ALBERT

Description: Crystal Growth & Design DOI: 10.1021/cg401230a

Description: A bstract Phosphatized and phosphatic small shelly fossils are a major source of information concerning the evolution of animals during the early Cambrian. Although progress has been made in understanding some of these fossils, many remain enigmatic, both with regard to their phylogenetic affinities and the overall morphology of the animal from which isolated sclerites came. Two unusual fossils from the upper lower Cambrian (Qiongzhusian or Atdabanian) Xihaoping Member of the Dengying Formation from Xiaowan, Xixiang County, southeastern Shaanxi Province, China are described herein. The first of these is a cap-shaped fossil we describe as Cambroskiadeion xiaowanense new genus and species. On its concave surface it bears a spine, the base of which is covered with numerous hemispherical verruculae. The long spine indicates that this was a sclerite rather than a univalved shell, although it remains unclear from what sort of animal it came. Similar fossils have been hypothesized to be halkieriid valves; although the rarity of halkieriid sclerites in the present samples argues against this view, it is possible these fossils are part of a similar multi-element skeleton. The second fossil is Acidocharacus longiconus Qin and Ding, 1988 ; it is known only from the Xihaoping Member and consists of a tall spine, often bearing barbs or bumps, attached to a rounded conical base. The base is covered with verruculae similar to those found on Cambroskiadeion . The function of these elements, and whether they were internal or external, remains unknown.

Description: The long and complex history of southern Africa makes it a geological nexus for understanding how crust forms, evolves and survives plate tectonic processes over billions of years. The goal of this study is to characterize the crustal thickness, composition, and Moho impedance contrasts across the Kaapvaal and Zimbabwe Cratons and surrounding mobile belts, which range in age from Archean to Palaeozoic. We use data gathered from the 1997–1999 Southern Africa Seismic Experiment, the Africa Array (2006–2007) and the Global Seismographic Network (1993–2009) to generate P -wave receiver function Gaussian-weighted common conversion point stacks across the region in order to provide a continuous 3-D image of crustal variations throughout southern Africa. We observe thickened crust associated with mobile belts and the intrusion of the Bushveld Complex relative to the less-deformed cratons. The southern Kaapvaal and eastern Zimbabwe Cratons have a well-defined Moho with an average depth of ~34 km and Vp / Vs of ~1.73, indicative of felsic average crustal composition. We explain the felsic composition observed in the Kaapvaal Craton in the context of significant crustal modification related to the deposition of the Ventersdorp lavas. We find that the Bushveld Province, the site of the world's largest layered mafic intrusion, has a thick (〉40 km) crust with a Vp / Vs 〉 1.8, indicative of a mafic average crustal composition. The magnitude of Moho conversions beneath the Bushveld Province is variable, with the lowest amplitude conversion appearing between the eastern and western limbs of the Bushveld Complex, indicative of mafic underplating beneath the region. In the Limpopo Belt and western Zimbabwe Craton, we observe low amplitude Moho conversions beneath the Okavango Dyke Swarm, and attribute this to the reworking of the crust by mafic underplating and intrusion during the Jurassic rifting of Gondwanaland. The Namaqua-Natal event thickened the crust and created a gradational transition from crust to mantle as seen by low amplitude Ps arrivals from receiver functions. Evidence for the presence of a mafic lower crust beneath the Namaqua-Natal Belt is observed in high Vp / Vs values (~1.8) and a high concentration of granulite xenoliths in kimberlite intrusions. In contrast to past interpretations for craton formation that suggest sharp Moho boundaries and low Vp / Vs ratios are characteristic of undisturbed cratons, we propose that these crustal properties are more controlled by tectonic events that later modify the existing cratonic crust. We cannot rule out secular crustal formation variations in the early Earth, but we propose that the southern African cratonic crust has been too heavily modified by later tectonic events to be used in arguments for secular variation, as may be the case for other cratons as well. Thus, it is important to consider the regional geological history of cratons to ensure that secular variation is not confused with the effects of later tectonic deformation and crustal modification.

Description: The Central Andes of southern Peru, Bolivia, Argentina and Chile (between 12°S and 42°S) comprise the largest orogenic plateau in the world associated with abundant arc volcanism, the Central Andean Plateau, as well as multiple segments of flat-slab subduction making this part of the Earth a unique place to study various aspects of active plate tectonics. The goal of this continental-scale ambient noise tomography study is to incorporate broad-band seismic data from 20 seismic networks deployed incrementally in the Central Andes from 1994 May to 2012 August, to image the vertically polarized shear wave velocity ( V sv ) structure of the South American Cordillera. Using dispersion measurements calculated from the cross-correlation of 330 broad-band seismic stations, we construct Rayleigh wave phase velocity maps in the period range of 8–40 s and invert these for the shear wave velocity ( V sv ) structure of the Andean crust. We provide a dispersion misfit map as well as uncertainty envelopes for our V sv model and observe striking first-order correlations with our shallow results (~5 km) and the morphotectonic provinces as well as subtler geological features indicating our results are robust. Our results reveal for the first time the full extent of the mid-crustal Andean low-velocity zone that we tentatively interpret as the signature of a very large volume Neogene batholith. This study demonstrates the efficacy of integrating seismic data from numerous regional broad-band seismic networks to approximate the high-resolution coverage previously only available though larger networks such as the EarthScope USArray Transportable Array in the United States.

Description: Cambrothyra ampulliformis Qian and Zhang, 1983, is a jar- or vase-shaped fossil known from the Lower Cambrian of Shaanxi and Hubei provinces, China. It has been interpreted as a protistan test or cyst or a metazoan sclerite. A large collection of specimens from the Xihaoping Member of the Dengying Formation in southern Shaanxi Province permits its detailed redescription. These fossils are highly variable in shape but this variation is continuous and does not support the current recognition of multiple species for this material. They were originally hollow with a restricted basal foramen and a calcareous wall probably composed of fibrous aragonite. All of these features support the identification of Cambrothyra as sclerites of a coeloscleritophoran, a problematic group of Cambrian scleritome-bearing metazoans. Furthermore, the walls of Cambrothyra sclerites contain numerous pores, a feature shared with other coeloscleritophorans. Cambrothyra resembles chancelloriids in particular due to the shared presence of a verruculose texture around the foramen and the absence of mirror-image pairs of asymmetric sclerites. However, unlike chancelloriids, which have rosette-like compound sclerites, the scleritome of Cambrothyra was dominated by isolated sclerites, with only a few pairs and clusters of sclerites and twin sclerites. Consequently, we hypothesize that Cambrothyra forms a clade with other chancelloriids, but represents a basal lineage that plesiomorphically retained isolated sclerites. The morphology of Cambrothyra sclerites, which shares features with both chancelloriids and halkieriids, thus supports the hypothesis that all coeloscleritophorans form a natural group.

Description: Phosphatized and phosphatic small shelly fossils are a major source of information concerning the evolution of animals during the early Cambrian. Although progress has been made in understanding some of these fossils, many remain enigmatic, both with regard to their phylogenetic affinities and the overall morphology of the animal from which isolated sclerites came. Two unusual fossils from the upper lower Cambrian (Qiongzhusian or Atdabanian) Xihaoping Member of the Dengying Formation from Xiaowan, Xixiang County, southeastern Shaanxi Province, China are described herein. The first of these is a cap-shaped fossil we describe asCambroskiadeion xiaowanensenew genus and species. On its concave surface it bears a spine, the base of which is covered with numerous hemispherical verruculae. The long spine indicates that this was a sclerite rather than a univalved shell, although it remains unclear from what sort of animal it came. Similar fossils have been hypothesized to be halkieriid valves; although the rarity of halkieriid sclerites in the present samples argues against this view, it is possible these fossils are part of a similar multi-element skeleton. The second fossil isAcidocharacus longiconusQin and Ding, 1988; it is known only from the Xihaoping Member and consists of a tall spine, often bearing barbs or bumps, attached to a rounded conical base. The base is covered with verruculae similar to those found onCambroskiadeion. The function of these elements, and whether they were internal or external, remains unknown.