J. P. Taylor

Paleogeographic reconstruction of a late Paleozoic arc collision zone, southern Mongolia

Permian sedimentary sequences exposed in southern Mongolia record final closure of the paleo–Asian Ocean and suturing of the terranes of northern China with the southern margin of a vast accretionary complex (the Altaids) in Mongolia. Detrital zircon U-Pb geochronology results presented here are the first of their kind in southern Mongolia. Geologic mapping, stratigraphic interpretations, and provenance data including U-Pb zircon geochronology suggest that sedimentary strata at two localities in southern Mongolia, Bulgan Uul and Nomgon, were once part of the same closing ocean basin. The Upper Permian sedimentary deposits at Bulgan Uul record an upward-shallowing marine succession that is unconformably overlain by Lower Triassic fluvial and alluvial strata. The Bulgan Uul marine succession is composed of distal turbidite fan deposits in the lowest portion of the section, with interbedded sandstone and limestone interpreted as shallow-marine deposits at the top of the section. Exposures of Permian-aged distal turbidite units at Nomgon are similar in stratigraphic architecture, sandstone provenance, and detrital zircon age distributions to those documented at Bulgan Uul. Paleocurrent measurements, sandstone provenance data, and U-Pb ages from detrital zircons collected from both study locations document southeastern transport directions for sediment derived from extinct Carboniferous and Ordovician–Silurian arcs of the southern Altaids. Results are consistent with depositional models for remnant ocean basins and indicate diachronous west-to-east closure of the paleo–Asian Ocean (a northern segment of Paleotethys) in the Late Permian. Finally, basin reconstructions place the coeval turbidite deposits at Nomgon to the southeast of Bulgan Uul during the Late Permian. These correlative turbidite successions at Nomgon are currently northeast of Bulgan Uul, offset by ∼250 km of left-lateral strike-slip faulting across the East Gobi fault zone.

Low-temperature thermal history and landscape development of the eastern Adirondack Mountains, New York: Constraints from apatite fission-track thermochronology and apatite (U-Th)/He dating

The Adirondack Mountains in northern New York State form an elongate, domal exposure of mainly high-grade metamor- phic tectonites in a mountainous setting with topographic relief of ~1 km. The origin of the Adirondack Mountains and this relief has long been enigmatic, since the Adirondacks presently lie within an intracratonic setting, inboard of the North American passive mar- gin and far from any active plate boundaries. Through the application of apatite fi ssion- track (AFT) thermochronology and apatite (U-Th)/He (AHe) dating within the eastern Adirondack Mountains, this study provides constraints on both the thermal and ero- sional effects of Mesozoic passage near a hot- spot and the timing of relief development. AFT thermochronology and AHe dating record relatively stable thermal conditions within the eastern Adirondacks from the Middle Jurassic into the Early Cretaceous. During the Early Cretaceous (ca. 130- 120 Ma), the region underwent heating as- sociated with progressive movement near the Great Meteor hotspot, resulting in the temporary establishment of an elevated geo- thermal gradient. Following regional heat- ing, cooling rates increased considerably (ca. 105-95 Ma), likely due to both thermal doming, producing an increase in erosion rate, and the relaxation of isotherms after passage near the hotspot. The regional distribution of AFT ages across the eastern Adirondacks reveals no systematic age gradient from core to periph- ery, which would be expected under con- ditions of persistent high relief during the decay of a crustal root over many tens to hun- dreds of millions of years. Instead, thermo- chronological data suggest that the present relief developed during the Late Cretaceous- Cenozoic through plateau dissection during periodic base-level changes.