T13D-1980 INVITED
Continental Collision and the STEP-wise Evolution of Convergent Plate Boundaries: The Mediterranean Region
This contribution focusses on terminal stage subduction, often triggered by continent-continent or arc- continent collision. The landlocked basin setting of the Mediterranean region provides unique opportunities to study terminal stage subduction and its consequences. We use seismic tomography results on lithosphere and upper mantle structure as a source of information on plate boundary structure, and concentrate on the lithospheric scale aspects. Combining this structural information with process-oriented numerical modelling studies and regional observations, we present a 3D model for convergent plate boundary evolution after collision, in which slab detachment and the formation of tear or STEP (Subduction-Transform-Edge-Propagator; see R. Govers and M.J.R. Wortel, EPSL, 236, 505- 523, 2005) faults are key elements. A STEP fault laterally decouples subducting lithosphere from non- subducting lithosphere in a scissor type of fashion. It enhances the ability of a slab to retreat through the mantle flow around the edge of the subducted slab. In this way collision and back-arc extension may occur in close proximity. In our study area this specifically pertains to collision along the north African margin, STEP formation in easterly direction, CCW rotation of the southern Apennines slab and the opening of the Tyrrhenian Sea. Vertical tearing of subducted lithosphere may play an important role as well, but is probably not crucial. Similar processes are likely to have occurred in the eastern Mediterranean. On the basis of the good agreement between the Mediterranean-based model and the evolution of the Tonga-Fiji region we expect that the model may shed light on other complex convergent plate boundary regions, as well. In summary: Upon continental (or arc-continent) collision, along-trench variations in lithospheric properties of the subducting lithosphere may lead to disruption and segmentation of the subduction system. Following slab detachment along limited segments of a convergent plate boundary, the development of STEP faults is expected. These faults contribute to an increase in arc curvature within plate boundary segments. This contributes to the sinuous geometry of long subduction systems such as in the western and southwest Pacific.
T13D-1981
3D modeling of subduction adjacent to deep continental roots: enhanced negative pressure in the mantle wedge, mountain building and continental motion
Subduction near to a deep continental root is modeled with finite elements. The root restricts asthenospheric corner flow, which increases mantle-wedge suction. The suction pulls the continental root toward the subduction zone, simultaneously promoting trenchward continental motion and compressive tectonics in the lithosphere between the root and the subduction zone. In a 2-D model with a root that extends to 250 km depth and lies 300 km from the mantle-wedge corner, we find two-fold increase in mantle-wedge suction and nearly a four-fold increase in continental compression (to 5e12 N/m) compared to a rootless reference model. In 3-D modeling, we find that distance to the subduction zone is more important than root width (in subduction strike direction) or reasonable variations in sub-lithospheric viscosity structure, with the root- enhanced suction effects diminishing rapidly as the wedge corner root distance increases beyond 400 km. When finite-width roots are wider than the distance to the mantle-wedge corner and closer than 400 km, compression of the continent between the root and the subduction zone is >50% that found in the infinite- width root.
T13D-1982
A Mass-balance Analysis of Subduction of the Adria Microplate in the Northern Apennines, Italy
The northern Apennines range is a northeast-tapering orogenic wedge, fashioned since mid-Oligocene time as the dominantly continental Adria plate has been consumed beneath the Corsica-Sardinia block to the west. Most of the northern Apennines wedge consists of materials (Triassic evaporites, Jurassic and Cretaceous carbonates, and Oligocene to Miocene clastic sediments) that have been structurally removed from the Adria plate and underplated beneath the Ligurian nappe. We compare the present-day mass of the wedge to the mass of material arriving at the subduction zone on the Adria plate. Balancing these volumes allows us to calculate the length of slab subducted, and to infer the position of the major decollement. As a surrogate for volume, we use area calculated in a cross section extending northeast from La Spezia, on the Tyrhennian coast, across the wedge to the present-day foredeep beneath the Po Plain near Bologna. This cross section generally coincides with the transect investigated by the multi-disciplinary RETREAT project. Our first approximation of the area of material accreted from the subducted Adria plate is 1816 km2. We include in our calculation the metamorphic rocks in the window of the Alpi Apuane, which are dominantly deeply buried and subsequently exhumed carbonates derived from the subducted Adria plate. We also include the section exposed in the vicinity of La Spezia that comprises upper Paleozoic basement rocks, overlying Mesozoic carbonates, and superjacent Cretaceous to Oligocene strata. These units are typically considered to be part of the Tuscan nappe, which structurally intervenes between the Ligurian nappe above and the rocks in the Alpi Apuane below. However, we exclude the Ligurian nappe and the volumetrically minor epi-Ligurian sediments resting above. Together these constitute the lid beneath which most of the material composing the wedge was added. To calculate the volume or area of material added from subducted Adria and the length of plate consumed, we use the width and thickness of classically defined upper Oligocene to upper Miocene sediments deposited in the foredeep that progressively migrated eastward in front of the advancing wedge, and also include the underlying sections of Mesozoic carbonates and evaporites. For the Macigno and underlying strata, we use 100 km width and 5.5 km thickness; for Cervarola, 80 km width and 5 km thickness; for Marnoso-arenacea, 120 km width and 6.5 km thickness. The cross-sectional area of these sequences approximates the area of the present-day wedge. We tentatively conclude that (1) during 30 Ma to present subduction, the main decollement was localized at or near the base of the Mesozoic and younger strata overlying crystalline basement of Adria, and (2) about 275 km of the Adria plate has been consumed.
T13D-1983
Sandbox and Numerical Experiments of Extensional Deformation at Retreating Subduction Zones
Many subduction zones show evidence for synconvergent horizontal extension in the overriding plate. This extension is widespread above the Apennines wedge, reaching to within ~50 km of the subduction front, along the Po and Adriatic flank of the range. This extension is commonly attributed to rollback of the overriding plate. However, Waschbush and Beaumont (1996, JGR) showed that for accreting subduction zones, faster rollback resulted in a larger accretionary flux. In their numerical experiments, the greater accretionary flux caused the forearc to always remain in contraction. They concluded that rollback was not a sufficient condition for forearc extension at actively accreting subduction zones. We present here results from sandbox and numerical modeling that show how this paradox might be resolved. The models presented by WB1996 had only one plate boundary, the subduction zone, which separated the subducting plate (or proplate) from the overriding plate (retroplate). Our modeling shows that the extensional deformation observed at many retreating subduction zones can only be manifested if there is a "medial plate", corresponding to a small rigid element underlying the forearc region. The result is two plate boundaries, a subduction boundary between the pro- and medial plates, and a divergent boundary between the medial and retro- plates. The width of the medial plate must be, at the least, greater than the thickness of the deforming crust above the medial plate, which means a width >40 km for the northern Apennines. This simple three- plate solution is able to produce a wide range of structural styles commonly observed at many accreting subduction zones, both retreating and advancing. An important question, still unresolved, is why this medial plate becomes stabilized beneath the forearc region. We speculate that underlying asthenospheric plays an important role, by providing a viscous resistance to separation across the subduction zone ("suction force") and to viscous tractions caused by asthenospheric corner flow above the slab.
T13D-1984
3D Thermochemical Numerical Model of a Convergent Zone With an Overriding Plate
We have created a new three dimensional thermochemical numerical model of a convergent zone, in which a viscoplastic oceanic plate subducts beneath a viscous overriding plate, using the finite element Geoscience research code Underworld. Subduction is initiated by mantle flow induced by the gravitational instability of a slab tip, and buoyancy of the overriding plate. A cold thermal boundary layer envelopes both plates, and is partially dragged into the mantle along with the subducting slab. The trench rolls back as the slab subducts, and the overriding plate follows the retreating trench without being entrained into the upper mantle. The model is repeated with the overriding plate excluded, to analyse the influence of the overriding plate. The overriding plate retards the rate of subduction. Maximum strain rates, evident along the trench in the absence of an overriding plate, extend to a greater depth within the subducted portion of the slab in the presence of an overriding plate.
T13D-1985
Three-dimensional, thermo-mechanical and dynamical analogue experiments of subduction: first results
We present a new analogue modeling technique developed to investigate the mechanics of the subduction process and the build-up of subduction orogenies. The model consists of a tank filled with water representing the asthenosphere and two lithospheric plates made of temperature-sensitive hydrocarbon compositional systems. These materials possess elasto-plastic properties allowing the scaling of thermal and mechanical processes. A conductive thermal gradient is imposed in the lithosphere prior to deformation. The temperature of the asthenosphere and model surface are imposed and controlled with an electric heater, two infrared ceramic heat emitters, two thermocouples and a thermo-regulator. This system allows an unobstructed view of the model surface, which is monitored using a stereoscopic particle image technique. This monitoring technique provides a precise quantification of the horizontal deformation and variations of elevation in the three-dimensional model. Convergence is imposed with a piston moving at a constant rate or pushing at a constant stress. The velocity is scaled using the dimensionless ratio of thermal conduction over advection. The experiments are first produced at a constant rate and the stress in the horizontal direction of the convergence is recorded. Then the experiment is reproduced with a constant stress boundary condition where the stress value is set to the averaged value obtained in the previous experiment. Therefore, an initial velocity allowing proper scaling of heat exchanges is obtained, but deformation in the model and spatial variations of parameters such as density or friction coefficient can produce variations of plate convergence velocity. This in turn impacts the strength of the model lithosphere because it changes the model thermal structure. In the first presented experiments the model lithosphere is one layer and the plate boundary is linear. The effects of variations of the subducting plate thickness, density and the lubrication of the interface between the plates are investigated.
T13D-1986
Thrust belt Formation followed by Back-Arc Extension: Mantle Dynamics from central North Island, New Zealand.
In central North Island a useful geological and geophysical data exists to examine the evolution from retro-arc compression to back-arc extension in the Neogene. We show that the switch from compression to extensional deformation is related in space and time with an event that rapidly removed much of the mantle lithosphere beneath central and western North Island at about 5 Ma. The geophysical smoking gun for this removal process is in two parts: a regional surface uplift at about 5 Ma, and a sharp east-west, lithospheric boundary across which there is a 7 km jump in Moho as determined by a common conversion point stack of receiver functions. Also associated with the boundary is sharp change in mantle properties such as seismic P-wave attenuation, isostatic gravity and Pn wave speeds. A cluster of earthquakes in the depth range 20-35 km tightly correlates with the Moho step. Thickening then removal of mantle lithosphere within central and western North Island is consistent with its geological history. About 20 my of compression and fold and thrust development within western North Island built much of the Taranaki (foreland) basin during the Miocene. This 600 km long and 200 km wide basin is now the principal source of hydrocarbons for New Zealand. Up to 100 km of shortening in both crust and mantle is estimated from deep seismic reflection profiling, subsidence curves and plate reconstructions. At the close of the Miocene thrusting ceased and a rapid (post 5 Ma) and regional (400 km wide) exhumation event began. Based on mudstone porosity data, a maximum 2.5 km of rock uplift (1 km of surface uplift) occurred and this gave shape to much of the North Island land mass as we see today. The edge of the foreland basin was exhumed by up to 1.5 km and oil wells drilled in the margin date this event as starting at 5 Ma. After 5 Ma a switch to extension occurred with present day back-arc extension occurring at rates between 8-20 mm/y. This extension is accompanied by volcanism and geothermal activity, and a heat output that is one of the highest measured for a continental region. We suggest that thickening and subsequent rapid removal of mantle lithosphere is an important process in controlling the switch from a compressional retro-arc to an extensional one.
T13D-1987
Crustal deformation in the northern Apennines, Italy, results from the RETREAT GPS network
To determine the present day velocity field across the Northern Apennines a GPS geodetic network was installed and measured annually from 2003 to 2007 as part of the RETREAT collaborative project. Each year additional sites were added to the network to improve coverage and in 2004 we started semi continuous observation of a handful of sites, logging data continuously over the summer months. Of these, two sites have now been logging data continuously since 2004 (COLD, RSMN) and one has been logging data since 2005 (SPEL). We analyze the data along with both regional and global CGPS stations from the EUREF, IGS, RING, ASI and other local networks using GAMIT/GLOBk version 10.3. The observed signal is small (only a few mm/yr relative to stable Eurasia) but the velocity field clearly reveals a complex pattern of deformation, related to syn-convergent extension processes. Along the thrust front we observe deformation perpendicular to the front with a few mm/yr of convergence across the Bologna front. We observe a small but significant extension (1-2 mm/yr) across the Apuane region on the Tyrrhenian side of the Apennines range. We see extension across the Alto Tiberina low angle normal fault of 2-3 mm/yr but additional extension seems to be taken up on older faults to the NW of the Alto Tiberina fault. In addition, a larger wavelength signal of few mm/yr NE motion is observed parallel to the coast on the Tyrrhenian side of the Apennines and seems to overprint other signals in the region.
T13D-1988
Resolution of the master structures partitioning shortening and extension in the northern Apennines, Italy
We integrate existing and new geologic data (RETREAT project), focused on the origin, growth, and activity of the mountain front at Bologna, Italy into a new model that explains Apennine orogenesis in the context of a slab rollback - upper plate retreat process. Data are assembled from river terraces and associated Pleistocene growth strata, watershed-averaged erosion rate studies, geodesy including releveling surveys, reinterpretation of published reflection lines, a new high resolution reflection line, recently compiled seismicity, and mantle anisotropy assembled from shear-wave splits. The Bologna mountain front is revealed to be an actively growing structure with rock uplift of ~1 mm/yr, cored by a blind mid-crustal flat-ramp structure that accommodates ongoing shortening driven by Adria subduction at a rate of ~2.5 mm/yr. Geologic, geomorphic, and geodetic data indicate relatively steady rock uplift and exhumation for the Apennine foreland over the past 1 Ma despite the different time scales of the observational window. Apennine extension is recognized both in the foreland, as high angle normal faults accommodating modest stretching in the carapace of the growing mountain front, and in the hinterland, with larger normal faults that accomplish crustal thinning as the upper plate retreats. Nearly all of the active normal faults are clustered near the topographic crest of the range where underlying mantle anisotropy changes from orogen normal to orogen parallel and the crust abruptly thins 25 km. This co-evolution of extension and shortening shares some notable characteristics with other, basement-involved collisional orogens including the early Tertiary Laramide orogeny in the American west and the Oligocene to Miocene evolution of the Alps. These geologic, geomorphic, geodetic, and geophysical observations argue that Apennine deformation is partitioned among a fold and thrust belt where shortening has recently stepped-down to a single, deep structure presumably coupled to the subduction interface, and a tight zone of active extension coupled to an asthenospheric nose at the trailing edge of the retreating upper plate.
T13D-1989
Anisotropic Middle Crust and Tectonic Fabric in Southern Apennines (Italy), a Receiver Function Study.
In the last years the growing amount of Receiver Function (RF) studies focusing on three-dimensional features showed that this technique is a powerful tool to detect anisotropy inside the crust providing constraints on the tectonic fabrics determination. In this study, we analyzed a data-set of about 2000 RF computed using teleseismic events recorded at 16 broadband seismic stations in Southern Apennines (Italy). The recent tectonic evolution of the region is complex and resulted from the collision between the Adria and Eurasia continental blocks. Local earthquakes and teleseismic tomographic images highlight the absence of a clear subducted plate in this area, supporting different interpretations, from slab window to vertical tear faults. Data reveal the structure of the crust, and accordingly with previous studies they trace out basin sequences and flysch units overriding limestones of platform origin. Our results outline the presence of anisotropy in the middle crust, in fact computed RFs show clear polarity reversal with azimuth at 3-4 s, corresponding to a depth of about 20 km. This feature requires a strong seismic anisotropy (up to 10-12%) in the middle crust, related to tectonic fabric. We observe a quite uniform NE-trending plunge of anisotropy axes beneath the Apennines belt, coherent with the plate motion, but for some stations it veers off. Into our preliminary interpretation, the today middle crust fabric is not that inherited by the Mesozoic extension of the Apulian continental margin (Tethys generation), but is generated by a plastic flux occurred under the rigid movements of platform limestones as a consequence of the Plio-Pleistocene continental collision.
T13D-1990
The force balance at convergent margins: implication for trench motions and mantle flow
The balance of the forces acting on convergent margins is the key to understand the dynamics of subduction and its control on plate motions, trench migration and the deformation in the overriding plate. In the subduction system, driving forces of slab pull and ridge push are resisted by viscous drag of the mantle around sinking slabs and at the base of the plates, modulating the rates at which lithosphere is entrained in the mantle at depth and migrates from ridge rise to trench. We present a model that includes the interaction of this set of essential forces, determining consistently the motions and their partitioning. The force balance is modulated according the thermal age of oceanic plates and allows matching the full range of trench motions, from rollback to advance, as shown by global compilations, and the flow in the mantle underneath, revealed by shear wave splitting in trench areas. Our results indicate that the ridge push force plays a relevant role in convergent margins migration and the evolution of the flow around trench zones.
T13D-1991
Geodynamic models of Late Oligocene subduction initiation in the Western Mediterannean
The present day tectonic setting of the Western Mediterranean, located in the convergence zone between
Africa and Europe, is the result of slab roll back and formation of extensional basins since Late Oligocene-
Early Miocene. Despite the important role of subduction in the evolution of the region, little attention has
been paid to the mechanisms involved in the initiation of subduction in the Late Oligocene-Early Miocene.
Two different scenarios have been proposed: (1) Initiation of subduction at a pre-existing fault along the
former Iberian passive margin including Corsica and Sardinia (PM). In this scenario, the pre-existing fault
resulted from a previous period of the rollback of the Alpine subduction zone along a STEP (Subduction-
Transform Edge Propagator) fault. (2) Polarity reversal of a southeast-dipping subduction zone (PR).
According to this scenario, the northwest-dipping subduction zone was initiated in the back-thrust belt of an
earlier southeast-dipping subduction zone, which was the extension of Alpine subduction extending
southwestward up to the Betics.
In this study, we investigate these scenarios using 2-D thermo-mechanical modeling. Mechanical properties
of the models are found to be particularly relevant in our study; we use elastic, (power law) viscous, and
brittle rheologies to represent temperature and pressure dependent rock behavior. Constraints on our
models come from published geological data and paleogeographic reconstructions for the Late Oligocene-
Early Miocene.
The results of PM model indicate that the main resistive force during subduction initiation process is the
elastic bending, which is overcome initially by tectonic forces such as ridge push and eventually by slab pull
when subduction reaches the self-sustaining stage. Results show that subduction can be initiated along
STEP faults with a shallow dip angle (< 70 degrees). Back-arc extension and formation of extensional
basins start soon after initiation ( ~ 4 Myr), and subduction becomes self-sustaining after less than 7
Myr. We find similar results for the importance of the elastic bending resistive force in the PR models. In PR
models, we investigate how various back-thrust belt dip angles, the back-thrust/trench distances, and suction
forces from the detached slab affect the model results.
T13D-1992
VEOX: Receiver Functions Across the Isthmus of Tehuantepec, Mexico
In 2007, 46 broadband stations were installed along the Isthmus of Tehuantepec, from Montepío, Veracruz, on the Gulf of Mexico, to San Mateo del Mar, Oaxaca, on the Pacific coast in a 250 km transect that is approximately perpendicular to the trench. We present preliminary results from receiver function analyses of teleseismic data from these stations, which we call the VEOX line. This is a continuation of a previous transect deployed as part of the Meso American Subduction Experiment (MASE). The results show a sharp Moho that is both shallower and thinner towards the coasts (~30 km) and deeper and thicker at the center (~45km). As expected, the Cocos plate plunges into the mantle with an approximately a constant dip; this can be seen from strong conversion signals at a depth of ~ 40km close to the trench. However, oceanic crust discontinuities in data from stations farther from the Pacific coast do not exhibit a strong signal in the receiver functions. Also the Moho under the Los Tuxtlas volcanic field near the Gulf of Mexico coast is not apparent in the data. Further results will help to delineate the geometry of the Cocos plate along this line and will allow the comparison of the subduction process in an entirely different setting than the first MASE line.
T13D-1993
Rayleigh wave dispersion on the Acapulco-Tampico transect in Mexico
In a previous study we have analyzed the data from 100 broadband stations installed from Acapulco to Tampico in Mexico over a period of 1.5 years. Those instruments were a part of the MASE (Middle America Subduction Experiment), and were deployed to build a geodynamical model of the subduction process in the Middle America Trench. The stations were installed with a spacing of 5-6km and provided a unique data set which would allow us to determine the upper mantle velocity structure in great details. The main focus was on a body wave tomography, receiver functions, and shear-wave splitting. The tomographic results in this area showed the presence of a flat slab under the western part of the array, and a steeply dipping slab beneath its center with its truncation at ~400km depth. However, the splitting analysis of the data revealed fast directions perpendicular to the trench not noticeably affected by the dipping slab. The purpose of this study is to identify the slab using a traditional fundamental mode surface wave interstation method. We will measure the dispersion of Rayleigh waves between periods of about 16 to 170s and will invert the measured dispersion curves to model the depth-dependence of the velocity structure within the upper part of the slab. Considering that Rayleigh waves at 170s are sensitive to shear-wave velocity structure down to about 300 km, this should enable us to reveal the upper part of the slab.
T13D-1994
Andean deformation in northern Sierras Pampeanas, NW Argentina
The Sierras Pampeanas of north-central Argentina are basement cored blocks that were exhumed and tilted along high angle reverse faults during the Andean compression. These basement blocks expose >1000 km by ca. 100 km of deformed and metamorphosed turbidites and Lower Paleozoic sedimentary rocks that record major deformational events in the tectonic evolution of the proto-Pacific South American margin. We present combined field, structural, and petrographic data to describe the brittle deformation in the Northern Sierras Pampeanas produced by the flat slab subduction of the Nazca under the South American plate. The flat slab region is characterized by the absence of modern arc volcanism and double verging foreland thrust belts. Structures related to thick-skinned deformation in northern Sierras Pampeanas include folds of variable scales, west-verging imbricated thrust sheets involving Upper-Precambrian to Cambrian through Tertiary rocks, fault bend folds, and kinks associated to the thrust structures. The core of the ranges reveals that bedding and Cambrian folds were affected by a pulsating and continuous deformation associated with a recent compressional event that we associate to the Andean Orogeny. Principal joint systems in the area of study preserve a predominant N-S orientation, suggesting E-W extension. Based on these observations, we propose the existence of an extensional stage in the tectonic evolution of the Andean foreland basin in this region.