Flat-slab Subduction Research Articles

Seismic imaging of the Northern Andean subduction zone from teleseismic tomography: a torn and fragmented Nazca slab

SUMMARY The Nazca-South America subduction zone in Ecuador is characterized by a complicated along-strike geometry as the slab transitions from flat slab subduction in the south, with the Peruvian flat slab, to what has been characterized as ‘normal’ dipping subduction beneath central Ecuador. Plate convergence additionally changes south to north as the trench takes on a convex shape. Highly heterogeneous bathymetry at the trench, including the aseismic oceanic Carnegie Ridge (CR), and sparse intermediate-depth seismicity has led many to speculate about the behaviour of the downgoing plate at depth. In this study, we present a finite-frequency teleseismic P-wave tomography model of the northern Andes beneath Ecuador and Colombia from 90 to 1200 km depth. Our model builds on prior tomography models in South America by adding relative traveltime residuals recorded at stations in Ecuador. The complete data set is comprised of 114 096 relative traveltime residuals from 1133 stations across South America, with the added data serving to refine the morphology of the Nazca slab in the mantle beneath the northern Andes. Our tomography model shows a Nazca slab with a fragmented along-strike geometry and the first teleseismic images of several proposed slab tears in this region. At the northern edge of the Peruvian flat slab in southern Ecuador, we image a shallow tear at 95–200 km depth that appears to connect mantle flow from beneath the flat slab to the Ecuadorian Arc. Beneath central Ecuador at the latitudes of the CR, the Nazca slab is continuous into the lower mantle. Beneath southern Colombia, the Malpelo Tear breaks the Nazca slab below ∼200 km depth.

Role of subduction dynamics on the unevenly distributed volcanism at the Middle American subduction system

A typical subduction of an oceanic plate beneath a continent is expected to be accompanied by arc volcanoes along the convergent margin. However, subduction of the Cocos plate at the Middle American subduction system has resulted in an uneven distribution of magmatism/volcanism along strike. Here we construct a new three-dimensional shear-wave velocity model of the entire Middle American subduction system, using full-wave ambient noise tomography. Our model reveals significant variations of the oceanic plates along strike and down dip, in correspondence with either weakened or broken slabs after subduction. The northern and southern segments of the Cocos plate, including the Mexican flat slab subduction, are well imaged as high-velocity features, where a low-velocity mantle wedge exists and demonstrate a strong correlation with the arc volcanoes. Subduction of the central Cocos plate encounters a thick high-velocity feature beneath North America, which hinders the formation of a typical low-velocity mantle wedge and arc volcanoes. We suggest that the presence of slab tearing at both edges of the Mexican flat slab has been modifying the mantle flows, resulting in the unusual arc volcanism.

Geology and geochronology of the Pinaya mineral district: Magmatism and porphyry-style mineralization in the southern extension of the Andahuaylas-Yauri belt, Southern Peru

The Pinaya district hosts porphyry Cu–Au mineralization in the Pinaya deposit and other mineralized centers. Cu–Au mineralization is contained in quartz stockworks with chalcopyrite and bornite associated with potassic alteration in early to intermineral daci-andesitic dikes and irregular intrusions emplaced within terrigenous facies of the Puno Group. Prograde, garnet-skarn formed in adjacent, carbonate-bearing host rocks, likely simultaneously with the potassic alteration of the productive intrusions. Both potassic and skarn associations were overprinted by progressive hydrolytic phases of sericite-clay and dickite-dominated weak and fracture-controlled advanced argillic alteration. The first was predominantly pyritic, but the second introduced localized chalcocite, digenite, and covellite in addition to pyrite in a high to very high-sulfidation environment. A later, porphyry-centered event of intermediate-sulfidation mineralization formed proximal quartz-adularia containing nominal amounts of precious metals and distal polymetallic veins. Supergene alteration and mineralization produced an irregular profile with mixed oxide-sulfides and supergene chalcocite.The age of the porphyry-related magmatism is bracketed by two U–Pb (zircon) dates of ∼28 Ma on early and intermineral porphyry phases. This age and the associated trace-element signature of the host intrusions (Sr, V, Sc) not only indicate that Pinaya was emplaced late in the evolution of the larger Andahuaylas-Yauri porphyry-skarn metallogenic belt, but also confirms that it can be confidently extended for ∼60 km to the south. It is proposed here that porphyry Cu–Au mineralization at Pinaya was emplaced at the transition from the flat-slab subduction that characterized the evolution of the Andahuaylas-Yauri batholith and related metallogeny to normal-slab subduction at 28 Ma.

Along-strike tectonic evolution of the Neogene Bermejo foreland basin and Eastern Precordillera thrust front, Argentina (30-32°S)

The Bermejo retroarc foreland basin system formed in flexural response to Cenozoic crustal thickening in the Andean orogenic system, specifically, the eastward propagation of the Precordillera fold-thrust belt and the basement-involved uplift of the Sierras Pampeanas. Previous work in the region has mainly focused on the mechanisms and expression of flat slab subduction and the structural geometry of the basement-involved Sierras Pampeanas and east-directed Precordillera fold-thrust belt at depth, advancing our understanding of the Bermejo basin history north of 31°S. However, the along strike evolution of the basin system to the south remains unresolved and contrasting tectonic models have proposed the Bermejo basin evolved synchronously versus asynchronously through time. Our study seeks to constrain the along-strike Neogene tectonics of the Bermejo basin via well-exposed Miocene fluvial stratigraphic intervals along the Eastern Precordillera, integrated with detrital zircon U–Pb geochronology provenance and detrital apatite (U–Th)/He thermochronology datasets. New data from two Neogene stratigraphic sections in the southern Bermejo basin constrain deposition between 13 and 6 Ma. Dominant fluvial-lacustrine mudstones, siltstones, and sandstones transition into fluvial-megafan deposits capped by alluvial fan conglomerate facies, tracking the eastward migration of Precordillera deformation. The cessation of sedimentation, and thermal history models of apatite (U–Th-Sm)/He thermochronology ages, indicates basin incorporation into the orogenic wedge by 6 Ma. When we compare our southern datasets with previous constraints from the northern Bermejo basin, we observe, from north to south: (1) a time-transgressive trend in basin initiation, (2) a ∼3 km decrease in stratigraphic thicknesses, and (3) older exhumation along the thrust front. These trends highlight the asynchronous nature of the Bermejo foreland basin system and along-strike variability of Precordillera thrust-front evolution.

Late Mesozoic impact of paleo-Pacific subduction on the North China craton revealed by apatite U-Pb and fission-track double dating and trace element analysis in the eastern Yanshan fold belt, northeastern Asia

The late Mesozoic tectonic evolution of the eastern North China craton was intimately related with the subduction of the paleo-Pacific plate. This study sheds light on Late Jurassic−Early Cretaceous basin-mountain coupling during paleo-Pacific subduction based on low-temperature thermochronology and geochemical analyses from the easternmost Yanshan fold belt, located at the northern edge of the craton. We performed apatite U-Pb and fission-track double dating and trace element analyses of basement and sedimentary rock samples, integrated with zircon and apatite (U-Th)/He analyses on bedrock samples. Our results revealed that the easternmost Yanshan fold belt experienced two stages of rapid cooling and exhumation in the Middle Jurassic−earliest Cretaceous (ca. 170−140 Ma) and in the Early Cretaceous (ca. 140−90 Ma). The Middle Jurassic−earliest Cretaceous exhumation (ca. 170−140 Ma) is mainly recorded by bedrock along the margins of intermontane basins of the eastern Yanshan fold belt. This exhumation event is consistent with compressional Yanshan movement during the Middle Jurassic−earliest Cretaceous coeval with paleo-Pacific flat-slab subduction. Subsequent Early Cretaceous rapid cooling is ascribed to the progressive evolution of the metamorphic core complexes and associated tectonic exhumation of major plutons of northeastern Asia, which were likely controlled by rollback of the paleo-Pacific slab. A middle Cretaceous (ca. 110−90 Ma) tectonic exhumation event is revealed by very low lag-time values in middle Cretaceous strata. Our findings illustrate the potential of a thermochronology approach that combines single-grain double dating and trace element analyses in synmagmatic orogenic systems, which may find application to other orogenic settings worldwide.

Crustal structure of the Lazufre volcanic complex and the Southern Puna from 3-D inversion of magnetotelluric data: Implications for surface uplift and evidence for melt storage and hydrothermal fluids

Abstract The Central Andes are unique in the global system of subduction zones in that a significant, high-altitude plateau has formed above a subduction zone. In this region, both subduction and the associated magmatism have been shown to vary in both space and time. Geophysical data have been invaluable in determining the subsurface structure of this region. Extensive seismic studies have determined the regional-scale distribution of partial melt in the crust and upper mantle. Magnetotelluric studies have been effective in providing independent constraints on the quantity and composition of partial melt in the crust and upper mantle. Geodetic studies have shown that a small number of volcanic centers exhibit persistent, long-term uplift that may indicate the formation of plutons or future eruptions. This paper describes a detailed study of the Southern Puna using magnetotelluric (MT) data. This region is located at the southern limit of the Central Andes in a region where a recent transition from flat-slab subduction to normal subduction has caused an increase in magmatism, in addition to hypothesized lithospheric delamination. It is also a region where an extensive zone adjacent to the volcanic arc is undergoing surface uplift, located near Volcán Lastarria and Cordon del Azufre (collectively called Lazufre). The main goals of the work are to define the crustal structure and to investigate processes that may cause surface uplift of relatively large regions not associated with active volcanism. As part of the PLUTONS project, MT data were collected on an east-west transect (approximately along 25°S) that extended across the Southern Puna, from Lazufre to north of Cerro Galan. The data were combined with previously collected MT data around Lazufre and inverted to give a 3-D resistivity model of the crust. The low resistivity of the crust resulted in limited sensitivity to mantle structure. A number of major crustal conductors were detected and included (1) a mid-crustal conductor extending eastward from the volcanic arc as far as the Salar de Antofalla; (2) an upper- to mid-crustal conductor located north of Cerro Galan; and (3) a conductor that rises westward from (1) and terminates directly beneath the region of surface uplift at Lazufre. These conductors are broadly coincident with the location of crustal low shear-wave anomalies. The conductive features were interpreted to be due to zones of partial melt stored in the crust, and petrological data were used to estimate melt fractions. Below Lazufre, it is likely that aqueous fluids contribute to the high conductivity, which is observed within the depth range of the inflation source, giving evidence that the surface uplift may be associated with both magmatic and hydrothermal processes.

Upper-plate response to ridge subduction and oceanic plateau accretion, Washington Cascades and surrounding region: Implications for plate tectonic evolution of the Pacific Northwest (USA and southwestern Canada) in the Paleogene

Abstract The interaction between subduction zones and oceanic spreading centers is a common tectonic process, and yet our understanding of how it is manifested in the geologic record is limited to a few well-constrained modern and ancient examples. In the Paleogene, at least one oceanic spreading center interacted with the northwestern margin of North America. Several lines of evidence place this triple junction near Washington (USA) and southern British Columbia (Canada) in the early to middle Eocene, and we summarize a variety of new data sets that permit us to track the plate tectonic setting and geologic evolution of this region from 65 to 40 Ma. The North Cascades segment of the voluminous Coast Mountains continental magmatic arc experienced a magmatic lull between ca. 60 and 50 Ma interpreted to reflect low-angle subduction. During this period of time, the Swauk Basin began to subside inboard of the paleo-trench in Washington, and the Siletzia oceanic plateau began to develop along the Farallon plate–Kula plate or Farallon plate–Resurrection plate spreading center. Farther east, peraluminous magmatism occurred in the Omineca belt and Idaho batholith. Accretion of Siletzia and ridge-trench interaction occurred between ca. 53 and 49 Ma, as indicated by: (1) near-trench magmatism from central Vancouver Island to northwestern Washington, (2) disruption and inversion of the Swauk Basin during a short-lived contractional event, (3) voluminous magmatism in the Kamloops-Challis belt accompanied by major E-W extension east of the North Cascades in metamorphic core complexes and supra-detachment basins and grabens, and (4) southwestward migration of magmatism across northeastern Washington. These events suggest that flat-slab subduction from ca. 60 to 52 Ma was followed by slab rollback and breakoff during accretion of Siletzia. A dramatic magmatic flare-up was associated with rollback and breakoff between ca. 49.4 and 45 Ma and included bimodal volcanism near the eastern edge of Siletzia, intrusion of granodioritic to granitic plutons in the crystalline core of the North Cascades, and extensive dike swarms in the North Cascades. Transtension during and shortly before the flare-up led to >300 km of total offset on dextral strike-slip faults, formation of the Chumstick strike-slip basin, and subhorizontal ductile stretching and rapid exhumation of rocks metamorphosed to 8–10 kbar in the North Cascades crystalline core. By ca. 45 Ma, the Farallon–Kula (or Resurrection)–North American triple junction was likely located in Oregon (USA), subduction of the Kula or Resurrection plate was established outboard of Siletzia, and strike-slip faulting was localized on the north-striking Straight Creek–Fraser River fault. Motion of this structure terminated by 35 Ma. These events culminated in the establishment of the modern Cascadia convergent margin.

Magmatic surge requires two-stage model for the Laramide orogeny

The Laramide orogeny is a pivotal time in the geological development of western North America, but its driving mechanism is controversial. Most prominent models suggest this event was caused by the collision of an oceanic plateau with the Southern California Batholith (SCB) which caused the angle of subduction beneath the continent to shallow and led to shut-down of the arc. Here, we use over 280 zircon and titanite Pb/U ages from the SCB to establish the timing and duration of magmatism, metamorphism and deformation. We show that magmatism was surging in the SCB from 90 to 70 Ma, the lower crust was hot, and cooling occurred after 75 Ma. These data contradict plateau underthrusting and flat-slab subduction as the driving mechanism for early Laramide deformation. We propose that the Laramide orogeny is a two-stage event consisting of: 1) an arc ‘flare-up’ phase in the SCB from 90-75 Ma; and 2) a widespread mountain building phase in the Laramide foreland belt from 75-50 Ma that is linked to subduction of an oceanic plateau.

Active thrust tectonics along the western slope of the Central Andes southernmost Pampean flat-slab segment (∼33°S, Chile): The Cariño Botado fault system

Along the western slope of the Central Andes in the southernmost part of the Pampean flat-slab subduction zone segment (∼33°S), crustal Quaternary west-vergent reverse faults are found, but little is known about them. To better understand and characterize these faults, grouped as the Cariño Botado thrust fault system (CBF), we undertook a multidisciplinary study focused on remote sensing and field geomorphic and geologic mapping, sedimentary relationships, and fault characterization. Optically Stimulated Luminescence (OSL) and UPb dating samples were obtained from faulted deposits to constrain the timing of fault motion. We combined the mapping, fault relationship descriptions, and age control to understand this system better and quantify tectonic rates here for the first time. The main CBF is best expressed with Late-Cenozoic hanging wall fault rocks over thrusting Quaternary coarse-grained alluvial fan deposits, and the fault zone is found over a large area and with an altered fault zone (∼0.5–2 km wide). The principal reverse fault segment of the CBF is well exposed in creeks, with an average dip of 40°E and an associated vertical throw (up to the east) of the hanging wall from ∼3.2 m up to ∼30 m. Five OSL samples, taken from tilted and untilted alluvial fan deposits adjacent to the CBF's main fault, yielded ages between 2.5 ± 0.2 ka and 9.0 ± 0.7 ka, supporting Holocene activity and ∼4.9 m of fault slip in the last ∼8.7 ka. Ninety-nine detrital zircons from fluvial terrace deposits in the study area were dated using UPb ICPMS, giving mainly Oligo-Miocene ages. The youngest ages of detrital zircons are Quaternary (n = 3; ∼0.9–4.4 Ma), which indicates that CBF activity could have started as early as ∼0.9 Ma producing an accumulated hanging wall uplift of ∼130 m. The alignment of the fault profiles, creek and river knickpoints, and alluvial and fluvial deposits allows the most recent activity of the main CBF to be traced between 5 km and up to 15 km along the fault trace. Based on fault length, slip characteristics, and earthquake scaling relationships, earthquakes up to Mw 7.5 could be associated with the fault. Because of the clear relationship between the river incision, distribution of fluvial terraces, variation in sinuosity and stream power, and the location of the faults, we suggest that the tectonic uplift controls the hanging wall fluvial incision from ∼0.9 Ma to the present. The Quaternary (at least 0.9 Ma to present) active CBF shares geometric, structural, and rupture timing similarities with the nearby better-characterized San Ramón Fault system. The latter is located ∼50 km to the south along the eastern border of Santiago, the capital city of Chile. Both fault systems share a comparable morpho-structural setting with a consistent average N-S trend. Significant seismic hazard is associated with both the CBF and the San Ramon System, which may be linked structurally and temporally regarding previous surface ruptures and major earthquakes, and therefore remain important research targets.

Geodynamic controls on the Paleo-Asian oceanic plate evolution: Constraints from Paleozoic intrusive suites along the northern margin of the North China Block

Identifying the spatio-temporal variations of arc-relation magmatism is critical for determining the polarity, geometry, localization, and onset of subduction zones on Earth. When and how the Paleo-Asian oceanic plate subducted beneath the northern margin of the North China Block (NCB) have been widely debated. Here we conducted systematic petrological, geochronological, and geochemical investigations on the Paleozoic magmatic rocks in the northern NCB. The Early Paleozoic magmatic rocks (ca. 490–420 Ma) are only distributed in the Bainaimiao arc and show typical continental arc geochemical characteristics, indicating a south dipping subduction zone. The Devonian alkaline rocks (ca. 400–370 Ma) constitute a ∼ 900 km-long magmatic belt in the northern margin of the NCB. Combined with a striking ca. 370–355 Ma magmatic quiescence and the absence of high-pressure rocks during that period, a flat-slab subduction model is proposed to better account for the tectonic evolution of the northern NCB. Subsequently, the Carboniferous to Permian calc-alkaline arc magmatism (ca. 355–250 Ma) began to emerge and showed northward younging and migration, suggesting slab retreat with roll-back of the subducting Paleo-Asian slab and again opening of mantle wedge along the northern NCB. Our findings therefore suggest that the Paleozoic magmatism in the northern NCB records an entire spatio-temporal evolution of the Paleo-Asian oceanic plate from early subduction angle deepening to flat-slab subduction, then slab rollback and finally normal subduction. This is tentatively interpreted as a combined effect from the Paleo-Asian oceanic plate subduction angle change and a migrating keel in the mantle wedge region.

Late Cretaceous tectonothermal events of the Gangdese belt, southern Tibet

Abstract The Gangdese belt of the southern Lhasa terrane (southern Tibet) records a Chileantype accretionary orogeny driven by subduction of Neotethyan oceanic lithosphere, prior to Indo-Asian collision and formation of the Tibetan Plateau. We present detailed structural analysis of outcrops and a drill core in the Jiama copper ore district along with 40Ar-39Ar cooling ages from white mica, plagioclase, and potassium feldspar and zircon U-Pb geochronology of granitoids and sandstone. These data add new constraints to the formation of a major angular unconformity, deformation along and within the footwall of the Gangdese décollement, and the coupling between deformation and magmatism. Structural analysis indicates that top- to- the- south motion along the décollement produced southvergent folding and thrusting of Upper Jurassic to Cretaceous strata in the Gangdese back-arc basin. A synthesis of new and compiled age data reveals that the décollement and associated south-vergent deformation occurred between ca. 90 and 65 Ma, contemporaneous with the formation of a major ca. 85–69 Ma angular unconformity between the overlying Paleocene–Eocene Linzizong Formation and the underlying Upper Cretaceous Shexing Formation. We posit that this deformation in the Gangdese belt resulted from flat-slab subduction of the Neotethyan oceanic slab beneath the southern margin of the Asian continent. A flat-slab subduction geometry is consistent with previously documented synchronous thrusting in the forearc and back-arc basins as well as the observed arc magmatic lull of the Gangdese belt between ca. 80 and 65 Ma.

Cretaceous source to sink system reconstruction of northeastern Asian continental margin: Insight from integrated detrital geochronology in NE China

The Cretaceous sedimentary successions in NE China are largely incomplete as the basins are inverted and younger strata are eroded. Except for the Songliao Basin, whose depositional record has remained relatively intact, the burial record to the east was interrupted in the late Early or Late Cretaceous. There is still controversy on the possible connection between the Songliao Basin and the eastern basin group, as well as the extent of the previously suggested “pan-Sanjiang” proto-basin that covered most current eastern satellite basins. To address these questions, we studied modern river catchments of variable sizes from sediment-supplying basement highs that cover most of eastern NE China to track the intense denudation events and provenance changes through the Cretaceous. We found a great inconsistency between detrital age patterns of the modern river sands and the spatial proportions of the source units. The detrital age distributions allow to re-evaluate the basement units that are mostly composed of the Triassic–Jurassic igneous suite and some metasedimentary units with Paleozoic ages, but the contribution from Proterozoic formations is negligible. Combining the newly dated modern catchment detrital U-Pb ages, the region-wide compilation of the basement emplacement ages, and the U-Pb ages in the Cretaceous formations in NE China, we systematically refine the Cretaceous provenance history. In the Early Cretaceous, the eastern satellite basins accumulated sediments from the Lesser Xing’an range (LXR) in the west, Zhangguangcai range (ZGCR) in the south, and the Nadanhada terrane (NT) in the east. The sediment of the southern Songliao Basin derived from the North China Craton (NCC) and from ZGCR but barely from the Great Xing’an Range (GXR). The Jiamusi uplift (JU) did not provide sediments to the eastern satellite basins and the Songliao Basin.In the Late Cretaceous, the eastern satellite basins received sediments from the exhumed JU but barely from ZGCR and LXR. The Songliao Basin still accumulated the sediments from the NCC, some south part of ZGCR, and GXR but minor from the central and northern ZGCR and LXR. Between the latest Early Cretaceous and early Late Cretaceous, a much wider proto-basin was suggested in NE China than previously assumed. It covered during its maximum extension period the current Songliao Basin, the eastern satellite basins, the LXR, ZGCR, JU, and partial GXR basement areas. The provenance change is induced by the Paleo-Pacific plate motion change, i.e., with the subduction direction change, the roll-back, and two-stage limited flat slab subduction trigged the formation of the basin-and-range system in NE China since the Early Cretaceous. Moreover, we suggest conducting modern sand provenance analysis to detect, verify or re-classify the ages of the zircon-bearing units, especially at a reconnaissance prospecting on areas covered by imprecise large-scale geological maps only to reveal the catchment better.

Silurian inverted Barrovian-type metamorphism in the Western Sierras Pampeanas (Argentina): a case of top to bottom heating?

AbstractThis paper focuses on one orogenic belt that formed during the Rinconada phase on the final stage of the Famatinian orogeny, between 445 and 410 Ma, which is well exposed at Sierra de Ramaditas and neighbouring ranges in western Argentina. The Ramaditas Complex is formed by metasedimentary and meta-ultrabasic rocks and amphibolites. This complex forms the upper nappe of a thrust stack resulting from westward thrusting. Deformation consists of an early high-temperature S1 foliation (stromatic migmatites), coeval with thrusting and metamorphism. Metamorphism attained peak P–T conditions of 6.0–6.9 kbar and 795–810 °C, at c. 440 Ma, i.e. coincident with the Rinconada orogenic phase. The lower unit and intermediate nappes crop out in the nearby sierras of Maz and Espinal and underwent low- to medium-grade Silurian metamorphism, respectively, together with the upper nappe, defining an inverted Barrovian-type metamorphism with T decreasing and P increasing downwards across the thrust stack (i.e. westward). We argue that the Rinconada orogenic phase developed near the continental margin of SW Gondwana, during a magmatic lull following accretion of the Precordillera terrane to the continental margin at c. 470 Ma. The active margin jumped to the west after accretion, and flat-slab subduction resumed in the early Silurian, provoking thrusting and imbrication of nappe stack under the still hot root (800–900 °C) of the older Famatinian magmatic arc. This ‘hot-iron’ process explains both the inverted Barrovian-type metamorphism and the missing overburden of 21 to 24 km implied by the P estimate.

Silurian to Cretaceous geological evolution of southern Mexico and its connection to the assembly and break-up of Western Equatorial Pangaea: geochronological constraints from the northern Sierra de Juárez Complex

Abstract The Sierra de Juárez Complex (SJC) of southern Mexico contains an extensive geological record from Precambrian to Cenozoic, involving Rodinia, NW Gondwana, western equatorial Pangaea, and eastern peninsular Mexico. It is thus critical for palinspastic reconstructions and lithotectonic correlations, mainly between the Mexican and NW South America terranes. In this contribution, we investigate the tectonic evolution of the northern SJC from Silurian to the Lower Cretaceous on the basis of fieldwork, petrography, and zircon U–Pb geochronology by laser ablation–inductively coupled plasma mass spectrometry. Our results allow us to constrain five main geological events: (1) Middle Paleozoic sedimentation along NW Gondwana during transtensional tectonics; (2) volcanosedimentary activity between 292 and 281 Ma in NW Gondwana during Rheic Ocean closure; (3) early-middle Permian metamorphism related to flat-slab subduction postdating Pangaea assembly; (4) Lower–Middle Jurassic anatexis and magmatism coeval with regional shearing at c. 175 Ma influenced by transtensional tectonics along eastern peninsular Mexico during Pangaea tenure; and (5) intermediate to acid magmatism between c. 136 and 129 Ma, correlated with the Zongolica continental arc in southern Mexico, followed by deep-crustal shearing related to either the formation of the extensional Chivillas basin or the Upper Cretaceous–Cenozoic contractional episode documented in the Cuicateco Terrane.

Mantle-derived high-K magmatic fluxes in northeast Iran arc: Constraints from zircon U-Pb-O-Hf and bulk rock major-trace elements and Sr-Nd-Pb isotopes

Most continental arcs are built up over a long time (≥100 myr), and while subduction may be ongoing throughout this interval, magmatism appears to be highly episodic. This episodic behaviour is characterized by high-flux magmatic events but an overall low rate of magmatism. The causes of high-flux magmatic events (“flare-ups”) are enigmatic in many continental arcs. Bulk-rock Sr, Nd, and Pb isotopes, as well as zircon O and Hf isotopes, imply that the mantle and the continental crust can be involved in magmatic flare-ups. However, the relative contributions of mantle vs. crust with changes in eruption rates can differ from arc to arc. The Cenozoic magmatic arcs of Iran, built on mature continental crust, are an excellent candidate for studying the geochemical-isotopic feedback of magmatic pulses to understand the triggers for a flare-up. Our new data constrain the timing of the flare-up in NE Iran to the Early to Middle Eocene (51–43 Ma). This flare-up is characterized by the outpouring of high-K calc-alkalic to shoshonitic magmas at ∼110 ± 8 km3/myr - km. Geochemical modelling using the “Arc Basalt Simulator version 3″ shows that the high-K trachybasalts, moderately to extremely depleted in high-field strength elements, can be derived from the shallower (3.0 GPa; 870 °C) to deeper parts (5.0–5.4 GPa; 965–980 °C) of a subducting slab with ∼1.0 to 5.5 % slab melt flux. Mixing modelling using Sr, Nd, and Pb isotope data indicates that the Torud mafic-intermediate magmatic rocks can be generated by adding ∼ 1% to <6% of slab components (50% AOC: 50% sediment) to an Indian MORB-like mantle. Our results suggest that the high magmatic fluxes in NE Iran were instigated mainly by Eocene slab steepening after Paleocene flat-slab subduction, resulting in enhanced upwelling and melting of a volatile-enriched asthenospheric mantle.

Basin formation, magmatism, and exhumation document southward migrating flat-slab subduction in the central Andes

The Central Andean Plateau is the largest plateau formed in a non-collisional setting and is taller, wider, has thicker crust, and has greater retroarc shortening than any other region of the modern Andes. It also hosts some of the thickest Cenozoic strata in the Andes, which were deposited synchronously with punctuated widening and narrowing of the magmatic arc. Multiple hypotheses have been advanced to explain these unique characteristics, yet the detailed, plateau-scale Cenozoic basin history needed to test these hypotheses remains unavailable and therefore has not been integrated into the established records of deformation and magmatism. Here we synthesize new detrital zircon maximum depositional ages (MDAs) and sediment provenance analysis of non-marine strata from ∼15°–16.5°S with existing records of sediment accumulation, deformation, and magmatism from ∼14°–24.5°S to develop a model of the Eocene–early Miocene evolution of the Altiplano-Puna plateau region. Stratigraphic correlations based on the new MDAs show a Paleocene–Eocene unconformity/condensed section followed by resumption of rapid sediment accumulation. Rapid sediment accumulation resumed at 46–43 Ma at 15–16°S, at 36 Ma at 18°S, and as young 19 Ma at ∼23°S. Provenance data presented herein indicate that Eastern Cordilleran detritus appeared in Altiplano strata at progressively younger ages to the south. The southward progression of this basin reorganization proceeded in lockstep with initiation of exhumation in the Eastern Cordillera, and also a lull in the magmatic arc followed by widespread volcanic flare-up. We present a model in which the observed stratigraphic hiatus, along with the magmatic lull and flare-up, orogenic widening, and high-magnitude shortening, are upper plate responses to shallowing and resteepening of an late Paleocene–early Miocene flat slab beneath the Altiplano-Puna plateau. In the proposed model initial slab flattening was driven by late Paleocene–early Eocene subduction of buoyant oceanic crust: potentially a Manihiki Plateau Conjugate. Subsequently, southward shallowing and resteepening of a late Eocene–early Miocene flat slab beneath the Altiplano-Puna Plateau was driven by subduction of an asperity on the Nazca Plate which we tentatively identify with the Juan Fernandez Ridge. The geographic coincidence of all of these features suggests that the flat slab hydrated and weakened the upper plate lithosphere, thereby facilitating later development of the suite of unique features which characterize the Central Andean Plateau.

The Mesozoic tectonic evolution of the East China Sea Basin: New insights from 3D seismic reflection data

The Mesozoic is a significant period in the East China Sea Basin (ECSB), since it recorded the tectonic transition from the Triassic Tethys tectonic domain to the subsequent Jurassic-Cretaceous Paleo-Pacific domain. However, the Mesozoic tectonic evolution of the East China Sea Basin is not well defined, and it has been highly debated in recent years. Here high-quality 3D seismic reflection data, information about fault timing, and fault maps were employed to analyze Mesozoic faulting and evolution of the ECSB. This study significantly improves our understanding of the Mesozoic tectonic evolution of the ECSB. Our results demonstrate that two sets of Mesozoic faults developed in the ECSB: (1) NW-striking faults are typical negative inversion structures and initially formed as low-angle thrusts as the result of multiphase plate convergence during Triassic. Then, the thrusts reactivated as extensional or transtensional faults during Cretaceous, which are interpreted to be related to NNW-SSE oblique extension associated with roll-back of the subduction zone of Paleo-Pacific plate. (2) The NE-striking faults are inferred to be thrust structures formed during the Early to Middle Jurassic. Flat-slab subduction of the Paleo-Pacific plate is introduced to explain the compression event during this stage. The NE-striking reactivated and newly-formed extensional faults were then developed extensively in the ECSB during the Cretaceous. The NNW-SSE oblique extension played a dominant role in this faulting event, which is related to roll-back of the subduction zone of Paleo-Pacific plate. Furthermore, the NE-striking faults continued to propagate cut through the Cenozoic sequence, since the strike of these faults is perpendicular to the Cenozoic extension stress field.

Magmatic record of changing Cordilleran plate-boundary conditions—Insights from Lu-Hf isotopes in the Mojave Desert

Abstract Belts of Cordilleran arc plutons in the eastern part of the Mojave crustal province, inboard from the southwestern North American plate boundary, record major magmatic pulses at ca. 180–160 and 75 Ma and smaller pulses at ca. 100 and 20 Ma. This cyclic magmatism likely reflects evolving plate-margin processes. Zircon Lu-Hf isotopic characteristics and inherited zircons for different-age plutons may relate magma sources to evolving tectonics. Sources similar in age to the bulk of the exposed Mojave crust (1.6–1.8 Ga) dominated the magmas. Rare zircons having εHf(t) values as low as −52 indicate that Cretaceous melt sources also included more ancient crustal components, such as Archean-derived detritus in supracrustal gneisses of the Vishnu basin. Some rocks signal contributions from mantle lithosphere (in the Miocene) or asthenosphere (middle Cretaceous). Temporal shifts in isotopic pattern in this sample of the Cordillera relate to cyclic pulses of magmatic flux. Hf-isotopic pull-downs suggestive of dominantly crustal sources characterize the Jurassic and Late Cretaceous flare-ups. The Late Cretaceous flare-up, occurring near the onset of flat-slab subduction, produced abundant Proterozoic xenocrystic zircon and Hf isotopes implicating derivation largely from heterogeneous deep Mojave crust. Isotopic pull-ups characterize the lower-flux middle Cretaceous and Miocene magmatic episodes. The middle Cretaceous pulse ca. 105–95 Ma produced Mojave crust signals but also the isotopically most juvenile magmatic zircons, ranging upward to barely positive εHf values and suspected to signal an asthenosphere contribution. This may point toward transtension or slab retreat causing 105–95 Ma backarc extension in the Mojave hinterland of the Cordillera. That possibility of backarc extension raises questions about the tectonic environment of the contemporaneous main Sierra Nevada high-flux arc closer to the continental margin.

Origin of the Sierras Pampeanas, Argentina: Flat-slab subduction and inherited structures

Abstract The Sierras Pampeanas (27°–33°S) in South America are characterized by basementcored uplifts and shortening that occurs &amp;gt;500 km from the nearest convergent margin. The deformation correlates spatially and temporally with an area of flat-slab subduction of the Nazca plate in the last 10 m.y. We use two-dimensional thermal-mechanical models to study the dynamics of Pampean flat-slab subduction and the origin of the Sierras Pampeanas. Models examine a geological time from ca. 12 Ma to present day, during which time the Juan Fernández Ridge subducted beneath South America. Models show that the buoyant ridge triggers slab flattening, resulting in regional continental compression through end loading at the plate margin. Deformation in the continental interior depends on the inherited structure of the continent, where surface uplifts and shortening are concentrated at preexisting weak zones. The inboard migration of deformation is controlled by surface topography caused by the buoyant ridge rather than basal shear from the growing flat slab. Deformation occurs prior to the passage of the ridge and is inhibited when the ridge is beneath the region owing to dynamic uplift.

Aseismic ridge subduction focused late Cenozoic exhumation above the Peruvian flat slab

Subduction of aseismic ridges and flat slab subduction are important processes that punctuate Cordilleran orogenesis and may enhance exhumation and rock uplift in the overriding plate. Distinguishing between the two drivers is often challenging, as many modern flat slabs spatially coincide with subducting buoyant ridges. The Peruvian flat slab is the largest region of active flat slab subduction on Earth, spanning over 1300 km of the subducting Nazca plate along the western margin of South America. The flat slab is associated with two seafloor ridges: the Nazca Ridge at the southern terminus and the fully subducted Inca Plateau in the north. These aseismic ridges are spatially confined with respect to the flat slab, allowing assessment of the relative roles of aseismic ridge interactions and flat slab subduction in driving upper plate exhumation. We present: (1) a regional compilation of geochronologic ages of Andean igneous rocks, which track the spatio-temporal evolution of Neogene magmatic arc cessation and hence slab flattening; (2) calculated geomorphic indices, which document landscape perturbations and climatic or lithologic changes, (3) a summary of erosion rates from river catchments on the western Andean slope, and; (4) a regional synthesis of thermochronologic ages that reflect the timing and magnitude of upper crustal cooling.Thermochronometric cooling ages systematically track the progressive passage of the Nazca Ridge, suggesting that the buoyant ridge focused exhumation in the overriding plate. Geomorphic indices demonstrate enhanced topography and steeper channels closer to the position of the subducted ridge. The spatial progression of basement block uplifts in Peru also coincides with the timing of ridge passage. In hinterland regions, >2 km of exhumation occurred since ca. 15 Ma above the Peruvian flat slab. For individual locations within the orogen, active rock uplift declines after ridge passage, suggesting that increased coupling is not maintained across the entirety of the flat slab. We argue that above broad zones of flat slab subduction, focused areas of aseismic ridge subduction concentrate upper-plate exhumation and uplift. These observations may be relevant to other flat slab systems, which exhibit a broad zone of arc shutoff with corridors of focused exhumation.