Diane Seward

Mesozoic and Cenozoic tectonics of the northern edge of the Tibetan plateau: fission-track constraints

Abstract Fission-track analysis on zircons and apatites yields new information about the timing of deformation of the northern Tibetan plateau. Ages on zircons, ranging from 221±22 to 96±4 Ma are indicative of a general late Triassic–early Jurassic cooling probably driven by the collision between the Qiantang and Kunlun blocks. Mid-Jurassic slow cooling is recorded also in the apatites in regions not affected by later Cenozoic deformation. This Jurassic denudation was followed by a period of sedimentation during the Cretaceous, except along the Altyn Tagh fault (ATF) zone, and in some restricted areas of the western and eastern Qilian Shan. This long and relatively quiet period ended at about 40±10 Ma along the major Altyn Tagh and Kunlun strike-slip fault zones, which were activated by the India–Asia collision. This first movement along lithospheric faults resulted in the eastward extrusion of the Tibet plateau, which was followed, in late Oligocene–Miocene times, by a major compression event, initiating the formation of the high relief of north Tibet. A final compressional event took place at 9–5 Ma and is well correlated with high sedimentation rates in the basins of this region. This compression induced continental subduction in the Kunlun ranges, the Altun Shan belt, and possibly the Qilian Shan belt.

Worldwide acceleration of mountain erosion under a cooling climate

Climate influences the erosion processes acting at the Earth’s surface. However, the effect of cooling during the Late Cenozoic era, including the onset of Pliocene–Pleistocene Northern Hemisphere glaciation (about two to three million years ago), on global erosion rates remains unclear. The uncertainty arises mainly from a lack of consensus on the use of the sedimentary record as a proxy for erosion and the difficulty of isolating the respective contributions of tectonics and climate to erosion. Here we compile 18,000 bedrock thermochronometric ages from around the world and use a formal inversion procedure to estimate temporal and spatial variations in erosion rates. This allows for the quantification of erosion for the source areas that ultimately produce the sediment record on a timescale of millions of years. We find that mountain erosion rates have increased since about six million years ago and most rapidly since two million years ago. The increase of erosion rates is observed at all latitudes, but is most pronounced in glaciated mountain ranges, indicating that glacial processes played an important part. Because mountains represent a considerable fraction of the global production of sediments, our results imply an increase in sediment flux at a global scale that coincides closely with enhanced cooling during the Pliocene and Pleistocene epochs.

Neogene kinematics of the central and western Alps: Evidence from fission-track dating

Zircon and apatite fission-track analysis has established the continuation of a zone of rapid change in mineral ages extending from the Simplon fault zone in the Simplon Alps, through the Rhone Valley, and along the southeastern side of the Mont Blanc and Belledonne massifs. Apatite fission-track ages to the northwest of this zone generally range from 1.4 to 6 Ma, whereas those to the southeast range from 6 to 13 Ma: corresponding values for zircon are 11-13 and 16-26 Ma, respectively. The zone corresponds to the position of the Frontal Pennine thrust in the Mont Blanc-Belledonne region. This structure, a prominent feature on recent deep seismic profiles, may have been reactivated in the Neogene with an important normal-fault component. This is suggested by apatite and zircon fission-track ages in the footwall that are younger than those in the hanging wall and by the observed elimination of the lower Pennine (or Simplon) nappes along this fault zone in the Rhone Valley-Mont Blanc-Belledonne region, resulting in the juxtaposition of North Penninic and Helvetic-Ultra-helvetic nappes at the current erosion level. The distribution of fission-track ages reflects this movement and the related Neogene exhumation of the Mont Blanc and Belledonne external massifs, which has continued until less than 1.4 m.y. ago (i.e., it is almost certainly still active)and has tilted the overlying nappe sequence.

Along-strike variations in the thermal and tectonic response of the continental Ecuadorian Andes to the collision with heterogeneous oceanic crust

Oblique to strike geological segmentation in the Andean chain has been previously recognised at various scales and is commonly attributed to changes in the convergence vectors of the oceanic and continental plates, as well as the upperplate expressions of differing along-strike subducted slab age, strength and composition. We present new white mica and biotite 40 Ar/ 39 Ar and zircon and apatite fission-track data from several traverses across the Cordillera Real of Ecuador in the northern Andes that reveal distinct along-strike differences in the timing of accelerated crustal cooling during the Cenozoic. The data record elevated cooling rates from temperatures of V380‡C during V65^55 and V43^ 30 Ma from all sampled regions of the Cordillera Real and at V15 Ma and since V9 Ma in the northern Cordillera Real. Each cooling period was probably driven by exhumation in response to the accretion and subduction of heterogeneous oceanic crust. Elevated cooling rates of up to V30^20‡C/Myr were initiated during the Palaeocene and Eocene^early Oligocene along the entire contemporaneous margin of Ecuador and were driven by the accretion of the oceanic Pallatanga Terrane and Pin ‹ on^Macuchi Block, respectively, onto northwestern South America. Both of these geological provinces originated at the southern parts of the leading and trailing boundaries of the Caribbean Plateau and accreted onto the margin during the approximately northeastward migration of the Plateau into its current position. Within Ecuador the development of higher topography and elevated cooling rates of up to 50‡C/Myr at V15 Ma and since V9 Ma are restricted to the region north of 1‡30PS and is situated above the postulated subducted flatslab section of the aseismic Carnegie Ridge. Plate convergence rate calculations suggest the Carnegie Ridge collided with the Ecuador Trench at V15 Ma, which caused the pre-existing coastal provinces to displace to the northeast, subsequently driving extension and marine ingressions in southern Ecuador and compression and uplift in northern Ecuador. fl 2001 Elsevier Science B.V. All rights reserved.

Neogene stratigraphy and Andean geodynamics of southern Ecuador

Abstract The present paper reviews Tertiary volcanic and sedimentary formations in the Inter-Andean region of southern Ecuador (between 2°S and 4°20′S) in order to develop a geodynamic model of the region. The formations occur in the southern shallow prolongation of the Inter-Andean Valley between the Cordillera Real to the east, and the Cordillera Occidental and Amotape–Tahuin Provinces to the west. One hundred fifty zircon fission-track analyses has established a detailed chronostratigraphy for the sedimentary and volcanic formations and several small intrusions. The Paleogene to early Miocene formations are dominated by intermediate and acidic volcanic and pyroclastic rocks. In addition, relics of Eocene continental sedimentary series have been identified. The Neogene sedimentary series lie unconformably on deformed and eroded metamorphic, sedimentary and volcanic formations. They were deposited in two stages, which are separated by a major unconformity dated at ≈10–9 Ma. (1) During the middle and early late Miocene (≈15–10 Ma) marginal marine deltaic, lagoonal, lacustrine and fluvial environments prevailed, which we group under the heading “Pacific Coastal sequences”. They presumably covered a greater surface area in southern Ecuador than their present occurrence in small topographic depressions. We suggest that they were deposited in the shallow marine Cuenca and Loja Embayments. Deposition in a marginal marine environment is also supported by the occurrence of brackish water ostracods and other fauna. (2) Above the regional (angular) unconformity, the coastal facies are overlain by late Miocene (≈9–5 Ma) continental alluvial fan and fluvial facies which are in turn covered by mainly airborne volcanic material. They represent the “Intermontane sequences” of the basins of Cuenca, Giron–Santa Isabel, Nabon, Loja and Malacatos–Vilcabamba. Sedimentologic and stratigraphic results are used to discuss the tectonic setting of Neogene sedimentation in the forearc and arc domain of the Ecuadorian subduction system. During the Pacific Coastal stage, northward displacement of the coastal forearc block along the Calacali–Pallatanga fault zone has driven crustal collapse in the Inter-Andean region. As a result, extensional subsidence drove the eastward ingression of shallow seas into the Cuenca and Loja Embayments from the Manabi and Progreso Basins to the west. Tectonic inversion in the forearc area during the early late Miocene (at ≈9.5 Ma) reflects the initiation of W–E oriented compression and uplift in the Inter-Andean region and the establishment of smaller Intermontane stage basins, which host the continental sequences. Coeval topographic rise of the Cordillera Occidental is indicated by the onset of clastic input from the west. The small Intermontane Basin of Nabon (≈8.5–7.9 Ma) formed during the period of maximum compression. The present data prove that the Neogene Andean forearc and arc area in southern Ecuador was a site of important but variable tectonic activity, which was presumably driven by the collision and coupling of the Carnegie Ridge with the Ecuadorian margin since ≈15–9 Ma.

Age of New Zealand Pleistocene substages by fission-track dating of glass shards from tephra horizons

Abstract Eleven tephras and two pumice-rich horizons interbedded in Pleistocene marine sediments of the Wanganui Basin have been dated using the fission-track method on glass shards. Ages of the lower boundaries of New Zealand Pleistocene marine Substages are 0.45 m.y. (Putikian), 1.06 m.y. (Okehuan), 1.55 m.y. (Marahauan). On the basis of sedimentation rates an estimation of 1.86 m.y. is obtained for the Plio-Pleistocene boundary (Hautawan).

Rb/Sr record of fluid-rock interaction in eclogites: The Marun-Keu complex, Polar Urals, Russia

Abstract Rb/Sr internal mineral isochrons in the eclogite facies Marun-Keu metamorphic complex, Polar Urals, Russia, date periods of fluid-rock interaction and record the metamorphic reaction history. The Marun-Keu complex consists of Late Proterozoic to Early Ordovician, mostly igneous rocks that experienced a subduction-related, non-pervasive eclogite facies metamorphism, followed by a local decompression-related amphibolite facies overprint, during the Uralian orogeny. Field observations show that metamorphic reactions as well as ductile deformation are controlled by local availability of a free fluid phase. Isotopic data reveals that availability of fluids similarly exerts control on isotope distribution. From a relic gabbro which has never been infiltrated by free fluids, a premetamorphic Rb/Sr age of 467 ± 39 Ma was obtained. Rb/Sr isochron ages for 14 samples of eclogite and amphibolite facies assemblages, sampled from within or close to metamorphic fluid veins, range from 352 ± 5 Ma to 360 ± 3 Ma. A Sm/Nd isochron for a metagranite yields an age of 354 ± 4 Ma. Taken together, the ages for both prograde and retrograde metamorphic assemblages overlap within analytical uncertainty and yield an average value of 355.5 ± 1.4 Ma, indicating that the metamorphic evolution and incipient exhumation of the Marun-Keu complex proceeded rapidly. The results demonstrate that assemblages preserve their Rb/Sr isotopic signatures as long as they remain devoid of free fluids, and that only fluid-rock interaction may cause Sr isotope redistribution. In addition, the data suggest local fluid-rock equilibrium, low fluid-rock ratios with overall fluid deficiency, and limited fluid mobility at depth. However, some fluids must have been mobile on the km-scale since they can be traced into the suprasubduction zone mantle wedge. Metasomatic veins in the Rai-Iz ophiolite yield a Rb/Sr mineral isochron age of 373.1 ± 5.4 Ma. They are interpreted as evidence for suprasubduction zone metasomatism in an oceanic setting, prior to subduction of the East European margin and associated formation of eclogites in the Marun-Keu complex. We propose that Rb/Sr mineral-isochron ages provide hygrochronological rather than thermochronological constraints. They define the cooling history only in combination with zircon and apatite fission track data. The straightforward interpretation of Rb/Sr mineral ages as cooling ages is obsolete.

Neogene tectonic evolution and exhumation of the southern Ecuadorian Andes: a combined stratigraphy and fission-track approach

Abstract Coastal marine and continental sedimentary facies of Middle to Late Miocene age are exposed in the Andes of southern Ecuador (Cuenca, Giron–Santa Isabel, Loja, Malacatos–Vilcabamba and Catamayo–Gonzanama Basins). The chronostratigraphy of the basin series was established by zircon fission-track dating on a total of 120 tephra layers. Subsequently, the timing of tectonic events was estimated through the well-dated stratigraphic sequences and intervening unconformities. Sedimentation from ≈15 to 9 Ma (termed Pacific Coastal Stage) was dominantly of coastal marine type, extending over an area far greater than the present basin perimeters. It ended when a period of east–west-oriented compression at ≈9.5–8 Ma exhumed the region, and sedimentation was then restricted to smaller basins (termed Intermontane Stage). These Late Miocene continental sediments were for the first time sourced from the west in the rising Western Cordillera. Apatite fission-track analysis was applied to some of the tephras in the Cuenca Basin and also to the older (Eocene, 42–35 Ma) Quingeo Basin series in order to quantify the basin histories with respect to timing and amount of burial and later exhumation. In the Quingeo Basin burial of the oldest sediments reached temperatures of ∼100°C at 18 Ma, when they started to cool down during a period of exhumation. This process preceded the Pacific Coastal Stage development of the other basins. In the Cuenca Basin, the oldest sediments were buried to temperatures of ca. 120°C by 9 Ma, when a period of inversion began and a phase of erosion was dominant. This timing correlates well with that estimated from structural evidence. At ca. 6 Ma the cooling rate slowed down and maybe even reverted to a small increase in temperature until 3 Ma, when the final stages of exhumation took place. Assuming a geothermal gradient of 35°C/km, total uplift for this part for Ecuador is about 6100 m over the last 9 million years. Assuming a steady state continuous movement, this means a mean rock uplift rate of ∼0.7 mm/yr and a surface uplift of 0.3 mm/yr to the Present.

Dynamic effects of aseismic ridge subduction: numerical modelling

The subduction of oceanic aseismic ridges, oceanic plateaus and seamount chains is a common process that takes place in a variety of tectonic settings and seems to coincide spatially and temporally with a gap of volcanic activity, shallow or even horizontal slab angles, enhanced seismic activity and various topographic features. In the present study we focus on these dynamic effects on the basis of 2D thermomechanical modelling incorporating effects of slab dehydration, mantle-wedge melting and surface topography development. In order to ascertain the impact of a moderate-size (200 × 18 km) aseismic ridge, 12 pairs of experiments (one for the case with a ridge, the other without) were carried out varying slab density and subducting- and overriding-plate velocities. By analysing pairs of experiments we conclude that subduction of a moderate-sized ridge does not typically result in strong slab flattening and related decrease of magmatic activity. This, in turn, suggests that, when slab flattening is indeed associated with the ridge subduction in nature, the slab itself should be in a nearly critical ( i.e ., transient from inclined to flat) state so that any local addition of positive buoyancy may strongly affect overall slab dynamics. Therefore, subducting ridges may serve as indicators of transient slab states in nature. Another important result from our study is the numerical quantification of strongly decreased magma production associated with flat slabs that may explain gaps in recent active volcanism at low-angle subduction margins. Lowering of magmatic rock production is caused by the absence of a hot mantle wedge above the flat slabs and does not directly depend on the mechanism responsible for the triggering of slab flattening. Finally we document several very distinct surface effects associated with the moderate-size ridge subduction such as local increase in elevation of overriding margin, enhancement of subduction erosion and landward trench displacement. Surface uplift may exceed the original ridge height due to additional uplift resulting from the overriding plate shortening. Topographic perturbations within the accretionary wedge domain are transient and have a tendency to relax after the ridge passes the trench. In contrast, the topographic high created in the continental portion of the overriding plate relaxes more slowly and may even be sustained for several millions of year after the ridge subduction.

Comparison of zircon and glass fission-track ages from tephra horizons

Tephras previously dated by the fission-track method used on glass shards have been redated using zircon crystals, which have greater track stability and consequently are expected to yield more reliable ages. The ages of the glass proved to be fortuitously correct in most cases, owing to two compensating errors–unrecognized annealing and inaccurate thermal neutron dose determination. Ages of glass shards should not be regarded as reliable until the glass used has been checked for annealing of the fission tracks. If annealing has taken place, a correction can be applied, but the method is very time consuming. The shards from the Borchers Ash, Kansas, suggested by some geologists as a glass age standard, are totally unsatisfactory because of proven annealing.