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Astronomically tuned cyclic sedimentary successions provide unprecedented insight into the temporal evolution of depositional systems and major geologic events. However, placing astronomically calibrated records into an absolute time frame with confidence requires independent and precise geochronologic constraints. Astronomical tuning of the precessionally modulated sedimentary cycles of the Mediterranean Basin deposited during the Messinian Salinity Crisis (5.96-5.33 Ma) has indicated an similar to 90 k.y. "Messinian gap", corresponding to the evaporative drawdown of the Mediterranean following the closure of the Mediterranean-Atlantic gateway. In the Messinian deposits, a volcanic ash dated by Ar-40/Ar-39 geochronology was used to anchor the sedimentary cycles to the insolation curve. However, the uncertainty of the Ar-40/Ar-39 date introduces a potential two-cycle (similar to 40 k.y.) uncertainty in the tuning. Using high-precision chemical abrasion-thermal ionization mass spectrometry (CA-TIMS) U-Pb geochronology on single zircon grains from two Messinian ash layers in Italy, we obtained dates of 5.5320 +/- 0.0046 Ma and 5.5320 +/- 0.0074 Ma with sub-precessional resolution. Combined with our astronomical tuning of the Messinian Lower Evaporites, the results refine the duration of the "Messinian gap" to at most 28 or 58 +/- 9.6 k.y., which correlates with either the TG12 glacial interval alone, or both TG12 and TG14 glacial intervals, supporting the hypothesis of a glacio-eustatic contribution in fully isolating the Mediterranean from the Atlantic Ocean. Our new U-Pb dates also allow us to infer a precessionally modulated cyclicity for the post-evaporitic deposits, and hence enable us to tune those successions to the insolation curve.
Structural and thermochronologic studies of the western margin of the central Andean Plateau show changing styles of deformation through time that give insights into tectonic evolution. In southwest Peru, uplift of the plateau proceeded in several distinct phases. First, NW striking, NE dipping reverse faults accommodated uplift prior to similar to 14-16 Ma. Subsequent uplift of the plateau relative to the piedmont (between the plateau and the Pacific Ocean) occurred between similar to 14 and 2.2 Ma and was accommodated by NW striking, SW dipping normal faults and subparallel monoclinal folds. The youngest phase of uplift affected the piedmont region and the plateau margin as a coherent block. Although the uplift magnitude associated with phase 1 is unknown, phases 2 and 3 resulted in at least 2.4-3.0 km of uplift. Up to 1 km of this may have occurred during phase 3. Geodynamic processes occurring in both the continental interior and the subduction zone likely contributed to uplift.
Sequences of coral reef terraces characterized by staircase morphologies and a homogeneous lithology make them appropriate to isolate the influence of uplift on drainage morphology. Along the northern coast of Sumba Island, Indonesia, we investigated the correlations between landscape morphology and uplift rates, which range from 0.02 to 0.6 mm.yr(-1). We studied eight morphometric indices at two scales: whole island (similar to 11,000 km(2)) and within sequences of reefal terraces (similar to 3000 km(2)). At the latter scale, we extracted morphometric indices for 15 individual catchments draining mostly the reefal terraces and for 30 areas undergoing specific ranges of uplift rates draining only the reefal terraces. Indices extracted from digital elevation models include residual relief, incision, stream gradient indices (SL and k(sn)), the hypsometric integral, drainage area, mean relief, and the shape factor. We find that SL, the hypsometric integral, mean relief and the shape factor of catchments positively correlate with uplift rates, whereas incision, residual relief, and k(sn) do not. More precisely, we find that only the areas that are uplifting at a rate faster than 03 mm.yr(-1) can yield the extreme values for these indices, implying that these extreme values are indicative of fast uplifting areas. However, the relationship is not bivalent because any uplift rate can be associated with low values of the same indices. For all indices, the transient conditions of the drainage influence the correlation with Pleistocene mean uplift rates, illustrating the necessity to extract morphometric indices with an appropriate choice of catchment scale. This type of analysis helps to identify the morphometric indices that are most useful for tectonic analysis in areas of unknown uplift, allowing for easy identification of short spatial variations of uplift rate and detection of areas with relatively fast uplift rates in unstudied coastal zones. (C) 2015 Elsevier B.V. All rights reserved.
Despite a long history of plate convergence at the western margin of the South American plate that has been ongoing since at least the Early Paleozoic, the southern Peruvian fore-arc displays little to no evidence of shortening. In the light of this observation, we assess the deformation history of the southern Peruvian fore-arc and its geodynamic implications. To accomplish this, we present a new structural and geo-thermochronological data set (zircon U-Pb, mica Ar-40/Ar-39, apatite and zircon fission-track and zircon (U-Th)/He analyses) for samples collected along a 400km long transect parallel to the trench. Our results show that the Mesoproterozoic gneissic basement was mainly at temperatures 350 degrees C since the Neoproterozoic and was later intruded by Jurassic volcanic arc plutons. Along the coast, a peculiar apatite fission-track age pattern, coupled with field observations and a synthesis of available geological maps, allows us to identify crustal-scale tilted blocks that span the coastal Peruvian fore-arc. These blocks, bounded by normal faults that are orthogonal to the trench, suggest post-60Ma trench-parallel extension that potentially accommodated oroclinal bending in this region. Block tilting is consistent with the observed and previously described switch in the location of sedimentary sources in the fore-arc basin. Our data set allows us to estimate the cumulative slip on these faults to be less than 2km and questions the large amount of trench-parallel extension suggested to have accommodated this bending.
Alluvial and transport-limited bedrock rivers constitute the majority of fluvial systems on Earth. Their long profiles hold clues to their present state and past evolution. We currently possess first-principles-based governing equations for flow, sediment transport, and channel morphodynamics in these systems, which we lack for detachment-limited bedrock rivers. Here we formally couple these equations for transport-limited gravel-bed river long-profile evolution. The result is a new predictive relationship whose functional form and parameters are grounded in theory and defined through experimental data. From this, we produce a power-law analytical solution and a finite-difference numerical solution to long-profile evolution. Steady-state channel concavity and steepness are diagnostic of external drivers: concavity decreases with increasing uplift rate, and steepness increases with an increasing sediment-to-water supply ratio. Constraining free parameters explains common observations of river form: to match observed channel concavities, gravel-sized sediments must weather and fine – typically rapidly – and valleys typically should widen gradually. To match the empirical square-root width–discharge scaling in equilibrium-width gravel-bed rivers, downstream fining must occur. The ability to assign a cause to such observations is the direct result of a deductive approach to developing equations for landscape evolution.
The Vicuna Pampa volcanic complex, at the SE edge of the arid Puna Plateau of the Central Andes, records the interplay between volcanic construction and degra-dational processes. The low-sloping Vicuna Pampa volcanic complex, with a 1200-m-deep, southeastward-opening depression, was previously interpreted as a collapse caldera based on morphological considerations. However, characteristic features associated with collapse calderas do not exist, and close inspection instead suggests that the Vicuna Pampa volcanic complex is a strongly eroded, broad, massif-type composite volcano of mainly basaltic to trachyandesitic composition. Construction of the Vicuna Pampa volcanic complex occurred during two distinct cycles separated by the development of the depression. The first and main cycle took place at ca. 12 Ma and was dominated by lava flows and subordinate scoria cones and domes. The second cycle, possibly late Miocene in age, affected the SW portion of the depression with the emplacement of domes. We interpret the central depression as the result of a possible sector collapse and subsequent intense fluvial erosion during middle to late Miocene time, facilitated by faulting, steepened topography, and wetter climate conditions compared to today. We estimate that similar to 65% of the initial edifice of similar to 240 km(3) was degraded. The efficiency of degradation processes for removing mass from the Vicuna Pampa volcanic complex is surprising, considering that today the region is arid, and the stream channels within the complex are predominantly transport limited, forming a series of coalesced, aggraded alluvial fans and eolian infill. Hence, the Vicuna Pampa volcanic complex records the effects of past degradation efficiency that differs substantially from that of today.
Southern Patagonia is a prime example of ongoing oceanic ridge collision and slab-window formation sustained over several million years. The impact of these phenomena on the thermal structure and exhumation of the crust have been mainly assessed with low-temperature thermochronology of bedrock samples. Here, we infer thermal histories from new and existing thermochronological data from the region of most recent ridge collision. In particular, we evaluate the potential far-reaching thermal effects of the evolving slab window, which have previously been considered responsible for patterns of late Miocene reheating associated with back-arc alkaline volcanism. Our model results define protracted cooling since similar to 15 Ma and stepwise exhumation since the late Miocene. The pattern of stepwise exhumation closely matches the onset of Patagonian glaciation at 7 Ma and the successive pulse of glacial incision coeval with neotectonic activity since 3-4 Ma that are also documented by independent geological and geomorphological evidence in the region. Importantly, our findings challenge the recently suggested lack of glacial erosion and incision since 5 Ma in this region. Furthermore, in contrast to previous modelling studies, we find that the available data do not evidence a previously proposed northward-propagating heating event associated with alkaline volcanism. We hypothesize that the anomalous alkaline volcanism in the Patagonian back-arc might be related to trench-orthogonal tears aligned with transform faults in the subducting plate. The substantial differences from the previous modelling procedure on some of the same samples is demonstrated to result from an important lack of convergence in model runs. (C) 2019 Elsevier B.V. All rights reserved.
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) similar to 110 to 90Ma, most likely during tectonic accretion and exhumation of metamorphic rocks from the subduction zone; (2) from similar to 60 to 40Ma, during the collision of the Kirehir and Anatolide-Tauride microcontinental domains with the Eurasian margin; (3) from similar to 0 to 25Ma, either during the early stages of the Arabia-Eurasia collision (soft collision) when the Arabian passive margin reached the trench, implying 70 to 530km of subduction of the Arabian passive margin, or during a phase of trench advance predating hard collision at similar to 20Ma; and (4) similar to 11Ma 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.
A valley-filling ignimbrite re-exposed through subsequent river incision at the southern margin of the Andean (Puna) plateau preserves pristine geological evidence of pre-late Miocene palaeotopography in the north western Argentine Andes. Our new Ar-40/(39) Ar dating of the Las Papas Ignimbrites yields a plateau age of 9.24 +/- 0.03 Ma, indicating valley-relief and orographic-barrier conditions comparable to the present-day. A later infill of Plio-Pleistocene coarse conglomerates has been linked to wetter conditions, but resulted in no additional net incision of the Las Papas valley, considering that the base of the ignimbrite remains unexposed in the valley bottom. Our observations indicate that at least 550 m of local plateau margin relief (and likely > 2 km) existed by 9 Ma at the southern Puna margin, which likely aided the efficiency of the orographic barrier to rainfall along the eastern and south eastern flanks of the Puna and causes aridity in the plateau interior.
Apatite and zircon (U-Th)/He ages from Ocona canyon at the western margin of the Central Andean plateau record rock cooling histories induced by a major phase of canyon incision. We quantify the timing and magnitude of incision by integrating previously published ages from the valley bottom with 19 new sample ages from four valley wall transects. Interpretation of the incision history from cooling ages is complicated by a southwest to northeast increase in temperatures at the base of the crust due to subduction and volcanism. Furthermore, the large magnitude of incision leads to additional three-dimensional variations in the thermal field. We address these complications with finite element thermal and thermochronometer age prediction models to quantify the range of topographic evolution scenarios consistent with observed cooling ages. Comparison of 275 model simulations to observed cooling ages and regional heat flow determinations identify a best fit history with <= 0.2 km of incision in the forearc region prior to similar to 14 Ma and up to 3.0 km of incision starting between 7 and 11 Ma. Incision starting at 7 Ma requires incision to end by similar to 5.5 to 6 Ma. However, a 2.2 Ma age on a volcanic flow on the current valley floor and 5 Ma gravels on the uplifted piedmont surface together suggest that incision ended during the time span between 2.2 and 5 Ma. These additional constraints for incision end time lead to a range of best fit incision onset times between 8 and 11 Ma, which must coincide with or postdate surface uplift.