Papers by Serge Lallemand
The Gagua Ridge, carried by the Philippine Sea Plate is subducting obliquely beneath the southern... more The Gagua Ridge, carried by the Philippine Sea Plate is subducting obliquely beneath the southernmost Ryukyu Margin. Bathymetric swath-mapping, performed during the ACT survey (Active Collision in Taiwan), indicates that, due to the high obliquity of plate convergence, slip partitioning occurs within the Ryukyu accretionary wedge. A transcurrent fault, trending N95°E, is observed at the rear of the accretionary wedge. Evidence of right lateral motion along this shear zone, called the Yaeyama Fault, suggests that it accommodates part of the lateral component of the oblique convergence. The subduction of the ridge disturbs this tectonic setting and significantly deforms the Ryukyu Margin. The ridge strongly indents the front of the accretionary wedge and uplifts part of the forearc basin. In the frontal part of the margin, directly in the axis of the ridge, localized transpressive and transtensional structures can be observed superimposed on the uplifted accretionary complex. As shown by sandbox experiments, these N330°E to N30°E trending fractures result from the increasing compressional stress induced by the subduction of the ridge. Analog experiments have also shown that the reentrant associated with oblique ridge subduction exhibits a specific shape that can be correlated with the relative plate motion azimuth.
Tectonophysics, 2001
Slip partitioning along oblique subduction zones has conventionally been examined by slip vector ... more Slip partitioning along oblique subduction zones has conventionally been examined by slip vector residuals determined from interface earthquakes. It is measured through contrasting the slip vector with respect to the local plate convergence direction de®ned on the surface of the Earth by the Euler vector. Interpretation of regional plate kinematics based on slip vector residuals thus de®ned may be misleading. Besides the apparent discrepancy attributable to the surface projection of vectors on an obliquely plunging surface, a more fundamental problem is the construction of the reference subduction¯ow ®eld for the subducted slab based upon the known plate convergence. It can be shown that for complicated slab geometry, the conventional practice of intuitively rotating the Euler kinematics onto the subducted slab leads to unrealistic intraplate deformation that invalidates the mechanical integrity of the oceanic lithosphere. Alternatively, we propose to calculate the velocity ®eld for the speci®c slab geometry based on the rationale that the subduction¯ow ®eld should be the one that endures the least amount of intraplate deformation. The calculated¯ow ®eld for the westernmost Ryukyu slab, that is in transition to the Philippine Sea plate (PSP)±Eurasian Plate (EP) collision near Taiwan, reveals quite distinct subduction¯ow ®eld. The particle paths associated with the calculated¯ow ®eld are in better accordance with the observed slip vectors while the corresponding strain-rate ®eld is consistent with the previously reported lateral compression seismogenic zone. Furthermore, it is speculated that the plate kinematics of the northwestern corner of PSP that is impinging the vicinity of the collision might have also undergone signi®cant readjustment to accommodate the potential intraplate deformation. It is noted that the westernmost subducted Ryukyu slab and the northwestern corner of PSP belong to a mechanically coherent tectonic unit. The 3D geometric constraints set up by the subducted slab and the ªunfriendlyº kinematic constraints imposed by the collision boundary both play important roles in con®guring the intraplate deformation within PSP. Manifestation in the local kinematics might indicate deviation from the general PSP±EP convergence, and is thus of crucial importance to tectonic interpretations of geophysical observations in this area. q
Geophysical Journal International, 2004
The maximum intersection (MAXI) method, which derives from the master station method (MSM), deter... more The maximum intersection (MAXI) method, which derives from the master station method (MSM), determines within a 3-D velocity model the absolute hypocentral location based on observed arrival times. First, the spatial node that better satisfies the arrival time differences computed at all station pairs, plus or minus an error tolerance value (in seconds), is defined as the preliminary hypocentral solution (PRED). Second, because PRED depends neither on the estimate of origin time nor on the residual root mean square (rms), residual outliers are objectively detected and cleaned out from the original data set without any iterative process or weighting. Third, a statistical minimization (residual rms) is conducted in a small domain around the PRED node, which results in a unique FINAL solution. The MAXI method is applied to the determination of earthquake hypocentres (with the proper station correction terms) in the southernmost extremity of the Ryukyu subduction zone, where several dense seismic clusters occur near the seismogenic plate interface. The location of earthquakes, recorded at both the Taiwanese and Japanese networks, is obtained for about a thousand events (between 1992 and 1997). The process uses a detailed 3-D velocity model based on multiple geophysical data sources obtained in the junction area between subduction and collision (east of Taiwan). The earthquake clustering and the significant drop in residual statistics (1.20, 0.80 and 0.35 s, for Taiwanese catalogue, MSM and MAXIM solutions respectively) indicate the accuracy of the method, which can be used to routinely determine absolute hypocentre location based on observed arrival times.
Earth and Planetary Science Letters, 1999
The North Andean convergent margin is a region of intense crustal deformation, with six great sub... more The North Andean convergent margin is a region of intense crustal deformation, with six great subduction earthquakes M w ½ 7:8 this century. The regional pattern of seismicity and volcanism shows a high degree of segmentation along strike of the Andes. Segments of steep slab subduction alternate with aseismic regions and segments of flat slab subduction. This segmentation is related to heterogeneity on the subducting Nazca Plate. In particular, the influence of the Carnegie Ridge collision is investigated. Four distinct seismotectonic regions can be distinguished: Region 1 -from 6ºN to 2.5ºN with steep ESE-dipping subduction and a narrow volcanic arc; Region 2 -from 2.5ºN to 1ºS showing an intermediate-depth seismic gap and a broad volcanic arc; Region 3 -from 1ºS to 2ºS with steep NE-dipping subduction, and a narrow volcanic arc; Region 4 -south of 2ºS with flat subduction and no modern volcanic arc. The Carnegie Ridge has been colliding with the margin since at least 2 Ma based on examination of the basement uplift signal along trench-parallel transects. The subducted prolongation of Carnegie Ridge may extend up to 500 km from the trench as suggested by the seismic gap and the perturbed, broad volcanic arc. These findings conflict with previous tectonic models suggesting that the Carnegie Ridge entered the trench at 1 Ma. Furthermore, the anomalous geochemical (adakitic) signature of the volcanoes in the broad Ecuador volcanic arc and the seismicity pattern are proposed to be caused by lithospheric tears separating the buoyant, shallowly subducting Carnegie Ridge from segments of steep subduction in Regions 1 and 3. It is further suggested that Carnegie Ridge supports a local 'flat slab' segment similar to that observed in Peru. The impact of the Carnegie Ridge collision on the upper plate causes transpressional deformation, extending inboard to beyond the volcanic arc with a modern level of seismicity comparable to the San Andreas fault system. The pattern of instrumental and historical seismicity indicates (1) great earthquakes on the northern and southern flanks of the colliding ridge, (2) a slight reduction in observed seismicity at the trench-ridge intersection, (3) increased stress far into the continent, and (4) a NNE displacement of the N. Andes block, to be further effects of the collision.
SPECIAL PAPERS-GEOLOGICAL SOCIETY OF AMERICA, 2002
Marine observations offshore of Taiwan indicate intense deformation of the Luzon arc-forearc comp... more Marine observations offshore of Taiwan indicate intense deformation of the Luzon arc-forearc complex, with episodic eastward migration of the active deformation front across the complex. This active tectonic domain absorbs a significant amount of shortening between the Eurasia margin and the Philippine Sea plate, which is moving toward N3l0E at about 8 cm/yr relative to Eurasia. Swath bathymetry and backscattering data, together with seismic reflection and geopotential data obtained during the ACT (Active Collision in ...
Marine Geophysical Research, Oct 1, 1998
The Gagua Ridge, carried by the Philippine Sea Plate, is subducting obliquely beneath the souther... more The Gagua Ridge, carried by the Philippine Sea Plate, is subducting obliquely beneath the southernmost Ryukyu Margin. Bathymetric swath-mapping, performed during the ACT survey (Active Collision in Taiwan), indicates that, due to the high obliquity of plate convergence, slip partitioning occurs within the Ryukyu accretionary wedge. A transcurrent fault, trending N95° E, is observed at the rear of the accretionary wedge. Evidence of right lateral motion along this shear zone, called the Yaeyama Fault, suggests that it ...
Bulletin de la Societe Geologique de France, 2002
In subduction zones undergoing oblique convergence, strain partitioning is often expressed by an ... more In subduction zones undergoing oblique convergence, strain partitioning is often expressed by an important deformation inducing strike-slip faulting. In accretionary wedges, parameters such as obliquity of the convergence and friction at the bottom of the wedge play an important role in the strain partitioning. The impact of these parameters is studied using sandbox experiments. Two backstop geometries have been designed to account for different geological settings. These experiments show that the wedge taper remains constant and ...
Taiwan represents the best area to study the processes associated with the growth of orogenic wed... more Taiwan represents the best area to study the processes associated with the growth of orogenic wedges. Because of the obliquity of the plate convergence and the geometry of the ocean-continent transition in the Eurasian plate, the progressive evolution from oceanic subduction to the subduction of the continental margin of China induces:(1) in the domain of incipient continental subduction to the south, major eastward backthrusting and shortening of the forearc domain between the former oceanic accretionary wedge and the Luzon arc ...
Geological Society, London, Special Publications, 2003
The Izu-Bonin-Mariana (IBM) forearc is characterized by the occurrence of boninite-like lavas. Th... more The Izu-Bonin-Mariana (IBM) forearc is characterized by the occurrence of boninite-like lavas. The study of the Cenozoic setting of the genesis of these boninitic lavas in light of modern geodynamic contexts in the Tonga and Fiji regions lead us to define three tectonic settings that favour the formation of boninites in back-arc basins in addition to previous settings that involve the presence of a mantle plume: (1) propagation at low angle between a spreading centre and the associated volcanic arc; (2) intersection at a high angle of an active spreading centre and a transform fault at the termination of an active volcanic arc; and (3) intersection at a right angle between an active spreading centre and a newly created subduction zone. A geodynamic model of the Philippine Sea Plate shows that boninites in the Bonin Islands are related to the second mechanism mentioned above, whereas Mariana forearc boninites are relevant to the third mechanism. In the early Eocene, the transform plate boundary, bounding the eastern margin of the Philippine Sea Plate at the location of the present-day Mariana arc evolved into a subduction zone that trends perpendicular to the active spreading centre of the West Philippine Basin, somewhere around 43-47 Ma. The presence of a mantle plume in the vicinity of the subduction zone bounding the northern IBM arc explains boninites that erupted in its northern part, but only in early Eocene time. boundary set at CaO/A1203 c. 0.75 (Crawford et aL 1989). Low-Ca boninites are interpreted as being produced from relatively more depleted sources than high-Ca boninites. Variations in CaO/AI203 values and TiO2 content recorded in several boninite suites (e.g.
JOURNAL OF GEOPHYSICAL RESEARCH, May 10, 1998
Abstract. Seismic reflection profiles from the sediment rich Alaska subduction zone image short, ... more Abstract. Seismic reflection profiles from the sediment rich Alaska subduction zone image short, frontally accreted, imbricate thrust slices and repeated se-quences of long, underthrust sheets. Rapid landward increases in wedge thickness, backthrusting, and uplift of the forearc are observed, suggesting underthrusting beneath the wedge. These features and a widely varying frontal wedge morphology are interpreted to be caused by different modes of accretion active concurrently along the trench at different locations. Episodic wedge ...
Comptes Rendus De L Academie Des Sciences Serie 2 Mecanique Physique Chimie Sciences De L Univers Sciences De La Terre, 1985
Deformation of both subducting and overriding at convergent plate boundaries tends to dissipate e... more Deformation of both subducting and overriding at convergent plate boundaries tends to dissipate energy that would otherwise be used to drive plate motions. For subducting plates, the magnitude of the bending deformation is not known because of poor constraints on slab strength. For overriding plates, back-arc orogeny results from upper plate shortening and frictional stresses on the plate interface that also resist subduction. Because both processes tend to slow oceanic plates, the observed plate motions can be used to constrain their importance relative to other plate-driving forces. We estimated the resistance from plate bending and back-arc orogeny for 207 globally-distributed subduction zone transects. For bending, we used new measurements of the bending curvature determined from slab seismicity. For orogeny, we estimated the resisting force based on the orography of the overriding plate. The effective viscosity of the bending slab and the effective frictional resistance along the subduction zone interface were considered to be unknown parameters. Using a global mantle flow model to predict plate motions under the influence of these effects, we constrain the viscosity of the bending slab to be at most ~ 300 times more viscous than the upper mantle; stronger slabs are intolerably slowed by the bending deformation. Weaker slabs, however, cannot transmit a pull force sufficient to explain rapid trenchward plate motions unless slabs stretch faster than seismically observed rates of ~ 10-15 s-1. The constrained bending viscosity (~ 2 × 1023 Pa s) is larger than previous estimates because slabs bend more gently than previously thought, with an average radius of curvature of 390 km that permits subduction of strong slabs. We find that back-arc orogeny also significantly affects plate motions, especially for plates such as the Nazca plate where orogenic uplift dominates the back arc region.
Comptes Rendus De L Academie Des Sciences Serie 2 Mecanique Physique Chimie Sciences De L Univers Sciences De La Terre, 1989
Geophysical Journal International, Nov 5, 2013
The Ryukyu subduction is known to generate very few earthquakes in its central segment contrarily... more The Ryukyu subduction is known to generate very few earthquakes in its central segment contrarily to its two extremities. We focus in this study on the southernmost part of the Ryukyu subduction zone offshore east Taiwan. Our first task was to build a homogeneous earthquake catalogue for the period 1900-2007. The new catalogue provides homogenized M W magnitudes and shows that several M W ≥ 7.0 earthquakes occurred offshore Hualien and Suao cities. We then focused on the 1920 June 5 earthquake (reported surface wave magnitude 8.1) previously located beneath the accretionary prism. The revised moment magnitude has been estimated in our catalogue at 7.7 ± 0.2. It is the biggest earthquake ever recorded in the Taiwan area but the fault that has produced this earthquake has not yet been identified with confidence. We relocated this event using regional phases (seismological bulletins archived at the Central Weather Bureau of Taiwan) about 50 km NNE and shallower of its former location, that is, within the Ryukyu Arc basement. According to earthquake bulletin information, revised magnitude, new hypocentral determination and known regional faults, we propose four potential active faults as candidates for the slip associated to this event: (1) the interplate seismogenic zone (ISZ), (2) an out-of-sequence thrust cutting through the forearc and branching on the ISZ, (3) a NS strike-slip fault cutting through the Ryukyu arc and (4) a N-S, westward dipping thrust fault, affecting the Philippine Sea Plate east of the Luzon Arc. The best compromise is to consider a rupture along the ISZ with a shallow nucleation possibly along a splay-fault followed by a downward and lateral propagation of the rupture that would explain the lack of significant seafloor motion and subsequent tsunami. We also estimate the maximum seismic coupling of the ISZ in the region east of Taiwan to about 0.4. In parallel, the evidences of aseismic slip occurring along the ISZ allow us to conclude that this region should only be affected by M < 8 earthquakes.
Agu Fall Meeting Abstracts, Dec 1, 2005
3-D laboratory models have been performed in order to investigate the way plates kinematics (subd... more 3-D laboratory models have been performed in order to investigate the way plates kinematics (subducting and overriding plate absolute motions and the resulting plate convergence rate) influences the geometry of the slab and the overriding plate deformation in subduction zones. In the experiments a viscous plate of silicone (subducting plate) is pushed beneath another plate, which is itself pushed toward or pulled away from the trench (overriding plate), and sinks into a viscous layer of glucose syrup (upper mantle). The subducting and overriding plate velocities explored the variability field of natural subduction plates kinematics. The overriding plate motion exerts a primary role in the control of slab geometries and overriding plate deformation rates. The experiments have revealed two different subduction behaviours: (Style I) the overriding plate moves toward the trench and shortens at high rates, the slab is flat and deflected when reaching the bottom of the box in a forward direction; (Style II) the overriding plates moves away from the trench and shortens at low rates the slab is steep and deflected on the box bottom in a backward direction. To a lesser extent, increasing subducting plate motion is associated to increasing slab dips and overriding plate shortening. Slab geometry and overriding plate deformation are less sensitive to the overall plate convergence rate. These laboratory models behaviours are consistent with statistical analysis performed on natural subduction zones, and enlighten the first order control exerted by the overriding plate absolute motion, on the geometry adopted by the slab and the way the overriding plate deforms.
Http Dx Doi Org 10 1080 00288306 2004 9515067, Sep 21, 2010
The Ruatoria margin indentation and its associated giant avalanche off East Cape, New Zealand, ha... more The Ruatoria margin indentation and its associated giant avalanche off East Cape, New Zealand, have been inferred to result from margin instability following oblique subduction of a large seamount. The earlier studies hypothesise that a diachronous seamount-wake trough formed the northern part of the indentation, and collapse between the oblique trough and an oversteepened margin front formed the southern indentation and giant avalanche. If correct, then the impacting seamount must now be landward of the indentation. New seismic profiles, supported by multibeam bathymetry and core samples, from landward of the Ruatoria Indentation, provide support for the passage of a large seamount deep beneath the continental shelf.
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Papers by Serge Lallemand