Tectonic Terranes Research Articles

South Tibetan Detachment System (STDS), NW Himalaya: A possible Cambro–Ordovician tectonic terrane boundary, and its Cenozoic remobilization

The South Tibetan Detachment System is an important extensional fault zone, separating the Greater Himalayan Sequence from the overlying Tethyan Himalayan Sequence, and is well exposed in the upper reaches of the Dhauliganga valley, NW Himalaya. This fault system is characterized by the occurrence of an extensive Cambro–Ordovician granite belt between Sutlej and Dhauliganga valleys, although only a few small granitoids intrude the high-grade mylonite gneiss of the Greater Himalayan Sequence in its immediate footwall. These bodies yielded U-Pb zircon crystallization ages between 498.92 ± 5.5 Ma and 486.54 ± 2.3 Ma. This work postulates that the South Tibetan Detachment System evolved as a major proto-tectonic marginal extensional terrane boundary during the Cambro–Ordovician Kurgiakh/Bhimphedian Orogeny, when it was the conduit for emplacement of the Cambro–Ordovician granite belt. Denudation of the Neoproterozoic Greater Himalayan Sequence and the Paleozoic granites on its footwall provided approximately ∼ 10 km thick sediments into the Tethyan Basin due to this fault system as a master growth fault. Reactivation of this fault system controlled further melting and emplacement of the Higher Himalayan Leucogranite belt during the Cenozoic. Zircon growth is observed in two distinct modes: pulsative from the Late Eocene to Early Oligocene, with peaks at 33.99 ± 1.07 Ma, 30.53 ± 0.32 Ma and 25.03 ± 0.54 Ma; and in the continuous mode from 23.68 ± 0.94 Ma to 13.30 ± 0.30 Ma, in the Miocene, for nearly 10.0 myr. These datasets reveal some of the oldest pulsative movements in the Late Eocene–Early Oligocene during crustal thickening, thrusting and associated metamorphism, followed by continuous extension during the Miocene. Data from the South Tibetan Detachment System are distinct in character, and do not support either its eastwards younging or diachronous movements along the Dhauliganga valley.

On the Rhodopean protoliths and their Middle Jurassic to Early Cretaceous evolution. A review

The Rhodope Massif represents a large high-grade metamorphic complex situated in the Balkan Peninsula, straddling the border between Bulgaria and Greece. Geological studies carried out during the last thirty years clearly established that the complex is composed of three major lithotectonic units characterized by differing protolith ages and Alpine thermotectonic evolution. The Rhodope Massif consists of Upper and Lower terranes representing continental fragments composed mainly of pre-Mesozoic magmatic rocks and covered by Early Mesozoic sedimentary sequences. These two entities are separated by an Intermediate Terrane comprising a Jurassic magmatic arc and its host rocks tectonically intercalated with Jurassic ophiolitic fragments together with the Early Mesozoic sediments. Detailed analyses of the previously published geochronological data revealed that the three main tectonic terranes were juxtaposed in the Late Jurassic–Early Cretaceous during the continental collision and northward subduction of the Lower and Intermediate terranes. These rocks experienced Middle Jurassic to Early Cretaceous high-pressure metamorphism, followed by high-temperature one accompanied by anatexis. At that time, the Upper Terrane was metamorphosed and deformed only partially. Being part of the European margin, this terrane represented an upper continental plate during the subduction, and therefore is not considered as a part of the Alpine Rhodope metamorphic complex. It is important to note that, although most of the present-day fabric and major tectonic zones in the Rhodope metamorphic complex are younger than Early Cretaceous, and therefore they postdate the geological evolution discussed in this study, they often reactivate the old boundaries between the main terranes and units.

Geodynamics and nature of the basement of the northern Lhasa Terrane: Insights from Early Cretaceous highly fractionated I-type granites in the Beila and Dongga areas, central Tibetan Plateau

Collisional orogenic belts serve as excellent places for investigating geodynamic process and growth mechanism of the continental crust. As one of the largest collisional orogenic belts worldwide, the Tibetan Plateau is formed by the amalgamation of multiple tectonic terranes. However, the nature and composition of the crustal basement beneath the northern Lhasa Terrane, located in central Tibetan Plateau, is still a matter of debate. Here, we investigated a series of Early Cretaceous granites in the Beila and Dongga areas, northern Lhasa Terrane. Zircon U-Pb dating reveals that the Beila and Dongga granites were emplaced at ca. 118 Ma and 110 Ma, respectively. These granites are characterized by high SiO2 and K2O, low TiO2 and P2O5 contents and low Mg#. They exhibit enrichment in light rare earth elements, significant negative Eu anomalies, and depletion in Ba and Sr. These geochemical features suggest that they belong to highly fractionated I-type granites. Both the Beila and Dongga granites display similar and enriched whole-rock Sr-Nd and zircon Hf isotope compositions, implying a likely common ancient crustal origin. Given the temporal sequences and redox conditions during their formation, it is highly probable that the Beila and Dongga granites formed during the syn-collision stage between the Lhasa and Qiangtang Terranes and the breakoff of the Bangong-Nujiang oceanic slab, respectively. Integrated with coeval magmatism in the central Tibetan Plateau, our new data reveal the presence of a regionally preserved ancient basement beneath the northern Lhasa Terrane, analogous to the equivalent basement beneath the central Lhasa Terrane.

Ni(Co) Laterite Deposits of Southeast Asia: A Review and Perspective

Southeast Asia has great potential for mineral exploration, and this region is well-known to host huge economic ore deposits located in complex tectonic terranes. Amongst these ore deposits, the Ni(Co) laterite deposits are mainly distributed in Indonesia, the Philippines, and Myanmar. There are two main types of Ni(Co) laterite deposits consisting of hydrous Mg silicate (or garnierite) and oxide ores, with limited development of clay silicate type. These deposits are influenced and controlled by the lithology of ultramafic bedrock, topography, climate, weathering, structures, and tectonic environment. The degree of bedrock serpentinization has an important influence on the grade of Ni laterite ore. Given the growing demand of modern society for Ni(Co) ore resources, deep research should be focused on a better understanding of the genesis of this laterite deposit and geological features of Ni(Co) ore, as well as its exploration applications in southeastern Asia. Improving current research and exploration methods by means of cutting-edge technologies can enhance the understanding of the Ni(Co) enrichment mechanism in weathered laterite and lead to the discovery of new deposits in Southeast Asia. Ni(Co) laterite deposits from this region, especially Indonesia and the Philippines, have the potential to be a source of scandium, rare earth elements, and platinum group elements.

The new global tectonic map—Analyses and implications

AbstractA full new tectonic model of the Earth is presented based a complete new tectonic interpretation carried out during a 10‐year project. It covers a huge amount of available geophysical and geological datasets, where ca. 11 000 tectonic elements such as faults and thrusts, transform faults, rift zones, passive margins and oceanic extension ridges and numerous other features have been mapped, and classified following the geological literature. The complete surface of the Earth has been subdivided into 1.180 larger and smaller tectonic terranes, in the domains of continental blocks, oceanic plates and mobile zones. Numerical multiscale statistical fabric analyses on orientation, length and area are presented on the digitised tectonic elements and the classified terranes using the first Digital Twin of the Earth. Graphical representation through maps with different projections and viewpoints, and planetary views, are presented to illustrate the new subdivision. Some important implications for plate tectonic reconstructions are being discussed.

Lateral Variations of Attenuation in the Crust of Alaska Using Lg Q Tomography

ABSTRACT We have conducted a crustal seismic (QLg) attenuation tomography study across Alaska using recordings from the EarthScope USArray from 2014 to 2019. The resolving power of the inversion is 150 × 150 km for most of Alaska, and it is 75 × 75 km in central and southern Alaska. Numerous fault systems and high mountain ranges are present across Alaska and accommodate compression in the north–south direction and shearing of southern Alaska toward the west. These mountain ranges include the Brooks range in the north, the Alaska range in central Alaska, and the Aleutian range in the southwest. The average LgQ for all of Alaska is significantly higher than in the western United States and Canada. This lower average attenuation impacts seismic hazard estimates for the region. According to the tomographic results, we see a significant variation of the QLg values from low to high across the southern part of the Brooks range. Also, we found higher attenuation in the southeast region of Alaska, where the Wrangell volcanoes are located. Moreover, we see an area of lower attenuation associated with weak frequency dependence in the south-central region of Alaska next to Anchorage. Another anomaly with lower attenuation can be seen extending from central Alaska to southeast Alaska, possibly associated with the Yukon–Tanana terrane. There are a few areas like southwest Alaska associated with the Togiak terrane and an area next to Fairbanks in Alaska’s interior that shows lower attenuation with lower frequency dependence and higher attenuation with higher frequency dependence, respectively, for low frequencies up to 3 Hz. Our model’s highest η zones (η≳95) are mostly confined to major tectonic terranes and other major tectonic elements such as faults and fractures. Regional variations in crustal attenuation can impact local seismic hazard estimates if incorporated into the hazard analysis.

The Curie depths of the United Arab Emirates: Implications for regional thermal structures and tectonic terranes

We have determined the depth of the Curie isotherm in the United Arab Emirates (UAE) using a modified centroid method based on the fractal distribution of sources. The derived spatial coverage of Curie depths and geothermal gradients are consistent with the geothermal gradients derived from the bottom hole temperatures measured in exploration wells and reveal three NNE–SSW to NE–SW-trending regions of shallow Curie depths. These are: (1) the north-western offshore region of the UAE with Curie depths of 11–25 km; (2) in central UAE ranging from the Shah field in south-central regions of the UAE to the northern offshore region of Dubai, with Curie depths of 10–25 km, high geothermal gradients of 22.1–55.3 °C/km, and high heat flows of 55.3–138.3 mW/m2; and (3) in the eastern region of the UAE from Al Ain to Dibba and the Gulf of Oman, with medium Curie depths of 25–30 km. These regions of shallow Curie depths are flanked by two NS to NE–SW-trending regions of deeper Curie depths between 35 and 50 km with low geothermal gradients and heat flows of 11.1–15.8 °C/km and 27.8–39.5 mW/m2 respectively. We interpreted these regional NE–SW trends to be related to tectonic terranes composed of magmatic arcs and microcontinental fragments that have amalgamated to form the crust beneath the UAE. In addition, many large hydrocarbon fields are located immediately above or on the margins of these shallow Curie depth zones, suggesting that the shallow Curie depth could have played a critical role in the maturation, migration, and entrapment of hydrocarbons in the UAE. Furthermore, since these zones delineate areas of high heat flow (>100 mW/m2) and high geothermal gradients (∼40 °C/km) they represent areas in which future geothermal exploration could be focused.

Fault controlled geometries by inherited tectonic texture at the southern end of the East African Rift System in the Makgadikgadi Basin, northeastern Botswana

One of the three narrow rift belts that mark the southern end of the East African Rift System (EARS) intersects the Makgadikgadi Basin of northeastern Botswana. Although tectonic activity in the region is known to have influenced the evolution of these pans, the interrelationship between shoreline geometry, fault strikes, and the intersection of the underlying tectonic terranes has yet to be fully realized. We analyzed faults and subsurface structures in the region of the pans using a field investigation in combination with satellite imagery and geophysical data, to constrain the influence that the regional tectonic regime has had on the formation of the present-day pan geometry. We find that pan shorelines are controlled by the intersection of three preferred fault orientations which can be understood in the context of the “older” terranes they overlie, namely the Magondi Belt and the Limpopo Belt. We propose that the pronounced curvature of the southern Magondi Belt has influenced the eastward curvature of the rift-related faults and was likely produced by the impingement of the developing fold belt on the Zimbabwe Craton. Furthermore, limited focal mechanism solutions data from earthquakes north and south of the pans suggests a change in regional extension direction from NW-SE to NE-SW. Determining the relationship between these fault orientations and the underlying tectonic terrains is an important step in understanding the formation of the Makgadikgadi Basin, and more broadly the current tectonic regime of Botswana. The evidence of fault-controlled shorelines within an evaporitic environment may also have implications for regional groundwater activity.

Enigmatic Sulawesi: The tectonic collage

Sulawesi has a complex tectonic history that is affected by major plate re-organisations during the Cenozoic resulting in an extension-dominated setting in an overall setting of convergence of the Indo-Australian, Pacific and Philippine Sea, and Eurasian plates. It is a complex collage of disparate tectonic terranes brought into juxtaposition by a variety of tectonic processes which have occurred at very fast rates. The island is subject to a variety of geohazards related to earthquake and volcanic activity.

Earthquake Occurrences of the Major Tectonic Terranes for the Arabian Shield and Their Seismic Hazard Implications

The Arabian Shield, which contains a group of diverse terranes accreted during the Late Proterozoic, has experienced considerable historical and recent earthquake activities. From north to south, the Midyan terrane, Tabuk-Neom area, Hijaz terrane, Jeddah terrane, and Asir terrane make up the western section of the Arabian shield. In order to determine the earthquake occurrences and earthquake recurrence characteristics in the study area, an earthquake dataset containing 2,991 seismic events recorded between 1941 and 2019 with magnitudes of 1.0 and 6.2 and depths between 0 and 50 km was examined. The data were compiled by combining phase readings and information, such as origin times, hypocenter parameters, and magnitudes reported by the International Seismological Centre. The maximum likelihood method has been applied to calculate the Gutenberg–Richter recurrence parameters (a- and b-values) and magnitudes of completeness (Mc). The range of b-values is 0.53–1.04, which indicates that the study region experienced different stress level accumulations that cause earthquakes with different magnitudes. The Hijaz terrane is characterized by a high b-value (1.04 ± 0.34), which indicates a relatively low stress regime that resulted from the earthquakes stress release. The Midyan terrane is characterized by a low b-value (0.53 ± 0.10), which could be indicative of a relatively higher stress regime associated with a dominantly extensional stress. Mc values were found to be 1.4 in Midyan and Jeddah terranes. The lower value of Mc at Midyan terrane demonstrates appropriate station distribution and high earthquake rates. However, at Jeddah terrane, the seismic activities are poorly detected that probably lead to the small value of Mc. Higher Mc are evident in Hijaz terrane (Mc = 2.3) and Tabuk-Neom (Mc = 2.4), where the station distribution is very poor. The maximum expected magnitudes (Mmax) are found to be 6.0 for Midyan terrane, 5.4 for Tabuk-Neom, 4.7 for Hijaz terrane, 4.8 for Jeddah terrane, and 7.7 for Asir terrane. The average recurrence intervals of earthquakes with the Mmax are ∼7, ∼20, ∼6, ∼120, and ∼200 years for each seismic terrain, respectively. The probability of occurrence and returned periods of different magnitudes in each region indicate that regions related to the Najd strike-slip fault system are the regions for large probable earthquake occurrences.

The effect of the Najd shear deformation on the Pan-African belt of the central Egyptian Nubian Shield: a synthesis for the post-collision tectonic events

This study concerns the structural setting of the central Egyptian-Nubian Shield (El Shalul area) utilizing field-structural, remote sensing, petrological and geochemical data. The exposed basement comprises ophiolitic-mélange, arc-related metavolcanics, metasediments, metagabbro-diorites and granitoids. The area experienced two stages of deformation, pre-Najd (∼850–630 Ma) and Najd-related (∼630–580 Ma). The pre-Najd stage is represented by the assembly of arc-terranes and their N-ward extrusion while the Najd-related stage encompasses three deformation phases. D1 is post-collision extensional event, depicting lateral spreading of tectonic terranes and NW-ejection of ophiolites. The emplacement of El Shalul granite (∼630 Ma) and deposition of molasse sediments in E-W and NW-SE extensional basins (Zeidun and Meesar) are D1- related. Following extension is a protracted phase of compression, shearing and transpression. It was commenced with NW-SE shortening (D2) and deformed the extensional basins by folding and thrusting with mild effect on the other basement units. Sinistral shearing (D3a) and transpression (D3b) along the NW-trending faults superseded the D2 compression, while dextral shearing (D3c) on the ENE-WSW and NE-SW faults overprinted the NW-SE penetrative structures and controlled the emplacement of post-granitic dykes. The significant conclusions of this study include (1) El Shalul granite complex is a large alkaline granitic sheet emplaced during a post-collision extensional regime and suffered the subsequent ∼630–580 Ma top-to-NW sinistral shearing and SW-directed thrusting (not a gneissic core complex) and (2) The NW-SE and NE-SW structural trends are not conjugated.

An integrated remote sensing, aeromagnetic and field study of the Ablah shear zone (Asir Terrane, Saudi Arabia): Structural styles and implications for the tectonic evolution

• Surface and subsurface structural mapping was carried out for the Ablah shear zone. • Northerly striking thrust faults define the positive flower structure that impacts the shallow rocks in the zone. • Shallow thrusting ends at 1.5 to 2 km depth and is converted into single thrust shear. • Contraction tectonic regime initiated the shear zone and was later superposed by dextral-transpression regime. An integration of merged remote-sensing data having different spatial/spectral resolutions with aeromagnetic data and detailed field investigation is used to identify the composite Neoproterozoic rock complexes and define the structural architecture of the Ablah Shear Zone (ASZ). The SPOT and Landsat-8 data are processed as color composite images allowed the separation of different lithologic units and associated structures. Aeromagnetic data were analyzed using the Total Magnetic Intensity (TMI) and Reduced to the Pole (RTP) methods. The resulted anomalies show seven N-S opposite-dipping thrust faults that affect the shallow crustal rocks and coalesce together at about 1.5 to 2 km depth where the rock magnetic polarities inversed. This thin-skinned thrusting was initiated by a contractional tectonic regime in the E-W direction and superposed later by a transpresssion tectonic regime in the ENE-WSW direction. This study deals with the thin-skinned thrusting deformation in the ASZ that could be occurred during the deformation of the other shear zones that affect the internal part of the Asir tectonic terrane. Two controversial tectonic meanings describe the tectonic evolution of the ASZ, the first one is the thin-skinned thrusting tectonic that indicating shallow earth s crust deformation and the second one is the occurrence of some ophiolitic slices that indicating suturing tectonic deformation. The absent of the complete ophiolitic section encourages us regarding the ASZ as a structural shearing zone rather than a suture zone.

Electrical resistivity structure across the Tethyan tectonic belt in western Yunnan, SW China

The Tethyan tectonic belt in western Yunnan, SW China, has witnessed a complex, multistage Tethyan evolution since the Paleozoic, including a late stage of continental collision between the Indian and Eurasian plates in the Cenozoic. Consequently, this belt has become a collage of Gondwana-derived microcontinental blocks and arc terranes wedged between two continents, including the Tengchong block, Baoshan block, Lincang block, and Simao block from west to east. To better understand the structure of these ancient continental blocks and the dynamic mechanism of the Indian-Eurasian continental collision, we investigate the electrical resistivity structure of the crust and uppermost mantle in the region. Magnetotelluric data were collected along a NW-SE striking transect crossing multiple tectonic terranes and were then modeled with a 3-D approach. The final resistivity model shows conductors beneath the Tengchong volcanic area, which may represent magma chambers that feed the active volcanism in the area. The electrical structure between the Tengchong area and the Wanding fault consists of layer-like lower crustal conductors along the transect, which may indicate mechanically weak layers where the tectonic escape of the Tibetan Plateau may occur via lower crustal flow. The Lincang block shows higher resistivity characteristics than the surrounding crust and demonstrates a trend of eastwardly napping resistors onto the Simao block, making up a crustal boundary of high-resistivity and low-resistivity bodies. This observation suggests that the Changning-Menglian suture between the Baoshan block and the Lincang block may be the boundary between Gondwana and Cathaysia.

The evolution of basal mantle structure in response to supercontinent aggregation and dispersal

Seismic studies have revealed two Large Low-Shear Velocity Provinces (LLSVPs) in the lowermost mantle. Whether these structures remain stable over time or evolve through supercontinent cycles is debated. Here we analyze a recently published mantle flow model constrained by a synthetic plate motion model extending back to one billion years ago, to investigate how the mantle evolves in response to changing plate configurations. Our model predicts that sinking slabs segment the basal thermochemical structure below an assembling supercontinent, and that this structure eventually becomes unified due to slab push from circum-supercontinental subduction. In contrast, the basal thermochemical structure below the superocean is generally coherent due to the persistence of a superocean in our imposed plate reconstruction. The two antipodal basal thermochemical structures exchange material several times when part of one of the structures is carved out and merged with the other one, similarly to “exotic” tectonic terranes. Plumes mostly rise from thick basal thermochemical structures and in some instances migrate from the edges towards the interior of basal thermochemical structures due to slab push. Our results suggest that the topography of basal structures and distribution of plumes change over time due to the changing subduction network over supercontinent cycles.

Fast Inversion of the Earthquake Rupture Processes with Complicated Velocity Structure: An Application to the Earthquake of 2017 Mw 6.5 Jiuzhaigou, China

Due to the limitation of seismic station coverage or the network transport interrupted when the earthquake occurred, an accurate seismic shakemap may not be released to the public quickly. When the near-source observed waveforms for the intensity prediction technology used are incomplete, we synthesize the seismic waveform into observation waveforms. An accurate seismic rupture process is necessary to synthesize virtual station observations. So, we should release the rupture process as soon as possible after a large earthquake. Most large earthquakes occur at the junction of two or three tectonic terranes. With violent tectonic movements, fault basins and uplift zones are distributed on the edge of the plateau. With complex structural conditions, the 1D layered half-space velocity structure model could not meet the requirement of earthquake rupture process inversion. It takes much time to calculate 3D Green’s function with a 3D velocity model for the complete waveform inversion of the earthquake rupture process. To rapidly invert the rupture process as accurately as possible, according to the geological conditions of the station, we calculated several Green’s function libraries in advance. We extracted Green’s functions from these libraries for each site based on the sites’ coordinates once an earthquake occurs. The time we spend in extracting Green’s functions from several Green libraries equals that we spend in extracting Green’s functions from one single library. The applicability of this method was tested in the 2017 Jiuzhaigou M6.5 earthquake with complex structural conditions in the mountain uplift zone. With our model, the time we spent in calculating the rupture process was almost the same as that we spent with the 1D velocity structure model, which was far less than that we could have spent in calculating 3D Green’s function. The degree of fitting between the synthetic data and the observation data of our model was much higher than the fitting of the 1D velocity model, which means that the earthquake rupture process we determined was more reliable.

Neoproterozoic Tectonic Events of Egypt

AbstractThe Egyptian Nubian Shield (ENS) represents the northwestern part of the Arabian‐Nubian Shield and the northern extension of the East African Orogen. The ENS is regarded as being formed due to northward‐directed escape tectonics. It is characterized by mild accretion and suture zones dominated by major strike‐slip zones with a commonly sinistral sense of movement; some shear zones display a dextral sense of shear. It is dominated by gneisses and migmatites in the south, arc volcaniclastic metasediments and highly dismembered ophiolites in the central parts, whereas its northern part is dominated by late‐ to post‐tectonic granitoids. In southern Sinai, the Neoproterozoic rocks are grouped into four complexes, namely Feiran–Solaf, Sa'al–Zaghra, Kid and Taba. The ENS ophiolites were formed between 730–750 Ma, mainly in a supra‐subduction zone setting. The ENS has undergone a Neoproterozoic deformation history involving three successive phases: (1) Early N–S shortening phase (D1), (2) Syn‐accretionary phase (D2) and (3) Post‐accretionary phase (D3). The initial island‐arc stage (780–730 Ma) is a N–S shortening phase initiated by collision between the Eastern Desert tectonic terrane to the north with both the Gebeit and Gabgaba terranes to the south (830–720 Ma). During the arc‐splitting and back‐arc spreading stage (730–620), voluminous syn‐tectonic granitoids intruded into the ENS (750–610 Ma). The E–W‐directed compressional/transpressional phase (620–450 Ma) led to the overall uplift of the central part of the ENS and consequently the development and exhumation of the core complexes in oblique convergent zones. The E–W intense shortening deformation resulted also in the formation of NW‐ and NE‐striking sinistral and dextral strike‐slip shear zones, respectively. The latest periods of the E–W‐directed compressional/transpressional regime were characterized by deposition of the molasse‐type Hammamat Sediments unconformably over the Dokhan Volcanics, or interbedded with them. The combined thrusting, folding and sinistral‐reverse shearing structures have been interpreted to resulted from the E–W‐directed compressional/transpressional phase in response to the oblique shortening of the Arabian‐Nubian Shield between East and West Gondwana.

Zircon Hf‐isotope constraints on the formation of metallic mineral deposits in Thailand

AbstractThe effects of the Sibumasu–Indochina Terranes collision created several kinds of mineral deposits in Thailand, which include porphyry–skarn copper–gold, epithermal gold and antimony, orogenic gold–antimony–tungsten and tin‐tungsten mineralization among others. The deposits show a distinct spatial zonal distribution and occur in specific tectonic terranes. Combining regional geological data and ore deposit distribution data with Hf‐isotopic data of zircons in igneous rocks can be used to investigate the relationship between crustal construction processes and metallogeny. In this study, we investigated the Sukhothai Fold Belt, which is composed of quartz monzodiorite, granodiorite, syenogranite, and monzogranite of I‐ and S‐type affinities. All granitoids were analyzed for zircon U–Pb geochronology and Lu–Hf isotopic analysis. The granitoids of the Sukhothai Fold Belt yielded U–Pb zircon ages ranging from ~243 to 202 Ma, which mark the timing of subduction to the syn‐collisional stage between the Sibumasu–Indochina terranes at ~243–237 Ma and the timing of post‐collision between the Sibumasu–Indochina terranes during 230–202 Ma. In addition, an age of ~43 Ma in the south of the Sukhothai Fold Belt may indicate intrusion during the sinistral movement of the Klaeng and Mae Ping fault zones resulted from the Indian–Eurasian plate collision. The Doi Tung quartz monzodiorite provided an age of ~350 Ma as a timing of formation of the Sukhothai Fold Belt. The negative and positive initial εHf values (−8.0 to +9.2) with two‐stage depleted mantle model ages (TDMC of 2.2–0.6 Ga) of zircons from the Sukhothai Fold Belt granitoids indicate that the sources of their magma derived from partial melting of old continental crust and young oceanic crust, which probably mixed with a mantle‐derived magma. A zircon Hf‐isotope compilation including the data obtained in this study and previously reported values was used to prepare a map that allows a comparison between magmatic source and mineral deposit distribution in Thailand. The spatial distribution of Hf isotopic data reveals a distinct zonation, with initial εHf values decreasing from the east to the west, that is, from the western margin of the Indochina Terrane or the Loei Fold Belt to the Sukhothai Fold Belt, the Inthanon Zone and the Sibumasu Terrane. The magmatic source for the granitoids in the Loei Fold Belt is dominated by mantle‐derived components, as shown by positive average initial εHf values (+1.0 to +12.7), and contributed to porphyry‐related skarn copper–gold and iron and epithermal gold mineralization. In contrast, magmas in the Sibumasu Terrane and the Inthanon Zone originated from melting of old crustal materials, as indicated by mostly negative average initial εHf values (−15.1 to +0.8), and are responsible for S‐type granite‐related tin‐tungsten mineralization. The average initial εHf values (−5.0 to +11.0) from the intrusions in the Sukhothai Fold Belt suggest mixed sources, including evolved and juvenile magmatic materials, which generated the orogenic gold deposits and other vein‐type antimony, tungsten, fluorite, and base metal deposits. These results imply a close spatial correlation between the source of magmatism in each tectonic terrane of Thailand and different metal ore deposits. These isotope maps can be used as a powerful tool in the exploration for various commodities at the regional scale.

A crustal magnetic model of Britain obtained by 3D inversion

The national baseline aeromagnetic survey of Britain allows a uniform assessment of the shallow and deep magnetic properties of the British tectonic terranes. The most significant is that associated with destruction of early Palaeozoic oceanic lithosphere across the Iapetus Suture separating Baltica and Avalonia from the Laurentian terranes. Here a formal 3D inversion of a continuous swathe of the data is considered. The study provides a uniform volumetric whole crust assessment extending for over 1000 km. Normally a 3D inversion of magnetic data is controlled using a variety of constraints however this is not appropriate at the crustal scale due to our increasingly imprecise knowledge of lithology at increasingly greater depths. The main crustal interface encountered occurs at the Curie isotherm depth. We demonstrate the behaviour of introducing different magnetic crustal depths and suggest the crustal ‘magnetic depth’ of our models can be independently constrained using global or regional studies of the deep geotherm. Static magnetic data have no inherent depth resolution. Here an empirical ‘1D depth’ weighting and a more formal ‘3D distance’ weighting are assessed. The inversion procedure is regularised to provide stable models appropriate to the data and their errors. To gain confidence when using such a ‘geologically-unconstrained’ inversion, we compare our 3D inversion results with an existing geologically-constrained 2.5D profile inversion across northern Britain. A surprising agreement in the 3D susceptibility magnitudes is observed. The chosen study area traverses 10 British terranes and images their tectonic fabric by way of non-magnetic zones (i.e. susceptibilities <0.0001 to 0.001 SI) and magnetic zones displaying geological relevance and tectonic significance at deeper crustal levels. Here we discuss the more significant 3D model features which, by virtue of a continuous crustal-scale assessment and fitting the data with a high degree of fidelity, provide additional structural insights.

Updated tectonic terrane boundaries of Botswana determined from gravity and aeromagnetic data

We used existing high-resolution gravity and aeromagnetic data to map the crustal units underneath Botswana. The thick sedimentary cover in this region has always been a challenge to understand the geological and tectonic configuration of unexposed crustal terranes using traditional geological methods. We utilized a standard physical mapping technique to convert previously obtained gravity and aeromagnetic data into apparent density and magnetic susceptibility maps, respectively, of the crustal tectonic terranes. Then, the derived maps were fused into a single apparent physical map to facilitate its geotectonic interpretation. The results showed that most of the tectonic boundaries were inaccurately mapped due to limited availability of high-resolution geophysical information. Moreover, we found that some terranes were missing from the current tectonic map. We used the apparent physical property map and the basement rocks information from literature to significantly update the tectonic map of Botswana, which shows the spatial extent of the geological units beneath the Kalahari sedimentary cover. The new map gives an insight into the geodynamics of Botswana crust.

Patterns of recent deformation of the western Maracaibo block, northern Colombia and western Venezuela, based on integration of geomorphic indices with regional geology

The Maracaibo block is a triangular, continental tectonic terrane that includes two isolated Andean ranges of northern Colombia and western Venezuela: the Sierra de Santa Marta Massif (SSMM; maximum elevation 5700 m) in the west and the Perija Range (PR; 3600 m) to the east. The Cesar-Rancheria Basin (CRB) is an intermontane basin that separates the two ranges. To establish patterns of recent deformation of this elevated region and to infer its tectonic mechanism, we have integrated the following results: (1) analysis of 350 stream profiles and calculations of geomorphic indices, including stream length-gradient (SL) index, ratio of valley-floor width to valley height (VF), and hysometric curves for 20 watersheds in both ranges and (2) interpretation of three seismic reflection profiles within the CRB and adjacent areas. We determine that the northeastern part of the SSMM is tectonically quiescent based on its concave stream profiles, low geomorphic indices, and few vertical-step knickpoints. In comparison, we find that the central, southern, and eastern parts of the SSMM show tectonic uplift and recent fault control based on slope-break knickpoints and values in steepness and geomorphic indices with possible additional controls from lithologies of varying erosional resistance. Correlations between steepness, SL indices, slope-break knickpoints, and topographic elevations of the SSMM and central PR all indicate recent deformation of these areas. We use seismic reflection profiles from the eastern part of the CRB to confirm the existence of late Quaternary faulting and folding in these geomorphologically active areas. We propose that active, southeastward shallow (approximately 10°–15°) subduction of the Caribbean plate along the base of the South American continental crust produces active crustal deformation within the southern and eastern SSMM. The central PR and eastern CRB are also being deformed by active strike-slip faults.