Abstract The Cenozoic tectonic evolution of the Gangdese arc, formed by juvenile crustal growth during the Mesozoic, remains ambiguous. Here, we conducted a petrological, geochronological and geochemical study of Oligocene (32–24 Ma) granitoids with protolith ages of 57–49 and 27 Ma from the eastern Gangdese arc. Geochemically, these rocks are divided into three groups. Group I has high CaO and Sr, and low REE contents, representing plagioclase-rich cumulates. Group II contains relatively high K 2 O, Pb and REE, and low Na 2 O contents, and is crystallized from evolved magmas. Group III has relatively high Al 2 O 3 and Sr, and low MgO, Y, Yb, Cr and Ni concentrations, and is thickened lower-crust-derived adakitic rocks. The inherited zircon magmatic cores of these rocks have distinctly different ε Hf ( t ) values (−9.88 to +8.50), whereas the magmatic rims have a narrow range of ε Hf ( t ) values (−3.29 to +5.22). The Hf isotopic homogenization indicates intensive mixing of melts derived from the old and juvenile crustal materials. We concluded that the Cenozoic magmatic or sedimentary rocks were buried into the thickened lower crust and melted to generate the Oligocene granitoids, and that the Gangdese arc experienced lasting Paleogene crustal thickening and reworking.

ABSTRACT Five genetic categories of sedimentary basins have been active within the Indus-Yarlung suture zone and in the neighboring High Himalaya since early Cenozoic time. These include: (1) the Xigaze forearc basin (Aptian–early Eocene), (2) the north Himalayan foreland basin (Paleocene–Eocene), (3) the Kailas extensional basin (Oligocene–Miocene), (4) the Liuqu wedge-top basin (early Miocene), and (5) a set of at least six rift and supradetachment basins that formed by arc-parallel extension (late Miocene–Pleistocene). The older basins (categories 1 and 2) were filled with predominantly deep-marine turbiditic deposits, which shoaled through time to subaerial (but very low) elevations. The other basins (categories 3–5) were filled with alluvial-fan, fluvial, and lacustrine sediments, and these formed at progressively higher elevations, culminating in category 5 basins at essentially modern (or slightly higher than modern) elevations (~4000–5000 m). Development of diverse basin types was a response to changing orientations and relative magnitudes of principal stresses in the upper crust of the suture zone and the northern Himalayan thrust belt. Through the Cenozoic, the orientation of maximum compressive principal stress (σ 1 ) changed from approximately horizontal and north-south (Paleocene–Eocene) to approximately vertical with least compressive principal stress (σ 3 ) oriented north-south (Oligocene–Miocene), to horizontal and north-south (early Miocene), to nearly vertical with σ 3 oriented approximately east-west (late Miocene–present). Tectonic stresses associated with the degree of coupling between the converging plates were also potentially important, especially during the Oligocene–Miocene, when the subducting Indian slab was rolling backward relative to the upper Eurasian plate, and during middle to late Miocene time, when the Indian slab was subducting nearly flat beneath the High Himalaya and southern Tibet. Preservation of these extensive sedimentary basins in an orogenic system that is generally being eroded rapidly and deeply stems from original basin-forming mechanisms that produced very large-scale basins (the forearc and early foreland basins) and subsequent evolution of the Himalayan thrust belt in a manner that has isolated High Himalayan basins behind an orographic barrier that protects them from erosion. Recent incision by trans-Himalayan and orogen-parallel suture-zone rivers, however, threatens future preservation of these High Himalayan basins (particularly categories 4 and 5).

In NW Argentina (~26°S), adjacent to the Puna Plateau of the central Andes, exhumation and deformation propagated from west to east across the Puna Plateau and Eastern Cordillera during early–late Cenozoic time. Presently existing data do not indicate a clear eastward younging trend in exhumation in the Precordillera and northern Sierras Pampeanas at the southeastern flank of the Puna Plateau at ~28°S. In this study, we mapped an ~80-km-wide transect at a latitude of ~28°S in the Sierra de Las Planchadas and the Fiambalá Basin. Apatite fission-track (AFT) ages from six samples from thrust fault hanging walls are between 20 and 14 Ma, and apatite helium (U-Th)/He ages from five samples range from 21 Ma west of the Sierra de Las Planchadas to 2 Ma in the Fiambalá Basin. Several samples record mixed apatite (U-Th)/He ages and pre-Cenozoic AFT ages, indicating partial resetting through the 120–60 °C temperature window and suggesting a depth of exhumation between ~6 and 3 km. Thermal modeling of AFT and (U-Th)/He ages indicates cooling from ca. 22 Ma until 2 Ma and a younging trend to the east. These ages are consistent with previously published AFT ages in the Fiambalá region and AHe ages in the Eastern Cordillera to the northeast and suggest Miocene exhumation and deformation in the Precordillera at 28°S and the Eastern Cordillera at 26°S. From a kinematic standpoint, the region of the Precordillera at 28°S may be considered a continuation to the south of the Eastern Cordillera at 26°S, implying a single continuous SW-NE–striking deformation front along this portion of the central Andes. Detrital zircon U-Pb ages from Upper Miocene strata in the Fiambalá Basin suggest that Permian–Miocene rocks in the Sierra de Las Planchadas are the primary source of the Miocene basin fill. This is consistent with previously published eastward-directed paleocurrent data and other provenance proxies and indicates that uplift of the Sierra de Las Planchadas began no later than ca. 9 Ma.

We analyzed the plant macro- and mesofossil records deposited in the Paleocene oil shales of the Boltysh crater (Ukraine) in terms of leaf morphology and its implication for reconstruction of the vegetation and paleoecology of the region. During the early Cenozoic, the Boltysh astrobleme formed a geothermal crater lake that accumulated sediments, preserving a record from the Paleocene to the early middle Eocene. These sediments contain fossil leaf fragments of ferns and angiosperms that grew close to the lake. The occurrence of the Mesozoic fern Weichselia reticulata is of importance. This discovery suggests the survival of this Jurassic to Cretaceous fern into the early Paleogene in the refugial geothermal ecosystem of the Boltysh crater area. Our finding is the youngest record of this fern, although it was a widespread and common element of secondary vegetation during the Cretaceous. The local survival of this fern may have been fostered by the unique combination of edaphic environmental factors of the Boltysh hydrothermal area. Other plant fossils include fragments of leaves that represent ferns likely belonging to lineages that diversified in the shadow of angiosperms, as well as remains of the flowering plants Pseudosalix , Sorbus , Comptonia , and ? Myrica leaf morphotypes.