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°S−46°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 (Mw=9.5) and 2010 (Mw=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 ~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~8 event if triggered to fail by stress transfer from the 2010 event.
► This paper presents new GPS data and models of the earthquake cycle deformation. ► Results suggest that some coseismic asperities may not be persistent. ► Strain accumulation likely evolves during the interseismic period. ► Variations of locking degree may correlate with structures of the incoming plate. ► At present the region is capable of producing a M~8 event.
The Central Pontides of N Turkey represents a mobile orogenic belt of the southern Eurasian margin that experienced several phases of exhumation associated with the consumption of different branches ...of the Neo‐Tethys Ocean and the amalgamation of continental domains. Our new low‐temperature thermochronology data help to constrain the timing of these episodes, providing new insights into associated geodynamic processes. In particular, our data suggest that exhumation occurred at (1) ~110 to 90 Ma, most likely during tectonic accretion and exhumation of metamorphic rocks from the subduction zone; (2) from ~60 to 40 Ma, during the collision of the Kirşehir and Anatolide‐Tauride microcontinental domains with the Eurasian margin; (3) from ~40 to 25 Ma, either during the early stages of the Arabia‐Eurasia collision (soft collision) when the Arabian passive margin reached the trench, implying 70 to 530 km of subduction of the Arabian passive margin, or during a phase of trench advance predating hard collision at ~20 Ma; and (4) ~11 Ma to the present, during transpression associated with the westward motion of Anatolia. Our findings document the punctuated nature of fault‐related exhumation, with episodes of fast cooling followed by periods of slow cooling or subsidence, the role of inverted normal faults in controlling the Paleogene exhumation pattern, and of the North Anatolian Fault in dictating the most recent pattern of exhumation.
Key Points
Punctuated rapid fault‐related exhumation along inverted normal faults, separated by phases of slow cooling or subsidence
~110–90 Ma: exhumation of HP metamorphics along the northern branch of the Neo‐Tethys; ~60–40 Ma: Kirşehir Block‐Eurasia collision
~40–25 Ma: initial stages of the Arabia‐Eurasia collision or trench advance; 4 ~11 Ma to present: onset of Anatolia westward motion
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 María Island (SMI; 37°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 °/ky. The inferred recurrence interval of strandline‐forming earthquakes is ∼180 years. Combining coseismic uplift and aseismic subsidence during an earthquake cycle, the net gain in strandline elevation in this environment is ∼0.4 m per event.
Forearc accretionary wedges are cyclic systems in which material is frontally and/or basally accreted. Material cycling involves underthrusting, subduction, underplating, exhumation, erosion, ...transfer to the trench and underthrusting again. In this study we present a novel, tectonochronologic approach to constrain long-term exhumation rates of basally accreted wedge complexes, based on isotopic dating of structural features, on petrological data and sandbox analogue simulations. Congruence between the structural inventory in nature and structures generated in scaled sandbox experiments allows detailed insights into wedge dynamics.
For the present-day surface material of the paleoaccretionary wedge of South-Central Chile (Valdivia area, 40°S), published U–Pb ages of detrital zircon place a maximum age of ∼278 Ma for subduction. Prograde metamorphism at transitional greenschist to blueschist facies conditions (420 °C, 8–9 kbar) was immediately followed by progressive penetrative deformation associated with basal accretion, dated at ∼250–245 Ma using Rb/Sr internal mineral isochrons. The accretion process involved duplex tectonics and antiformal stacking, with formation of near-horizontal mylonitic shear zones at around 241 Ma. Continuous basal accretion at depth gave rise to an extensional tectonic regime at higher structural levels. Both semi-ductile, small-scale extensional shear zones and post-kinematic vein mineralizations yield Rb/Sr ages of ∼235 Ma. Tension gashes, representing the latest isotopically dateable stage of structural evolution, were formed at ∼210 Ma, at conditions of ∼230 °C at 1.5–3 kbar, as constrained by fluid inclusion data. Zircon fission track data indicate final cooling to below ∼200 °C at 186±24 Ma. The results suggest continuous basal accretion for at least 50 Ma, with long-term average exhumation rates of 0.6±0.2 mm/a, most probably outbalanced by similar long-term average erosion rates. Changing plate boundary conditions at about 210–200 Ma terminated the accretion process, as evident from a dramatic decrease of exhumation rates at that time. Since then, the paleoaccretionary wedge remained stably in place despite its delicate geotectonic position within the Andean active margin.
The tectonochronologic approach complements thermochronologic and geomorphologic methods of exhumation research as it provides direct constraints on mass flux rates even for high-temperature increments of
P–
T trajectories.
The active plate margin of South America is characterized by a frequent occurrence of large and devastating subduction earthquakes. Here we focus on marine sedimentary records off Southern Chile that ...are archiving the regional paleoseismic history over the Holocene and Late Pleistocene. The investigated records – Ocean Drilling Program (ODP) Site 1232 and SONNE core 50SL – are located at ~
40°S and ~
38°S, within the Perú–Chile trench, and are characterized by frequent interbedded strata of turbiditic and hemipelagic origin. On the basis of the sedimentological characteristics and the association with the active margin of Southern Chile, we assume that the turbidites are mainly seismically triggered, and may be considered as paleo-megaearthquake indicators. However, the long-term changes in turbidite recurrence times appear to be strongly influenced by climate and sea level changes as well. During sea level highstands in the Holocene and Marine Isotope Stage (MIS) 5, recurrence times of turbiditic layers are substantially higher, primarily reflecting a climate-induced reduction of sediment availability and enhanced slope stability. In addition, segmented tectonic uplift changes and related drainage inversions likely influenced the postglacial decrease in turbidite frequencies. Glacial turbidite recurrence times (including MIS 2, MIS 3, cold substages of MIS 5, and MIS 6), on the other hand, are within the same order of magnitude as earthquake recurrence times derived from the historical record and other terrestrial paleoseismic archives of the region. Only during these cold stages sediment availability and slope instability were high enough to enable recording of the complete sequence of large earthquakes in Southern Chile. Our data thus suggest that earthquake recurrence times on the order of 100 to 200 years are a persistent feature at least during the last glacial period.
This study focuses on the present‐day deformation mechanisms of the south central Chile margin, at the transition zone between two megathrust earthquake segments defined from historical data: the ...Valdivia and Concepción sectors. New GPS data and finite‐element models with complex geometries constrained by geophysical data are presented to gain insight into forearc kinematics and to address the role of upper plate faults on contemporary deformation. GPS vectors are heterogeneously distributed in two domains that follow these two earthquake segments. We find that models which simulate only interseismic locking on the plate interface fail to reproduce surface deformation in the entire study area. In the Concepción domain, models that include a crustal‐scale fault in the upper plate better reproduce the GPS observations. In the Valdivia domain, GPS data show regional‐scale vertical axis rotations, which could reflect postseismic deformation processes at the edge of the Mw 9.5 earthquake that ruptured in 1960 and/or activity of another crustal fault related to motion of a forearc sliver. Our study suggests that upper plate faults in addition to earthquake cycle transients may exert an important control on the surface velocity of subduction zone forearcs.
We present results from a vertical array of accelerometers that was recently installed in Bishkek (Kyrgyzstan) with the long-term aim of recording strong motion data. Taking advantage of recordings ...of a Mb 4.7 earthquake that occurred 40 km from the array site during the installation phase, we provide results of some preliminary data analysis. First, estimates of the S-wave velocity and
Q
s structure are deduced by the inversion of the deconvolved wavefield between the sensors in the borehole. Furthermore, the application of the nonstationary ray decomposition Kinoshita (Earth Planets Space 61:1297-1312,
2009
) allowed at least three reflectors in the shallow velocity structure below the array to be identified. The complex nature of the wavefield (with up-going, down-going waves, and converted phases) due to the coarse, unconsolidated subsoil structure is highlighted by means of numerical simulations of ground motion.
The vibroseis reflection profiling component of the URSEIS '95 experiment provides a high-resolution crustal-scale image of the unextended southern Uralide orogen. A marked lateral and vertical ...variation of reflectivity throughout the entire crust differentiates the former margin of the East European craton to the west from accreted terranes to the east. Between these regions is a less reflective zone corresponding to the Magnitogorsk volcanic arc and the crustal root zone. Continuous reflections and reflective domains that correlate at the surface with major tectonic features define a bivergent orogen in which the Paleozoic collisional structure has been largely preserved.
Uplifted Neogene marine sediments and Quaternary fluvial terraces in the Mut Basin, southern Turkey, reveal a detailed history of surface uplift along the southern margin of the Central Anatolian ...plateau from the Late Miocene to the present. New surface exposure ages (10Be, 26Al, and 21Ne) of gravels capping fluvial strath terraces located between 28 and 135m above the Göksu River in the Mut Basin yield ages ranging from ca. 25 to 130ka, corresponding to an average incision rate of 0.52 to 0.67mm/yr. Published biostratigraphic data combined with new interpretations of the fossil assemblages from uplifted marine sediments reveal average uplift rates of 0.25 to 0.37mm/yr since Late Miocene time (starting between 8 and 5.45Ma), and 0.72 to 0.74mm/yr after 1.66 to 1.62Ma. Together with the terrace abandonment ages, the data imply 0.6 to 0.7mm/yr uplift rates from 1.6Ma to the present. The different post-Late Miocene and post-1.6Ma uplift rates can imply increasing uplift rates through time, or multi-phased uplift with slow uplift or subsidence in between. Longitudinal profiles of rivers in the upper catchment of the Mut and Ermenek basins show no apparent lithologic or fault control on some knickpoints that occur at 1.2 to 1.5km elevation, implying a transient response to a change in uplift rates. Projections of graded upper relict channel segments to the modern outlet, together with constraints from uplifted marine sediments, show that a slower incision/uplift rate of 0.1 to 0.2mm/yr preceded the 0.7mm/yr uplift rate. The river morphology and profile projections therefore reflect multi-phased uplift of the plateau margin, rather than steadily increasing uplift rates. Multi-phased uplift can be explained by lithospheric slab break-off and possibly also the arrival of the Eratosthenes Seamount at the collision zone south of Cyprus.
► Uplifted marine sediments and fluvial terraces reveal topographic growth in S Turkey. ► Post-1.6Ma uplift rates of 0.6–0.7mm/yr exceed post-8Ma rates of 0.25mm/yr. ► River profile projections show low uplift rates prior to an increase at 1.6Ma. ► Data suggest a pulsed history of uplift, rather than steadily increasing uplift. ► Topographic growth likely related to both seamount collision and slab break-off.
The paleogeography of the Altaids and its kinematic and tectonic evolution during the final collision and amalgamation of Eurasia is still poorly known. Addressing this problem, a paleomagnetic study ...has been undertaken on Paleozoic sedimentary rocks from the Karatau, Southern Kazakhstan. Stepwise thermal demagnetization reveals the presence of a high‐temperature component of magnetization in most samples. Fold tests indicate a syn‐folding age of magnetic remanence acquisition at three of the five areas studied. Directional data of Devonian and Permian rocks yield a positive fold test, implying a primary magnetization. Resulting prefolding paleolatitudes for Permian and Devonian rocks show the proximity of the Karatau to Baltica during those times. Syn‐ and post‐folding magnetizations result in paleolatitudes for Karatau, which intersect the paleolatitude curve based on the Baltica apparent polar wander path (APWP), at times, which can be correlated to major deformational events at ~280 Ma, ~260 Ma, and ~230 Ma, respectively. We interpret this with complicated pattern of remagnetization events accompanying deformation, which can include syn‐folding remagnetization events and areas of primary magnetic signals. Additionally, the differences between reference declinations based on the APWP for Baltica and observed declinations suggest successive counterclockwise rotational reorganization of the Karatau during the late Paleozoic to Early Mesozoic, with maximal rotation values of ~65° with respect to Baltica. The remagnetization events are correlated with latest intracontinental stages of orogenic evolution in the Ural mountains and thus the Paleozoic amalgamation of the Eurasian continent and suggest synchronous and coherent tectonic evolution in the Urals and Karatau mountains.
Key Points
Syn‐and post folding magnetic acquisition ages were found in the Karatau.
Remagnetization events can be correlated with major deformational phases.
Structural and tectonic coherence of Karatau and Ural mountains.
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