The history of tectonism on Venus: A stratigraphic analysis
Authors:
Ivanov et al
Abstract:
The surface of Venus displays several tectonized terrains in which the morphologic characteristics of the original materials are almost completely erased by superposed tectonic structures whose large dimensions (»100 km) suggest formation related to mantle convection. The characteristics of these tectonized terrains are in contrast to volcanic units in which tectonic structures are less significant or absent and thus do not obscure the volcanic character of the units. We describe the temporal distribution of tectonized terrains, their stratigraphic relationships with volcanic units, and how these outline the major episodes in the geological evolution of Venus. Five major tectonized units make up ~20% of the planet: 1) tessera (t, 7.3%), 2) densely lineated plains (pdl, 1.6%), 3) ridged plains/Ridge belts (pr/rb, 2.4%), 4) groove belts (gb, 8.1%), and 5) rift zones (rz, 5.0%). Clear relationships of relative age are often seen among the tectonic and volcanic units at the global scale and define three contrasting regimes of volcanic and tectonic resurfacing. The majority of tectonized terrains (t through gb) are the products of tectonic resurfacing and are embayed by the vast volcanic plains and, thus, are older. There are no units with either mildly- or non-tectonized surfaces that interleave the tectonic terrains, which would be expected if the tectonic resurfacing operated only during specific repetitive phases in discrete regions. These tectonized terrains (t through gb) thus define a tectonically dominated regime of resurfacing that occurred at a global-scale near the beginning of the observable geological history of Venus. This ancient tectonic regime began with formation of tessera and was followed by formation of pdl and pr/rb. Groove belts formed near the end of this regime. Branches of groove belts compose the tectonic components of many coronae, suggesting that these features are genetically related (e.g., mutual development of mantle diapirs and zones of extension) and that coronae may have punctuated the final stages of the ancient tectonic regime. This regime was followed by emplacement of the vast volcanic plains, such as shield and regional plains, the surfaces of which are extensively deformed by the global network of wrinkle ridges. Emplacement of the plains defines the second, volcanically dominated regime, representing a time when surface tectonic deformation related to the mantle convection waned. Rift zones are the stratigraphically youngest manifestations of regional-scale tectonic deformation on Venus. Rifts are spatially and temporarily associated with the youngest lava flows and often cut the crest areas of large, but isolated, dome-shaped rises. Structures of rift zones always cut the surface of the vast plains, which means that rifts are separated in time from the ancient tectonic regime, post-date the regional plains, and represent a new phase of tectonism that was contemporaneous with the late volcanism of lobate plains. Rift zones and lobate plains define the third, network rifting-volcanism regime, of resurfacing that was related to late stages of evolution of the dome-shaped rises.
A unique condition for early diversification of small shelly fossils in the lowermost Cambrian in Chengjiang, South China: Enrichment of phosphorus in restricted embayments
Authors:
1. Tomohiko Sato (a)
2. Yukio Isozaki (a)
3. Takahiko Hitachi (a)
4. Degan Shu (b)
Affiliations:
a. Department of Earth Science and Astronomy, University of Tokyo, Tokyo 153-8902, Japan
b. Early Life Institute, Northwest University, Xi’an 710069, China
Abstract:
In the earliest Cambrian the major diversification of small shelly fossils (SSFs) was the first episode of the so-called Cambrian Explosion. In order to clarify the background environmental conditions of this event, we examined lowermost Cambrian strata with bedded phosphorites in the Chengjiang area, South China. The lowermost Cambrian (the Zhongyicun Mb of the Zhujiaqing Fm) in eastern Yunnan is composed of bedded phosphorites and phosphatic limestones with diverse SSFs. We investigated 3 sections within the Chengjiang area at Hongjiachong, Maotianshan, and Xiaolantian. Detailed lithostratigraphic analysis of outcrops and drill cores at Hongjiachong indicates that the Zhongyicun Mb consists of 5 distinct units, A to E in ascending order. The presence of 15 genera of SSFs in 20 horizons shows that the Zhongyicun Mb yields two distinct SSF assemblages of Fortunian age (earliest Cambrian; 541-529 Ma); i.e. the first assemblage with simple-shape SSFs (Anabarites, Protohertzina) from the basal Unit A, and the second assemblage with various molluscan shells (Paracarinachites, Ocruranus-Eohalobia) from Units C-E. As well as abundant phosphate grains, all SSFs occur as clastic grains, suggesting that phosphorite was primarily formed in extreme shallow-water settings, as were the small shelly animals. We established that the first occurrence of the second SSF assemblage is at least 5 m lower (ca. 1-2 m.y. earlier in age) than previously reported in Chengjiang, and we speculate that the major diversification in SSF assemblage likely occurred during the Fortunian, at least before ca. 534 Ma. Judging from the rift-related tectonic setting and relevant paleogeography of western South China, we further speculate that the Zhongyicun Mb was primarily deposited in restricted embayments in the Kangdian rift basin, and that the rapid SSF diversification in the Fortunian occurred in a unique setting in highly phosphorus-rich seawater.
