The Greater Caucasus Mountains contain the highest peaks in Europe and define, for over 850 km al... more The Greater Caucasus Mountains contain the highest peaks in Europe and define, for over 850 km along strike, the leading edge of the second-largest active collisional orogen on Earth. However, the mechanisms by which this range is being constructed remain disputed. Using a new database of earthquake records from local networks in Georgia, Russia, and Azerbaijan, together with previously published hypocenter locations, we show that the central and eastern Greater Caucasus Mountains are underlain by a northeast-dipping zone of mantle seismicity that we interpret as a subducted slab. Beneath the central Greater Caucasus (east of 45°E), the zone of seismicity extends to a depth of at least 158 km with a dip of ∼40°NE and a slab length of ∼130–280 km. In contrast, beneath the western GC (west of 45°E) there is a pronounced lack of events below ∼50 km, which we infer to reflect slab breakoff and detachment. We also observe a gap in intermediate-depth seismicity (45–75 km) at the western e...
ABSTRACT The Greater Caucasus Mountains contain the highest peaks in Europe and define, for over ... more ABSTRACT The Greater Caucasus Mountains contain the highest peaks in Europe and define, for over 850 km along strike, the leading edge of the second-largest active collisional orogen on Earth. However, the mechanisms by which this range is being constructed remain disputed. Using a new database of earthquake records from local networks in Georgia, Russia, and Azerbaijan, together with previously published hypocenter locations, we show that the central and eastern Greater Caucasus Mountains are underlain by a northeast-dipping zone of mantle seismicity that we interpret as a subducted slab. Beneath the central Greater Caucasus (east of 45°E), the zone of seismicity extends to a depth of at least 158 km with a dip of ∼40°NE and a slab length of ∼130–280 km. In contrast, beneath the western GC (west of 45°E) there is a pronounced lack of events below ∼50 km, which we infer to reflect slab breakoff and detachment. We also observe a gap in intermediate-depth seismicity (45–75 km) at the western end of the subducted slab beneath the central Greater Caucasus, which we interpret as an eastward-propagating tear. This tear coincides with a region of minimum horizontal convergence rates between the Lesser and Greater Caucasus, as expected in a region of active slab breakoff. Active subduction beneath the eastern Greater Caucasus presents a potentially larger seismic hazard than previously recognized and may explain historical records of large magnitude (M 8) seismicity in this region.
1] The Greater Caucasus are the northernmost extent of the Arabia-Eurasia collision and are thoug... more 1] The Greater Caucasus are the northernmost extent of the Arabia-Eurasia collision and are thought to represent the main locus of shortening within the central portion of the collision zone between 40 and 48 E. Recent work suggests that in detail, since the Plio-Pleistocene, much of the shortening in the eastern portion of the Caucasus system has been focused within the Kura fold-thrust belt along the southeastern margin of the Greater Caucasus. Here we present new field mapping and stratigraphic investigations of the eastern termination of the Kura fold-thrust belt in Azerbaijan to better constrain the structural geometries, magnitude of shortening, and initiation age for this portion of the fold-thrust belt. Our work suggests that this area of the fold-thrust belt exhibits significant along-strike variations in structural style and evolution and can effectively be divided into two distinct domains at~48 E. The western domain is characterized by a subcritical median surface slope and isolated folds and thrusts propagating out of sequence, whereas the eastern domain is dominated by a single duplex structure and a history of in-sequence development in a critically tapered wedge. We hypothesize that these variations result from changes in relative rates of syn-tectonic sedimentation, erosion, and convergence velocity along strike. We find that within the western domain, the fold-thrust belt has accommodated~12 km of total shortening. An unconformity within the western domain brackets the initiation age of this portion of the fold-thrust belt to between 1.8 and 0.88 Ma yielding permissible average shortening rates of between 6.7 and 13.6 mm/yr. Comparison of these average shortening rates to the geodetically measured shortening rate of 8 mm/yr indicates that since initiation, the fold-thrust belt has accommodated 83-100% of convergence between the Greater and Lesser Caucasus at this longitude. Citation: Forte, A. M., E. Cowgill, I. Murtuzayev, T. Kangarli, and M. Stoica (2013), Structural geometries and magnitude of shortening in the eastern Kura fold-thrust belt, Azerbaijan: Implications for the development of the Greater Caucasus Mountains, Tectonics, 32,
Relative ages of late Cenozoic stratigraphy throughout the Caspian region are referenced to regio... more Relative ages of late Cenozoic stratigraphy throughout the Caspian region are referenced to regional stages that are defined by changes in microfauna and associated extreme (>1000 m) variations in Caspian base level. However, the absolute ages of these stage boundaries may be significantly diachronous because many are based on the first occurrence of either transgressive or regressive facies, the temporal occurrence of which should depend on position within a basin. Here, we estimate the degree of diachroneity along the Akchagyl regional stage boundary within the Caspian basin system by presenting two late Miocene-Pliocene aged measured sections, Sarica and Vashlovani, separated by 50 km and exposed within the Kura fold-thrust belt in the interior of the Kura Basin. The Kura Basin is a western subbasin of the South Caspian Basin and the sections presented here are located >250 km from the modern Caspian coast. New U-Pb detrital zircon ages from the Sarica section constrain the maximum depositional age for Productive Series strata, a lithostratigraphic package considered correlative with the 2-3 Myr-long regional Eoakchagylian or Kimmerian stage that corresponds to a period of extremely low (>500 m below the modern level) Caspian base level. This new maximum depositional age from the Productive Series at Sarica of 2.5 AE 0.2 Ma indicates that the regionally extensive Akchagyl transgression, which ended the deposition of the Productive Series near the Caspian coast at 3.2 Ma, may have appeared a minimum of 0.5 Myr later in the northern interior of the Kura Basin than at the modern Caspian Sea coast. The results of this work have important implications for the tectonic and stratigraphic history of the region, suggesting that the initiation of the Plio-Pleistocene Kura fold-thrust belt may have not been as diachronous along strike as previously hypothesized. More generally, these results also provide a measure of the magnitude of diachroneity possible along sequence boundaries, particularly in isolated basins. Comparison of accumulation rates between units in the interior of the Kura subbasin and the South Caspian main basin suggest that extremely large variations in these rates within low-stand deposits may be important in identifying the presence of subbasins in older stratigraphic packages.
The Greater Caucasus Mountains contain the highest peaks in Europe and define, for over 850 km al... more The Greater Caucasus Mountains contain the highest peaks in Europe and define, for over 850 km along strike, the leading edge of the second-largest active collisional orogen on Earth. However, the mechanisms by which this range is being constructed remain disputed. Using a new database of earthquake records from local networks in Georgia, Russia, and Azerbaijan, together with previously published hypocenter locations, we show that the central and eastern Greater Caucasus Mountains are underlain by a northeast-dipping zone of mantle seismicity that we interpret as a subducted slab. Beneath the central Greater Caucasus (east of 45°E), the zone of seismicity extends to a depth of at least 158 km with a dip of ∼40°NE and a slab length of ∼130–280 km. In contrast, beneath the western GC (west of 45°E) there is a pronounced lack of events below ∼50 km, which we infer to reflect slab breakoff and detachment. We also observe a gap in intermediate-depth seismicity (45–75 km) at the western e...
ABSTRACT The Greater Caucasus Mountains contain the highest peaks in Europe and define, for over ... more ABSTRACT The Greater Caucasus Mountains contain the highest peaks in Europe and define, for over 850 km along strike, the leading edge of the second-largest active collisional orogen on Earth. However, the mechanisms by which this range is being constructed remain disputed. Using a new database of earthquake records from local networks in Georgia, Russia, and Azerbaijan, together with previously published hypocenter locations, we show that the central and eastern Greater Caucasus Mountains are underlain by a northeast-dipping zone of mantle seismicity that we interpret as a subducted slab. Beneath the central Greater Caucasus (east of 45°E), the zone of seismicity extends to a depth of at least 158 km with a dip of ∼40°NE and a slab length of ∼130–280 km. In contrast, beneath the western GC (west of 45°E) there is a pronounced lack of events below ∼50 km, which we infer to reflect slab breakoff and detachment. We also observe a gap in intermediate-depth seismicity (45–75 km) at the western end of the subducted slab beneath the central Greater Caucasus, which we interpret as an eastward-propagating tear. This tear coincides with a region of minimum horizontal convergence rates between the Lesser and Greater Caucasus, as expected in a region of active slab breakoff. Active subduction beneath the eastern Greater Caucasus presents a potentially larger seismic hazard than previously recognized and may explain historical records of large magnitude (M 8) seismicity in this region.
1] The Greater Caucasus are the northernmost extent of the Arabia-Eurasia collision and are thoug... more 1] The Greater Caucasus are the northernmost extent of the Arabia-Eurasia collision and are thought to represent the main locus of shortening within the central portion of the collision zone between 40 and 48 E. Recent work suggests that in detail, since the Plio-Pleistocene, much of the shortening in the eastern portion of the Caucasus system has been focused within the Kura fold-thrust belt along the southeastern margin of the Greater Caucasus. Here we present new field mapping and stratigraphic investigations of the eastern termination of the Kura fold-thrust belt in Azerbaijan to better constrain the structural geometries, magnitude of shortening, and initiation age for this portion of the fold-thrust belt. Our work suggests that this area of the fold-thrust belt exhibits significant along-strike variations in structural style and evolution and can effectively be divided into two distinct domains at~48 E. The western domain is characterized by a subcritical median surface slope and isolated folds and thrusts propagating out of sequence, whereas the eastern domain is dominated by a single duplex structure and a history of in-sequence development in a critically tapered wedge. We hypothesize that these variations result from changes in relative rates of syn-tectonic sedimentation, erosion, and convergence velocity along strike. We find that within the western domain, the fold-thrust belt has accommodated~12 km of total shortening. An unconformity within the western domain brackets the initiation age of this portion of the fold-thrust belt to between 1.8 and 0.88 Ma yielding permissible average shortening rates of between 6.7 and 13.6 mm/yr. Comparison of these average shortening rates to the geodetically measured shortening rate of 8 mm/yr indicates that since initiation, the fold-thrust belt has accommodated 83-100% of convergence between the Greater and Lesser Caucasus at this longitude. Citation: Forte, A. M., E. Cowgill, I. Murtuzayev, T. Kangarli, and M. Stoica (2013), Structural geometries and magnitude of shortening in the eastern Kura fold-thrust belt, Azerbaijan: Implications for the development of the Greater Caucasus Mountains, Tectonics, 32,
Relative ages of late Cenozoic stratigraphy throughout the Caspian region are referenced to regio... more Relative ages of late Cenozoic stratigraphy throughout the Caspian region are referenced to regional stages that are defined by changes in microfauna and associated extreme (>1000 m) variations in Caspian base level. However, the absolute ages of these stage boundaries may be significantly diachronous because many are based on the first occurrence of either transgressive or regressive facies, the temporal occurrence of which should depend on position within a basin. Here, we estimate the degree of diachroneity along the Akchagyl regional stage boundary within the Caspian basin system by presenting two late Miocene-Pliocene aged measured sections, Sarica and Vashlovani, separated by 50 km and exposed within the Kura fold-thrust belt in the interior of the Kura Basin. The Kura Basin is a western subbasin of the South Caspian Basin and the sections presented here are located >250 km from the modern Caspian coast. New U-Pb detrital zircon ages from the Sarica section constrain the maximum depositional age for Productive Series strata, a lithostratigraphic package considered correlative with the 2-3 Myr-long regional Eoakchagylian or Kimmerian stage that corresponds to a period of extremely low (>500 m below the modern level) Caspian base level. This new maximum depositional age from the Productive Series at Sarica of 2.5 AE 0.2 Ma indicates that the regionally extensive Akchagyl transgression, which ended the deposition of the Productive Series near the Caspian coast at 3.2 Ma, may have appeared a minimum of 0.5 Myr later in the northern interior of the Kura Basin than at the modern Caspian Sea coast. The results of this work have important implications for the tectonic and stratigraphic history of the region, suggesting that the initiation of the Plio-Pleistocene Kura fold-thrust belt may have not been as diachronous along strike as previously hypothesized. More generally, these results also provide a measure of the magnitude of diachroneity possible along sequence boundaries, particularly in isolated basins. Comparison of accumulation rates between units in the interior of the Kura subbasin and the South Caspian main basin suggest that extremely large variations in these rates within low-stand deposits may be important in identifying the presence of subbasins in older stratigraphic packages.
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