Frontal Accretion Research Articles

Surface processes versus kinematics of thrust belts: impact on rates of erosion, sedimentation, and exhumation – Insights from analogue models

Abstract The mechanical equilibrium of an orogenic wedge is maintained thanks to interactions between tectonic processes and surface processes. To better constrain the influence of erosion and sedimentation on the evolution of orogens, we performed a series of analogue models based on the tapered wedge principle, varying the amounts of erosion and sedimentation. The models develop by frontal accretion in the foreland basin and by simple underthrusting and subsequent underplating in the hinterland. The variations in rates of erosion and sedimentation strongly modify the extent, the morphology, the structures, the timing of development and the material paths in the different models. Under certain conditions, entire structural units can be formed and subsequently eroded out of the geological record, leading to important underestimations when restoring sections. Particles located in the converging lower-plate or in the upper-plate show complex uplift paths related to tectonic stages. The correlation between models and three Alpine tectonic cross-sections emphasizes the role of erosion and sedimentation on the dynamics and development of the orogen and adjacent Molasse basin. Along strike changes in the present structure of the orogen could be explained in part by differences in surface processes.

Interactions between tectonics, erosion, and sedimentation during the recent evolution of the Alpine orogen: Analogue modeling insights

On the basis of a section across the northwestern Alpine wedge and foreland basin, analogue modeling is used to investigate the impact of surface processes on the orogenic evolution. The basis model takes into account both structural and lithological heritages of the wedge. During shortening, erosion and sedimentation are performed to maintain a critical wedge. Frontal accretion leads to the development of a foreland thrust belt; underplating leads to the formation of an antiformal nappe stack in the internal zones. Important volumes of analogue materials are eroded out of the geological record, which in the case of the Alps suggests that the original lengths and volumes may be underestimated. The foreland basin evolves differently depending on the amounts of erosion/sedimentation. Its evolution and internal structuring is governed by the wedge mechanics, thought to be the main controlling mechanism in the development of the Molasse basin in a feedback interaction with surface processes.

Late Cenozoic metamorphic evolution and exhumation of Taiwan

The Taiwan mountain belt is composed of a Cenozoic slate belt (Hsuehshan Range units, HR, and Backbone Slates, BS) and of accreted polymetamorphic basement rocks (Tananao Complex, TC). Ongoing crustal shortening has resulted from the collision between the Chinese continental margin and the Luzon volcanic arc, which initiated ∼6.5 Ma ago. The grade and age of metamorphism and exhumation are a key record of the development of the orogenic wedge. Because the Taiwan mountain belt is mostly composed by accreted sediments lacking metamorphic index minerals, quantitative constraints on metamorphism are sparse. By contrast, these rocks are rich in carbonaceaous material (CM) and are therefore particularly appropriate for RSCM (Raman Spectroscopy of CM) thermometry. We apply this technique in addition to (U‐Th)/He thermochronology on detrital zircons to assess peak metamorphic temperatures (T) and the late exhumational history respectively, along different transects in central and southern Taiwan. In the case of the HR units, we find evidence for high metamorphic T of at least 340°–350°C and locally up to 475°C, and for relative rapid exhumation with zircon (U‐Th)/He ages in the range of 1.5–2 Ma. Farther east, the BS were only slightly metamorphosed (T < 330 °C), and zircons are not reset for (U‐Th)/He. From the eastern BS to the inner TC schists, T gradually increases from ∼350°C up to ∼500°C following an inverted metamorphic gradient. Available geochronological constraints and the continuous thermal gradient from the BS to the basement rocks of the TC suggest that the high RSCM T of the TC were most probably acquired during the last orogeny, and were not inherited from a previous thermal event. Zircons yield (U‐Th)/He ages of ∼0.5–1.2 Ma. Peak metamorphic T and the timing of exhumation do not show along‐strike variations over the TC in the studied area. In contrast, exhumation is laterally diachronous and decreases southward in the case of the HR units. In particular, our data imply that the HR units have been exhumed by a minimum of 15 km over the last few Ma. In the case of the BS, they show far less cumulated exhumation and much slower cooling rates. We propose that most of the deformation and exhumation of the Taiwan mountain belt is sustained through two underplating windows located beneath the Hsuehshan Range and the TC. Our data show significant departures from the predictions of the prevailing model in Taiwan, which assumes a homogeneous critical wedge with dominant frontal accretion. Our study sheds new light on how the mountain belt has grown as a possible result of underplating mostly.

Mountain building in Taiwan: A thermokinematic model

The Taiwan mountain belt is classically viewed as a case example of a critical wedge growing essentially by frontal accretion and therefore submitted to distributed shortening. However, a number of observations call for a significant contribution of underplating to the growth of the orogenic wedge. We propose here a new thermokinematic model of the Taiwan mountain belt reconciling existing kinematic, thermometric and thermochronological constraints. In this model, shortening across the orogen is absorbed by slip on the most frontal faults of the foothills. Crustal thickening and exhumation are sustained by underplating beneath the easternmost portion of the wedge (Tananao Complex, TC), where the uplift rate is estimated to ∼6.3 mm a−1, and beneath the westernmost internal region of the orogen (Hsueshan Range units, HR), where the uplift rate is estimated to ∼4.2 mm a−1. Our model suggests that the TC units experienced a synchronous evolution along strike despite the southward propagation of the collision. It also indicates that they have reached a steady state in terms of cooling ages but not in terms of peak metamorphic temperatures. Exhumation of the HR units increases northward but has not yet reached an exhumational steady state. Presently, frontal accretion accounts for less than ∼10% of the incoming flux of material into the orogen, although there is indication that it was contributing substantially more (∼80%) before 4 Ma. The incoming flux of material accreted beneath the TC significantly increased 1.5 Ma ago. Our results also suggest that the flux of material accreted to the orogen corresponds to the top ∼7 km of the upper crust of the underthrust Chinese margin. This indicates that a significant amount (∼76%) of the underthrust material has been subducted into the mantle, probably because of the increase in density associated with metamorphism. We also show that the density distribution resulting from metamorphism within the orogenic wedge explains well the topography and the gravity field. By combining available geological data on the thermal and kinematic evolution of the wedge, our study sheds new light onto mountain building processes in Taiwan and allows for reappraising the initial structural architecture of the passive margin.

Frontal accretion: An internal clock for bivergent wedge deformation and surface uplift

We propose a conceptual model of frontal accretion within bivergent wedges, which is based on two‐dimensional sandbox simulations and the analysis of particle displacement fields. Each frontal accretion cycle consists of a thrust initiation, an underthrusting phase, and a reactivation phase. The location and magnitude of deformation within a bivergent wedge and its associated surface uplift vary systematically with the phase of the frontal accretion cycle and are thus predictable. Therefore the frontal accretion cycle can be considered as an internal clock for wedge‐scaled deformation and surface uplift. We further demonstrate that the geometry of the deformation front and the spatial distribution of surface uplift can be used to infer the currently active phase within a frontal accretion cycle. Surface uplift of the axial zone and the retrowedge may reach up to half of the thickness of the incoming sedimentary layer during a frontal accretion cycle. We estimate cycle duration to range between 104 and 105 years, which is thus similar to the time frame of climatic cycles. The proposed conceptual model provides an explanation for the commonly observed transience of deformation and surface uplift and for the discrepancy between geodetic, paleoseismologic, and geologic estimates of fault slip.

Sensitivity of shear zones in orogenic wedges to surface processes and strain softening

We investigate the internal deformation of orogenic wedges growing by frontal accretion with a two-dimensional numerical model. Our models are limited to crustal deformation and assume a horizontal detachment as observed for various natural orogens (e.g. Alaska and Costa Rica). The model wedges develop as a result of convergence of a brittle sediment layer in front of a strong backstop. We find that our reference model develops in-sequence forward-thrusts which propagate upward from the basal detachment. For this reference model we investigate the sensitivity of shear zone activity to surface processes and strain softening. Model results show that diffusive or slope dependent erosion enhances material transport across the wedge and slows down forward propagation of the deformation front. Frictional strain softening focuses deformation into narrow shear zones and enhances displacement along them. This has also been postulated for natural thrusts such as the Glarus thrust in the Swiss Alps and the Moine thrust in the Scottish Caledonides. A second series of models investigates the effects of regularly spaced weak inclusions within the sediment layer which simulate remnants of previous deformation phases. These inclusions facilitate and focus internal deformation, influence the thrust dip and thrust vergence and enable thrust reactivation in the internal part of the wedge. Our results show that inactive thrusts in the internal part of the wedges may be reactivated in models with diffusive surface processes, strain softening or weak inclusions. Thrust reactivation occurs as models seek to maintain their critical taper angle. First order characteristics of our numerical models agree well with natural orogenic wedges and results from other numerical and analogue models.

Structural contacts in subduction complexes and their tectonic significance: the Late Palaeozoic coastal accretionary wedge of central Chile

Understanding the contact between the very low-grade metagreywacke of the Eastern Series and high-pressure metamorphosed schist of the Western Series in the Late Palaeozoic accretionary wedge of central Chile is fundamental for the understanding of the evolution of ancient accretionary wedges. We show the progressive development of structures and finite strain from the least deformed rocks in the eastern part of the Eastern Series of the accretionary wedge to high-pressure schist of the Western Series at the Pacific coast. Upright chevron folds of sedimentary layering are associated with an axial-plane foliation, S 1 . As the F 1 folds became slightly overturned to the west, S 1 was folded about west-vergent open F 2 folds and an S 2 axial-plane foliation developed. Near the contact between the Western and Eastern Series S 2 represents a penetrative subhorizontal transposition foliation. Towards the structurally deepest units in the west the transposition foliation becomes progressively flattened. Finite-strain data as obtained by R f /ϕ analysis in metagreywacke and X-ray texture goniometry in phyllosilicate-rich rocks show a smooth and gradual increase in strain magnitude from east to west. Overturned folds and other shear-sense indicators show a uniform top-to-the-west shear sense in moderately deformed rocks, whereas the shear sense is alternating top-to-the-west and top-to-the-east in the strongly flattened high-pressure rocks of the Western Series near the Pacific coast. We interpret the progressive structural and strain evolution across the contact between the two series to reflect a continuous change in the mode of accretion in the subduction wedge. Initially, the rocks of the Eastern Series were frontally accreted to the pre-Andean margin before c . 300 Ma. Frontal accretion caused horizontal shortening, and upright folds and subvertical axial-plane foliations developed. At c . 300 Ma the mode of accretion changed and the rocks of the Western Series were underplated below the Andean margin. This basal accretion caused a major change in the flow field within the wedge and gave rise to vertical shortening and the development of the penetrative subhorizontal transposition foliation. Subsequent differential exhumation was resolved gradually over a wide region, implying that exhumation was not tectonically controlled.

Punctuated thrust deformation in the context of doubly vergent thrust wedges: Implications for the localization of uplift and exhumation

Research Article| June 01, 2007 Punctuated thrust deformation in the context of doubly vergent thrust wedges: Implications for the localization of uplift and exhumation M. Naylor; M. Naylor 1Edinburgh University, School of GeoSciences, Grant Institute, West Mains Road, Edinburgh EH9 3JW, UK Search for other works by this author on: GSW Google Scholar H.D. Sinclair H.D. Sinclair 1Edinburgh University, School of GeoSciences, Grant Institute, West Mains Road, Edinburgh EH9 3JW, UK Search for other works by this author on: GSW Google Scholar Geology (2007) 35 (6): 559–562. https://doi.org/10.1130/G23448A.1 Article history received: 18 Oct 2006 rev-recd: 29 Jan 2007 accepted: 07 Feb 2007 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 M. Naylor, H.D. Sinclair; Punctuated thrust deformation in the context of doubly vergent thrust wedges: Implications for the localization of uplift and exhumation. Geology 2007;; 35 (6): 559–562. doi: https://doi.org/10.1130/G23448A.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 Understanding uplift and exhumation in orogenesis requires integrating the effects of both climatic and tectonic forcing. Punctuated thrust activity is widely documented from field studies in a range of mountain belts. The potential for climatic forcing of thrust activity has been a subject of recent debate. Here we use a modeling approach to analyze the behavior of individual thrust units in the context of asymmetric doubly vergent thrust wedges. The model predicts that rates of surface uplift, frontal accretion, and exhumation should be punctuated on a time scale linked to thrust sheet geometry and convergence rates. This time scale ranges from 0.1 to 5 m.y. for various settings, and should be calculated before external forcings such as climate are invoked. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Investigating the kinematics of mountain building in Taiwan from the spatiotemporal evolution of the foreland basin and western foothills

The Taiwanese range has resulted from the collision between the Luzon volcanic arc and the Chinese continental margin, which started about 6.5 Myr ago in the north, and has since propagated southward. The building of the range has been recorded in the spatiotemporal evolution of the foreland basin. We analyze this sedimentary record to place some constraints on the kinematics of crustal deformation. The flexure of the foreland under the load of the growing wedge started with a 1.5 Myr long phase of rapid subsidence and sedimentation, which has migrated southward over the last 3.5 Myr at a rate of 31 +10/−5 mm/yr, reflecting the structural evolution of the range and the growth of the topography during the oblique collision. Isopachs from the Toukoshan (∼0 to 1.1 Ma) and Cholan (∼1.1 to 3.3 Ma) formations, as well as the sedimentation rates retrieved from a well on the Pakuashan anticline, indicate that the foreland basement has been moving toward the center of mass of the orogen by ∼45–50 mm/yr during the development of the basin. From there, we estimate the long‐term shortening rate across the range to 39.5–44.5 mm/yr. By considering available data on the thrust faults of the foothills of central Taiwan, we show that most (if not all) the shortening across the range is accommodated by the most frontal structures, with little if any internal shortening within the wedge. The range growth appears therefore to have been essentially sustained by underplating rather than by frontal accretion. In addition, only the upper ∼7 to 9 km of the underthrusted crust participates to the growth of the orogen. This requires that a significant amount of the Chinese passive margin crust is subducted beneath the Philippine Sea plate.

Morphotectonics domains and structural styles in the Makran accretionary prism, offshore Iran

The Makran accretionary prism is regarded as one of the most extensive subduction complex on Earth. It provides an ideal example of accretionary prism to study processes related to subduction at plate boundaries such as frontal accretion and underplating. The rear portion of the wedge is uplifted and extended by normal faulting and ductile flow. Spectacular shale diapirs and mud volcanoes are present all along the external part of the prism and can be seen on the regional 2D-seismic section presented in this study. The Himalayan Turbidites sequence is the main detachment level for the imbricate fan and extensional faults of the Makran accretionary prism. It is also assumed to be the main source for rising shale diapirs and mud volcanoes along imbricated thrusts within the wedge. Evidence of active sediment remobilization is prevalent in the mid-slope morphotectonic province of the accretionary prism. It is proposed that the initiation of diapirism appears to be spatially coincident with the onset of underplating processes in the rear portion of the prism. The rapid uplift of the prism and the onset of extensional faults favour the extrusion of overpressured sediments and fluids/gas along thrust faults on the seaward side of the prism. The extensional faults above the deep zone of underplating have been mildly inverted implying episodic alternation of compression and extension. The geometry of the growth strata preserved in the limbs of the fault propagation folds in the outer part of the Makran accretionary prism reveals limb rotation as the main folding mechanism with uplift rate of the growing folds exceeding the sedimentation rate suggesting active thrusting at the wedge front. The growth strata patterns also indicate a synchronous activity of many of the thrusts of the imbricate fan system that exhibit a sequence of thrusting propagating toward the foreland.

Thrust front segmentation induced by differential slab retreat in the Apennines (Italy)

AbstractNew seismic reflection data reveal that the south‐eastward prolongation of the Apennine thrust front in the central Adriatic Sea is most likely located in a more external position with respect to the traditional interpretation. In this new interpretation, the Apennine thrust front shows a segmented geometry in correspondence to the Tremiti lithospheric transfer zone. The available data suggest that the north‐eastward flexural retreat of the subducting Adriatic lithosphere and the related frontal accretion of the Apennine accretionary prism have acted during the last 5 Ma and may be considered still lively in both the Po plain and in the Central Adriatic domain, north of the Tremiti lineament. By analysing the tectonic evolution of the Apulo‐Adriatic areas, it may be argued that the differential slab retreat between the central and southern sectors of the subducting Adriatic plate has been able to induce the segmentation of the Apennine thrust front.

Inversion of forearc basins in south-central Chile caused by rapid glacial age trench fill

Research Article| September 01, 2006 Inversion of forearc basins in south-central Chile caused by rapid glacial age trench fill Daniel Melnick; Daniel Melnick 1GeoForschungsZentrum Potsdam, Telegrafenberg, 14473 Potsdam, Germany Search for other works by this author on: GSW Google Scholar Helmut P. Echtler Helmut P. Echtler 1GeoForschungsZentrum Potsdam, Telegrafenberg, 14473 Potsdam, Germany Search for other works by this author on: GSW Google Scholar Geology (2006) 34 (9): 709–712. https://doi.org/10.1130/G22440.1 Article history 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 Daniel Melnick, Helmut P. Echtler; Inversion of forearc basins in south-central Chile caused by rapid glacial age trench fill. Geology 2006;; 34 (9): 709–712. doi: https://doi.org/10.1130/G22440.1 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 This study examines the response of a forearc to the increase in sediment flux to the trench caused by the onset of glacial denudation in the Patagonian Andes. We investigated shelf-coastal basins in south-central Chile, which generally comprise Eocene–early Miocene nearshore facies overlain by late Miocene–early Pliocene deep-water siltstones and by late Pliocene–Quaternary nearshore deposits. Seismic profiles and coastal exposures reveal Eocene–early Pliocene extension followed by ongoing late Pliocene compression evidenced from growth strata adjacent to seismically active reverse faults. The onset of major global cooling ca. 6 Ma triggered glacial denudation in the uplifted high Andes. Exhumed material transported along the steep and humid Andean western slope increased trench sedimentation rates and caused continuous accretion and subduction of terrigenous material. We interpret forearc basin inversion as a response to a decrease in slope and basal friction of the wedge caused by frontal accretion and subduction of water-rich material, respectively, in order to reach a critical taper. This process lifted the shelf ∼1.5 km during the middle Pliocene. The Juan Fernández Ridge and Chile Rise confined >2 km trench fill between 45 and 34°S, limiting accretion and basin inversion. Glacial age trench fill and the steady decrease in plate convergence rate shifted this segment of the margin from erosive to accretionary during the Pliocene. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Asymmetric growth of the Pyrenees revealed through measurement and modeling of orogenic fluxes

Orogenic asymmetry is ubiquitous in terms of topography, structure and metamorphism, but the relative influences of climate and tectonics in determining such asymmetry remain contentious. This study demonstrates that for the case of the European Pyrenees, orogenic asymmetry is not defined by inherited structural asymmetry, nor by orographic asymmetry of precipitation, but is the simple consequence of the inherent tectonic growth of a doubly-vergent orogen. The Pyrenees record approximately 60 Myr of upper crustal accretion, erosion and sedimentation that developed from Late Cretaceous to early Miocene times. Approximately 165 km of plate convergence was accommodated by four phases of orogenic growth that can be summarized in terms of their generic orogenic development as follows: 1) Early inversion of passive margin extensional faults, initiating the pro-wedge. 2) Growth by a combination of frontal accretion and underplating to the pro-wedge; symmetrical and steady record of erosion and foreland basin sedimentation. 3) Cessation of pro-wedge frontal accretion with propagation of the retro-wedge deformation front. 4) Accelerated growth of pro-wedge antiformal stack induced by underplating; accelerated erosion over antiformal stack and increase in sediment discharge to pro-foreland basin; stabilization of outer pro-wedge by sediment blanketing. Discrete element model experiments are used to explore the natural tendency of the system towards asymmetry. Model output demonstrates that during active frontal accretion to the pro-wedge of an orogen, the maxima in erosional denudation is offset towards the pro-wedge relative to the maximum depths of exhumation. This asymmetry is fundamentally driven by the progressive reduction in advection velocities of material within the orogen from the pro-wedge into the retro-wedge. Hence, for an efficient erosional regime, a dynamic coupling between erosion and structural uplift of the pro-wedge will reduce activity of the retro-wedge and hence drive asymmetry in terms of low temperature thermochronometry, erosion and sedimentation. The models also demonstrate the tendency for pro-wedge growth through frontal accretion to slow. However, the cessation of frontal accretion as recorded by the South Pyrenean Thrust Belt (phases 3 and 4) requires additional forcing; it is proposed that the thick wedge-top sedimentation recorded at this time, increased taper angles and hence stabilized the wedge. Accretionary influx into the orogen at this time was accomplished through underplating. These processes governing asymmetry do not require a spatially variable climate or a laterally heterogeneous underthrust plate.

Thermal structure and exhumation history of the Lesser Himalaya in central Nepal

The Lesser Himalaya (LH) consists of metasedimentary rocks that have been scrapped off from the underthrusting Indian crust and accreted to the mountain range over the last ∼20 Myr. It now forms a significant fraction of the Himalayan collisional orogen. We document the kinematics and thermal metamorphism associated with the deformation and exhumation of the LH, combining thermometric and thermochronological methods with structural geology. Peak metamorphic temperatures estimated from Raman spectroscopy of carbonaceous material decrease gradually from 520°–550°C below the Main Central Thrust zone down to less than 330°C. These temperatures describe structurally a 20°–50°C/km inverted apparent gradient. The Ar muscovite ages from LH samples and from the overlying crystalline thrust sheets all indicate the same regular trend; i.e., an increase from about 3–4 Ma near the front of the high range to about 20 Ma near the leading edge of the thrust sheets, about 80 km to the south. This suggests that the LH has been exhumed jointly with the overlying nappes as a result of overthrusting by about 5 mm/yr. For a convergence rate of about 20 mm/yr, this implies underthrusting of the Indian basement below the Himalaya by about 15 mm/yr. The structure, metamorphic grade and exhumation history of the LH supports the view that, since the mid‐Miocene, the Himalayan orogen has essentially grown by underplating, rather than by frontal accretion. This process has resulted from duplexing at a depth close to the brittle‐ductile transition zone, by southward migration of a midcrustal ramp along the Main Himalayan Thrust fault, and is estimated to have resulted in a net flux of up to 150 m2/yr of LH rocks into the Himalayan orogenic wedge. The steep inverse thermal gradient across the LH is interpreted to have resulted from a combination of underplating and post metamorphic shearing of the underplated units.

Upward delamination of Cascadia Basin sediment infill with landward frontal accretion thrusting caused by rapid glacial age material flux

The Cascadia convergent margin is a first‐order research target to study the impact of rapid sedimentation processes on the mechanics of frontal subduction zone accretion. The near‐trench part of the accretionary prism offshore Washington is affected by strongly increased glacial age sedimentation and fan formation that led to an outstanding Quaternary growth rate with landward vergent thrust faulting that is rarely observed elsewhere in accretionary wedges. Multichannel seismic reflection data acquired on the ORWELL project allows us to study the structure and dynamics of the atypical frontal accretion processes. We performed a kinematical and mechanical analysis of the frontal accretion structures, and developed a dynamic Coulomb‐wedge model for the landward‐verging backthrust formation. Backthrusting results from heterogeneous diffuse strain accumulation in the mechanically heterogeneous Cascadia basin sediment succession entering the subduction zone, and strain partitioning along a midlevel detachment that is activated by gravitational loading caused by rapid glacial age sedimentation. These complex deformation processes cause the passive “upward” delamination of the upper turbidite beds from the basal pelagic carbonate section similar to triangle‐zone formation and passive backthrust wedging in foreland thrust belts caused by rapid burial beneath syntectonic sediment deposits. The deformation mechanism at the tectonic front of the Cascadia margin is an immediate response to the strongly increased late Pleistocene sediment flux rather than to atypical physical boundary conditions as generally thought.

Controls on the strength of coupling among climate, erosion, and deformation in two‐sided, frictional orogenic wedges at steady state

Many important insights regarding the coupling among climate, erosion, and tectonics have come from numerical simulations using coupled tectonic and surface process models. However, analyses to date have left the strength of the coupling between climate and tectonics uncertain and many questions unanswered. We present an approximate analytical solution for two‐sided orogenic wedges obeying a frictional rheology, and in a condition of flux steady state, that makes explicit the nature and sensitivity of the coupling between climate and deformation. We make the simplifying assumption that the wedge grows in a self‐similar fashion consistent with Airy isostasy such that topographic taper is invariant with orogen width, tectonic influx rate, and climate. We illustrate first how and why the form of the erosion rule matters to orogen evolution and then derive a physically based orogen‐scale erosion rule. We show that steady state orogen width, crest elevation, and crustal thickness are controlled by the ratio of accretionary flux to erosional efficiency to a power dictated by the erosion process. Remarkably, we show that for most combinations of parameters in the erosion law, rock uplift rate is more strongly controlled by erosional efficiency than it is by the accretionary flux. Further, assuming frontal accretion with no underplating, the spatial distribution of erosional efficiency dictates the relative rock uplift rates on the pro‐wedge and retro‐wedge and the time‐averaged trajectories of rocks through the orogen. The restriction to invariant frictional properties is conservative in these respects; systems subject to positive feedback between erosion and rheology will exhibit even stronger coupling among climate, erosion, and deformation than shown here.

Amphibious seismic survey images plate interface at 1960 Chile Earthquake

The southern central Chilean margin at the site of the largest historically recorded earthquake in the Valdivia region, in 1960 (Mw = 9.5), is part of the 5000‐km‐long active subduction system whose geodynamic evolution is controversially debated and poorly understood. Covering the area between 36° and 40°S, the oceanic crust is segmented by prominent fracture zones. The offshore forearc and its onshore continuation show a complex image with segments of varying geophysical character, and several fault systems active during the past 24 m.y.In autumn 2001, the project SPOC was organized to study the Subduction Processes Off Chile, with a focus on the seismogenic coupling zone and the forearc. The acquired seismic data crossing the Chilean subduction system were gathered in a combined offshore‐onshore survey and provide new insights into the lithospheric structure and evolution of active margins with insignificant frontal accretion.

Frontal accretion and thrust wedge evolution under very oblique plate convergence: Fiordland Basin, New Zealand

ABSTRACTA thrust wedge with unusual geometry has developed under very oblique (50–60°) convergence between the Pacific and Australian Plates, along the 240‐km length of the Fiordland margin, New Zealand. The narrow (25 km‐wide) wedge comprises three overlapping components, lying west of the offshore section of the Alpine Fault, and straddles a change of > 30° in the regional strike of the plate boundary. Swath bathymetry, marine seismic reflection profiles, and dated samples together reveal the stratigraphy, structure, and evolution of the wedge and the underthrusting, continental, Caswell High (Australian Plate). Lateral variations in the composition and structure of the accretionary wedge, and the depth of the décollement thrust, result partly from variations in crustal structure and basement relief of the underthrust plate, and from associated variations in the thickness of turbidites available for frontal accretion. In the southern Fiordland Basin the underthrust plate is undergoing flexural uplift and extension, and a thick turbidite section is available for accretion. Along‐strike, a structurally elevated portion of the underthrust plate is very obliquely colliding with the central part of the accretionary wedge, the turbidite section available for accretion is condensed, and structural inversion occurs in the underthrust plate. Growth of the thrust wedge is inferred to have commenced in the Pliocene prior to 3 ± 1 Ma, but much of the wedge developed in the Quaternary. The spatial distribution of thrusting has varied through time, with most late Quaternary shortening occurring on structures within 10 km of the right‐stepping deformation front. Estimates of the magnitude and rates of deformation indicate that the wedge accommodates a significant component of the oblique convergence between the Pacific and Australian Plates. Shortening of up to 7.3 ± 1.4 km and 9.1 ± 1.8 km within the southern and central parts of the wedge, respectively, represent about 5–15% of the total 70–140 km of shortening predicted across the plate boundary since 6.4 Ma, and about 10–30% since 3 Ma. Late Quaternary shortening rates of the order of 1–5 mm yr−1, estimated across both the northern and southern parts of the wedge, represent about 10–50 and 5–21% of the total NUVEL‐1 A shortening across the plate boundary at these respective latitudes, implying that most shortening is occurring onshore. Furthermore, possible oblique‐slip thrusting within the wedge may be accommodating boundary‐parallel displacement of 0–6 mm yr−1, representing 0–17% of the total predicted within the plate boundary.

Mechanical decoupling and basal duplex formation observed in sandbox experiments with application to the Western Mediterranean Ridge accretionary complex

Sandbox experiments of accretionary wedges were performed incorporating a thin weak layer of micro glass beads. The impact of heterogeneous sedimentary input on wedge mechanics, evolution and mass transfer was investigated. We report the first experimentally documented growth of basal duplexes. These occurred for high basal friction conditions, with restricted output of the lower section. The upper and lower sections were completely decoupled due to the intervening layer of glass beads, with frontal accretion occurring in the upper section simultaneously with basal duplex formation and underplating of subsequent generations of duplexes. IMERSE multichannel seismic reflection data from the Western Mediterranean Ridge (WMR) image Tertiary clastics beneath a thick section of Messinian evaporites. The base of the evaporites is identified as the primary décollement for deformation in the frontal part of the accretionary complex. Constriction of the channel of subducting Tertiary sediments, as well as internal deformation observed as arcward-dipping reflectors argue for basal underplating and/or two different active décollements. We propose an evolution of the WMR in accordance with the sandbox experimental results. A weak mid-level detachment (base of evaporites) combined with a strong basal detachment produce mechanical decoupling and basal accretion of toeward-verging duplexes.

Frontal accretion along the western Mediterranean Ridge: the effect of Messinian evaporites on wedge mechanics and structural style

In the context of the IMERSE project, several crossings of the deformation front of the western Mediterranean Ridge were made in the region of the Sirte Abyssal Plain, the Messina Abyssal Plain and the intervening region. In this paper, we present seismic images and interpretations across the deformation front, with particular emphasis on the role the Messinian evaporites have played in controlling the accretionary tectonics of the thin frontal portion of the wedge. The seismic images show that the basal detachment generally is located at the base of the evaporites. From a consideration of the mechanics of the wedge, for both Coulomb and plastic rheologies, we show that the low wedge taper (c. 2°) requires that the detachment is characterised by extreme fluid overpressuring (within 2% of lithostatic in places) and that the basal yield stress (less than 1 MPa) is lower than that of a wet salt décollement zone. This supports the seismic interpretation that the detachment occurs in overpressured sediments beneath the impermeable evaporites. Lateral variations in the accretionary style can be related to lateral variations in evaporite thickness, the effectiveness of the evaporite as an impermeable seal and to local relief on the subducting plate; these factors control the escape of fluids from beneath the evaporites and hence fluid pressure and basal yield stresses.