Terrane Boundary Research Articles

The 2020 Mw 6.4 Koryak Highlands earthquake illustrates hidden seismic hazards in the northern Pacific Cordillera

Summary On 9th January 2020, a Mw 6.4 earthquake struck the central Koryak Highlands of eastern Siberia, northeast of the diffuse triple junction between the North American, Pacific and Eurasian plates. The largest earthquake recorded in the central Koryak Highlands to date, it provides an excellent opportunity to study the little-known active tectonics of this remote, sparsely instrumented region. We mapped coherent, coseismic surface deformation with Sentinel 1 Interferometric Synthetic Aperture Radar (InSAR), making this one of the highest-latitude earthquakes to be captured successfully with satellite radar, in spite of the rugged, snow-covered terrain. Elastic dislocation modelling, teleseismic back-projections, calibrated hypocentral relocations, and teleseismic moment tensor solutions are used to resolve a left-lateral fault trending northwestwards, proximal but perpendicular to a regional geological suture zone, the Khatyrka-Vyvenka Thrust. The earthquake probably ruptured unilaterally northwestwards along a 20 km long segment that appears indistinct in the local topography, and likely generated no surface rupture. We interpret that these observations are indicative of a structurally immature fault zone and estimate a seismogenic zone thickness of 10–15 km. The Koryak Highlands earthquake illustrates how terrane boundaries within cordilleran belts may continue to accommodate tectonic strain long after accretion, resulting in significant earthquakes even along hidden faults.

Vorticity Estimation of the Terrane Boundary Shear Zone of the Eastern Ghats Mobile Belt, and its Tectonic Significance

ABSTRACT The Terrane Boundary Shear Zone (TBSZ) of the Eastern Ghats Mobile Belt on the NW front against Bastar Craton shows northwesterly vergent thrust kinematics; and on the northern front with Singhbhum Craton shows dextral strike-slip tectonics. The kinematic vorticity numbers are as follows: the NW front, Wm(RGN) is 0.70–0.83 and Wm (Rs/ θ ) is 0.82–1.0; and the northern front, Wm(RGN) is 0.60–0.81 and Wm(Rs/θ) is 0.61–1.0. The estimates suggest that the Wm (Rs/ θ ) values are larger which indicates that the last incremental strain is dominated by simple shear. Further, both fronts are deformed by transpressional to simple shear deformation. However, the northern boundary shear zone has a transtensional component suggesting the TBSZ has undergone shear zone perpendicular extension. The development of the Mahanadi rift valley on the northern front may be related to such extension.

Evidence of High‐Pressure Metamorphism Along the Mahanadi Shear Zone in the Eastern Ghats Province, Eastern India: Implications on Tectonics and Continental Assembly Involving India and East Antarctica

ABSTRACTA suite of mafic granulite enclaves within mylonitised felsic gneiss occurring along the E‐W trending Mahanadi Shear Zone of the Eastern Ghats Province preserves evidence of high‐pressure metamorphism. Garnet‐clinopyroxene‐bearing mafic granulite contains a mineral assemblage of garnet + clinopyroxene + plagioclase + quartz + rutile which was formed after dehydration melting of a hornblende‐bearing protolith during M1 metamorphism that peaked at 1.1–1.4 GPa, 760°C–840°C. The retrograde stage (M1R) is marked by the formation of hornblende and symplectic intergrowth of clinopyroxene + plagioclase + orthopyroxene after garnet at 0.8–0.9 GPa, 760°C–810°C, suggesting an isothermal decompression type P–T path. The whole rock trace element and REE characteristics suggest a MORB‐OIB protolith for the mafic granulites. The host felsic gneiss has a granitic protolith which was emplaced in an arc setting. The rocks exposed south of the Mahanadi Shear Zone in the Phulbani domain are represented by granulites with contrasting metamorphic characteristics. The garnet‐orthopyroxene‐bearing mafic granulite within coarse‐grained charnockite and the aluminous granulite within felsic gneiss show evidence of biotite dehydration melting. The peak M1 assemblage in the aluminous granulite is represented by the assemblage spinel + garnet + quartz + plagioclase + K‐feldspar which was stable at 0.70–0.74 GPa, 904°C–935°C. M1R in this rock is characterised by coronas of garnet and sillimanite over spinel and the formation of matrix biotite at 707°C–806°C by near‐isobaric cooling. Similar isobaric cooling has been documented from the formation of garnet, clinopyroxene and quartz coronas on orthopyroxene in mafic granulite and garnet and quartz coronas on clinopyroxene, wollastonite and calcite in calc‐silicate granulite. The juxtaposition of lower crustal rocks showing clockwise and counterclockwise P–T paths across the Mahanadi Shear Zone implies a paired metamorphic character in a subduction–collision setting. Zircon U‐Pb and monazite U‐Th‐total Pb data show a complex history of the rock suite. The enclave suite of rocks within the Mahanadi Shear Zone underwent peak M1 metamorphism at ca. 980–960 Ma which was followed by decompression to a shallower level by ca. 960 Ma when the host granitic magma crystallised. Rocks occurring in the Phulbani domain (southernly placed crustal domain), on the other hand, underwent ultrahigh temperature metamorphism at shallower crustal levels broadly at the same time. We argue that the southern Phulbani domain of the Eastern Ghats Province, India, collided with the Angul‐Prydz domain of the Rayner Province, East Antarctica which eventually caused underthrusting of the former below the latter across the Mahanadi Shear Zone. In the context of the Eastern Ghats‐Rayner reconstruction, this indicates the closure of the intervening Mawson Sea. A second metamorphic event (M2) reworked the exhumed deep crustal rocks at ca. 900 Ma during the final docking of the Eastern Ghats‐Rayner belt against cratonic India. Our results clearly show that the Angul domain is an exotic block, and the Mahanadi Shear Zone is a terrane boundary shear zone suturing discrete domains of the Rayner‐Eastern Ghats orogen.

Southeastern extension of Singhbhum Shear Zone, Eastern Indian Shield: A detailed appraisal

Trans-crustal, northerly dipping, arcuate Singhbhum Shear Zone (SSZ) constituting the interface of Proterozoic North Singhbhum Mobile Belt (NSMB) and Singhbhum Craton (SC) is established over a strike length of about ∼200 km from Baharagorato Chakradharpur in the west beyond which it bifurcates around the wedge shaped Chakradharpur granite. Based on geological mapping and meso to micro scale structural analysis, present work examines the SSZ’s southeastern extension beyond Baharagora, indicating its southward continuation for about 50 km to Dubukidihi within the NSMB lithopackage. Bangriposi Shear Zone (BSZ) which defines the terrane boundary between the SC and NSMB in this sector, splays towards north as Bangriposi West Shear Zone (BWSZ) and both transects the Dhanjori Basin longitudinally. BSZ and BWSZ are interpreted to be cogenetic with SSZ. NSMB in this area comprises of pelitic and psamopelitic rocks intruded by mafic–ultramafic magmatism and granitoids. Considering a north to south compression regime during accretion of NSMB with SC, the study area, which forms the interface between the craton and mobile belt, is at an acute to obtuse angle with the direction of overall stress field. It is marked by several high strain zones characterized by N-S shear band cleavages with moderate easterly dips, and northerly pitching stretching/mineral lineation. Consistent westerly vergent folds on mylonitic plane and orientation of stretching lineation indicates top to WSW movement in the shear zones. Tectonically interleaved mafic–ultramafic schist within the Romapahari granite are sites of copper mineralization with enhanced nickel concentration pointing towards future exploration sites for epigenetic copper, uranium mineralization which is so characteristic of SSZ.

Evidence of an Archean-Paleoproterozoic micro-continent remobilized during the Neoproterozoic from the western Hoggar (Tuareg Shield, Algeria): Geodynamic evolution and implication on gold mineralization

The western boundary of the Tirek terrane in the Hoggar contains the two largest gold deposits in Algeria, namely the Amesmessa and Tirek gold fields. New geological mapping of this area and results of a combined petrological, geochemical and geochronological study (U-Pb zircon geochronological (LA-)ICP-MS method) reveals a Paleoproterozoic TTG basement, dated at 1965 ± 18 Ma (inherited zircons from 2771 to 2006 Ma), overlain by a supracrustal unit made up of quartzites and metapelites. Detrital zircons from metapelites yielded ages ranging from Archaean to Paleoproterozoic (2710 Ma–2050 Ma). Furthermore, sills and dykes of sub-alkaline orthogneisses that intrude the previous lithologies have been dated at 1843 ± 19 Ma. On the other hand, the migmatitic granitic-granodioritic batholith, which covers 50 % of the studied area and intrudes all units, has a subduction magmatism affinity and is dated at 663 ± 4 Ma.The study area was affected by a HT-LP metamorphic overprint during a clockwise P-T evolution with a peak conditions around 800–850 °C at 5 kbar followed by a decompression, then a cooling reaching the P-T conditions of 600 °C and 3 kbar. Uranium-Pb dating of monazite yields an age of 578 ± 5 Ma, which is interpreted to reflect the time of peak metamorphic overprint synchronous with the emplacement of synkinematic granites along shear zones with transcurrent movement. During this event, the western border of the studied area, the East Ouzzalian Shear zone, was the site of magmatic intrusions as well as the circulation of various fluids. This promoted an important fluid/rock interaction event, leading to extraction of gold by fluid percolation through the rocks and its deposition along N-S/NE-SW quartz vein ore shoots. The comparison with the western terranes of the Tuareg Shield (In Ouzzal, Iforas, Tassendjanet, and Kidal terranes) suggests that Tirek and these terranes represent relics of a single Archean-Paleoproterozoic continent. Currently, the Archean lower crust cross out in the central part of this micro-continent. The latter is mainly affected by Paleoproterozoic granulite facies metamorphism, unlike the Paleoproterozoic middle crust, which is exposed on either side and was strongly remobilized during the Pan-African orogeny (850–570 Ma), allowing gold mineralization during the later stages of this event.

Magnetotelluric evidence for the formation of the layered Sask Craton by flat slab subduction

Long-period magnetotelluric (MT) data were collected at 56 locations over the Sask Craton in 2021 and 2022. The data were combined with existing broadband data and inverted to produce a 3-D resistivity model of the Sask Craton and Trans-Hudson Orogen (THO). The model reveals a number of northeast striking electrically conductive crustal structures that extend into the mantle lithosphere. In the mantle lithosphere, these conductors coalesce into a single large low resistivity anomaly in the depth range 70–85 km termed the Northern Sask Craton (NSC) conductor. The resistivity of the NSC conductor is attributed to sulfides deposited along an interface between a flat slab that was accreted to the base of the pre-THO Sask Craton lithosphere during closure of the Manikewan Ocean. Kimberlites have erupted along the margin of the NSC conductor. The boundary of the conductor likely represents deep-seated faults and mantle terrane boundaries formed during flat slab subduction that allowed the ascent of kimberlite melts. The resistivity of the northeast-trending conductors can be interpreted as due to graphite and sulfides precipitated by past fluid or melt flow during ocean closure and orogensis. A number of these conductors are located beneath known mineral districts and trends and may represent source pathways for regional base and precious metal deposits. Other conductors may represent possible, previously unknown, regions hosting mineralization. Many of these conductors are associated with major regional faults and shear zones, which may be deep-seated features that helped to guide both kimberlites and mineralizing fluids. Of the prominent northeast-trending conductors west of the Sask Craton, one corresponds to the previously reported North American Central Plains (NACP) conductor. The new model shows that this conductor abruptly terminates at 54 °N and is not observed farther south in the model. This shows that the NACP is not as spatially continuous as previously suggested, suggesting that the tectonic processes that formed the THO were not as uniform along-strike as shown in existing tectonic models. The connection of one of the northeast-trending anomalies to the NSC conductor suggests that a previously unrecognized phase of east-dipping subduction may have occurred beneath the Sask Craton as the THO was formed.

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.

Suturing of the Archean Bastar craton with the Eastern Ghats Province to form the Greater Indian Landmass: Insights from geochemistry, U-Pb geochronology and phase equilibria modelling

The contact between the Archean Bastar craton (BC) and Proterozoic Eastern Ghats Province (EGP), central India, is marked by a suture zone termed Terrane Boundary Shear Zone (TBSZ). BC in this area is largely composed of hornblende-biotite granite with some mafic dykes. Rocks in the TBSZ include quartzofeldspathic (leptynite) gneiss, garnet-orthopyroxene-bearing granitoid, mafic granulites (Group A of cratonic affinity, and Group B of EGP affinity), Mg-Al granulite and an isolated exposure of orthopyroxene-bearing gneiss. Detailed geochemical analysis shows remarkable similarity between Hbl-Bt granite and Grt-Opx-bearing granitoid, with A-type affinity, and between mafic dykes and Group A mafic granulites. However, the Opx-bearing gneiss is geochemically distinct having I-type affinity, similar to TTG gneisses described from BC. Metamorphic phase equilibria analysis and trace element modelling shows that (i) melting of Opx-bearing gneiss would produce a ferroan granitic melt resembling the Hbl-Bt granite, (ii) metamorphism at appropriate P-T conditions would convert the granite to Grt-Opx-bearing granitoid and the mafic dyke to Group A mafic granulite. U-Pb geochronology of zircon constrains emplacement ages of the magmatic precursors of Opx-bearing gneiss and Grt-Opx-bearing granitoid as ca. 2.73 and ca. 2.5 Ga, respectively. These rocks were subjected to an early granulite facies metamorphism, followed by an amphibolite facies metamorphism, shearing and hydrous fluid flux. Geochronological data shows that the latter event took place at ca. 0.52 Ga, while the earlier granulite facies event can only be tentatively suggested to be of late Stenian/Tonian age. Collating all the evidence (including published geophysical and geochronological data), we suggest that the initial collision between BC and EGP took place during late Stenian/Tonian time as a consequence of formation of the Greater Indian Landmass, a part of Rodinia supercontinent. The TBSZ, probably initiated due to the late Stenian/Tonian collision, was reactivated and reworked tectonothermally at ca. 0.52 Ga, caused by far field stress effect of the Kuunga orogeny, which was strong enough to obliterate most of the imprints of the late Stenian/Tonian orogeny.

Seismic and Potential Field Constraints on Upper Crustal Architecture of Inner Bering Shelf, Offshore Southwestern Alaska

Southwestern Alaska encompasses a group of fault-bounded tectonostratigraphic terranes that were accreted to North America during the Mesozoic and Paleogene. To characterize the offshore extension of these terranes and several significant faults identified onshore, we reprocessed three intersecting multichannel deep seismic reflection profiles totaling ∼750 line-km that were shot by the R/V Ewing across part of the inner Bering continental shelf in 1994. Since the uppermost seismic section is often contaminated by high amplitude water layer multiples from the hard and shallow seafloor, the migrated reflection images are supplemented with high-resolution P wave velocity models derived by traveltime tomography of the recorded first-arrivals to depths of up to 2000 m. Additionally, other geophysical datasets such as seismicity, well logs, high resolution satellite-altimetry gravity, air-borne magnetics, ship-board gravity and magnetics, are also incorporated into an integrated regional interpretation. We delineate the offshore extension of the major mapped geological elements, including the Togiak fault (TGF), East Kulukak fault (EKF), Chilchitna fault (CF), Lake Clark fault (LCF), Togiak terrane (TT), Goodnews terrane (GT), Peninsular terrane (PT), Northern Kahiltna flysch (NKF) and Southern Kahiltna flysch (SKF) deposits, and the regional suture zone. The geophysical evidence from this study suggest that the major faults and terrane boundaries of southwestern Alaska, excluding the LCF, not only extend beneath the Bering shelf offshore but also appear to rotate, forming a trend parallel to the inactive Beringian margin. The LCF extends offshore but likely terminates in the northeastern part of the Bristol Bay basin. Additionally, the constraints from seismicity data indicates that while the major faults in southwestern Alaska exihibit some activity onshore, they remain dormant in the offshore region. These new findings will contribute to a better understanding of terrane accretion processes and existing fault models of southwestern Alaska.

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

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

Syntectonic Shear-induced Emplacement of Crystallizing Granite Magmas Evident from Magmatic Shear Sense, Mafic Schlierens, and Microgranular Enclaves in the Mesoproterozoic A-type Kanigiri Pluton, Nellore Schist Belt, Southeast India

ABSTRACT The observations on field-based mesoscale magmatic structures suggest that the A-type Kanigiri granite (KG) pluton, Nellore Schist Belt (NSB) has undergone a long plutonic history as evidenced by the continuum of deformation from early magmatic to the ductile regime at the waning stage of pluton evolution. The linear alignment of the KG pluton, which lies sub-parallel to the regional Terrane Boundary Shear Zone (TBSZ), and the long-standing deformation regime indicate the genetic link with this shear zone. The formation of mafic schlierens in the KG pluton owes to the mechanical crystal flow-sorting process during the replenishment of KG magma, which is further induced by the injection of crystal-charged microgranular enclave (ME) magmas in the crystallizing felsic magma chamber. The outflown ME magma blobs from the walls of the conduit during the invasion trigger the local turbulence, resulting the swirling of early crystallized mafic crystals of the host KG magma. This kind of swirling in the host KG magma must have been formed at an early stage of crystallization i.e., crystal-poor condition of the KG melts. When the MEs flow out, gravity causes them to interact with the host KG magma and attempt to dissolve into it. This results in schlieren rims formed around the MEs. The magmatic shearing observed in the KG pluton results from an active crystal-mush environment due to accumulated strain caused by the syntectonic movement as evidenced by the magmatic sense of shears. The locally formed slickensides are sub-magmatic origin during the upward pushing of replenishing ME and another pulse of KG magma. The presence, spread and prevalence of magmatic structures like ME magma globules, magma flowage, mafic schlierens, slickensides and shear sense indicators in the KG pluton suggest that the KG pluton underwent dynamic magma emplacement and evolution due to a shear zone, probably the TBSZ, which acted upon KG pluton in a syntectonic environment.

Early Paleozoic accretionary history of the Pearya terrane: New insights from igneous and detrital zircon signatures of the Kulutingwak Formation, Ellesmere Island, Nunavut, Canada

AbstractThe juxtaposition of the composite Pearya terrane and the northern Laurentian margin at Ellesmere Island, Nunavut, Canada, has significant ramifications for the Paleozoic tectonic history of the circum-Arctic region. Published tectonic models rely upon interpretation of the subduction-related Kulutingwak Formation as an indicator of Ordovician and/or Silurian accretion (Trettin, 1998). New igneous and detrital zircon U-Pb and Lu-Hf isotopic data from 16 samples collected in the Yelverton Inlet–Kulutingwak Fiord region of northern Ellesmere Island suggest that the Kulutingwak Formation of Trettin (1998) contains structural blocks derived from both the Pearya terrane and Silurian strata associated with the ancestral Laurentian margin. Data from this study demonstrate a complex provenance history for rocks within the Petersen Bay, Kulutingwak Fiord, and Emma Fiord fault zones, with age probability peaks of ca. 470 Ma, 650 Ma, and 960–980 Ma that suggest affinity with the Pearya terrane, and age probability peaks of ca. 1800 Ma and 2700 Ma that indicate connections to the Laurentian margin. The combination of these signatures in Kulutingwak Formation rocks suggests that the Pearya terrane was proximal to the northern Laurentian margin by Late Ordovician time. Silurian and younger strike-slip displacement on the major fault zones resulted in the incorporation of blocks derived from the Pearya terrane basement and Silurian clastic rocks into the Kulutingwak Formation. Silurian displacement along these strike-slip faults, which are integral components of the Canadian Arctic transform system, is recorded by syndepositional deformation structures in the Danish River Formation and prevented the transition from soft to hard collision of the Pearya terrane. The two-stage model for the Pearya terrane—accretion followed by significant translation—provides a process for developing complex steep terrane boundaries with contentious displacement histories that are common in accretionary orogens.

Geodynamic Settings of Late Paleozoic–Early Mesozoic Granitoid Magmatism at the Arctic Continental Margins: Insights from New Geochronological and Geochemical Data from the Taimyr Peninsula

Despite significant progress in Arctic geological studies, a number of principal questions concerning the Paleozoic collisional events remain unanswered. Therefore, the Taimyr Peninsula, representing the only outcropped high Arctic region where magmatic complexes, formed by Hercynian collision between the Siberian Craton and the Kara Block, are well exposed, is crucially important. In this paper we report new geochemical and geochronological data for intrusions in the poorly studied northeastern part of the Taimyr Peninsula. The obtained results in combination with published data show that supra-subduction magmatism at the southern active margin of the Kara Block continued from ca. 345 to 285 Ma (Early Carboniferous to Early Permian), and was followed by a post-collisional magmatic pulse that affected the whole Taimyr across terrane boundaries at ca. 280 Ma in the Early Permian. After cessation of the post-collisional magmatism at ca. 265 Ma, the Taimyr experienced extension, and voluminous magmatic series associated with a Siberian mantle plume were formed between 251 and 228 Ma during the Triassic. The studied post-collisional and plume-related intrusions of the Northeastern Taimyr are generally classified as evolved high-K I-type granites with adakitic affinity. The latter is a regional feature because the majority of the analyzed plume-related granitoids are geochemically similar to high potassium continental adakites. It is suggested that the adakitic geochemical characteristics of the plume-related granitoids resulted from melting of hydrated mafic lower crustal protoliths and were controlled by the source lithology. Comparison of the new results with data available for adjacent areas allows for correlation of terranes on a regional scale and sheds light on the evolution of the Arctic continental margins in general. In the Early–Middle Paleozoic, the Kara Block was part of a continental terrane that formed at the northern edge of Baltica as a result of Neoproterozoic Timanian orogeny. In the Early Carboniferous, the southern margin of Kara turned into an active margin, while its inferred continuation in the eastern Uralian margin of Baltica remained a passive margin until the Early Permian. This discrepancy can be explained by dextral displacement of Kara relative to Baltica that took place in the Early Carboniferous and was later accommodated by the formation of the Taimyr collisional belt in the course of the Early Permian collision between Kara and Siberia. After collision, the Taimyr was incorporated into the northern Eurasian margin as an uplifted block that experienced surface erosion and supplied clastic material in surrounding basins.

Seismic probing of buried ancient terrane boundaries - insight into Fennoscandia’s Palaeoproterozoic continental formation

Terrane boundaries are usually mapped from structures, lithologies and ages of the bedrock geology. Geophysical studies occasionally used to image terrane boundaries at depth typically yield localised information. In this contribution we demonstrate that it is possible to map Palaeoproterozoic terrane boundaries over large areas in northwest Fennoscandia through modelling of the crustal structure using receiver function inversion. Our new model suggests the existence of three “arms” of thick crust interpreted as deep expressions of Palaeoproterozoic terrane boundaries. We also observe previously unknown high velocity lower crust that we relate to Palaeoproterozoic rifting. The deep structural information shows that crustal structures can be preserved over billions of years and yields a three-dimensional crustal terrane model. This model improves our understanding of Palaeoproterozoic tectonics and continent formation processes.

Pan-African granitic magmatism of the Kaoko Belt: Tectonic perspective from its South American connection and insights into the crustal architecture of SW Gondwana

The Kaoko Belt in northwestern Namibia formed during the assembly of Gondwana in the Neoproterozoic to Cambrian Pan-African orogenic cycle. The belt has a complex tectonic architecture, in which a variety of basement units and metasedimentary associations became juxtaposed along shear zones and experienced widespread intrusive magmatism, generally confined to its western domain. We present a large new dataset of whole-rock geochemical and isotope data, as well as U-Pb zircon ages, encompassing a wide variety of granitoids and subordinate basic rocks across the Kaoko Belt. This is complemented by whole-rock isotope analyses from various basement and metasedimentary units and by a compilation of data from the literature. The new dataset strengthens previous models that correlate the granitoids in the Kaoko Belt to the coeval Florianópolis Batholith in the South American Dom Feliciano Belt, which we use to shed new light on the tectonic organization and evolution of the Pan-African orogenic cycle in the Neoproterozoic. The similarities in the compositional range of granitoids from both sub-domains of the Western Kaoko Belt, namely the Orogen Core and the Coastal Terrane, suggest that the entire zone comprises a heterogeneous but comparable and consistently diverse association of rocks. The comparison to South America underscores that, while the granitoids have “crust-like” isotopic signatures, these are in sharp contrast with those of the mostly Archean to Paleoproterozoic basement rocks of both the Kaoko and Dom Felicano belts. This suggests that the magmatic sources of the intrusive magmatism of the Kaoko Belt are likely a mixture of autochthonous crustal melting with an important mantle-derived input at some point after the Paleoproterozoic. This input was probably driven by accretionary orogeny and/or rifting that took place in the Tonian, Ediacaran, or both. The presence of more juvenile associations in the western domain of the Kaoko Belt may indicate an isotopic boundary within the belt that mirrors the tectonic configuration of the Dom Feliciano Belt in South America. This isotopic border, which would correspond to the Puros Shear Zone, could have acted as a terrane boundary from the earliest stages of the tectonic evolution of the Kaoko Belt.

Geochemical evidences of subduction along the Achankovil terrane boundary shear zone, Southern Granulite Terrane, South India

Geochemical evidences of subduction along the Achankovil terrane boundary shear zone, Southern Granulite Terrane, South India

The Manhattan project: Isotope geochemistry and detrital zircon geochronology of schists in New York City, USA

The geology of New York City (USA) consists primarily of metasedimentary rocks that were deformed during the series of orogenies between ca. 470 Ma and ca. 300 Ma that culminated in the amalgamation of Pangea. The rocks in New York City play a key role in understanding the tectonic history of these orogenies because they lie at a critical location at the boundary between the Northern and Southern Appalachian Mountains. The primary question addressed here is where these metasedimentary rocks originated prior to the assembly of Pangea. Through detrital zircon and whole-rock Nd isotope analyses, we show that all the metasedimentary rocks of New York City, mapped as the Manhattan Schist and the Hartland Group, are primarily derived from Laurentia as indicated by detrital zircon populations dominated by 1200−900 Ma grains and εNd values between −7 and −13. The results presented here do not necessitate an exclusively Laurentian source for the detrital material found in New York City, but the data strongly suggests protoliths represent sedimentary units that are primarily derived from the Laurentian margin. Another important result from this study is the limited contributions from any rift volcanics and/or Gondwanan material(s). There is some subtle variability across our zircon sample suite, but there is no convincing evidence for major changes in bulk provenance signal that would be consistent with derivation from vastly different continental sources for these rocks. The shared provenance signal observed here is counter to the previous suggestions that a major terrane boundary, often called Cameron’s Line, exists in New York City, separating Laurentian rocks from those of a Gondwanan affinity.

A newly discovered 2030–2010 Ma magmatic suite records the dawn of Proterozoic extension on the southern margin of the Yilgarn Craton

Uranium–lead zircon geochronology, zircon lutetium–hafnium isotope analyses and whole-rock geochemistry are used to investigate concealed basement geology and the location of Archean–Paleoproterozoic terrane boundaries in the northeastern Albany–Fraser Orogen (AFO). We identify a new major component of the AFO, and the first compelling evidence for ca. 2030–2010 Ma A-type felsic magmatism in western Australia. Prior to this study, the oldest rocks in the AFO that formed by extension and reworking of the southern Yilgarn Craton margin had protolith emplacement ages of ca. 1810–1800 Ma; our results extend this record by ca. 200 Myr and provide new insights into the crustal and tectonic evolution of the AFO. This phase of magmatism marks the start of a long-lived and widespread Paleoproterozoic extensional regime that prevailed across the West Australian Craton (WAC) for ∼ 400 Myr. Extension in the WAC was coeval with localised magmatism, volcanism and volcano-sedimentary basin formation in the North and South Australian cratons. Globally, our findings also correlate with the onset of prolonged extension and the generation of a vast tract of oceanic crust around the periphery of Nuna. We posit a petrogenetic link between ca. 2030–2010 Ma felsic rocks in the AFO and age–isotope-equivalent ultramafic–mafic rocks in the eastern Yilgarn Craton that are located along-strike and are interpreted to reflect a possible far-field and late-stage magmatic expression of the ca. 2060–2055 Ma Bushveld superplume.

Suture Reactivation, Slip Partitioning, and a Protracted Strike‐Slip Rate Gradient in the Denali Fault System, Southern Alaska, USA

AbstractActive strike‐slip fault systems commonly display along‐strike Quaternary slip rate gradients associated with fault bends and splay faults, which generate surface uplift by dip‐slip faulting or distributed “off fault” deformation. By analogy, the documentation of long‐term (107 yr) slip gradients on some continental strike‐slip fault systems implies long‐term coevolution of strike‐slip and dip‐slip fault systems. Here we leverage the observed ≥33 Myr right‐lateral slip gradient on the Denali fault, Alaska, USA to investigate the role of splay thrust systems in accommodating the slip gradient. We focus on the Broxson Gulch thrust system, which splays southwestward from the Denali fault in the eastern Alaska Range. Apatite and zircon (U‐Th)/He and fission‐track cooling ages from metasedimentary and metaplutonic rocks intersected by the thrust system record an along‐strike decrease in cooling ages commensurate with an increase in late Oligocene‐Neogene bedrock exhumation and shortening with proximity to the Denali fault. The dominant structure in the Broxson Gulch thrust system is the Valdez Creek fault, which is an upper crustal reactivation of the Valdez Creek shear zone–the main Late Cretaceous suture between western North America and outboard accreted arc terranes. After reactivation of the Valdez Creek shear zone at ca. 30 Ma, the thrust system grew by south‐vergent imbrication of the upper crust along thrust and reverse faults until at least 6 Ma. Incorporating results from the Broxson Gulch thrust system into the regional structural evolution of the Denali fault system reveals significant spatiotemporal heterogeneity in shortening adjacent to the Denali fault. Moreover, nearly all of the late Oligocene‐Neogene shortening south of the Denali fault was focused along reactivated terrane boundaries inherited from Mesozoic assembly of the North American Cordillera, and the spatial distribution of the inherited structures appears to control slip partitioning behavior of the Denali fault system across time scales ranging from 101 (historic seismicity) to 107 yr. The slip partitioning behavior of the Denali fault system highlights the mechanical importance of inherited structures leading to protracted shortening on splay thrust systems, which siphon slip from the master strike‐slip fault. We contend that the weakness of nearby reactivated terrane boundaries should be considered among other mechanisms commonly evoked to explain the partitioning behavior of continental strike‐slip fault systems (e.g., stress field rotation, obliquity angle, and strength of master strike‐slip fault).

Formation of Archean greenstone belts: Insights from an assemblage-scale study in the western Superior craton

Archean greenstone belts contain the oldest volcanic rocks on Earth and can provide critical insights into Earth’s early evolution. This study chooses the Sturgeon Lake greenstone belt in the easternmost Western Wabigoon terrane (WWT) of the Superior craton to illustrate an option for better understanding the formation of Archean greenstone belts. Our dataset includes outcrop observations, bulk compositions of 77 mafic to felsic volcanic samples, and in-situ U-Pb, Lu-Hf, and trace elements of zircon from 14 intermediate to felsic volcanic samples. The results revealed that this greenstone belt had broadly continuous volcanism from ca. 2780 to 2725 Ma, followed by a tectonic quiescence prior to a ca. 2707–2703 Ma volcanism at the terrane boundary. The intermediate volcanic rocks are characterized by variable Th/Nb and highly depleted Hf (zircon ƐHf(i) = +3 to + 6), interpreted to be derived from mantle sources ranging from those of normal mid-ocean ridge basalts (N-MORB) to those of enriched MORB (E-MORB), which commonly assimilated various felsic crustal components. The mafic volcanic rocks are dominated by low La/Sm basalts with negligible Nb, Ta anomalies and limited range of Th/Nb, forming a mantle array on the Th/Yb-Nb/Yb plot and consistent with oceanic crust of N-MORB to enriched N-MORB affinities. Some basalts from the ca. 2738–2725 Ma assemblages, however, gave high La/Sm, negative Nb and Ta anomalies, and limited range of Th/Nb plotting above and parallel to the mantle array. These basalts might be associated with passive subcretion and rollback of oceanic crust of the stagnant-lids tectonics or subduction of oceanic lithosphere as in modern-style plate tectonics, both of which are supported by trace element compositions of zircon that indicate hydrous, oxidized, large-ion lithophile element-enriched, and medium- to high-pressure magmas. Identification of three magma-series would not be possible if the data were lumped together, which is commonly seen in terrane- or craton-scale studies that sometimes propose contrasting interpretations of rock origins and Archean geodynamic processes. This detailed approach of distinguishing rock types and zooming into the subunits of a greenstone belt offers a practical solution to more accurately characterize Archean volcanic rocks and to better understand the early Earth.