Daniel L. Farber

Geological Setting and Age of Australopithecus sediba from Southern Africa

From Australopithecus to Homo Our genus Homo is thought to have evolved a little more than 2 million years ago from the earlier hominid Australopithecus. But there are few fossils that provide detailed information on this transition. Berger et al. (p. 195; see the cover) now describe two partial skeletons, including most of the skull, pelvis, and ankle, of a new species of Australopithecus that are informative. The skeletons were found in a cave in South Africa encased in sediments dated by Dirks et al. (p. 205) to about 1.8 to 1.9 million years ago. The fossils share many derived features with the earliest Homo species, including in its pelvis and smaller teeth, and imply that the transition to Homo was in stages. A new species of Australopithecus, about 1.9 million years old, shows many derived features with Homo, helping to reveal its evolution. We describe the geological, geochronological, geomorphological, and faunal context of the Malapa site and the fossils of Australopithecus sediba. The hominins occur with a macrofauna assemblage that existed in Africa between 2.36 and 1.50 million years ago (Ma). The fossils are encased in water-laid, clastic sediments that were deposited along the lower parts of what is now a deeply eroded cave system, immediately above a flowstone layer with a U-Pb date of 2.026 ± 0.021 Ma. The flowstone has a reversed paleomagnetic signature and the overlying hominin-bearing sediments are of normal polarity, indicating deposition during the 1.95- to 1.78-Ma Olduvai Subchron. The two hominin specimens were buried together in a single debris flow that lithified soon after deposition in a phreatic environment inaccessible to scavengers.

Spin crossover in ferropericlase at high pressure: a seismologically transparent transition?

An iron spin transition has no effect on the seismologic properties of lower-mantle minerals. Seismic discontinuities in Earth typically arise from structural, chemical, or temperature variations with increasing depth. The pressure-induced iron spin state transition in the lower mantle may influence seismic wave velocities by changing the elasticity of iron-bearing minerals, but no seismological evidence of an anomaly exists. Inelastic x-ray scattering measurements on (Mg0.83Fe0.17)O-ferropericlase at pressures across the spin transition show effects limited to the only shear moduli of the elastic tensor. This explains the absence of deviation in the aggregate seismic velocities and, thus, the lack of a one-dimensional seismic signature of the spin crossover. The spin state transition does, however, influence shear anisotropy of ferropericlase and should contribute to the seismic shear wave anisotropy of the lower mantle.

Role of plate kinematics and plate-slip-vector partitioning in continental magmatic arcs: Evidence from the Cordillera Blanca, Peru

New structural and geochronological data from the Cordillera Blanca batholith in the Peruvian Andes, coupled with Nazca–South American plate-slip-vector data, indicate that oblique convergence and associated strike-slip partitioning strongly influenced continental magmatic arc evolution. Both the strain field and mode of magmatism (plutonism vs. volcanism) in the late Miocene Peruvian Andes were controlled by the degree to which the arc-parallel component of the plate slip vector was partitioned into the arc. Strong strike-slip partitioning at ca. 8 Ma produced arc-parallel sinistral shear, strike-slip intercordilleran basins and east-west–oriented tension fractures that facilitated emplacement of the Cordillera Blanca batholith (ca. 8.2 ± 0.2 Ma). Periods during which the strike-slip component was not partitioned into the arc (ca. 10 and ca. 7 Ma) were associated with roughly arc-normal contraction and ignimbrite volcanism. The data thus support the contention that contraction within continental magmatic arcs favors volcanism, whereas transcurrent shear favors plutonism. The tie between oblique convergence and batholith emplacement in late Miocene Peruvian Andes provides a modern analogue for batholiths emplaced as the result of transcurrent shear in ancient arcs.

Active detachment faulting above the Peruvian flat slab

The Cordillera Blanca detachment fault in the Peruvian Andes is, to our knowledge, the first active detachment to be documented above a modern flat slab. Crustal detachment has unroofed the ca. 8 Ma Cordillera Blanca batholith, now the backbone of the highest mountain range in Peru. Large-magnitude slip along the fault was thermally enhanced by emplacement of the batholith, the penultimate magmatic event prior to flattening of the Nazca slab. However, extensional models based on arc magmatism and crustal thickening alone do not adequately explain the scale or structural asymmetry of a series of young, deep-seated, west-dipping normal faults across Peru. Here we show that the onset of detachment faulting coincided with subduction of the aseismic Nazca Ridge and consequent flattening of the Nazca slab. We propose that slab buoyancy from ridge subduction triggered extensional collapse of the prethickened continental crust, and that this buoyancy drove footwall uplift that exceeds basin subsidence. The west-dipping asymmetry of late Cenozoic extensional faults in Peru may be controlled by a preexisting crustal anisotropy (older thrusts), and/or formation of Riedel-like shears kinematically linked to the flat Nazca slab.

Post-Middle Oligocene origin of paleomagnetic rotations in Upper Permian to Lower Jurassic rocks from northern and southern Peru

Abstract We report paleomagnetic data from 28 sites of Upper Permian to Lower Jurassic strata from northern and southern Peru. In northern Peru (6°S), a stable magnetic component from six Permo-Triassic sites passes fold and reversal tests. The overall mean pole agrees well with Late Permian to Triassic poles from cratonal South America, suggesting this part of Peru has experienced neither significant rotation nor latitudinal transport since the Permo-Triassic. In southern Peru (13 to 16°S), thermal demagnetization isolates stable magnetic components in 16 of 22 Upper Permian to Lower Jurassic sites collected along the transition between the Altiplano and the Eastern Cordillera. These 16 sites are rotated 14 to 147° counterclockwise and pass an inclinations-only fold test. Within the same structural zone, three other Permo-Triassic sites as well as 10 Paleocene sites also show important counterclockwise rotations [Roperch and Carlier, J. Geophys. Res. 97 (1992) 17233–17249; Butler et al., Geology 23 (1995) 799–802]. The large magnitude and exclusively counterclockwise sense of rotation suggest that the tectonic regime included an important sinistral shear component. No correlation exists between rotation amount and rock age, suggesting the rotations are post-Paleocene in age. Because the rotations occur along the fringe of the Eastern Cordillera, they were likely produced during its structural formation, hence from the Late Oligocene to Present. Sinistral shear acting in the northern part of the Bolivian Orocline appears much more pronounced than that north of the Abancay Deflection, which likely arises from differences in convergence obliquity.

An in situ Raman spectroscopic study of Na 2 Si 2 O 5 at high pressures and temperatures; structures of compressed liquids and glasses

Abstract Raman spectra of Na2Si2O5 glass and Na2Si2O5 liquid have been collected in situ to pressures of 16 GPa at 300 K and 10 GPa at 885 K. In glass compressed at 300 K, bands associated with intertetrahedral bridging 0 atoms decrease in intensity and ultimately become unresolvable above 8 GPa. Over this same pressure range, new peaks appear in the glass between 600 and 800 cm-1; these new features are consistent with the formation of SiO5 or SiO6 polyhedra through the destruction of nonbridging O atoms. Between ∼10 and 16 GPa, band intensities shift, the total integrated Raman scattering intensity of the glass decreases by more than a factor of ten, and the pressure dependence of mode shifts changes. These changes are consistent with a new densification mechanism initiated in the glass above ∼ GPa, probably from the formation of linkages between two or more highly coordinated Si atoms. No vibrations attributed to bridged silica tetrahedra are observed in the liquid above 8 GPa, indicating that at least 50% of the Si in the liquid at 8 GPa is highly coordinated. Upon decompression of thermally quenched samples at 300 K, bands associated with bridged tetrahedra reappear at pressures below 1 GPa. Similarly, glasses that have been compressed only at 300 K and then decompressed have bands associated with bridged tetrahedra. The formation of bands associated with bridged tetrahedra on decompression both in glasses quenched from liquids formed at high-pressure and in glasses compressed at 300 K demonstrates that these species arise from structural reorganizations during decompression. Such reorganizations plausibly involve the breakdown of highly coordinated Si polyhedra and the resultant formation of bridged tetrahedra. The nearly complete decomposition ofthe high-pressure structure in glasses on decompression documents that in situ high-pressure measurements are crucial in deriving accurate constraints on the structure of silicate liquids at high pressure.

Pressure-induced coordination changes in alkali-germanate melts - An in situ spectroscopic investigation

The structure of liquid Na2Ge2O5�H2O, a silicate melt analog, has been studied with Raman spectroscopy to pressures of 2.2 gigapascals. Upon compression, a peak near ∼240 wavenumbers associated with octahedral GeO6 groups grows relative to a peak near ∼500 wavenumbers associated with tetrahedral GeO4 groups. This change corresponds to an increase in octahedral germanium in the liquid from near 0% at ambient pressures to >50% at a pressure of 2.2 gigapascals. Silicate liquids plausibly undergo similar coordination changes at depth in the Earth. Such structural changes may generate decreases in the fusion slopes of silicates at high pressures as well as neutrally buoyant magmas within the transition zone of the Earths mantle.

Paleomagnetic evidence for rapid vertical-axis rotation in the Peruvian Cordillera ca. 8 Ma

Paleomagnetic results from 31 Neogene sites in the Peruvian Andes yield primary magnetizations, as demonstrated by positive fold and reversal tests. Strata dated as 18–9 Ma record a significant counterclockwise rotation (−11° ± 5°), whereas unconformably overlying younger strata (7–6 Ma) are not rotated. The age of rotation thus is between 9 and 7 Ma, a period that coincides with the widespread Quechua 2 deformation phase. Moreover, eight independent studies on 107–9 Ma rocks from Peru between 9°S and 15°S reveal similar and significant rotations (−15° ± 6°). This suggests that the region rotated during a 2 m.y. period of deformation ca. 8 Ma, when the Andes underwent rapid uplift and important deformation commenced in the Subandean zone.

Anomalous pressure evolution of the axial ratio c/a in hcp cobalt: Interplay between structure, magnetism, and lattice dynamics

We performed angle-dispersive x-ray diffraction measurements on hydrostatically compressed hcp cobalt to 90GPa. Near 75GPa, we document an inversion in the pressure derivative of the axial ratio c∕a with no discontinuity in the volume and lattice parameters compression curves. These results are also reproduced by ab initio calculations. Our study indicates significant interactions among structure, magnetism and elasticity, suggesting that the collapse of the magnetic moment is responsible for the observed anomaly in c∕a, as well as for the anomalies in the elastic and vibrational properties of hcp Co at high pressure.

Achieving accuracy in spectroradiometric measurements of temperature in the laser-heated diamond anvil cell: Diamond is an optical component

We present theoretical calculations of the optical effects of the diamond anvil on the accuracy of spectroradiometric temperature measurements made with the laser-heated diamond anvil cell. Considering the dual effects of wavelength-dependent index of refraction and wavelength-dependent absorbance, we find that systematic errors can be minimized by using low numerical aperture optics and by characterizing wavelength-dependent absorbance for each anvil as part of system response measurement. Quantitatively, failure to observe these guidelines can lead to systematic errors of hundreds of degrees. This physical effect may be one part of the origin of the discrepancy of experimental measurements of the melting temperature of iron at megabar pressures.