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The Subandean fold and thrust belt of Bolivia constitutes the easternmost part of the Andean orogen that reflects thin-skinned shortening and eastward propagation of the Andean deformation front. The exact interplay of tectonics, climate, and erosion in the deposition of up to 7.5 km of late Cenozoic strata exposed in the Subandes remains unclear. To better constrain these relationships, we use four W-E industry seismic reflection profiles, eight new zircon U-Pb ages from Mio-Pliocene sedimentary strata, and cross-section balancing to evaluate the rates of thrust propagation, shortening, and deposition pinch-out migration. Eastward thrusting arrived in the Subandean belt at similar to 12.4 +/- 0.5 Ma and propagated rapidly toward the foreland unit approximately 6 Ma. This was followed by out-of- sequence deformation from ca. 4 to 2.1 Ma and by renewed eastward propagation thereafter. Our results show that the thrust-front propagation- and deposition pinch-out migration rates mimic the sediment accumulation rate. The rates of deposition pinchout migration and thrust propagation increased three- and two fold, respectively (8 mm/a; 3.3 mm/a) at 86 Ma. The three-fold increase in deposition pinch-out migration rate at this time is an indication of enhanced erosional efficiency in the hinterland, probably coupled with flexural rebound of the basin. Following the pulse of pinch-out migration, the Subandean belt witnessed rapid similar to 80 km eastward propagation of thrusting to the La Vertiente structure at 6 Ma. As there is no evidence for this event of thrust front migration being linked to an increase in shortening rate, the enhanced frontal accretion suggests a shift to supercritical wedge taper conditions. We propose that the supercritical state was due to a drop in basal strength, caused by sediment loading and pore fluid overpressure. This scenario implies that climate-controlled variation in erosional efficiency was the driver of late Miocene mass redistribution, which induced flexural rebound of the Subandean thrust belt, spreading of a large clastic wedge across the basin, and subsequent thrust-front propagation.
Stable isotope altimetry is a useful tool for estimating paleoelevation in sedimentary records. Yet questions remain regarding how source moisture, climate, and local topography can influence these estimates. Here we present stable isotope altimetry results on late Quaternary pedogenic carbonates of known elevation on both flanks of the Andean orogen at 33 degrees S. We measured delta O-18 values of pedogenic carbonates and river water samples from small drainages at regular elevation increments within the Rio Aconcagua (Chile) and Rio Mendoza (Argentina) catchments. The delta O-18 values of river waters correlate well with elevation and show similar isotopic gradients between the Chilean (-3.7 parts per thousand/km) and Argentine (-4.8 parts per thousand/km) sides of the range. Uncertainties associated with scatter in the river water data and assumptions about the temperature of carbonate formation indicate that elevation estimates have 1 sigma errors of 350-450 m. We estimate the isotopic composition of soil water from pedogenic carbonates on both sides of the range by assuming mean annual temperatures based the modern temperature lapse rate from meteorological station data. Combined, our data show that stable isotope altimetry produces reasonable estimates of modern elevation, with the majority of our samples (60%) within the 1 sigma uncertainties and 77% within 2 sigma.
Lower Cretaceous early syn-rift facies along the eastern flank of the Eastern Cordillera of Colombia, their provenance, and structural context, reveal the complex interactions between Cretaceous extension, spatio-temporal trends in associated sedimentation, and subsequent inversion of the Cretaceous Guatiquia paleo-rift. South of 4 degrees 30'N lat, early syn-rift alluvial sequences in former extensional footwall areas were contemporaneous with fan- delta deposits in shallow marine environments in adjacent hanging-wall areas. In general, footwall erosion was more pronounced in the southern part of the paleorift. In contrast, early syn-rift sequences in former footwall areas in the northern rift sectors mainly comprise shallow marine supratidal sabkha to intertidal strata, whereas hanging-wall units display rapid transitions to open-sea shales. In comparison with the southern paleo-rift sector, fan-delta deposits in the north are scarce, and provenance suggests negligible footwall erosion. The southern graben segment had longer, and less numerous normal faults, whereas the northern graben segment was characterized by shorter, rectilinear faults. To the east, the graben system was bounded by major basin-margin faults with protracted activity and greater throw as compared with intrabasinal faults to the west. Intrabasinal structures grew through segment linkage and probably interacted kinematically with basin-margin faults. Basin-margin faults constitute a coherent fault system that was conditioned by pre-existing basement fabrics. Structural mapping, analysis of present-day topography, and balanced cross sections indicate that positive inversion of extensional structures was focused along basin-bounding faults, whereas intrabasinal faults remained unaffected and were passively transported by motion along the basin-bounding faults. Thus, zones of maximum subsidence in extension accommodated maximum elevation in contraction, and former topographic highs remained as elevated areas. This documents the role of basin-bounding faults as multiphased, long-lived features conditioned by basement discontinuities. Inversion of basin-bounding faults was more efficient in the southern than in the northern graben segment, possibly documenting the inheritance and pivotal role of fault-displacement gradients. Our observations highlight similarities between inversion features in orogenic belts and intra-plate basins, emphasizing the importance of the observed phenomena as predictive tools in the spatiotemporal analysis of inversion histories in orogens, as well as in hydrocarbon and mineral deposits exploration.
Tectonic and climatic control on evolution of rift lakes in the Central Kenya Rift, East Africa
(2009)
The long-term histories of the neighboring Nakuru-Elmenteita and Naivasha lake basins in the Central Kenya Rift illustrate the relative importance of tectonic versus climatic effects on rift-lake evolution and the formation of disparate sedimentary environments. Although modem climate conditions in the Central Kenya Rift are very similar for these basins, hydrology and hydrochemistry of present-day lakes Nakuru, Elmenteita and Naivasha contrast dramatically due to tectonically controlled differences in basin geometries, catchment size, and fluvial processes. In this study, we use eighteen C-14 and Ar-40/Ar-39 dated fluvio-lacustrine sedimentary sections to unravel the spatiotemporal evolution of the lake basins in response to tectonic and climatic influences. We reconstruct paleoclimatic and ecological trends recorded in these basins based on fossil diatom assemblages and geologic field mapping. Our study shows a tendency towards increasing alkalinity and shrinkage of water bodies in both lake basins during the last million years. Ongoing volcano-tectonic segmentation of the lake basins, as well as reorganization of upstream drainage networks have led to contrasting hydrologic regimes with adjacent alkaline and freshwater conditions. During extreme wet periods in the past, such as during the early Holocene climate optimum, lake levels were high and all basins evolved toward freshwater systems. During drier periods some of these lakes revert back to alkaline conditions, while others maintain freshwater characteristics. Our results have important implications for the use and interpretation of lake sediment as climate archives in tectonically active regions and emphasize the need to deconvolve lacustrine records with respect to tectonics versus climatic forcing mechanisms.
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.
The seismicity of the Kenya rift is characterized by high-frequency low-magnitude events concentrated along the rift axis. Its seismic character is typical for magmatically active continental rifts, where igneous material at a shallow depth causes extensive grid faulting and geothermal activity. Thermal overprinting and dike intrusion prohibit the buildup of large elastic strains, therefore prohibiting the generation of large-magnitude earthquakes. On 6 January 1928, the M-S 6.9 Subukia earthquake occurred on the Laikipia-Marmanet fault, the eastern rift-bounding structure of the central Kenya rift. It is the largest instrumentally recorded seismic event in the Kenya rift, standing in contrast to the current model of the rift's seismic character in which large earthquakes are not anticipated. Furthermore, the proximity of the ruptured fault and the rift axis is intriguing: The rift-bounding structure that ruptured in 1928 remains seismically active, capable of generating large-magnitude earthquakes, even though thermally weakened crust and better oriented structures are present along the rift axis nearby, prohibiting any significant buildup of elastic strain. We excavated the surface rupture of the 1928 Subukia earthquake to find evidence for preceding ground-rupturing earthquakes. We also made a total station survey of the site topography and mapped the site geology. We show that the Laikipia-Marmanet fault was repeatedly activated during the late Quaternary. We found evidence for six ground-rupturing earthquakes, including the 1928 earthquake. The topographic survey around the trench site revealed a degraded fault scarp of approximate to 7.5 m in height, offsetting a small debris slide. Using scarp-diffusion modeling, we estimated an uplift rate of U = 0.09-0.15 mm/yr, constraining the scarp age to 50-85 ka. Assuming an average fault dip of 55 degrees-75 degrees, the preferred uplift rate (0.15 mm/yr) accommodates approximately 10%-20% of the recent rate of extension (0.5 mm/yr) across the Kenya rift.
Foreland basin development in the Andes of central Colombia has been suggested to have started in the Late Cretaceous through tectonic loading of the Central Cordillera. Eastward migration of the Cenozoic orogenic front has also been inferred from the foreland basin record west of the Eastern Cordillera. However, farther east, limited data provided by foreland basin strata and the adjacent Eastern Cordillera complicate any correlation among mountain building, exhumation, and foreland basin sedimentation. In this study, we present new data from the Medina Basin in the eastern foothills of the Eastern Cordillera of Colombia. We report sedimentological data and palynological ages that link an eastward-thinning early Oligocene to early Miocene syntectonic wedge containing rapid facies changes with an episode of fast tectonic subsidence starting at ca. 31 Ma. This record may represent the first evidence of topographic loading generated by slip along the principal basement-bounding thrusts in the Eastern Cordillera to the southwest of the basin. Zircon fission-track ages and paleo-current analysis reveal the location of these thrust loads and illustrate a time lag between the sedimentary signal of topographic loading and the timing of exhumation (ca. 18 Ma). This lag may reflect the period between the onset of range uplift and significant removal of overburden. Vitrinite reflectance data document northward along-strike propagation of the deformation front and folding of the Oligocene syntectonic wedge. This deformation was coupled with a nonuniform incorporation of the basin into the wedge-top depozone. Thus, our data set constitutes unique evidence for the early growth and propagation of the deformation front in the Eastern Cordillera, which may also improve our understanding of spatiotemporal patterns of foreland evolution in other mountain belts.
Though orogen-parallel shortening and vertical extension have dominated the tectonic evolution of the central Andes, a significant kinematic shift from horizontal contraction to extension appears to have occurred within the high Puna-Altiplano Plateau, with the establishment of extension oblique to the orogen since late Miocene time. We present data from the southern margin of the Puna Plateau, NW Argentina, where new normal faults have been documented in the Fiambala, Punta Negra, and La Quebrada areas. The unifying characteristics of these areas are that young normal faults reactivate or crosscut older thrust and reverse faults. The relationship between the faults and the late Miocene- Pliocene Punaschotter conglomerate suggests that the extensional faulting must be younger than 3.5 to 7 Ma. Existing data are incomplete but indicate that similar horizontal extension has occurred in many regions throughout the Puna- Altiplano Plateau, while shortening continues along the plateau margins. Given the spatial and temporal distribution of this late Miocene to Pliocene kinematic shift, both lithospheric loss in the Puna Plateau and plateau-wide gravitational extensional spreading enhanced by slowing of plate convergence rate could be responsible. The young, disorganized, horizontal extension in the Andes today may be the precursor to more pronounced extension such as observed on the Tibetan Plateau since mid-Miocene time.
Neighbouring faults can interact, potentially link up and grow, and consequently increase the seismic and related natural hazards in their vicinity. Despite evidence of Quaternary faulting, the kinematic relationships between the neighbouring Mosha Fasham Fault (MFF) and the North Tehran Thrust (NTT) and their temporal evolution in the Alborz mountains are not well understood. The ENE-striking NTT is a frontal thrust that delimits the Alborz mountains to the south with a 2000 m topographic front with respect to the proximal Tehran plain. However, no large instrumentally recorded earthquakes have been attributed to that fault. In contrast, the sigmoidally shaped MFF is a major strike-slip fault, located within the Alborz Mountains. Sinistral motion along the eastern part of the MFF is corroborated by microseismicity and fault kinematic analysis, which documents recent transtensional deformation associated with NNE-SSW oriented shortening. To better understand the activity of these faults on different timescales, we combined fault- kinematic analysis and geomorphic observations, to infer the kinematic history of these structures. Our fault kinematic study reveals an early dextral shear for the NTT and the central MFF, responsible for dextral strike-slip and oblique reverse faulting during NW-oriented shortening. This deformation regime was superseded by NE-oriented shortening, associated with sinistral-oblique thrusting along the NTT and the central-western MFF, sinistral strike-slip motion along subsidiary faults in the central MFF segment, and folding and tilting of Eocene to Miocene units in the MFF footwall. Continued thrusting along the NTT took place during the Quaternary. However, folding in the hanging wall and sinistral stream-offsets indicate a left-oblique component and Quaternary strike-slip reactivation of the eastern NTT- segment, close to its termination. This complex history of faulting under different stress directions has resulted in a composite landscape with inherited topographic signatures. Our study shows that the topography of the hanging wall of the NTT reflects a segmentation into sectors with semi-independent uplift histories. Areas of high topographic residuals and apparent high uplift underscore the fault kinematics. Combined, our data suggest an early mechanical linkage of the NTT and MFF fault systems during a former dextral transpressional stage, caused by NW-compression. During NE-oriented shortening, the NTT and MFF were reactivated and incorporated into a nascent transpressional duplex. The youngest manifestation of motion in this system is sinistral transtension. However, this deformation is not observed everywhere and has not yet resulted in topographic inversion.