<p>Estimating the rate and the pattern of active deformation of slow-slip structures in intracontinental regions has always been a challenging task. Central Portugal is one of those intracontinental regions where the convergence of Eurasian and Nubian plates governs its active deformation. The NE-striking Lower Tagus Valley (LTV) is a locus of active deformation and several historical earthquakes. The eastern and western margins of the LTV are fault-controlled zones (Lower Tagus Valley Fault Zone; LTVFZ), characterized by the predominant strike-slip component. The ~80 km long LTVFZ is one of the most significant intraplate structures in mainland Portugal, and its seismic activity may pose a considerable threat in densely populated urban and industrial areas developed along the LTV. However, the spatio-temporal seismic history along the main structures of LTV is still poorly constrained. In this study, we investigate the geomorphologic features along the Eastern LTVFZ using high-resolution digital aerial orthophotos, high-resolution topographic data extracted from airborne Light Detection and Ranging (LiDAR) data sets, drone-derived high-resolution topographic data, and very high-resolution orthophotos acquired by a small unoccupied aerial system. Removing vegetation cover by LiDAR data leads to access to bare earth surface models that are essential to recognize subtle geomorphic features and constrain their offsets. Accordingly, several cumulative left-lateral displacements were measured along a 20-km stretch of the Eastern LTVFZ. The smallest measured offsets range between 2 and 3 meters that may correspond to the coseismic slip during the most recent surface faulting.&#160; &#160;&#160;&#160;</p><p>To specify the contribution of the Eastern LTVFZ to the regional seismic hazard, we investigate its seismic history through three paleoseismic trenches excavated across the fault near the city of Almeirim. The stratigraphic units, structural features, and geological relations were first logged in the field and then evaluated using the high-resolution, rectified seamless trench-wall photomosaics. Several vertical to steep fault strands exposed in the trench walls cut through the late Pleistocene and Holocene alluvial deposits, recording the signature of several strong paleoearthquakes. Stratigraphic analysis and optically stimulated luminescence dating suggest that the most recent surface faulting has occurred sometime in the middle-late Holocene. The horizontal displacement of this earthquake was measured at two localities nearby the trench site, both in the field and on the very high-resolution orthophotos, and amounts to 2 to 3 meters of the on-fault sinistral offset. The evidence of an older earthquake has been preserved in the late marine isotope stage (MIS) 3 deposits, and at least two even older earthquakes recorded in a sequence of alluvial deposits that predate MIS 4. Although the Eastern LTVFZ may be characterize by low slip rates and instrumentally a quiescent structure, it remains capable of generating morphogenic large-magnitude earthquakes of Mw 7 to 7.5 with millennial recurrence intervals. Such seismic behavior challenges the reliability of assessing seismic hazard over slow-slipping faults across intraplate settings in the lack of geological information.</p>
This study uses quartz‐based optically stimulated luminescence dating to determine when a massive fortification wall (W11186) was constructed in the archaeological site of Tall Zarʿa in the Jordan Valley. A total of 11 samples of sediments were taken from a trench on the north side of the wall (extramuros). The extracted quartz grains had good luminescence characteristics and were identified as well bleached by comparison with feldspar infrared‐stimulated luminescence (IRSL) ages. The optical ages showed that the deposition against the wall occurred in two different periods. The first was around 3.20 ± 0.07 ka ago (Iron Age), and the second was around 2.28 ± 0.08 ka (Hellenistic period). Human activity was evident because of the intermixing of cultural material with the presumed naturally deposited units.
<p>We demonstrate how assumptions about strath terrace formation affect the interpretation of climatic control on landscapes, calculation of incision and rock uplift rates, and recommend strategies for geochronological sampling and interpretation. An innovative approach to OSL dating terrace gravels allows us to produce a detailed ~200 kyr history of punctuated river incision and strath terrace formation spanning two stratigraphic landform levels in the High Atlas Mountains (NW Africa). Extensive preservation and exposure of strath-top gravels, within a post-orogenic setting unaffected by eustatic influences, enables the derivation of rates of base-level fall, integrated over periods of strath-top deposition, metastable equilibrium, and incision, that are consistent with independently constrained regional rock uplift rates. Combining a punctuated river incision model with our well-constrained terrace formation history allows us to demonstrate how assumptions concerning Quaternary river incision and deposition can lead to the problematic Sadler Effect, an apparent dependence of incision rates on measured time interval. Subsequently, we demonstrate that an approach to reinterpreting previously published data using the punctuated incision model, even when combined with limited terrace age data, results in more consistent and parsimonious conclusions about rates of river incision, rock uplift and base-level lowering across the mountain belt. Our recommendations for sampling strategies to constrain rock uplift rates require samples to be taken just above the strath surface, and in addition towards the top of the deposit for river incision rates. In a setting with punctuated river incision and strath terrace formation, both rock uplift and incision rates require burial dates, as exclusive use of abandonment ages will not yield constraints on accurate rates of rock uplift or incision. Furthermore, we find that only with multiple along-stream locations and multiple burial dates in each terrace deposit, could a reliable climatic signal be extracted; this signal would not have shown up in terrace abandonment ages such as those derived from cosmogenic exposure dates. The demonstrated effects of assumptions about strath terrace formation, and the recommended approaches for sampling and interpretation, have implications for those attempting to constrain palaeoclimatic, tectonic, and geomorphic histories from strath terrace records in regions exhibiting punctuated river incision.</p>
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