Quantifying the P–T–t conditions of north–south Lhasa terrane accretion: new insight into the pre‐Himalayan architecture of the Tibetan plateau

An integrated field, petrological and geochronological study of the Basong Tso region of south‐eastern Tibet has constrained the timing and P–T conditions of north–south Lhasa terrane accretion and provides new insight into the tectonothermal evolution of the Tibetan plateau. Two distinct high‐grade metamorphic belts are recognized in the region: a southern belt (the Basong Tso complex) that consists of sheared schist and orthogneiss; and a northern belt (the Zhala complex) that comprises paragneiss and granite. Combined pseudosection modelling and U–Pb geochronology of monazite and zircon indicates that the Basong Tso complex records peak metamorphic conditions of 9 ± 0.5 kbar and 690 ± 25 °C at c. 204–201 Ma, whereas the Zhala complex experienced peak metamorphic conditions of 5.0 ± 1.0 kbar and 740 ± 40 °C at c. 198–192 Ma. Microstructural analysis suggests that the two belts share a common early prograde history, after which the Basong Tso complex attained peak conditions following rapid burial, and the Zhala complex approached peak conditions along an isobaric path. Overall it is inferred that the Basong Tso and Zhala complexes represent the lower and upper structural levels of an evolving orogen that underwent Barrovian‐type metamorphism following collision (M1), followed by Buchan‐style overprinting at higher structural levels due to heat advection by syn‐tectonic granites (M2). Mylonitization (sensu lato) of the Basong Tso complex and juxtaposition of the two units occurred after attainment of peak conditions. The dominance of Mesozoic regional metamorphism across most of the Tibetan plateau indicates that Cenozoic crustal thickening processes, where present, are only manifested at depth.     

Late Quaternary Floodplain History of the Brazos River in East-Central Texas

The floodplain along a 75-km segment of the Brazos River, traversing the Gulf Coastal Plain of Texas, has a complex late Quaternary history. From 18,000 to 8500 yr B.P., the Brazos River was a competent meandering stream that migrated from one side of the floodplain to the other, creating a thick layer of coarse-grained lateral accretion deposits. After 8500 yr B.P., the hydrologic regime of the Brazos River changed. The river became an underfit meandering stream that repeatedly became confined within narrow and unstable meander belts that would occasionally avulse. Avulsion occurred four times; first at 8100 yr B.P., then at 2500 yr B.P., again around 500 yr B.P., and finally around 300 yr B.P. The depositional regime on the floodplain also changed after 8500 yr B.P., with floodplain construction dominated by vertical accretion. Most vertical accretion occurred from 8100 to 4200 yr B.P. and from 2500 to 1250 yr B.P. Two major and three minor periods of soil formation are documented in the floodplain sequence. The two most developed soils formed from 4200 to 2500 yr B.P. and from around 1250 to 500 yr B.P. These changes on the floodplain appear to be the result not of a single factor, but of the complex interplay among changes in climate, sediment yield, and intrinsic floodplain variables over time.     

Plate tectonics in the Eastern Alps

The weight of the geological evidence, which includes the recognition of a late Cretaceous paired metamorphic belt, suggests that a southward dipping subduction zone existed in the Eastern Alps. On this basis a new plate tectonic model is presented for the post-Palaeozoic evolution of that orogen.     

Geoarchaeological investigations of St. Ann's Bay,Jamaica: The search for the Columbus Caravels and an assessment of 1000 years of human land use

A complex interbedded sequence of shallow bay, alluvial fan, and lagoon/marsh facies was revealed during geoarchaeological investigations of St. Ann's Bay, Jamaica. Eighteen radiocarbon ages and time-diagnostic artifacts were obtained from these sediments. From these data, the landscape evolution of St. Ann's Bay, Jamaica, during the last 4800 years is reconstructed. The landscape reconstructions for St. Ann's Bay are used to identify areas where the two caravels abandoned by Christopher Columbus in A. D. 1504 might be located. In addition, two periods of rapid sedimentation, one from A. D. 1000 to 1300 (lagoon filling) and the other between A. D. 1700 and modern times (bay filling), are attributed to human land use around St. Ann's Bay. The earlier period of increased sedimentation corresponds to the introduction and expansion of native peoples to the north side of the island, who brought with them slash-and-burn agriculture. The later period of increased sedimentation in the bay corresponds to the intensification of agricultural use of the area by the Spanish and British. © 1993 John Wiley & Sons, Inc.     

The impact of fluvial processes and landscape evolution on archaeological sites and settlement patterns along the San Xavier reach of the Santa Cruz River,Arizona

The Santa Cruz arroyo in the Tucson Basin, Arizona, has undergone major environmental changes over the last 8000 years. the Holocene stratigraphy along a 15 km segment of the arroyo, known as the San Xavier reach, which traverses the San Xavier Indian Reservation was investigated in detail. the Holocene alluvial sequence reveals that aggradation occurred until 8000 yr B.P. within a braided stream, was followed by a major period of channel erosion and widening from 8000 to 5500 yr B.P., which, in turn, was followed by vertical aggradation of the floodplain and five short periods of channel cutting. Broad climatic changes are correlated with major changes in the fluvial regime and landscape. However, the cycles of arroyo cutting and filling, during the semiarid climate of the last 2500 years, were probably the result of the creation of unstable internal geomorphic conditions, flooding, and human impacts on the floodplain. the alluvial history of the San Xavier reach has had a pronounced effect on the preservation and completeness of the archaeological record. Periods of erosion have created absences or gaps in the archaeological record and deep burial has removed some of the preserved archaeological remains from view. Environmental changes on the floodplain also influenced late prehistoric Hohokam settlement and subsistence patterns in the San Xavier region.     

Dating the geologic history of Oman’s Semail ophiolite: insights from U-Pb geochronology

Eclogites from the deepest structural levels beneath the Semail ophiolite, Oman, record the subduction and later exhumation of the Arabian continental margin. Published ages for this high pressure event reveal large discrepancies between the crystallisation ages of certain eclogite-facies minerals and apparent cooling ages of micas. We present precise U-Pb zircon (78.95 ± 0.13 Ma) and rutile (79.6 ± 1.1 Ma) ages for the eclogites, as well as new U-Pb zircon ages for trondhjemites from the Semail ophiolite (95.3 ± 0.2 Ma) and amphibolites from the metamorphic sole (94.48 ± 0.23 Ma). The new eclogite ages reinforce published U-Pb zircon and Rb-Sr mineral-whole rock isochron ages, yet are inconsistent with published interpretations of older ⁴⁰Ar/³⁹Ar phengite and Sm-Nd garnet dates. We show that the available U-Pb and Rb-Sr ages, which are in tight agreement, fit better with the available geological evidence, and suggest that peak metamorphism of the continental margin occurred during the later stages of ophiolite emplacement.