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Lago Laja is a late Quaternary volcanic‐dammed lake located near the drainage divide of the south central Andes. Field observations, lake reflection seismic profiles, bathymetry, and remote sensing data reveal an active fault system that runs parallel to the volcanic arc along the axis of the Main Cordillera, the Lago Laja fault system (LLFS). Normal faults of this extensional system cut late Pleistocene volcanics, <7.1 ka still water lacustrine sediments, 6.3 ka pyroclastic deposits, and Holocene alluvial fans. We divide the LLFS in three segments on the basis of fault geometry, width, and slip magnitude. The underwater faults of the central segment in the lake's deepest part have the maximum Holocene vertical slip rate of >2.7 mm/yr. Since 7.1 ka, the LLFS accounts for ∼0.7% of arc‐normal extension at an average minimum rate of 1.2 mm/yr and strain rate of ∼10−14 s−1. Seismites and surface ruptures evidence M>6 paleoearthquakes. The Main Cordillera at ∼37°S is a large‐scale pop‐up structure uplifted by thrusting along its foothills. In this light, we interpret extension in the axial and highest part of the Andes as incipient synorogenic gravitational collapse in response to uplift and crustal thickening. Thermal weakening due to elevated heat flow and postglacial lithospheric rebound and unbending have probably contributed to the arc‐limited collapse and Holocene acceleration of deformation rates. The lack of significant strike‐slip offsets along the LLFS as well as along both foothills‐thrust systems at 37°S contrasts with the intra‐arc dextral fault zone south of 38°S. Regional structural data indicates that north of 38°S, diffusely distributed strain reflects low partitioning of oblique subduction, while to the south deformation is localized in a discrete strike‐slip fault zone along the volcanic arc, reflecting a higher degree of partitioning. We relate this strain partitioning gradient to favorable fault orientations in the fore arc north of the Arauco Peninsula, a major seismotectonic boundary.
Major earthquakes ( M > 8) have repeatedly ruptured the Nazca-South America plate interface of south-central Chile involving meter scale land-level changes. Earthquake recurrence intervals, however, extending beyond limited historical records are virtually unknown, but would provide crucial data on the tectonic behavior of forearcs. We analyzed the spatiotemporal pattern of Holocene earthquakes on Santa Maria Island (SMI; 37 degrees S), located 20 km off the Chilean coast and approximately 70 km east of the trench. SMI hosts a minimum of 21 uplifted beach berms, of which a subset were dated to calculate a mean uplift rate of 2.3 +/- 0.2 m/ky and a tilting rate of 0.022 +/- 0.002 degrees/ky. The inferred recurrence interval of strandline-forming earthquakes is similar to 180 years. Combining coseismic uplift and aseismic subsidence during an earthquake cycle, the net gain in strandline elevation in this environment is similar to 0.4 m per event
The Andean orogen is the most outstanding example of mountain building caused by the subduction of oceanic below continental lithosphere. The Andes formed by the subduction of the Nazca and Antarctic oceanic plates under the South American continent over at least ~200 million years. Tectonic and climatic conditions vary markedly along this north-south–oriented plate boundary, which thus represents an ideal natural laboratory to study tectonic and climatic segmentation processes and their possible feedbacks. Most of the seismic energy on Earth is released by earthquakes in subduction zones, like the giant 1960, Mw 9.5 event in south-central Chile. However, the segmentation mechanisms of surface deformation during and between these giant events have remained poorly understood. The Andean margin is a key area to study seismotectonic processes because of its along-strike variability under similar plate kinematic boundary conditions. Active deformation has been widely studied in the central part of the Andes, but the south-central sector of the orogen has gathered less research efforts. This study focuses on tectonics at the Neogene and late Quaternary time scales in the Main Cordillera and coastal forearc of the south-central Andes. For both domains I document the existence of previously unrecognized active faults and present estimates of deformation rates and fault kinematics. Furthermore these data are correlated to address fundamental mountain building processes like strain partitioning and large-scale segmentation. In the Main Cordillera domain and at the Neogene timescale, I integrate structural and stratigraphic field observations with published isotopic ages to propose four main phases of coupled styles of tectonics and distribution of volcanism and magmatism. These phases can be related to the geometry and kinematics of plate convergence. At the late Pleistocene timescale, I integrate field observations with lake seismic and bathymetric profiles from the Lago Laja region, located near the Andean drainage divide. These data reveal Holocene extensional faults, which define the Lago Laja fault system. This fault system has no significant strike-slip component, contrasting with the Liquiñe-Ofqui dextral intra-arc system to the south, where Holocene strike-slip markers are ubiquitous. This contrast in structural style along the arc is coincident with a marked change in along-strike fault geometries in the forearc, across the Arauco Peninsula. Thereon I propose that a net gradient in the degree of partitioning of oblique subduction occurs across the Arauco transition zone. To the north, the margin parallel component of oblique convergence is distributed in a wide zone of diffuse deformation, while to the south it is partitioned along an intra-arc, margin-parallel strike-slip fault zone. In the coastal forearc domain and at the Neogene timescale, I integrate structural and stratigraphic data from field observations, industry reflection-seismic profiles and boreholes to emphasize the influence of climate-driven filling of the trench on the mechanics and kinematics of the margin. I show that forearc basins in the 34-45°S segment record Eocene to early Pliocene extension and subsidence followed by ongoing uplift and contraction since the late Pliocene. I interpret the first stage as caused by tectonic erosion due to high plate convergence rates and reduced trench fill. The subsequent stage, in turn, is related to accretion caused by low convergence rates and the rapid increase in trench fill after the onset of Patagonian glaciations and climate-driven exhumation at ~6-5 Ma. On the late Quaternary timescale, I integrate off-shore seismic profiles with the distribution of deformed marine terraces from Isla Santa María, dated by the radiocarbon method, to show that inverted reverse faulting controls the coastal geomorphology and segmentation of surface deformation. There, a cluster of microearthquakes illuminates one of these reverse faults, which presumingly reaches the plate interface. Furthermore, I use accounts of coseismic uplift during the 1835 M>8 earthquake made by Charles Darwin, to propose that this active reverse fault has been mechanically coupled to the megathrust. This has important implications on the assessment of seismic hazards in this, and other similar regions. These results underscore the need to study plate-boundary deformation processes at various temporal and spatial scales and to integrate geomorphologic, structural, stratigraphic, and geophysical data sets in order to understand the present distribution and causes of tectonic segmentation.
We analyze the role of megathrust geometry on slip estimation using the 1960 Chile earthquake (M-W = 9.5) as an example. A variable slip distribution for this earthquake has been derived by Barrientos and Ward (1990) applying an elastic dislocation model with a planar fault geometry. Their model shows slip patches at 80-110 km depth, isolated from the seismogenic zone, interpreted as aseismic slip. We invert the same geodetic data set using a finite element model (FEM) with precise geometry derived from geophysical data. Isoparametric FEM is implemented to constrain the slip distribution of curve-shaped elements. Slip resolved by our precise geometry model is limited to the shallow region of the plate interface suggesting that the deep patches of moment were most likely an artifact of the planar geometry. Our study emphasizes the importance of fault geometry on slip estimation of large earthquakes.
This work explores the control of fore-arc structure on segmentation of megathrust earthquake ruptures using coastal geomorphic markers. The Arauco-Nahuelbuta region at the south-central Chile margin constitutes an anomalous fore- arc sector in terms of topography, geology, and exhumation, located within the overlap between the Concepcion and Valdivia megathrust segments. This boundary, however, is only based on similar to 500 years of historical records. We integrate deformed marine terraces dated by cosmogenic nuclides, syntectonic sediments, published fission track data, seismic reflection profiles, and microseismicity to analyze this earthquake boundary over 10(2) -10(6) years. Rapid exhumation of Nahuelbuta's dome-like core started at 4 +/- 1.2 Ma, coeval with inversion of the adjacent Arauco basin resulting in emergence of the Arauco peninsula. Here, similarities between topography, spatiotemporal trends in fission track ages, Pliocene-Pleistocene growth strata, and folded marine terraces suggest that margin-parallel shortening has dominated since Pliocene time. This shortening likely results from translation of a fore-arc sliver or microplate, decoupled from South America by an intra-arc strike-slip fault. Microplate collision against a buttress leads to localized uplift at Arauco accrued by deep-seated reverse faults, as well as incipient oroclinal bending. The extent of the Valdivia segment, which ruptured last in 1960 with an M-w 9.5 event, equals the inferred microplate. We propose that mechanical homogeneity of the fore-arc microplate delimits the Valdivia segment and that a marked discontinuity in the continental basement at Arauco acts as an inhomogeneous barrier controlling nucleation and propagation of 1960-type ruptures. As microplate-related deformation occurs since the Pliocene, we propose that this earthquake boundary and the extent of the Valdivia segment are spatially stable seismotectonic features at million year scale.
We observed vertically displaced coastal and river markers after the 27 February 2010 Chilean earthquake [moment magnitude (Mw) 8.8]. Land-level changes range between 2.5 and -1 meters, evident along an ~500-kilometers- long segment identified here as the maximum length of coseismic rupture. A hinge line located 120 kilometers from the trench separates uplifted areas, to the west, from subsided regions. A simple elastic dislocation model fits these observations well; model parameters give a similar seismic moment to seismological estimates and suggest that most of the plate convergence since the 1835 great earthquake was elastically stored and then released during this event.
We use Global Positioning System (GPS) velocities and kinematic Finite Element models (FE-models) to infer the state of locking between the converging Nazca and South America plates in South-Central Chile (36 degrees S -46 degrees S) and to evaluate its spatial and temporal variability. GPS velocities provide information on earthquake-cycle deformation over the last decade in areas affected by the megathrust events of 1960 (M-w = 9.5) and 2010 (M-w = 8.8). Our data confirm that a change in surface velocity patterns of these two seismotectonic segments can be related to their different stages in the seismic cycle: Accordingly, the northern (2010) segment was in a final stage of interseismic loading whereas the southern (1960) segment is still in a postseismic stage and undergoes a prolonged viscoelastic mantle relaxation. After correcting the signals for mantle relaxation, the residual GPS velocity pattern suggests that the plate interface accumulates slip deficit in a spatially and presumably temporally variable way towards the next great event. Though some similarity exist between locking and 1960 coseismic slip, extrapolating the current, decadal scale slip deficit accumulation towards the similar to 300-yr recurrence times of giant events here does neither yield the slip distribution nor the moment magnitude of the 1960 earthquake. This suggests that either the locking pattern is evolving in time (to reconcile a slip deficit distribution similar to the 1960 earthquake) or that some asperities are not persistent over multiple events. The accumulated moment deficit since 1960 suggests that highly locked patches in the 1960 segment are already capable of producing a M similar to 8 event if triggered to fail by stress transfer from the 2010 event.
Coastal uplift and tsunami effects associated to the 2010 M(w)8.8 Maule earthquake in Central Chile
(2011)
On February 27, 2010 at 03:34:08 AM an M(w)8.8 earthquake, with epicenter located off Cobquecura (73.24 degrees W; 36.29 degrees S), severely hit Central Chile. The tsunami waves that followed this event affected the coastal regions between the cities of Valparaiso and Valdivia, with minor effects as far as Coquimbo. The earthquake occurred along the subduction of the Nazca oceanic plate beneath the South American plate. Coseismic coastal uplift was estimated through observations of bleached lithothamnioids crustose coralline algae, which were exposed after the mainshock between 34.13 degrees S and 38.34 degrees S, suggesting the latitudinal distribution of the earthquake rupture. The measured coastal uplift values varied between 240 +/- 20 cm at sites closer to the trench along the western coast of the Arauco peninsula and 15 +/- 10 cm at sites located farther east. A maximum value of 260 +/- 50 cm was observed at the western coast of Santa Maria Island, which is similar to the reported uplift associated with the 1835 earthquake at Concepcion. Land subsidence values on the order of 0.5 m to 1 m evidenced a change in polarity and position of the coseismic hinge at 110-120 km from the trench. In four sites along the coast we observed a close match between coastal uplift values deduced from bleached lithothamnioids algae and GPS measurements. According to field observations tsunami heights reached ea. 14 m in the coastal area of the Maule Region immediately north of the epicenter, and diminished progressively northwards to 4-2 m near Valparaiso. Along the coast of Cobquecura, tsunami height values were inferior to 2-4 m. More variable tsunami heights of 6-8 m were measured at Dichato-Talcahuano and Tirua-Puerto Saavedra, in the Biobio and Arauco regions, respectively, to the south of the epicenter. According to eyewitnesses, the tsunami reached the coast between 12 to 20 and 30 to 45 minutes in areas located closer and faraway from the earthquake rupture zone, respectively. Destructive tsunami waves arrived also between 2.5 and 4.5 hours after the mainshock, especially along the coast of the Biobio and Arauco regions. The tsunami effects were highly variable along the coast, as a result of geomorphological and bathymetric local conditions, besides potential complexities induced by the main shock.
Surface uplift at the northern margin of the Central Anatolian Plateau (CAP) is integrally tied to the evolution of the Central Pontides (CP), between the North Anatolian Fault (NAF) and the Black Sea. Our regional morphometric and plate kinematic analyses reveal topographic anomalies, steep channel gradients, and local high relief areas as indicators of ongoing differential surface uplift, which is higher in the western CP compared to the eastern CP and fault-normal components of geodetic slip vectors and the character of tectonic activity of the NAF suggest that stress is accumulated in its broad restraining bend. Seismic reflection and structural field data show evidence for a deep structural detachment horizon responsible for the formation of an actively northward growing orogenic wedge with a positive flower-structure geometry across the CP and the NAF. Taken together, the tectonic, plate kinematic, and geomorphic observations imply that the NAF is the main driving mechanism for wedge tectonics and uplift in the CP. In addition, the NAF Zone defines the boundary between the extensional CAP and the contractional CP. The syntectonic deposits within inverted intermontane basins and deeply incised gorges suggest that the formation of relief, changes in sedimentary dynamics, and > 1 km fluvial incision resulted from accelerated uplift starting in the early Pliocene. The Central Pontides thus provide an example of an accretionary wedge with surface-breaking faults that play a critical role in mountain building processes, sedimentary basin development, and ensuing lateral growth of a continental plateau since the end of the Miocene.