Flat-slab Subduction Research Articles

Cyclical processes in the North American Cordilleran orogenic system

Research Article| June 01, 2015 Cyclical processes in the North American Cordilleran orogenic system P.G. DeCelles; P.G. DeCelles 1Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA Search for other works by this author on: GSW Google Scholar S.A. Graham S.A. Graham 2Department of Geological and Environmental Sciences, Stanford University, Stanford, California, 94305, USA Search for other works by this author on: GSW Google Scholar Author and Article Information P.G. DeCelles 1Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA S.A. Graham 2Department of Geological and Environmental Sciences, Stanford University, Stanford, California, 94305, USA Publisher: Geological Society of America Received: 26 Nov 2014 Revision Received: 13 Mar 2015 Accepted: 16 Mar 2015 First Online: 09 Mar 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 © 2015 Geological Society of America Geology (2015) 43 (6): 499–502. https://doi.org/10.1130/G36482.1 Article history Received: 26 Nov 2014 Revision Received: 13 Mar 2015 Accepted: 16 Mar 2015 First Online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation P.G. DeCelles, S.A. Graham; Cyclical processes in the North American Cordilleran orogenic system. Geology 2015;; 43 (6): 499–502. doi: https://doi.org/10.1130/G36482.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract To understand linkages among processes that combined to build the North American Cordillera, we synthesize its tectono-magmatic history from the California accretionary wedge to the retroarc foreland of Wyoming. At this latitude, the Cordilleran magmatic arc experienced high-flux events (HFEs) at ca. 160–150 Ma and ca. 105–90 Ma. Retroarc shortening provided the main source of HFE magmas, which in turn created eclogitic arc roots that later foundered into the mantle and cleared the sub-arc region of excess mass, and provided space to accommodate renewed retroarc shortening. The forearc, arc, and retroarc regions all responded variably to this cycle of tectono-magmatic processes, and Laramide flat-slab subduction may have both disrupted and been enhanced by events within this cycle. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Geometry and kinematics of the Andean thick-skinned thrust systems: Insights from the Chilean Frontal Cordillera (28°–28.5°S), Central Andes

The structure of the Chilean Frontal Cordillera, located over the Central Andes flat-slab subduction segment (27°–28.5°S), is characterized by a thick-skinned deformation, affecting both the pre-rift basement and the Mesozoic and Cenozoic infill of the NNE-SSW Lautaro and Lagunillas Basins, which were developed during the Pangea-Gondwana break-up. The compressive deformation show a complex interaction between Mesozoic rift structures and thrust systems, affecting a suite of Permo-Triassic (258–245 Ma) granitic blocks. We used a combination of geological mapping, new structural data, balanced and restored cross sections and geochronological data to investigate the geometry and kinematics of the Andean thick-skinned thrust systems of the region. The thrust systems include double-vergent thick-skinned thrust faults, basement-cored anticlines and minor thin-skinned thrusts and folds. The presence of Triassic and Jurassic syn-rift successions along the hanging wall and footwall of the basement thrust faults are keys to suggest that the current structural framework of the region should be associated with the shortening of previous Mesozoic half grabens. Based on this interpretation, we propose a deformation mechanism characterized by the tectonic inversion of rift-related faults and the propagation of basement ramps that fold and cut both, the early normal faults and the basement highs. New U–Pb ages obtained from synorogenic deposits (Quebrada Seca and Doña Ana formations) indicate at least three important compressive pulses. A first pulse at ∼80 Ma (Late Cretaceous), a second pulse related to the K-T phase of Andean deformation and, finally, a third pulse that occurred during the lower Miocene.

Repeated slab advance–retreat of the Palaeo-Pacific plate underneath SE China

Southeast China, an important part of the circum–Pacific magmatic–metallogenetic belt, was characterized by late Mesozoic extensive magmatism and related metallogenesis. It is now generally accepted that this magmatism was related to subduction of the Palaeo-Pacific plate, and a series of tectonic models such as normal subduction, shallow subduction, and flat-slab subduction have been suggested. Here we propose a new tectonic model involving repeated slab advance–retreat of the Palaeo-Pacific plate on the basis of new geochronological and geochemical data of Late Triassic to Early Jurassic mafic rocks and Early Jurassic A-type granites in southern Jiangxi and western Fujian provinces. The results indicate that Late Triassic (ca. 228 Ma) mafic rocks are shoshonitic, formed in a post-collisional regime of the Tethyan tectonic domain. Early Jurassic (ca. 197–191 Ma) mafic rocks are sodic, emplaced in a continental arc setting coupled with the subduction of the Palaeo-Pacific plate. Early Jurassic (ca. 189 Ma) granites, occurring as a NNE-trending belt, belong to the A2 group and formed in an extension setting caused by slab break-off. There are an other four A-type granite belts in southeast China, i.e. the Late Triassic, Late Jurassic, Early Cretaceous, and Late Cretaceous A-type granite belts, respectively. Late Triassic (229–221 Ma) A-type granites occur as an ENE-trending belt and were coincident with the Late Triassic mafic magmatism. Late Jurassic (163–153 Ma), Early Cretaceous (136–124 Ma), and Late Cretaceous (101–91 Ma) A-type granite belts, together with the Early Jurassic (189 Ma) A-type granite belt, are all NNE-trending, parallel to the present coastline. The Late Jurassic belt is located further inland, on the west side of the Early Jurassic belt. The Early Cretaceous belt almost overlaps the Early Jurassic belt and the Late Cretaceous belt is located at the coastal area of southeast China. Integrating these observations, we propose a repeated slab-advance–retreat model for the late Mesozoic magmatic evolution of southeast China. Palaeo-Pacific plate subduction underneath southeast China initiated in the Late Triassic Rhaetian and reached southern Jiangxi by ca. 197 Ma, followed by slab rollback during 197–191 Ma and by slab break-off at ca. 189 Ma. Then slab advance was reestablished with the northwestward subduction approaching southern Hunan at ca. 178 Ma. From ca. 174 Ma, slab rollback reinitiated and gradually migrated from inland to the coastal area. This repeated slab-advance–retreat model is helpful to further understand the geodynamic mechanism of the late Mesozoic tectono-magmatism and related metallogenesis of southeast China.

Detrital zircon record of Neogene exhumation of the central Alaska Range: A far-field upper plate response to flat-slab subduction

Far-field deformation in the upper plate of many ancient and modern convergent margins is commonly attributed to flat-slab subduction processes. Recent studies have suggested that the high topography of the central Alaska Range (Alaska, USA), for example, is the far-field deformational response to Neogene shallow subduction processes in southern Alaska. In this study, we analyze six stratigraphically controlled detrital zircon samples from Miocene–Pliocene sandstone of the Tanana foreland basin to evaluate the temporal and spatial links between exhumation of the central Alaska Range and shallow subduction. The Tanana basin, located north of the central Alaska Range, contains ∼2 km of Neogene strata. Lower Miocene strata from the basin have detrital zircon ages with dominant Precambrian populations. These grain ages are attributed to sediment derivation from the Yukon composite terrane source area located at the northern edge and north of the Alaska Range. During the early–middle Miocene the provenance of the Tanana basin switched to a distinctly Mesozoic source signal. These Mesozoic-age grains are attributed to sediment derived from source areas within and south of the Alaska Range. The early–middle Miocene change in provenance from predominantly Precambrian to Mesozoic sediment sources, in the context of previously published basin analysis studies, is interpreted to represent the onset of Neogene uplift of the central Alaska Range. The timing of exhumation of the central Alaska Range is important not only for understanding the tectonics of the highest mountain range in North America, but additionally because recent modeling studies predict that this range is one of the first expected products of Neogene flat-slab subduction along the southern Alaska convergent plate boundary. Results of our study provide a needed chronologic framework to fully evaluate the temporal and spatial upper plate responses to shallow subduction processes.

Detrital zircon U–Pb geochronology, Hf isotopes and geochemistry constraints on crustal growth and Mesozoic tectonics of southeastern China

In situ U–Pb geochronological, Hf isotopic and REE geochemical analyses of detrital zircons from Mesozoic sandstones in central Jiangxi and east Hunan of the South China Block (SCB) are used to provide not only information about crustal evolution process, but more importantly new constraints on sedimentary provenances as well as Mesozoic basin evolutions in central SCB. A total of 417 concordant zircon analyses define five U–Pb age populations at ca. 2.6–2.3Ga, ca. 2.0–1.7Ga, ca. 1.0–0.7Ga, ca. 500–400Ma and ca. 300–200Ma. Integrated analyses of zircon U–Pb ages and Hf isotopes of detrital zircons reveal five episodes of juvenile continental crust growth: ca. 2.5Ga, ca. 1.7Ga, ca. 850Ma, ca. 440Ma and ca. 230Ma, with all but the ca. 2.5Ga episode likely represent that of the SCB.None of the three samples from T3–J1 strata showed strong ca. 1850Ma and ca. 230Ma peaks suggesting that the T3–J1 sediments probably sourced from the Yangtze Block. In contrast, stronger peaks of ca. 1850Ma and ca. 250–230Ma in post-J1 strata relative to that of T3–J1 strata suggest a dominantly local Cathaysian provenance. In addition, the distinct low εHf(T) values for zircons of ca. 430Ma from T3–J1 strata in comparison with higher εHf(T) values for that from post-J1 strata also support aforementioned viewpoint. Such sharp changes between the pre-J1 and post-J1 strata coincide with the remarkable change in regional palaeogeography from a broad shallow marine basin in the Late Triassic–Early Jurassic time to a basin-and-range-style province in the Middle Jurassic. The characteristics and time evolution of detrital zircons from the studied area are consistent with the flat-slab subduction model which involves the development of a broad sag basin during the T3–J1 time, and a rapid regional uplift in the Cathaysia Block caused by the reinitiating of normal subduction along the coastal region at ca. 190Ma.

Density, temperature, and composition of the North American lithosphere—New insights from a joint analysis of seismic, gravity, and mineral physics data: 2. Thermal and compositional model of the upper mantle

AbstractTemperature and compositional variations of the North American (NA) lithospheric mantle are estimated using a new inversion technique introduced in Part 1, which allows us to jointly interpret seismic tomography and gravity data, taking into account depletion of the lithospheric mantle beneath the cratonic regions. The technique is tested using two tomography models (NA07 and SL2013sv) and different lithospheric density models. The first density model (Model I) reproduces the typical compositionally stratified lithospheric mantle, which is consistent with xenolith samples from the central Slave craton, while the second one (Model II) is based on the direct inversion of the residual gravity and residual topography. The results obtained, both in terms of temperature and composition, are more strongly influenced by the input models derived from seismic tomography, rather than by the choice of lithospheric density Model I versus Model II. The final temperatures estimated in the Archean lithospheric root are up to 150°C higher than in the initial thermal models obtained using a laterally and vertically uniform “fertile” compositional model and are in agreement with temperatures derived from xenolith data. Therefore, the effect of the compositional variations cannot be neglected when temperatures of the cratonic lithospheric mantle are estimated. Strong negative compositional density anomalies (<−0.03 g/cm3), corresponding to Mg # (100 × Mg/(Mg + Fe)) >92, characterize the lithospheric mantle of the northwestern part of the Superior craton and the central part of the Slave and Churchill craton, according to both tomographic models. The largest discrepancies between the results based on different tomography models are observed in the Proterozoic regions, such as the Trans Hudson Orogen (THO), Rocky Mountains, and Colorado Plateau, which appear weakly depleted (>−0.025 g/cm3 corresponding to Mg # ∼91) when model NA07 is used, or locally characterized by high‐density bodies when model SL2013sv is used. The former results are in agreement with those based on the interpretation of xenolith data. The high‐density bodies might be interpreted as fragments of subducted slabs or of the advection of the lithospheric mantle induced from the eastward‐directed flat slab subduction. The selection of a seismic tomography model plays a significant role when estimating lithospheric density, temperature, and compositional heterogeneity. The consideration of the results of more than one model gives a more complete picture of the possible compositional variations within the NA lithospheric mantle.

Seismic velocity structure and anisotropy of the Alaska subduction zone based on surface wave tomography

AbstractSouthcentral Alaska is a complex tectonic region that transitions from subduction of Pacific crust to flat slab subduction—and collision—of overthickened Yakutat crust. Because much of the Yakutat crust has been subducted, seismic imaging is needed in order to understand the crustal and upper mantle structural framework for this active tectonic setting. Here we use teleseismic Rayleigh waves to image large‐scale variations in shear wave structure. Our imaging technique employs a two‐plane wave representation with finite frequency sensitivity kernels. Our 3‐D isotropic model reveals several features: the subducting Pacific/Yakutat slab, slow wave speeds characterizing the onshore Yakutat collision zone, slow wave speeds of the Wrangell subduction zone, and a deep tomographic contrast at the eastern edge of the Pacific/Yakutat slab. We produce anisotropic phase velocity maps that exhibit variations in the fast direction of azimuthal anisotropy. These maps show the dominance of the Yakutat slab on the observed pattern of anisotropy. West of the Yakutat slab the fast directions are approximately aligned with the plate convergence direction. In the region of the Yakutat slab the pattern is more complicated. Along the margins of the slab the fast directions are roughly parallel to the margins. We identify notable differences and similarities with published SKS splitting measurements. Integrative modeling using 3‐D anisotropy models and different seismic measurements will be needed in order to establish a detailed 3‐D anisotropic velocity model for Alaska. This study provides a large‐scale starting point for such an effort.

Assimilating lithosphere and slab history in 4-D Earth models

We develop methods to incorporate paleogeographical constraints into numerical models of mantle convection. Through the solution of the convection equations, the models honor geophysical and geological data near the surface while predicting mantle flow and structure at depth and associated surface deformation. The methods consist of four constraints determined a priori from a plate history model: (1) plate velocities, (2) thermal structure of the lithosphere, (3) thermal structure of slabs in the upper mantle, and (4) velocity of slabs in the upper mantle. These constraints are implemented as temporally- and spatially-dependent conditions that are blended with the solution of the convection equations at each time step. We construct Earth-like regional models with oceanic and continental lithosphere, trench migration, oblique subduction, and asymmetric subduction to test the robustness of the methods by computing the temperature, velocity, and buoyancy flux of the lithosphere and slab. Full sphere convection models demonstrate how the methods can determine the flow associated with specific tectonic environments (e.g., back-arc basins, intraoceanic subduction zones) to address geological questions and compare with independent data, both at present-day and in the geological past (e.g., seismology, residual topography, stratigraphy). Using global models with paleogeographical constraints we demonstrate (1) subduction initiation at the Izu–Bonin–Mariana convergent margin and flat slab subduction beneath North America, (2) enhanced correlation of model slabs and fast anomalies in seismic tomography beneath North and South America, and (3) comparable amplitude of dynamic and residual topography in addition to improved spatial correlation of dynamic and residual topography lows.

Temporal variations in the influence of the subducting slab on Central Andean arc magmas: Evidence from boron isotope systematics

The Pampean flat-slab segment in the southern Central Andes represents an ideal setting at which to investigate how changes in the tectonic configuration of a subduction zone (convergence angles and rates, seamount subduction and shallowing slab angle) affects the recycling of subducted components to arc magmas. To constrain sources, particularly of slab-derived fluids and their contribution to arc magmatism, boron isotope and select major and trace element compositions were determined for pyroxene- and zircon-hosted melt inclusions obtained from a suite of Paleocene to Miocene arc magmatic rocks, from the southern Central Andes. Considerable changes in δB11 values and boron concentrations are observed with time. Significantly lower δB11 values (average =−1.9±2.2‰(1σ)) and B/Nb ratios (average =3.3±1.3(1σ)) were obtained for melt inclusions from Oligocene arc rocks (∼24 Ma) compared to those from the Paleocene (∼61 Ma) (averages =+1.6±0.8‰ and 17.8±1.4(1σ), respectively) and the Miocene (∼18 Ma) (averages =+4.7±1.9‰ and 11.9±5.5 (1σ), respectively).A slab-derived fluid with a δB11 composition of +1.5‰, primarily derived from altered oceanic crust on the down-going slab, affected the source of the Paleocene arc magma. The source of the Oligocene arc magmas received less boron derived from the subducting slab (≲1% fluid addition) than the Paleocene and Miocene arc magmas (up to 3.5% fluid addition). This is consistent with a greater depth to the slab-mantle interface and is potentially related to the widening of the volcanic arc and more distal position of these samples relative to the trench during this time period. The higher δB11 values (up to ∼9‰) obtained for the Miocene melt inclusions record an increase in the influence of serpentinite-derived fluids on the source of arc magmas after ∼19.5 Ma. This is approximately coeval with the subduction of the Juan Fernandez Ridge (JFR), suggesting that the oceanic lithosphere associated with the subducting JFR in the Early Miocene was hydrated and serpentinised, similar to the present day ridge. As serpentinisation increases the buoyancy of the slab this finding supports the link between the intersection of the JFR with the Andean margin and the onset of flat-slab subduction.

The role of thrust faulting in the formation of the eastern Alaska Range: Thermochronological constraints from the Susitna Glacier Thrust Fault region of the intracontinental strike-slip Denali Fault system

Horizontal-slip along restraining bends of strike-slip faults is often partitioned into a vertical component via splay faults. The active Susitna Glacier Thrust Fault (SGTF), as shown by its initiation of the 2002 M7.9 Denali Fault earthquake, lies south of, and intersects the dextral strike-slip Denali Fault. Geochronology and thermochronology data from samples across the SGTF constrain the region's tectonic history and the role of thrusting in the formation of the eastern Alaska Range south of the Denali fault. U-Pb zircon ages indicate intrusion of plutons in the footwall (~57 Ma) and hanging wall (~98 Ma). These U-Pb zircon ages correlate to those from the Ruby Batholith/Kluane Terrane ~400 km east along the Denali Fault, supporting geologic correlations and hence constraints on long-term slip rates. 40Ar/39Ar mica and K-feldspar data from footwall and hanging wall samples (~54 to ~46 Ma) reflect cooling following magmatism and/or regional Eocene metamorphism related to ridge subduction. Combined with apatite fission track data (ages 43–28 Ma) and thermal models, both sides of the SGTF acted as a coherent block during the Eocene and early Oligocene. Contrasting apatite (U-Th)/He ages across the Susitna Glacier (~25 Ma footwall, ~15 Ma hanging wall) suggest initiation of faulting during the middle Miocene. Episodic cooling and exhumation is related to thrusting on known or hypothesized faults that progressively activate due to varying partition of strain along the Denali Fault associated with changing kinematics and plate interaction (Yakutat microplate collision, flat-slab subduction and relative plate motion change) at the southern Alaskan plate margin.

Neogene paleoelevation of intermontane basins in a narrow, compressional mountain range, southern Central Andes of Argentina

The topographic growth of mountain ranges at convergent margins results from the complex interaction between the motion of lithospheric plates, crustal shortening, rock uplift and exhumation. Constraints on the timing and magnitude of elevation change gleaned from isotopic archives preserved in sedimentary sequences provide insight into how these processes interact over different timescales to create topography and potentially decipher the impact of topography on atmospheric circulation and superposed exhumation. This study uses stable isotope data from pedogenic carbonates collected from seven different stratigraphic sections spanning different tectonic and topographic positions in the range today, to examine the middle to late Miocene history of elevation change in the central Andes thrust belt, which is located immediately to the south of the Altiplano-Puna Plateau, the world's second largest orogenic plateau. Paleoelevations are calculated using previously published local isotope-elevation gradients observed in modern rainfall and carbonate-formation temperatures determined from clumped isotope studies in modern soils. Calculated Neogene basin paleoelevations are between 1 km and 1.9 km for basins that today are located between 1500 and 3400 m elevation. Considering the modern elevation and δ18O values of precipitation at the sampling sites, three of the intermontane basins experienced surface uplift between the end of deposition during the late Miocene and present. The timing of elevation change cannot be linked to any documented episodes of large-magnitude crustal shortening. Paradoxically, the maximum inferred surface uplift in the core of the range is greatest where the crust is thinnest. The spatial pattern of surface uplift is best explained by eastward migration of a crustal root via ductile deformation in the lower crust and is not related to flat-slab subduction.

Influence of Peruvian flat-subduction dynamics on the evolution of western Amazonia

Convection in the Earth's mantle is mainly driven by cold, dense subducting slabs, but relatively little is known about how 3D variations in slab morphology and buoyancy affect mantle flow or how the surface above deforms in response (i.e. dynamic topography). We investigate this problem by studying the dynamics of an active region of flat-slab subduction located in Peru in South America. Here the slab geometry is well known, based on the regional seismicity, and we have observations from the local geological record to validate our models. Of particular interest is the widespread subsidence and deposition of the Solimões Formation across western Amazonia that coincided with the development of the Peruvian flat-slab during the Mid-Late Miocene. This formation covers an extensive area from the foredeep to the Purus Arch located ∼2000km away from the trench. Close to the Andes the preservation of several kilometers of sedimentary thicknesses can be easily accounted for by flexure. Based on an estimate of the Andean loading we predict 2.8 to 3.6 km of accommodation space that spans 100 km. The spatial and temporal history of the Solimões Formation however, particularly the thick distal foreland accumulations up to 1.2 km deep, can only be matched with the addition of a longer-wavelength dynamic source of topography. Following the transition from normal to flat subduction, we predict over 1 km of dynamic subsidence (∼1500km wide) that propagates over 1000 km away from the trench, tracking the subduction leading edge. This is followed by a pulse of dynamic uplift over the flat segment behind it. We therefore propose that a combination of uplift, flexure and dynamic topography during slab flattening in Peru is responsible for the sedimentation history and landscape evolution of western Amazonia that eventually led to the configuration of the Amazon Drainage Basin we know today.

Present-day geodynamics of the northern North American Cordillera

Diffuse continental deformation results from interactions at plate boundaries, buoyancy forces generated by gradients in gravitational potential energy, and loads applied to the base of the lithosphere. Using finite element models, we calculate a deviatoric stress field associated with buoyancy forces, and then perform an iterative inversion to calculate deviatoric stress fields associated with boundary forces in the northern North American Cordillera. Our results reveal the presence of two distinct geodynamic domains. In the outboard domain, approximately equal magnitudes of boundary and buoyancy forces can account for the observed deformation along the Aleutian megathrust. In contrast, large boundary forces related to subduction of the Pacific and Yakutat slabs dominate the force-balance in south-central Alaska and combine with relatively small buoyancy forces to reproduce the observed kinematic indicators. In the inboard domain, encompassed by interior and northern Alaska and western Canada, boundary and buoyancy forces alone cannot reproduce the observed deformation. Therefore, we infer that deviatoric stresses due to basal tractions from a deeper mantle convection cell contribute to surface deformation in the inboard domain. Low effective lithospheric viscosity in south-central Alaska and the balancing effect of an independent geodynamic system driven by basal tractions in northern Alaska combine to confine the anomalously large Yakutat-related boundary deviatoric stresses to south-central Alaska. Deviatoric stresses associated with flat-slab subduction of the Yakutat microplate are a factor of two greater than boundary force estimates for the Andean and Indian–Eurasian convergent margins, where buoyancy and boundary forces are roughly equal in magnitude and dominate the force-balance.

Overriding plate controls on subduction evolution

AbstractGeologic and geophysical observations indicate that the thickness, density, and strength of the lithosphere vary on the Earth. However, the role of the overriding plate lithosphere properties on the evolution and morphology of subduction is not well understood. This paper presents time‐dependent numerical models of subduction that vary the overriding plate thickness, strength, and density and allow for a plate interface that evolves with time via an anisotropic brittle failure rheology. We examine the effect of these parameters on subduction evolution, in particular, the emergence of (a) asymmetric versus symmetric subduction, (b) trench retreat versus advance, (c) subduction zone geometry, (d) slab stagnation versus penetration into the lower mantle, and (e) flat slab subduction. Almost all of the models presented result in sustained asymmetric subduction from initiation. Trench advance occurs in models with a thick and or strong overriding plate. Slab dip, measured at a depth below the plate boundary interface, has a negative correlation with an increase in overriding plate thickness. Overriding plate thickness exerts a first‐order control over slab penetration into the lower mantle, with penetration most commonly occurring in models with a thick overriding plate. Periods of flat slab subduction occur with thick, strong overriding plates producing strong plate boundary interface coupling. The results provide insight into how the overriding plate plays a role in establishing advancing and retreating subduction as well as providing an explanation for the variation of slab geometry across subduction zones on Earth, where similar patterns of evolution are observed.

Structural evolution of the early Permian Nambucca Block (New England Orogen, eastern Australia) and implications for oroclinal bending

The Paleozoic to early Mesozoic southern New England Orogen of eastern Australia exhibits a remarkable ear-shaped curvature (orocline), but the geodynamic processes responsible for its formation are unclear. Oroclinal bending took place during the early Permian, simultaneously with the deposition of the rift-related Sydney, Gunnedah, and Bowen basins, which bound the oroclines to the west. The Nambucca Block is another early Permian rift basin, but it is situated in the core of the oroclinal structure. Here we present new stratigraphic, structural, and geochronological data from the Nambucca Block in an attempt to better understand its tectonic history and relationships to the formation of the oroclines. We recognized four phases of folding and associated structural fabrics (S1–4), with the second phase (S2) dated at 275–265 Ma by 40Ar/39Ar geochronology of muscovite. This age overlaps with independent constraints on the timing of oroclinal bending, suggesting that the earlier two phases of deformation in the Nambucca Block (F1 and F2) were associated with orocline formation. We propose that oroclinal bending involved three major stages. The first stage (<300 Ma) was associated with variations in rates of trench rollback and formation of rift basins in a hot extensional back-arc setting. This was followed by a second stage of oroclinal bending, possibly linked to dextral wrench faulting, which involved ~ N-S contraction (F1). Subsequent deformation at 275–265 Ma involved formation of nappe-style structures (F2). This phase of contractional deformation may have resulted from an increased plate coupling that was possibly linked to flat-slab subduction.

Flat slab subduction, trench suction, and craton destruction: Comparison of the North China, Wyoming, and Brazilian cratons

The mechanisms of growth and destruction of continental lithosphere have been long debated. We define and test a unifying plate tectonic driving mechanism that explains the numerous petrological, geophysical, and geological features that characterize the destruction of cratonic lithospheric roots. Data from three Archean cratons demonstrate that loss of their roots is related to rollback of subducted flat slabs, some along the mantle transition zone, beneath the cratons. During flat slab subduction dehydration reactions add water to the overlying mantle wedge. As the subducting slabs roll back, they suck in mantle material to infill the void space created by the slab roll back, and this fertile mantle becomes hydrated. The roll-back causes concomitant lithospheric thinning of the overlying craton so the flux of newly hydrated mantle material inevitably rises causing adiabatic melting, generating new magmas that gradually destroy the roots of the overlying craton through melt–peridotite reactions. Calculated fluxes of new mantle material beneath cratons that have lost their roots range from 2.7 trillion to 70 million cubic kilometers, which is sufficient to generate enough melt to completely replace the affected parts of the destroyed cratons. Cratonic lithosphere may be destroyed in massive quantities through this mechanism, warranting a re-evaluation of continental growth rates with time.

Late Cretaceous–early Eocene Laramide uplift, exhumation, and basin subsidence in Wyoming: Crustal responses to flat slab subduction

AbstractLow‐angle subduction of the Farallon oceanic plate during the Late Cretaceous–early Eocene is generally considered as the main driver forming the high Rocky Mountains in Wyoming and nearby areas. How the deformation was transferred from mantle to upper crust over the great duration of deformation (~40 Myr) is still debated. Here, we reconstruct basin subsidence and compile paleoelevation, thermochronology, and provenance data to assess the timing, magnitude, and rates of rock uplift during the Laramide deformation. We reconstruct rock uplift as the sum of surface uplift and erosion constrained by combining paleoelevation and exhumation with regional stratigraphic thickness and chronostratigraphic information. The amount (and rate) of rock uplift of individual Laramide ranges was less than 2.4–4.8 km (~0.21–0.32 mm/yr) during the early Maastrichtian‐Paleocene (stage 1) and increased to more than ~3 km (~0.38–0.60 mm/yr) during the late Paleocene–early Eocene (stage 2). Our quantitative constraints reveal a two‐stage development of the Laramide deformation in Wyoming and an increase of rock uplift during stage 2, associated with enhanced intermontane basin subsidence. Exhumation and uplift during stage 1 is consistent with eastward migration of Cordilleran deformation associated with low‐angle subduction, whereas the change in exhumation during stage 2 seems to follow a southwestward trend, which requires an alternative explanation. We here suggest that the increase of rock uplift rate during the late Paleocene–early Eocene and the southwestward younging trend of uplift may be a response to the rollback and associated retreating delamination of the Farallon oceanic slab.

A Late Triassic tectonothermal event in the eastern Acatlán Complex, southern Mexico, synchronous with a magmatic arc hiatus: The result of flat-slab subduction?

Basement rocks exposed in the Acatlan Complex of the Mixteca terrane in southern Mexico record two tectonothermal events: (1) a Devonian–Mississippian (ca. 365–318 Ma) event, recording extrusion and exhumation of high-pressure rocks; and (2) an Early to Middle Permian (ca. 289–263 Ma) event, involving N-S dextral shearing, transtensional deformation, and local S-vergent thrusting in a magmatic arc environment. We document an additional, regionally significant, tectonothermal event during the Middle to Late Triassic recorded by 40 Ar/ 39 Ar step-heating laser-probe ages ranging from ca. 239 and 219 Ma (≡ cooling from ca. 525 °C to 300 °C) for amphibole, muscovite, and biotite from: (1) the Carboniferous Amarillo unit, consisting of medium-grade, metasedimentary rocks intruded by mafic dikes; and (2) the Pennsylvanian–Middle Permian, low-grade, clastic-calcareous, arc-related Tecomate Formation. U-Pb laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) data yield an age of 339 ± 6 Ma for the youngest population of detrital zircon grains in the Amarillo unit. Lithogeochemical and Sm-Nd isotopic data for the Amarillo unit dikes are very similar to those of other Carboniferous meta-igneous rocks in the eastern and southwestern part of the Acatlan Complex, displaying affinities transitional between mid-ocean-ridge basalt (MORB) and continental tholeiites, and initial ɛ Nd ( t = 339 Ma) values from -6.6 to +6.4, indicating both depleted and enriched mantle sources, as well as variable contamination by continental crust or by subduction-derived fluids. The 40 Ar/ 39 Ar cooling ages coincide with an apparent hiatus in magmatic activity in southern Mexico, which is inferred to record a change from steep to flat subduction.

Petrogenesis of Early Carboniferous adakitic dikes, Sawur region, northern West Junggar, NW China: Implications for geodynamic evolution

Adakitic dikes are widespread in the Sawur region, northern West Junggar, northwestern China. Zircon U–Pb analyses of the dikes have yielded consistent ages of ca. 334Ma. The dikes are characterized by relatively high SiO2 (55.5–61.8wt.%), Al2O3 (14.5–16.3wt.%), and Sr (468–1005ppm), and low Y (10.1–14.1ppm) and Yb (0.93–1.39ppm) contents, with high Sr/Y (34–74) ratios and slight Eu anomalies, which are analogous to those of slab-derived adakites. In addition, the dikes are relatively MgO-rich (1.75–3.57wt.%; Mg#=44–56), with high Th/Yb ratios and positive εNd (334Ma) (+6.2 to +6.5) and εHf (334Ma) (+11.3 to +15.8) values. The data suggest that the dikes were generated by partial melting of subducted oceanic crust and overlying sediments. However, these Early Carboniferous adakitic rocks and associated I-type granites in the northern part of West Junggar are petrochemically distinct from the Late Carboniferous adakitic plutons from the southern part of West Junggar. The latter are associated with charnockites, A-type granites, tholeiites, and Nb-enriched, alkaline basalts, as well as magnesian dikes, which were generated by ridge subduction during the Late Carboniferous. The distinct lithological association of the Early Carboniferous adakites is consistent with flat slab subduction, which was characterized by anomalous heating of the slab and consequential partial melting to generate adakitic magma. After generation of the Early Carboniferous adakites, the subducting slab cooled, reverted to a more normal subduction angle in the late Early Carboniferous, and was associated with more typical I-type granite arc magmatism.

Quaternary tectonics and seismic potential of the Andean retrowedge at 33–34°S

Abstract The Andean retrowedge, located between 33°S and 34°S, lies in the transition region of the Pampean flat-slab subduction zone to the north and a normal subduction zone to the south. Neotectonic structures and shallow seismicity are very common north of this segment and become progressively less frequent southwards. The Frontal Cordillera and the Cerrilladas Pedemontanas are the main morphostructures involved in the Quaternary deformation of this region. The Frontal Cordillera is a thick-skinned fold-and-thrust belt uplifted since Late Miocene time. The Cerrilladas Pedemontanas are low-relief hills that represent the mild inversion of the Cuyo Triassic rift depocentre since Pliocene time. Middle Miocene–Holocene synorogenic strata cover the Cuyo basin and surrounding foreland areas. The Quaternary tectonic evolution of this area has been established through integration of new data from fieldwork in the Frontal Cordillera piedmont with subsurface information and previously published data. Mean Late Pleistocene uplift rates ranging between 0.21 and 0.92 mm a −1 and earthquake minimum moment magnitudes ( M w ) of c. 6.4–6.7 have been estimated for the morphostructural units analysed in this manuscript.