwana and is thought to have opened the newly proposed Chilean Ocean. The development of major rift systems took place throughout Laurussia and Siberia. Late Devonian rifting was associated with volcanic activity, especially prominent in the Viluy rift in Siberia. The deposition, during the Late Devonian time, is characterized by the existence of large carbonate platforms with reefs on large continents and synorogenic flysch in collisional areas. The sea-level dropped towards the Devonian- Carboniferous boundary. The climate was undergoing change from greenhouse to icehouse. The following plate tectonic events could have influenced the extinction of biotas at the Frasnian-Famennian boundary: 1) The very extensive basaltic volcanism in Laurussia and Siberian and along the arcs. 2) The closure of the Rheic Ocean and the development of the Variscan orogeny. 3) Intensive spreading in the Paleotethys and the development of numerous subductions and volcanic arcs. These events resembled a transition from a rift to a drift phase, during the Permian–Mesozoic break-up of Pangaea. This break-up was associated with other biota extinctions. • The paper presents 10 paleogeographic Late Devonian maps. • Late Devonian was a time of the onset of Variscan orogeny. Spreading of the Paleotethys Ocean was the main extensional event. • Late Devonian rifting was associated with volcanic activity, especially prominent in the Viluy rift in Siberia. • The plate tectonic events could have influenced the extinction of biotas.

Tectonics of the Northern Urals and Western Siberia: General History of Development

A review of the materials obtained over the past decade on the geology and tectonics of the North Urals and its continuation in the basement of the West-Siberian Plate allows us to clarify and supplement characteristics of the geological features of the region, and substantiate our new conclusions. We suggested more precise vertical division of complexes developed in the region into structural stages: Archean‒Paleoproterozoic, Riphean‒Vendian (Timanides), Paleozoic‒Early Jurassic (Uralides, which consist of the Uralide sensu stricto and the Old Kimmerian sub-stages), platform, and the neo-Orogenic stages. New data were presented in support of the initially unified island-arc nature of the Schuchya, Voikar, and Khulga areas of the Tagil arc sensu lato, while an alternative interpretation of the origin of zircon clusters from volcanic-sedimentary island-arc complexes, for which the mantle but not microcontinental source was supposed, is given. A complex orocline structure of the Ural‒Pai-Khoi bend of the primarily linear island arc due to the two-stage collision of the arc with the continent in the Early Carboniferous and then in the Early Jurassic, is characterized. Using PUT sesmic transect and drilling data allowed us to clarify the synform structure of the axial part and the eastern limb of the Tagil synclinorium and its transition to the East Urals zone. New data have been obtained confirming the existence of a northwestern tectonic boundary under the central part of the Yamal Peninsula, which corresponds to the contact of the Paleozoides and the submerged part of the Siberian Platform. An abnormally high concentration of hydrocarbon deposits on the Yamal Peninsula are associated with the presence of rift junction intensively developed over the superplume branch and high-density heat flow, which characterizes the zone of high geodynamic activity permeable to deep fluid-gas flows.

Chapter 3 Tsunami hazard with reference to the UK

Abstract Tsunami present a significant geohazard to coastal and water-body marginal communities worldwide. Tsunami, a Japanese word, describes a series of waves that, once generated, travel across open water with exceptionally long wavelengths and with very high velocities before shortening and slowing on arrival at a coastal zone. Upon reaching land, these waves can have a devastating effect on the people and infrastructure in those environments. With over 12 000 km of coastline, the British Isles is vulnerable to the tsunami hazard. A significant number of potential tsunami source areas are present around the entire landmass, from plate tectonic boundaries off the Iberian Peninsula to the major submarine landslides in the northern North Sea to more localized coastal cliff instability which again has the potential to generate a tsunami. Tsunami can be generated through a variety of mechanisms including the sudden displacement of the sea floor in a seismic event as well as submarine and onshore landslides displacing a mass of water. This review presents those impacts together with a summary of tsunami triggers and UK case histories from the known historic catalogue. Currently, apart from some very sensitive installations, there is very little in the UK in the way of tsunami management and mitigation strategies. A situation that should be urgently addressed both on a local and national level.

Meso-Cenozoic uplift of the Taihang Mountains, North China: evidence from zircon and apatite thermochronology

AbstractThe Taihangshan Mountain Belt, in the central North China Craton, represents an important crustal and tectonic boundary. To explore the complex tectonic evolution of this area during the Mesozoic–Cenozoic, we gathered zircon and apatite (U–Th)/He thermochronology data along a vertical transect (elevation of 630−1584 m) of the northern part of the Taihang Mountain Belt. From our data, we observed three separate rapid cooling phases that occurred at 100 Ma, 50−40 Ma and 27 Ma. Combined with previously published geochronological ages, we suggest that the uplift of the Taihang Mountain Belt initiated during the Jurassic and experienced multiphase rapid uplift from the Cretaceous to the Cenozoic. The early Cretaceous rapid cooling/uplifting events are widespread in the North China Craton and are caused by the collision between the Okhotomorsk Block and the East Asian continental margin. The Eocene and Oligocene rapid cooling events correspond to the initial rifting and thermal subsidence of the Bohai Bay Basin, indicating a coupling between the creation of basins and mountains.

Clockwise rotation of the Tarim basin driven by the Indian plate impact

Tarim basin is usually regarded as a rigid tectonic block that resists the northward expansion of the Tibetan plateau that is associated with ongoing northward motion of the Indian plate. The structural complexity of its tectonic boundaries with adjacent units inferred from seismological studies suggests, however, that this simple view of a static block under north-south compression needs revision. Here, we demonstrate (from Global Position System data) that Tarim basin is rotating clockwise (at a rate of 0.461°/Myr) around a (virtual) vertical axis within the unit. This rotation logi-cally results from the asymmetric nature of the India-Asia collision zone as inferred from seismological studies and can explain the otherwise puzzling pattern of lithospheric interactions along the Tarim basin borders as well as the diachro-nous seawater retreat and the closing of the westerly moisture pathway that results in aridification and desertification of Tarim.

S-type granites in the western Superior Province: a marker of Archean collision zones

Detailed field observations indicate that Neoarchean S-type granites were emplaced along and (or) proximal to some terrane (tectonic) boundary zones in the western Superior Province, southeastern Manitoba. These S-type granites are characterized by the presence of at least one diagnostic indicator mineral, such as sillimanite, cordierite, muscovite, garnet, and tourmaline. They are medium- to high-K calc-alkaline, moderately to strongly peraluminous (ANKC >1.1), and contain >1% CIPW normative corundum. Compared with more voluminous, older I-type granitoids in tonalite–trondhjemite–granodiorite suites in the region, the S-type granites occur as relatively small intrusions and have high (SiO2 >72 wt.%) contents with a small silica range (SiO2 = 72.2–81.2 wt.%), but a large range of Zr/Hf (17.1–43.8) and Nb/Ta (0.3–16.0) ratios. These geochemical characteristics suggest that the S-type granites were derived from partial melting of heterogeneous sedimentary rocks deposited as synorogenic flysch that underwent burial and crustal thickening during terrane collision. Although the S-type granites display geochemical variations in individual and between different plutons, their low Sr (<400 ppm) and Yb (<2 ppm) contents and low Sr/Y (<40) and La/Yb (<20) ratios are consistent with a partial melting process that left a granulite-facies residue consisting of plagioclase, pyroxene, and ± garnet. The S-type granites display low zircon saturation temperatures (mostly <800 °C) and low emplacement pressures (<300 MPa), similar to strongly peraluminous leucogranites formed in the Himalayas. Therefore, we propose that the Neoarchean S-type granites in the western Superior Province, whose source rocks were deposited between colliding tectonic blocks between 2720 and 2680 Ma, may serve as a geological marker of some Archean terrane boundary zones.

The Generation of Plate Tectonics From Grains to Global Scales: A Brief Review

AbstractThe physics of rock deformation in the lithosphere governs the formation of tectonic plates, which are characterized by strong, broad plate interiors, separated by weak, localized plate boundaries. The size of mineral grains in particular controls rock strength and grain reduction can lead to shear localization and weakening in the strong ductile portion of the lithosphere. Grain damage theory describes the competition between grain growth and grain size reduction as a result of deformation, and the effect of grain size evolution on the rheology of lithospheric rocks. The self‐weakening feedback predicted by grain damage theory can explain the formation of mylonites, typically found in deep ductile lithospheric shear zones, which are characteristic of localized tectonic plate boundaries. The amplification of damage is most effective when minerallic phases, like olivine and pyroxene, are well mixed on the grain scale. Grain mixing theory predicts two coexisting deformation states of unmixed materials undergoing slow strain rate, and well‐mixed materials with large strain rate; this is in agreement with recent laboratory experiments, and is analogous to Earth's plate‐like state. A new theory for the role of dislocations in grain size evolution resolves the rapid timescale of dynamic recrystallization. In particular, a toy model for the competition between normal grain growth and dynamic recrystallization predicts oscillations in grain size with periods comparable to earthquake cycles and postseismic recovery, thus connecting plate boundary formation processes to the human timescale.

Rock Engineering: Where is the Laboratory?

This paper is based on an invited lecture presented at the ARMA (ARMA-American Rock Mechanics Association) 2018 Rock Mechanics/Geomechanics Symposium, Seattle, WA. June 17–20, 2018. At the time of the invitation, the author was preparing an “Appendices” to a book (Sikora (2018) Charles Fairhurst—The Long Shadow published (via Amazon) by Itasca Consulting Group) to be published by Itasca Consulting Group—but page limitations required that the “Appendices” be eliminated. The invitation from ARMA provided an ideal opportunity to present the essence of the notes to professional colleagues. This paper attempts to expand on the presentation in Seattle. At the International Society for Rock Mechanics (ISRM) in 1962, Dr. Muller emphasized the central importance of large-scale discontinuities and anisotropy in rock engineering, and the need to establish a discipline distinct from the continuum fields of elasticity and plasticity. With the benefit of over 50 years of hindsight, it is clear that both elasticity and plasticity are important in rock mechanics—but discontinuities, especially on the scale of engineering projects, can be critical. Having been involved with the development of rock mechanics at the University of Minnesota since the late 1950s; with Professor Cundall as a faculty colleague since 1972; and with the founding of Itasca Consulting Group in 1981, the topic of discontinuities in rock has been a prominent long-standing concern to the ‘Minnesota group’. Theoretical developments in mechanics are often stimulated by experimental observations in classical ‘bench-scale’ laboratories. Thus, elasticity theory was stimulated by Hooke’s experiments (1678) and plasticity by Tresca’s experiments (1864). Even if it was possible to construct a laboratory to test ‘specimens’ of a rock mass on a scale sufficient to include large discontinuities, separation from the rock mass would remove in situ forces from the specimen, resulting in unknown changes to the specimen. What are the options to establish the constitutive behavior of the rock mass? Where is the laboratory? This paper discusses past attempts to answer this question and suggests a direction for the future.Rock in situ is unlike any other material encountered in engineering. Typically, it will vary in age from several hundreds of millions to as much as a few billions of years. Rocks of different composition and mechanical properties are often adjacent to each other. Subject to changing tectonic forces and gravity over this period, the rock mass is mechanically complex, and usually contains systems of fractures and mechanical interfaces, varying from grain boundaries to tectonic plate boundaries. Within this range, discontinuities comparable in size to the dimensions of engineering projects in rock are of particular concern to designers. The International Society for Rock Mechanics [Recently re-named International Society for Rock Mechanics and Rock Engineering. The acronym (ISRM) has been retained] was formed in 1962 to focus attention on the need to develop mechanic-based design procedures to give due consideration to such discontinuities. The paper reflects on approaches taken to address this concern in the almost 6 decades since formation of ISRM. Early efforts concentrated on testing of large physical models in a laboratory, plus a variety of efforts to incorporate discrete discontinuities into continuum mechanics. Particular attention is given to the development of the Discrete Element Method (DEM), introduced by Cundall (Proc Symp Int’l Soc Rock Mech 2:129–132, 1971). Examples of the application of DEM to practical design problems and conclusions drawn from them are discussed. In some cases, results show important differences with the current procedures and empirical rules. Although most of the examples shown are drawn from mining, references are made to applications in other engineering fields, especially Civil Engineering and recent developments in Enhanced Geothermal Systems. Currently, the principal limitation to widespread application of DEM to rock engineering design problems is computational speed. This problem is one faced in many scientific and engineering disciplines, so it is anticipated that solutions will be developed in the coming several years. In the meantime, simpler representations of discrete fracture systems are used to develop valuable general insights to inform practical designs. Recent leadership by the US Department of Energy in rock mechanics research through FORGE and SubTER [Frontier Observatory for Research in Geothermal Energy (FORGE); Subsurface Science, Technology, Engineering, and R&D Crosscut (SubTER).] offers hope that the importance of subsurface engineering to the US is being recognized. A problem, mentioned recently by Hoek (2018) that needs to be addressed in the United States is that of developing an engineering workforce capable of applying analytical and numerical techniques sensibly to design in rock.

3‐D Modeling of Vertical Gravity Gradients and the Delimitation of Tectonic Boundaries: The Caribbean Oceanic Domain as a Case Study

AbstractGeophysical data acquisition in oceanic domains is challenging, implying measurements with low and/or nonhomogeneous spatial resolution. The evolution of satellite gravimetry and altimetry techniques allows testing 3‐D density models of the lithosphere, taking advantage of the high spatial resolution and homogeneous coverage of satellites. However, it is not trivial to discretise the source of the gravity field at different depths. Here, we propose a new method for inferring tectonic boundaries at the crustal level. As a novelty, instead of modeling the gravity anomalies and assuming a flat Earth approximation, we model the vertical gravity gradients (VGG) in spherical coordinates, which are especially sensitive to density contrasts in the upper layers of the Earth. To validate the methodology, the complex oceanic domain of the Caribbean region is studied, which includes different crustal domains with a tectonic history since Late Jurassic time. After defining a lithospheric starting model constrained by up‐to‐date geophysical data sets, we tested several a‐priory density distributions and selected the model with the minimum misfits with respect to the VGG calculated from the EIGEN‐6C4 data set. Additionally, the density of the crystalline crust was inferred by inverting the VGG field. Our methodology enabled us not only to refine, confirm, and/or propose tectonic boundaries in the study area but also to identify a new anomalous buoyant body, located in the South Lesser Antilles subduction zone, and high‐density bodies along the Greater, Lesser, and Leeward Antilles forearcs.

Simplifying Age Progressions within the Cook‐Austral Islands using ARGUS‐VI High‐Resolution 40Ar/39Ar Incremental Heating Ages

AbstractThe Cook‐Austral islands do not exhibit a simple linear age progression from a single point of active volcanism, in contrast to other Pacific island chains. Explaining this anomaly, however, has been difficult due to the low reliability of the existing K/Ar ages for these islands. Here we present 56 new incremental heating 40Ar/39Ar age determinations for eight of the Cook‐Austral islands. We show that these 40Ar/39Ar ages are on average 10–40% different and generally older than the K/Ar ages for the same samples, are more reproducible within single lava flows, exhibit less scatter among data from a single island, and are more reliable than published K/Ar age determinations. Our new data result in two clearly defined and matching age‐progressive trends, with origins at the Macdonald and Arago seamounts. This supports the hypothesis that the Macdonald and Rurutu tracks formed by two contemporaneous hotspots aligned in the direction of plate motion. Compared to other volcanic chains on the Pacific plate, the Cook‐Austral hotspot tracks record angular rotational plate velocities (0.96 ± 0.05 to 1.09 ± 0.04°/Ma) similar to Hawaii (1.15°/Ma) but faster than Samoa (0.63°/Ma). This may imply that hot spot location relative to tectonic boundaries may have an effect on the age progressions recorded by volcanic chains. Specifically, the Cook‐Austral and Hawaii hot spots have over the last 30 Myr been located truly intraplate and thus far away from any tectonic boundary, while Samoa has been proximal to the active Tonga‐Kermedec subduction zone.

Quaternary evolution of the Suluova Basin: implications on tectonics and palaeonvironments of the Central North Anatolian Shear Zone

The Suluova Basin is a prominent member of the wide transtensional Amasya Shear Zone located at the central part of the North Anatolian Shear Zone. This basin is crucial and provides well-resolved data to understand the evolution of transtensional tectonic zones as well as the morphological and paleoenvironmental changes of North Anatolia during the Quaternary. Analysis of detailed stratigraphical sections, faulting data, and mammal paleontology reveals that the Suluova Basin has started to evolve as a closed half-graben along the NW–SE-trending, SW-dipping basin bounding fault zone with normal slip in the early Quaternary. Initial sedimentation mode of the basin was dominated by alluvial fan facies associations. Progressive basin subsidence resulted in an expansion of a freshwater lake at the basin depocenter as faults propagated westwards. Further extensions in the basin were caused to initiate the E–W-trending southern tectonic boundary. Newly created accommodation space hosted a vast freshwater lake during the Calabrian (∼1.8–0.78 Ma) acting as a refugia for a rich faunal assemblage of large and small land mammals. The conditions prior to the onset of Middle Pleistocene (MIS19, ∼0.79 Ma) is marked with increasing regional erosion where paleo-Lake Suluova was captured by the regional river system. Synchronously, the next phase of the shear zone formation was introduced with E–W-trending dextral and NE–SW-trending sinistral strike-slip faults, cross-cutting the former basin structure, forming new depocenters. These faults are still active with noticeable seismic activity and comprise future risks for the major cities of the region.