image: Figure 5 from A. Roark et al. (A) Orientation of the sampling transects through specimen MID2 for stable isotopes (outer image; sample billet) and trace elements (inset image; thin section). (B) The resulting isotopic and trace-element data for specimen MID2. view more
Credit: A. Roark et al. and GSA Bulletin
Boulder, Colo., USA - Does assembling a mega-continent necessarily lead to a mega-monsoon? Can you tell by looking at seashells?
This study by Andy Roark and colleagues tested predictions that the supercontinent Pangea underwent strong monsoons, or massive seasonal changes in wind direction, during times of high sea level (i.e., interglacial) by analyzing the chemistry of fossil brachiopod shells. The shells were deposited in a shallow sea in present-day West Virginia and Ohio in the latest Pennsylvanian (~300 million years ago).
By carefully micro-sampling each shell along its direction of growth and analyzing for stable carbon and oxygen isotopes and trace elements, the team reconstructed a record of seasonal variation during the lifetimes of the organisms. The data showed that the region experienced minimal seasonal variation in temperature and rainfall; in other words, at most a very weak monsoon.
These results may help resolve a paleoclimate debate about the relationship between monsoonality and sea level and provide a glimpse of changing seasons on an ancient mega-continent.
FEATURED ARTICLE
Low seasonality in central equatorial Pangea during a late Carboniferous highstand based on high-resolution isotopic records of brachiopod shells
A. Roark et al., Dept. of Geology and Geophysics, Texas A&M University, MS 3115, College Station, Texas 77843, USA. This article is online at http://dx.doi.org/10.1130/B31330.1.
Other GSA BULLETIN articles are highlighted below:
Coupling of marine and continental oxygen isotope records during the Eocene-Oligocene transition
N.D. Sheldon et al., Dept. of Earth and Environmental Sciences, University of Michigan, Ann Arbor, Michigan 48109, USA. This article is online at http://dx.doi.org/10.1130/B31315.1.
About 34 million years ago, permanent ice sheets grew on Antarctica as Earth's climate went through a major cooling event that spanned the Eocene-Oligocene boundary. Oceanic temperatures cooled relatively uniformly across the planet, whereas temperatures on land varied considerably, with some regions cooling much more significantly than others. In this paper for GSA Bulletin, authors Nathan D. Sheldon and colleagues present a new Eocene-Oligocene continental paleoclimatic record from the Isle of Wight (UK) that shows, for the first time, a close correspondence between the timing of oceanic and continental changes. The new record, based on the oxygen isotopic composition of freshwater snails, demonstrates significant amplification of the terrestrial signal relative to the oceanic signal, while simultaneously suggesting that the there is a strong connection between Northern and Southern Hemisphere climatic changes via the global hydrological cycle. Comparison with other continental records also supports that idea of significant regional differences in the magnitude of climatic change during major global changes.
Low seasonality in central equatorial Pangea during a late Carboniferous highstand based on high-resolution isotopic records of brachiopod shells
A. Roark et al., Dept. of Geology and Geophysics, Texas A&M University, MS 3115, College Station, Texas 77843, USA. This article is online at http://dx.doi.org/10.1130/B31330.1.
Does assembling a mega-continent necessarily lead to a mega-monsoon? This study by Andy Roark and colleagues tested predictions that the supercontinent Pangea underwent strong monsoons, or massive seasonal changes in wind direction, during times of high sea level (i.e., interglacial) by analyzing the chemistry of fossil brachiopod shells. The shells were deposited in a shallow sea in present-day West Virginia and Ohio in the latest Pennsylvanian (~300 million years ago). By carefully micro-sampling each shell along its direction of growth and analyzing for stable carbon and oxygen isotopes and trace elements, the team reconstructed a record of seasonal variation during the lifetimes of the organisms. The data showed that the region experienced minimal seasonal variation in temperature and rainfall; in other words, at most a very weak monsoon. These results may help resolve a paleoclimate debate about the relationship between monsoonality and sea level and provide a glimpse of changing seasons on an ancient mega-continent.
Integrated stratigraphic, geochemical, and paleontological late Ediacaran to early Cambrian records from southwestern Mongolia
E.F. Smith et al., Dept. of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts 02138, USA. This paper is online at http://dx.doi.org/10.1130/B31248.1.
The Precambrian-Cambrian boundary, a major evolutionary and environmental transition in Earth history, is spectacularly exposed in the Zavkhan Terrane in western Mongolia. This paper by E.F. Smith and colleagues presents integrated detailed geological mapping, lithostratigraphy, sequence stratigraphy, and chemostratigraphy from across the Zavkhan Terrane that allows for data from this locality to be incorporated with data globally. A new facies and age model are constructed, allowing for accurate correlation across the basin and shifting first appearance datums globally in the fossil record. Rather than interpreting global fossil first appearances during the early Cambrian as rates of taxonomic origination, the authors show that the first appearances of these fossils are controlled by regional sedimentation and taphonomy.
Structure and evolution of volcanic plumbing systems in fold-and-thrust belts: A case study of the Cerro Negro de Tricao Malal, Neuquén Province, Argentina
Derya Gürer et al., Dept. of Earth Sciences, University of Utrecht, P.O. Box 80021, 3508 TA Utrecht, The Netherlands. This paper is online at http://dx.doi.org/10.1130/B31341.1.
Large parts of magma ascent in volcanic plumbing systems remain mysterious. For example, the detailed mechanisms through which magmas reach Earth's surface in compressional tectonic settings are unclear, given that compression is intuitively assumed to impede magma ascent. To unravel magma ascent mechanisms in compressional settings, Derya Gürer and colleagues performed an extensive, detailed structural field study of the exhumed Cerro Negro intrusive complex, Neuquén Province, Argentina, which was emplaced in a fold-and-thrust belt. Their study demonstrates that local stresses, such as outer-arc stretching at folds' apex, are more effective on magma transport and emplacement than regional compressional stresses. Their results highlight dynamic and complex processes that demonstrate that magma ascent and transport is not only governed by regional tectonic stresses, as commonly assumed. This study has potentially large implications on our understanding of the widespread occurrence of volcanism in the Andean orogen and of mud volcanoes formed in fold-and-thrust belts worldwide.
Systematic mineralogical diversity in A-type granitic intrusions: Control of magmatic source and geological processes
Angeliki Papoutsa et al., Dept. of Earth Sciences, Dalhousie University, Halifax, Nova Scotia B3H 4R2, Canada. This paper is online at http://dx.doi.org/10.1130/B31245.1.
A-type granites comprise a very distinct group of granites, the origin of which has been the subject of debate for decades. In this paper, Angeliki Papoutsa and colleagues examine the differences in mineral composition within several such granite bodies that formed at the same time along the same major fault zone. Three granite types are present based on their dominant mica or amphibole minerals and each type has important differences in bulk rock chemistry and estimated conditions in the parent magma, such as temperature, pressure, water, and fluorine content. Each type can be linked to specific sources of magma and conditions of emplacement.
Post-tectonic landscape evolution of a coupled basin and range: Pinaleño Mountains and Safford Basin, Southeastern Arizona
M.C. Jungers and A.M. Heimsath, School of Earth and Space Exploration, Arizona State University, ISTB4, Room 795, 781 E. Terrace Road Tempe, Arizona 85287, USA. This article is online at http://dx.doi.org/10.1130/B31276.1.
M.C. Jungers and A.M. Heimsath reconstruct the incision history of the Gila River in Safford Basin and a record of upland erosion rates over the last three million years for the Pinaleño Mountains, one of Arizona's "Sky Islands" in its iconic Basin and Range physiographic province. Erosion rates since regional extensional tectonics ceased have been remarkably moderate at 25-60 m per million years, and they seem to be largely a function of how quickly the Gila River incised through basin fill during the integration of the modern Gila River drainage network. The dates and rates presented in the paper are the result of novel applications of a suite of cutting edge cosmogenic nuclide geochronology techniques.
High-resolution temporal and stratigraphic record of Siletzia's accretion and triple junction migration from non-marine sedimentary basins in central and western Washington
M.P. Eddy et al., Dept. of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA. This article is online at http://dx.doi.org/10.1130/B31335.1.
Approximately 50 million years ago a large basaltic terrane (Siletzia) was accreted to western Washington and Oregon. This paper by Michael P. Eddy and colleagues places precise temporal constraints on the timing of this collision and demonstrates that it was followed by increased strike-slip faulting. This activity likely took place at a triple-junction between tectonic plates, and the documentation of this setting will help constrain past plate positions.
Application of actualistic sand petrofacies in hydrogeology: An example from the northern Sacramento Valley, California, USA
R.V. Ingersoll et al., Dept. of Earth, Planetary, and Space Sciences, University of California, Los Angeles, California 90095-1567, USA. This paper is online at http://dx.doi.org/10.1130/B31334.1.
Diverse detrital source areas surrounding the Sacramento Valley of northern California and the abundance of deep water wells from which sand samples can be acquired make this an ideal natural laboratory in which to apply actualistic sand petrofacies methods in solving hydrogeologic subsurface problems. The methods utilized herein should find application in other tectonically active regions with contrasting sediment source terranes. The present study demonstrates the utility of actualistic sand petrofacies in the analysis of both individual wells and aquifers to promote understanding of complex subsurface relations. The simultaneous study of sand composition and three-dimensional geologic relations provides additional insights regarding the interconnectivity (or lack thereof) of aquifers. Additional work to refine source areas, in combination with detailed textural analysis, would help to better define aquifer architecture and improve water-resource assessment, development and management.
Effects of lateral friction on the structural evolution of fold-and-thrust belts: Insights from sandbox experiments with implications for the origin of landward-vergent thrust wedges in Cascadia
J. Zhou et al., State Key Laboratory of Petroleum Resource and Prospecting, China University of Petroleum, Beijing 102249, China. This paper is online at http://dx.doi.org/10.1130/B31320.1.
Although the origin of seaward-vergent and dually vergent fold-and-thrust belts (FTBs) have been satisfactorily explained by the classic Coulomb wedge theory, the origin of uncommon landward-vergent FTBs have not been adequately explained by the classic Coulomb wedge theory up to now. Various factors are inferred to influence the structural evolution of FTBs, but the effects of lateral friction have rarely been given sufficient attention. This recent sandbox models reveal that in the presence of a weak or ductile basal décollement, the direction of lateral friction can dictate not only the deformation propagation direction and the surface slope direction but also the structural vergence of FTBs. An extremely weak basal décollement and landward directed lateral friction are probably the two key factors responsible for the origin of landward-vergent FTBs such as those in Cascadia as well as those in analogue models.
Synchronous opening of the Rio Grande rift along its entire length at 25-10 Ma supported by apatite (U-Th)/He and fission-track thermochronology, and evaluation of possible driving mechanisms
J.W. Ricketts et al., Dept. of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131, USA. This paper is online at http://dx.doi.org/10.1130/B31233.1.
Although the Rio Grande rift is one of the premier continental rifts of the world, there is still no agreed upon model for why it formed when and where it did. In this study, J.W. Ricketts and colleagues combine new apatite (U-Th)/He data with existing apatite fission-track data from rocks that were exhumed during extension, and these constraints suggest synchronous extension along the length of the rift from 25 to 10 million years ago. These timing constraints are used along with regional tectonic and volcanic constraints to test previous models for formation of the rift, and they find significant flaws in each model. Therefore, Ricketts and colleagues propose a new geodynamic model that involves detachment of part of the Farallon slab beneath the present-day Rio Grande rift. This event intensified and focused asthenospheric upwelling along a N-S trend beneath Colorado and New Mexico, allowing the North American lithosphere to become critically weakened so that extension occurred primarily from 25 to 10 million years ago.
Tectonic evolution from subduction to arc-continent collision of the Junggar ocean: Constraints from U-Pb dating and Hf isotopes of detrital zircons from the North Tianshan belt, NW China
X. Zhang et al., Dept. of Earth Sciences, The University of Hong Kong, Pokfulam Road, Hong Kong, China. This paper is online at http://dx.doi.org/10.1130/B31230.1.
This paper by X. Zhang and colleagues presents a good example of how to utilize the U-Pb and Lu-Hf isotopic data of detrital zircons from sedimentary rocks to trace subduction-related tectonic processes and timing of arc-continent collision. The Junggar ocean was a Paleozoic ocean situated between the Yili and Central Tianshan blocks, but how this ocean was finally closed through subduction still remains unknown. U-Pb and Lu-Hf isotopic data of detrital zircons from sandstones in the North Tianshan belt indicate that most of the 541-440 Ma zircons possess low negative eHf(t) values, while the <440-300 Ma zircons exhibit dominantly positive eHf(t) values, which can be linked to the Junggar oceanic slab rollback since ca. 440 Ma. This event, subsequently, gave rise to the opening of the South Tianshan back-arc basin/ocean between the Central Tianshan and Tarim blocks. The Junggar ocean was finally closed at ca. 300 Ma in association with arc-continent collision, followed by post-collisional magmatism.
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