Detrital Zircon Fission-track Ages Research Articles

Provenance of Neogene deposits of Barreiras Formation in the southeastern Brazilian continental margin

The Barreiras Formation records continental-to-shallow–water marine sequences deposited during the Paleogene–Neogene periods in the Brazilian continental margin. In northern Rio de Janeiro and southern Espirito Santo states, the Barreiras Formation preserve alluvial depositional system. The sediments were derived from a source located in the hinterland of the southeastern Brazilian continental margin, where rocks formed or reworked notably during the Gondwana supercontinent assembly are exposed. Detrital zircon U–Pb ages are mostly late Neoproterozoic, dominantly close to 608 Ma. Source rocks of this age occurred within the Ribeira and Aracuai belts and were formed during their main magmatic activity. U–Pb zircon age distribution is similar to the southern and central Ribeira belt. Zircon fission-track ages occur in four main groups, between 429 and 358 Ma, 351 and 274 Ma, 270 and 171 Ma, and 167 and 127 Ma. Complex variation in the detrital zircon fission-track ages is related to the continental crust thermal evolution in the source areas. Older ages also occur between 534 and 433 Ma (Cambrian-to-Silurian periods) and are chrono-correlated to post-orogenic processes after the Gondwana supercontinent agglutination. Zircon fission-track ages between 429 and 274 Ma (Silurian to Permian periods) are related to the formation of the Pangea supercontinent, whereas the predominant zircon fission-track age group, between 270 and 171 Ma (Permian to Jurassic periods), is chrono-correlated to both orogeneses in the Gondwana supercontinent west margin and the Pangea supercontinent breakup. Zircon fission-track ages from 167 to 127 Ma are in the same period as the opening of the North and South Atlantic oceans. Data indicate that the thermal evolution of the source region either during the Gondwana supercontinent and South America Platform stages is complex.

Erosional and depositional history of the Atlantic passive margin as recorded in detrital zircon fission-track ages and lithic detritus in Atlantic Coastal plain sediments

Comparison of fission-track (FT) ages of detrital zircons recovered from Atlantic Coastal Plain sediments to FT ages of zircons from bedrock in source terranes in the Appalachians provides a key to understanding the provenance of the sediments and, in turn, the erosional and depositional history of the Atlantic passive margin. In Appalachian source terranes, the oldest zircon fission-track (ZFT) ages from bedrock in the western Appalachians (defined for this paper as the Appalachian Plateau, Valley and Ridge, and far western Blue Ridge) are notably older than the oldest ages from bedrock in the eastern Appalachians (Piedmont and main part of the Blue Ridge). The age difference is seen both in ZFT sample ages and in individual zircon grain ages and reflects differences in the thermotectonic history of the rocks. In the east, ZFT data indicate that the rocks cooled from temperatures high enough to partially or totally reset ZFT ages during the Paleozoic and (or) Mesozoic. The majority of the rocks are interpreted to have cooled through the ZFT closure temperature (∼235 °C) at various times during the late Paleozoic Alleghanian orogeny. In contrast, most of the rocks sampled in the western Appalachians have never been heated to temperatures high enough to totally reset their ZFT ages. Reflecting their contrasting thermotectonic histories, nearly 80 percent of the sampled western rocks yield one or more zircon grains with very old FT ages, in excess of 800 Ma; zircon grains yielding FT ages this old have not been found in rocks in the Piedmont and main part of the Blue Ridge. The ZFT data suggest that the asymmetry of zircon ages of exposed bedrock in the eastern and western Appalachians was in evidence by no later than the Early Cretaceous and probably by the Late Triassic. Detrital zircon suites from sands collected in the Atlantic Coastal Plain provide a record of detritus eroded from source terranes in the Appalachians during the Mesozoic and Cenozoic. In Virginia and Maryland, sands of Early Cretaceous through late early Oligocene age do not yield any old zircons comparable in age to the old zircons found in bedrock in the western Appalachians. Very old zircons yielding FT ages >800 Ma are only encountered in Coastal Plain sands of middle early Miocene and younger age. Miocene and younger fluvial-deltaic deposits associated with the major mid-Atlantic Coastal Plain rivers that now head in the western Appalachians (the Hudson, Delaware, Susquehanna, Potomac, James, and Roanoke) contain abundant clasts of fossiliferous chert and quartzite and other distinctive rock types derived from Paleozoic rocks of the western Appalachians. These distinctive clasts have not been reported in older Coastal Plain sediments. The ZFT and lithic detritus data indicate that the drainage divide for one or more east-flowing mid-Atlantic rivers migrated west into the western Appalachians, and the river(s) began transporting western Appalachian detritus to the Atlantic Coastal Plain, sometime between the late early Oligocene and middle early Miocene. By no later than late middle Miocene most if not all of the major rivers that now head west of the Blue Ridge were transporting western Appalachian detritus to the Coastal Plain. Prior to the drainage divide migrating into the western Appalachians, the ZFT data are consistent with the dominant source of Atlantic Coastal Plain sediments being detritus from the Piedmont and main part of the Blue Ridge, with possible input from distant volcanic sources. The ZFT data suggest that the rapid increase in the rate of siliciclastic sediment accumulation in middle Atlantic margin offshore basins that peaked in the middle Miocene and produced almost 30 percent of the total volume of post-rift siliciclastic sediments in the offshore basins began in the early Miocene when Atlantic river(s) gained access to the relatively easily eroded Paleozoic sedimentary rocks of the western Appalachians.

Detrital-zircon fission-track ages from the Lower Cenozoic sediments, NW Himalayan foreland basin: Clues for exhumation and denudation of the Himalaya during the India-Asia collision

Detrital-zircon fission-track (FT) ages from the Lower Cenozoic Sub-Himalayan foreland basin reflect the progressive effects of crustal thickening and exhumation on the Himalayan source rocks as a consequence of the India-Asia collision. The oldest stratum, the transgressive marine Paleocene-Eocene Subathu Formation (57–41.5 Ma) contains ca. 50 Ma detrital-zircon P1 peak, which was derived from the Indus Tsangpo Suture Zone and the Ladakh Batholith of the Asian plate. A dominant 302.4 ± 21.9 Ma peak with a few 520 Ma grains in this formation has been derived by erosion of the zircon partial-annealing zone (ZPAZ) of 240–180 °C. As the first imprint of the collision, this zone affected the Himalayan Proterozoic basement and its Tethyan sedimentary cover. Since the detritus in the Subathu has been derived both from the Indian and Asian plates, the possible suturing of these plates took place during the Subathu sedimentation. A sudden change in the provenance is recorded in the detrital-zircon FT cooling ages in the Oligo-Miocene Dagshai and Kasauli Formations, which have dominant 30 and 25 Ma P1 peaks, respectively. We interpret a distinct unconformity spanning ~10 m.y. between the Subathu and Dagshai Formations. Since ca. 30 Ma, molassic sedimentation coincides with shifting of the source rocks to the Himalayan metamorphic belt. This belt has sequentially undergone three distinct cooling and exhumation pulses after the ultrahigh-pressure–high-pressure (UHP-HP) metamorphism (53–50 Ma) in the extreme north and two widespread M1 and M2 metamorphisms (40–30 and 25–15 Ma) in the middle parts. These events appear to be largely responsible for the deposition of the ca. 30 Ma zircon Himalayan peak and ca. 25 and 15 Ma young Himalayan peaks, respectively; the latter appears within the Lower Siwalik Subgroup (13–11 Ma). During the Lower Siwalik deposition, pre-Himalayan peaks gradually decrease with the intensification of the Himalayan events in source rocks. In spite of uninterrupted fluvial sedimentation in the Dagshai-Kasauli–Lower Siwalik sequences since 30 Ma, breaks of ~5–7 m.y. in the zircon FT ages reveal pulsative cooling and exhumation in the well-identified source areas. Although cooling and exhumation of the Himalayan source rocks remained almost uniform during the Eocene, source heterogeneity is reflected in fluvial sedimentation since 37 Ma from Pakistan to Nepal in response to the India-Asia collision.

Petrographical, geochemical and geochronological constraints on igneous clasts and sediments hosted in the Oligo-Miocene Bakony Molasse, Hungary: evidence for a Paleo-Drava River system

The provenance of igneous clasts and arenitic sediment enclosed within the Bakony Molasse was studied using geochemical and geochronological methods. The majority of igneous clasts were eroded from the Oligocene Periadriatic magmatic belt. A part of the andesite material has Eocene formation age. Rhyolitic pebbles originated from Permian sequences of the Greywacke zone or the Gurktal Alps. Apatite fission track (FT) ages from the sandstone matrix (age clusters at ~75 and ~30 Ma) are typical for the Austroalpine nappe pile and for the cooling ages of Periadriatic magmatic belt. Variscan detrital zircon FT ages indicate source areas that had not suffered Alpine metamorphism, such as the Bakony Mountains, Drauzug and the Southern Alps. Another group of detrital zircon grains of Late Triassic–Jurassic FT age (mean: ~183 Ma) marks source zones with Mesozoic thermal overprint such as the Gurktal Alps and some Austroalpine regions. Zircon grains with Oligocene FT age (mean: ~34.7 Ma) were derived from the Periadriatic intrusives and their contact zones. On the basis of the new data, we propose that the ancestor of the recent Drava River had already existed in Oligo-Miocene time and distributed eroded material of the southern Eastern Alps to the east.

Thermochronology of the Torlesse accretionary complex, Wellington region, New Zealand

The Torlesse Complex comprises several Mesozoic accretionary prism complexes together forming continental basement over large parts of New Zealand. This study focuses on the thermal history of relatively low grade graywacke rocks exposed in a transect in southern North Island that crosses the structural grain of the Torlesse Complex, including its older and younger parts. Zircon fission track (FT) ages for the Late Triassic Rakaia Terrane, which is the most inboard of the accretionary complexes, are partially annealed, some possibly reset, and may indicate early Cretaceous (134±10 Ma) cooling from maximum temperatures (Tmax), probably related to imbrication of younger complexes of the Pahau Terrane. Numerical modeling of the zircon FT ages and published 40ArA/39Ar muscovite and biotite ages for the Rakaia Terrane suggest Tmax values of 265–310°C and exhumation from depths of 10–12 km. The rocks underlying the Aorangi Range and involving the youngest accretionary complex have experienced much lower Tmax values of ≤210° and ≥110°C, bracketed by reset apatite FT ages and detrital zircon FT ages. The occurrence of a circa 100 Ma component of zircon FT ages in both the weakly and highly indurated rocks beneath the Aorangi Range, as well as in remnants of an overlying Albian accretionary slope basin (Whatarangi Formation), imply multistorey accretion and incorporation of sediment into the youngest prism. This circa 100 Ma zircon FT age component also places a maximum age on the termination of Mesozoic subduction beneath the New Zealand region. The occurrence of reset apatite FT ages across the whole of the Wellington transect indicates that at least 4 km of exhumation occurred during the late Miocene.

Tracking crustal processes by FT thermochronology in a forearc high (Hikurangi margin, New Zealand) involving Cretaceous subduction termination and mid-Cenozoic subduction initiation

Raukumara Peninsula is a forearc high on the Pacific margin of New Zealand. Its basement comprises two sedimentary terranes that formed in subduction prisms during the Late Jurassic–mid-Cretaceous, marking the end of subduction along the Pacific margin of eastern Gondwanaland. The cover includes allochthonous Late Cretaceous–Oligocene passive margin slope to basin facies emplaced into the forearc during the Early Miocene initiation of the current subduction regime, as well as autochthonous Neogene sediments. Apatite and zircon fission track (FT) data are reported and interpreted for the basement and cover sediments. The detrital zircon FT ages constrain the maximum depositional ages of the low grade (zeolite and pumpellyite facies) basement. Pahau Terrane is no older than 152±13 Ma, this zircon FT mode and older ones (198±8 Ma, 232±58 Ma) recording cooling/denudation in the source areas, probably including Rakaia Terrane. Waioeka Terrane, located oceanward of Pahau Terrane, is no older than 125±2 Ma, being the age of volcanic-derived zircons, and supports Early Cretaceous dinoflagellate ages reported previously. Omaio petrofacies comprises the youngest part of the Waioeka Terrane, and has a zircon FT age of 108±6.3 Ma. This indicates that basement sequences accumulated in the northeast contemporaneously with cover sequences (Koranga Fm) in the southwestern part of the peninsula. These zircon FT ages together with the results of thermal history modelling of certain Waioeka Terrane samples are consistent with migration of a thrust system in a subduction prism environment during the Late Jurassic–Cretaceous until about 85 Ma, as proposed earlier, when Mesozoic subduction terminated. Apatite FT data indicate for all basement samples a phase of Neogene heating and cooling associated with basin formation, sediment accumulation and subsequent uplift and erosion, resulting in almost complete removal of the Neogene stratigraphic record over western parts of the peninsula. Forward modelling of the data imply rapid earliest Miocene downwarping of the crust in Bay of Plenty, coinciding with initiation of subduction of Pacific plate, followed by emplacement of a 1.5–2.0 km thick wedge of East Coast Allochthon from the northeast. Early Miocene sediments accumulated in a foredeep along the front (SW margin) of the allochthon, probably sourced from a block of Pahau basement west of Whakatane Fault, which was exhumed and denuded by 4 km contemporaneously. During the late-Early to Middle Miocene the basin depocentre migrated further to the southwest, while areas previously downwarped (northeast) were uplifted and eroded. The depocentre migration and subsequent inversion is a pattern that affected most of North Island, and is related to mechanical or dynamical interaction of the two plates as the slab of Pacific oceanic lithosphere was emplaced beneath North Island. This may be a general feature of young subduction zones.