No abstract

An explosive episode of biological diversification occurred near the beginning of the Cambrian period. Evolutionary rates in the Cambrian have been difficult to quantify accurately because of a lack of high-precision ages. Currently, uranium-lead zircon geochronology is the most powerful method for dating rocks of Cambrian age. Uranium-lead zircon data from lower Cambrian rocks located in northeast Siberia indicate that the Cambrian period began at approximately 544 million years ago and that its oldest (Manykaian) stage lasted no less than 10 million years. Other data indicate that the Tommotian and Atdabanian stages together lasted only 5 to 10 million years. The resulting compression of Early Cambrian time accentuates the rapidity of both the faunal diversification and subsequent Cambrian turnover.

Five isotope-enhanced geologic transects in the southern Annapurna Range of central Nepal elucidate structural geometries near the Main Central thrust. Whole-rock Nd isotopes and U-Pb ages of detrital zircons unambiguously distinguish Greater Himalayan (hanging wall) and Lesser Himalayan (footwall) metasedimentary rocks. ε Nd (0) values for lower Lesser Himalayan rocks typically range from -20 to -26, whereas Greater Himalayan rocks usually have ε Nd (0) values of -19 to -12. Lower Lesser Himalayan rocks yield detrital zircons with an age peak at ca. 1880 Ma and no detrital zircons younger than ca. 1550 Ma. In contrast, Greater Himalayan rocks yield detrital zircons with a prominent broad peak of ages at ca. 1050 Ma and no detrital zircons younger than ca. 600 Ma. The protolith boundary between Greater and Lesser Himalayan rocks is up to 1 km farther south than usually mapped on the basis of lithology. Field and microstructural observations indicate the presence of a top-tothe-south ductile shear zone superimposed on this boundary, confi rming this shear zone as the Main Central thrust. No evidence exists for large-scale structural mixing of Greater and Lesser Himalayan rocks along the Main Central thrust in the Annapurna Range.

North Aegean continental lithosphere was thickened by southwest vergent thrusting and continental subduction within the Alpine collisional orogen but has subsequently been greatly extended on a northeast‐southwest axis in the back arc of the Hellenic subduction zone. Crosscutting relationships with two granodiorite bodies emplaced at ∼31–33 Ma, the Xanthi and eastern Vrondou plutons, constrain a pre‐mid‐Oligocene origin of Alpine convergent structures in northeastern Greece. Post‐Alpine thinning of the north Aegean nappe pile began in earliest Miocene time and has been accommodated by a succession of distinct structural systems. The earliest of these, the “Symvolon shear zone”, appears to represent a midcrustal, coaxial rupture of the Falakron marble series, a carbonate platform >5000 m thick that was subducted northeastward beneath high‐grade rocks of the Rhodope metamorphic province in late Alpine time. Zircon and titanite U‐Pb dates and hornblende 40Ar/39Ar dates obtained in this study constrain the intrusion and incipient mylonitization of the Symvolon or “Kavala” granodiorite within the Symvolon shear zone at ∼21–22 Ma. Following its emplacement, the Symvolon body resided at temperatures between 300°C and 500°C for 5–7 m.y., during which coaxial deformation may have continued within a widening Symvolon rupture. The Strymon Valley detachment, a regionally south‐west dipping low‐angle normal fault, succeeded the Symvolon shear zone in middle Miocene time. Southwestward displacement of the Serbo‐Macedonian gneiss complex by as much as 80 km in the hanging wall of this detachment facilitated the unroofing of the Rhodope metamorphic core complex, including the Falakron marble series and several Tertiary plutons, in its footwall. Biotite and K‐feldspar 40Ar/39Ar dates from 11.1 ± 0.2 Ma to 15.5 ± 0.3 Ma yielded by Symvolon granodiorite samples document the cooling of the Rhodope core complex below 150°C–300°C during its southwestward‐progressive exhumation in the footwall of the Strymon Valley detachment.

Abstract— 40Ar‐39Ar analyses of a total of 26 samples from eight shock‐darkened impact melt breccias of H‐chondrite affinity (Gao‐Guenie, LAP 02240, LAP 03922, LAP 031125, LAP 031173, LAP 031308, NWA 2058, and Ourique) are reported. These appear to record impacts ranging in time from 303 ± 56 Ma (Gao‐Guenie) to 4360 ± 120 Ma (Ourique) ago. Three record impacts 300–400 Ma ago, while two others record impacts 3900–4000 Ma ago. Combining these with other impact ages from H chondrites in the literature, it appears that H chondrites record impacts in the first 100 Ma of solar system history, during the era of the “lunar cataclysm” and shortly thereafter (3500–4000 Ma ago), one or more impacts ˜300 Ma ago, and perhaps an impact ˜500 Ma ago (near the time of the L chondrite parent body disruption). Records of impacts on the H chondrite parent body are rare or absent between the era of planetary accretion and the “lunar cataclysm” (4400‐4050 Ma), during the long stretch between heavy bombardment and recent breakup events (3500‐1000 Ma), or at the time of final breakup into meteorite‐sized bodies (<50 Ma).