The Central and Eastern Anatolian plateaus are integral parts of the world's third largest orogenic plateau. In the past decade, geophysical surveys have provided insights into the crust, ...lithosphere, and mantle beneath Eastern Anatolia. These observations are now accompanied by recent surveys in Central Anatolia and new data constraining the timing and magnitude of uplift along its northern and southern margins. Together with predictions from geodynamic models on the effects of various processes on surface deformation and uplift, the observations can be integrated to identify probable mechanisms of Anatolian Plateau growth.
A changeover from shortening to extension along the southern margin of Central Anatolia that is coeval with the start of uplift can be most easily associated with oceanic slab break-off and tearing. This interpretation is supported by tomography, deep seismicity (or lack thereof), and gravity data. Based on the timing of uplift, geophysical and geochemical observations, and model predictions, slab break-off likely occurred first beneath Eastern Anatolia in middle to late Miocene time, and propagated westward toward Cyprus by the latest Miocene. Alternatively, the break-off near Cyprus could have occurred in late Pliocene to early Pleistocene time, in association with collision of the Eratosthenes Seamount (continental fragment) with the subduction zone. Uplift at the northern margin of Central Anatolia appears to result from crustal shortening starting in the late Miocene or early Pliocene, which has been linked to the broad restraining bend of the North Anatolian Fault. The uplift history of the interior of Central Anatolia since the late Miocene is unclear, although shortening there appears to have ended by the late Miocene, followed by NE–SW extension. This change in the deformation style broadly coincides with faster retreat of the Hellenic trench as well as uplift of the northern and southern margins of Central Anatolia.
These different events throughout the plateau may be linked, as faster retreat of the Hellenic trench has been predicted to occur after slab break-off, which could have induced extension of Central Anatolia and helped to form the North Anatolian Fault through accelerated westward movement of Anatolia relative to Eurasia. Correlative geochronologic evidence that we summarize here supports the hypothesis that the geodynamic activity throughout the Aegean–Anatolian domain starting in latest Miocene to early Pliocene time defines a series of events that may all be linked to slab break-off.
Alluvial rivers aggrade, incise, and adjust their sediment‐transport rates in response to changing sediment and water supply. Fluvial landforms, such as river terraces, and downstream stratigraphic ...archives may therefore record information about past environmental change. Using a physically based model describing sediment transport and long‐profile evolution of alluvial rivers, we explore how their responses to environmental change depend on distance downstream, forcing timescales, and whether sediment or water supply is varied. We show that amplitudes of aggradation and incision, and therefore the likelihood of terrace formation, are greater upstream and in shorter and/or wetter catchments. Aggradation and incision, and therefore terrace ages, may also lag behind environmental change. How sediment‐transport rates evolve depends strongly on whether water or sediment supply is varied. Diverse responses to environmental change could arise in natural alluvial valleys, controlled by their geometry and hydrology, with important implications for paleo‐environmental interpretations of fluvial archives.
Plain Language Summary
Rivers carry sediment from upland regions, where it is produced by weathering and erosion, to low‐lying areas where it is deposited and stored. This process is sensitive to climate: for example, wetter conditions enhance rivers' capacity to transport sediment, causing rivers to cut into their beds, which in turn remobilizes sediment that is transported downstream. Changing climate may also lead to enhanced sediment production and supply to rivers, causing them to accumulate sediment in their beds. Climate cycles can therefore produce alternating periods of sediment accumulation and removal, resulting in ancient floodplains being abandoned and preserved alongside modern rivers (“terraces”), and in variations in the amounts of sediment delivered downstream. Terraces and patterns of sediment accumulation downstream could therefore provide valuable records of past climatic change. We develop a mathematical model that describes the effects of climatic change on river‐bed height and sediment‐transport rates. We show how the amount and timing of river in‐filling and down‐cutting, and the timescales over which they occur, depend on the rate of climatic change, on river length and associated drainage‐basin area, and on the amount of water it carries. These predictions will facilitate more detailed interpretations of terrace records along rivers and of the sedimentary deposits that they leave behind.
Key Points
Alluvial rivers respond in diverse ways to cyclical climatic change depending on their geometry and hydrology and on the forcing frequency
Terrace formation is favored upstream and in shorter and wetter catchments and may lag appreciably behind environmental change
Variation in sediment output lags variation in sediment supply but can lead and be amplified with respect to variation in water supply
Investigating rock‐uplift variations in time and space provides insights into the processes driving mountain‐belt evolution. The Apennine Mountains of Italy underwent substantial Quaternary rock ...uplift that shaped the present‐day topography. Here, we present linear river‐profile inversions for 28 catchments draining the eastern flank of the Northern‐Central Apennines to reconstruct rock‐uplift histories. We calibrated these results by estimating an erodibility coefficient (K) from incision rates and catchment‐averaged erosion rates obtained from cosmogenic‐nuclide data, and we tested whether a uniform or variable K produces a rock‐uplift model that satisfactorily fits independent geochronological constraints. We employ a landscape‐evolution model to demonstrate that our inversion results are reliable despite substantial seaward lengthening of the catchments during uplift. Our findings suggest that a rock‐uplift pulse started around 3.0–2.5 Ma, coinciding with the onset of extension in the Apennines, and migrated southward at a rate of ∼90 km/Myr. The highest reconstructed rock‐uplift rates (>1 km/Myr) occur in the region encompassing the highest Apennine massifs. These results are consistent with numerical models and field evidence from other regions exhibiting rapid rock‐uplift pulses and uplift migration related to slab break‐off. Our results support the hypothesis of break‐off of the Adria slab under the central Apennines and its southward propagation during the Quaternary. Moreover, the results suggest a renewed increase in rock‐uplift rates after the Middle Pleistocene along the Adriatic coast, coeval with recent uplift acceleration along the eastern coast of southern Italy in the Apulian foreland.
Plain Language Summary
Rivers that drain mountainous regions store information on the history of mountain growth. Specifically, variations in the slope of rivers with distance can be interpreted as variations in the rate at which mountains grow through time. Because changes in slope may also occur due to changes in the hardness of rocks underlying the rivers, we must calibrate and correct our interpretations based on changes in rock type. We use 28 rivers draining the eastern flank of the Northern‐Central Apennines of Italy to reconstruct temporal and spatial variations in the history of mountain growth. We find that a pulse of growth started between 3 and 2.5 million years ago and then migrated southwards through time. This southward movement is most likely associated with tectonic plate dynamics beneath the mountain belt, as the plate plunging beneath the Apennines tore progressively southward.
Key Points
Spatio‐temporal rock uplift histories inferred from linear inversion of river profiled
Impact of variable erodibility coefficient and downstream catchment lengthening on inversion results
Quaternary uplift history of the Apennine belt and implications on the driving mechanisms
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
Snow cover reduces cosmogenic nuclide production rates in bedrock. Corrections for snow cover can be more than 10% in mountainous, mid-latitude regions where many glacial chronologies have been ...constructed using cosmogenic nuclide surface dating of landforms. Most published snow corrections use historic climate data of limited duration that are not likely to reflect adequately the full range of snow conditions over the time of exposure. We present a model for describing the impact of snow burial on long-term exposure histories of landforms. The model applies an energy balance approach to snowpack evolution and incorporates both historic and long-term climate proxy data. Attenuation of cosmogenic fast neutrons is modeled alternatively as a simple exponential decrease with increased shielding or as a thin surface layer with constant production followed by an exponential decrease with increasing depth. The choice of attenuation model has little effect on the modeled results for the Cairngorms but will have a more significant effect in regions characterized by thinner, less dense snowpacks. Spatial variability in snow cover is modeled as a function of elevation only, ignoring local variability in snow accumulation as a result of slope aspect, wind redistribution and local topography. Thus, model results reveal general spatial and temporal trends in snow shielding effects, rather than site-specific corrections.
Applications to data from the Cairngorm Mountains of Scotland show that the constant-plus-exponential (CPE) production rate-depth profile reduces but does not eliminate snow-shielding effects. Under present-day conditions, snow at 900 m in the Cairngorm Mountains reduces average production rates by 6% using the CPE profile and 9% with the exponential profile (EP). Long-term climate simulations from 15.5 ka through today produce larger snow shielding effects, mainly because they predict an increased proportion of precipitation as snowfall during the Younger Dryas. At 900 m, this long-term simulation reduces average cosmogenic isotope production rates by 12% (CPE) and 14% (EP). These results indicate that snow-shielding corrections based on historic climate records may be a potential source of systematic error in midlatitude mountainous regions.
Epigenetic gorges in fluvial landscapes Ouimet, W. B.; Whipple, K. X.; Crosby, B. T. ...
Earth surface processes and landforms,
November 2008, Letnik:
33, Številka:
13
Journal Article
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 40Ar/39Ar, apatite and zircon fission‐track and zircon (U‐Th)/He analyses) for samples collected along a 400 km long transect parallel to the trench. Our results show that the Mesoproterozoic gneissic basement was mainly at temperatures ≤350°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‐60 Ma 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 2 km and questions the large amount of trench‐parallel extension suggested to have accommodated this bending.
Key Points
New geo‐thermochronological data set along a 400 km long trench‐parallel profile
Southern Peruvian fore‐arc deformation history and its linkage to oroclinal bending
Crustal‐scale block tilting occurs contemporaneously to oroclinal bending
Sr isotope records from marginal marine basins track the mixing between seawater and local continental runoff, potentially recording the effects of sea level, tectonic, and climate forcing in marine ...fossils and sediments. Our 110 new 87Sr/86Sr analyses on oyster and foraminifera samples from six late Miocene stratigraphic sections in southern Turkey, Crete, and Sicily show that 87Sr/86Sr fell below global seawater values in the basins several million years before the Messinian Salinity Crisis, coinciding with tectonic uplift and basin shallowing. 87Sr/86Sr from more centrally located basins (away from the Mediterranean coast) drop below global seawater values only during the Messinian Salinity Crisis. In addition to this general trend, 55 new 87Sr/86Sr analyses from the astronomically tuned Lower Evaporites in the central Apennines (Italy) allow us to explore the effect of glacio‐eustatic sea level and precipitation changes on 87Sr/86Sr. Most variation in our data can be explained by changes in sea level, with greatest negative excursions from global seawater values occurring during relative sea level lowstands, which generally coincided with arid conditions in the Mediterranean realm. We suggest that this greater sensitivity to lowered sea level compared with higher runoff could relate to the inverse relationship between Sr concentration and river discharge. Variations in the residence time of groundwater within the karst terrain of the circum‐Mediterranean region during arid and wet phases may help to explain the single (robust) occurrence of a negative excursion during a sea level highstand, but this explanation remains speculative without more detailed paleoclimatic data for the region.
Key Points
Negative Sr isotope excursions in Miocene Mediterranean marine sediments
Sr isotope excursions primarily correspond to sea level fall or basin uplift
Groundwater effects on river and basin water geochemistry are likely important
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.
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