The August GSA TODAY science article presents a GIS study that leads to new interpretations of the relative effects of tectonic uplift and erosionally induced isostatic rebound on the landscape development of the Colorado Plateau.
Highlights are provided below. Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to GEOLOGY in stories published. Contact Ann Cairns at acairns@geosociety.org for copies of articles and for additional information or other assistance.
GEOLOGY
Glacier readvance during the late glacial (Younger Dryas?) in the Ahklun Mountains, southwestern Alaska.
Jason P. Briner et al., Institute of Arctic and Alpine Research and Department of Geological Sciences, University of Colorado, Boulder, Colorado 80303, USA. Pages 679–682.
New evidence from a remote Alaskan mountain range shows that glaciers in southwestern Alaska responded to climate change that occurred far away in the North Atlantic region near the end of the last Ice Age. This finding, thanks to dating techniques applied to newly discovered glacier deposits, adds to a growing body of evidence suggesting that there is a close connection between North Atlantic and North Pacific climates. This study, along with other recent data, suggests that atmospheric circulation and ocean-surface currents may communicate changes from one ocean across the continents to the other. With a better understanding of how local climates are affected by distant climate perturbations, we may be able to better predict spatial patterns of future climate changes.
Coastal landsliding and catastrophic sedimentation triggered by Cretaceous-Tertiary bolide impact: A Pacific margin example?
Cathy Busby et al., Department of Geological Sciences, University of California, Santa Barbara, California 93106, USA. Pages 687–690.
The authors report the first Pacific margin example of a paleolandslide triggered by the Chicxulub Cretaceous-Tertiary (K-T) bolide impact. This bolide impact is held responsible for the mass extinction at the end of the age of dinosaurs. Huge landslide deposits have been recognized in Cretaceous-Tertiary boundary deposits of the Gulf of Mexico, the Caribbean Sea, and along the Atlantic offshore of North and South America. These have been interpreted to be the result of seismic shaking caused by the impact, which was probably a 13 on the Richter scale, with vertical ground motion in excess of 1 m within 7000 km of the impact. The giant landslides stick out like a sore thumb along the Atlantic margin, where general tectonic stability makes giant landslides rare, but they could easily be overlooked on the tectonically active Pacific margin. Furthermore, the Cretaceous-Tertiary boundary is not commonly preserved in marine deposits of the Pacific margin, possibly because of widespread tectonic uplift at that time. The authors' new Pacific margin sequence provides evidence of giant landslides and catastrophic sedimentation at distances of >1800 km from the bolide impact site. We present evidence that seismicity was protracted enough to trigger a series of collapse and resedimentation events.
15 k.y. paleoclimatic and glacial record from northern New Mexico.
Peter Fawcett et al., Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131, USA. Pages 723–726.
A suite of sediment cores taken from an alpine bog in the Sangre de Cristo Mountains of northern New Mexico have provided new insights into climate change in the region over the last 15 k.y. Correlation of well-dated changes in sediment character in the bog with glacial landforms shows that the region has experienced several glacial advances and several other cold periglacial periods. The glacial episodes include a mountain glacier at the end of the last Ice Age, a cirque glacier during a cold reversal known as the Younger Dryas ca. 11.5 ka, and a younger cirque glacier advance 4 ka during a period known worldwide as the Neoglacial. The periglacial climate episodes begin in the middle Holocene ca. 5 ka and end with an event correlated with the Little Ice Age. Collectively these glacial and periglacial episodes correlate in time with cold climates recorded in other parts of North America, the North Atlantic Ocean, and Europe and suggest that these climate changes are at least hemispheric in extent.
Bookshelf faulting in Nicaragua.
Peter C. La Femina et al., Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, Florida, 33149, USA. Pages 751–754.
Convergent plate margins mark the location where an oceanic plate subducts beneath a continental plate, causing large earthquakes and volcanoes. Over long periods of geological time, this process creates and modifies much of the continental crust on which we live. Subduction direction is often not perpendicular to the trend of the coastline or the trench offshore, resulting in lateral stresses that slide relatively thin slivers or blocks of crust parallel to the coast. The crustal deformation mechanisms that accommodate this process are still not well understood. Recent studies suggest that approximately 14 mm/yr of northwest-directed motion of a 100-km-wide forearc sliver occurs along the Central America volcanic arc in Nicaragua in response to oblique subduction. However, there is little evidence for the expected northwest-trending strike-slip faults that ought to accommodate this motion. Instead, northeast-trending faults are common. The authors present a new tectonic model for the accommodation of trench-parallel motion of the coastal block along the Central America volcanic arc in Nicaragua. This model is based on new seismic data and new interpretations of fault- and focal- mechanism data, and suggests that trench-parallel motion is accommodated by bookshelf faulting along faults that are orthogonal to the trench. These findings are important in terms of the seismic hazard analyses for Nicaragua and possibly along most of the Central America volcanic arc, because earthquakes on northeast-trending faults caused most earthquake fatalities during the last century, including the 1972 Managua earthquake that caused ~12,000 fatalities.
Control of regional sea level by surface uplift and subsidence caused by magmatic underplating of Earth's crust.
John Maclennan, Laboratoire de Géosciences Marines, Institut de Physique du Globe de Paris, 4 Place Jussieu, 75005 Paris, France, and Bryan Lovell, Bullard Laboratories, Department of Earth Sciences, Madingley Road, Cambridge CB3 0EZ, UK. Pages 675–678.
The level of the sea against the land has changed constantly throughout geological time: coastlines move back and forth as the sea advances and retreats in tides measured in millions of years. There has long been debate about the extent to which this is controlled by synchronous worldwide changes in the level of the oceans, as contrasted with vertical movements of individual landmasses that are of only regional extent. The issue has been of interest to oil companies as well as academics, because sea level exercises a fundamental control over the development of oil- and gas-bearing rocks on continental margins. In the 1970s an Exxon team led by Peter Vail presented an emphatic and hugely influential case for global control of sea level. Since then, the dominance of global controls has been disputed, particularly for those periods of geological time lacking big ice sheets that vary in volume and hence affect global sea level. The establishment of an alternative model to the global orthodoxy requires a testable fundamental mechanism operating on a regional scale. In this article, the authors present a quantitative model for geologically rapid regional uplift and subsidence. They propose a link between surface elevation and episodic emplacement and crystallization of magma deep within Earth's crust. Surface effects include uplift, subsidence, and volcanic activity.
Paleosol barometer indicates extreme fluctuations in atmospheric CO2 across the Cretaceous-Tertiary boundary.
Lee Nordt et al., Department of Geology, Baylor University, Waco, Texas 76798, USA. Pages 703–706.
Variations in atmospheric CO2 concentration (pCO2) have profoundly influenced paleoclimates in the geological records, and may well affect our future with continued fossil fuel emissions. Stable carbon isotopes from soils buried in ancient landscapes were used to track fluctuations in atmospheric pCO2 across the Cretaceous-Tertiary boundary, the time when dinosaurs became extinct. The authors' results detect an atmospheric CO2 spike shortly before the boundary event, capturing the influence of widespread volcanic activity associated with the collision of the Indian tectonic plate with the Asian continent. This event created a temporary global greenhouse climate. With reduction in volcanic activity, atmospheric pCO2 dropped precipitously at the Cretaceous-Tertiary boundary, creating a CO2 crisis. Did this help lead to the demise of the dinosaurs?
Differential incision of Grand Canyon related to Quaternary faulting--Constraints from U-series and Ar/Ar dating.
Joel Pederson et al., Department of Geology, Utah State University, Logan, Utah 84322, USA. Pages 739–742.
The most common question people ask when visiting Grand Canyon--perhaps the world's most famous geologic feature--is, "how and when was the canyon formed?" We know that the Colorado River flowing through the bottom of the canyon is responsible for excavating this awe-inspiring feature, but scientists are still working out exactly how and when the river has done this work and how it has been influenced by factors such as faulting and climate change. For the past few decades, the predominant scientific view has been that erosion of the canyon took place after ca. 6 Ma and was largely finished by ca. 1.2 Ma. In contrast to this, recent research, including that presented in this article, indicates that the Colorado River has continued to incise Grand Canyon over the last ~500 k.y. and that the rate of incision varies along the length of the canyon depending upon location relative to active faults. Specifically, river incision rates calculated downstream of the Toroweap fault in western Grand Canyon are about half the ~140-meters-per-million-year incision rate calculated for the main part of Grand Canyon upstream of the fault--the part of the canyon seen by most visitors. We hypothesize that this differential incision is due to movement along the Toroweap fault, which we measure at ~95 meters per million years. An overall picture is coming into focus in which much of the regional incision of the Colorado Plateau (including Grand Canyon) has been caused by the Colorado River forming into its present-day path to the sea ca. 6 Ma. New research indicates this erosion is continuing at "healthy" rates and has been modified by local faulting and regional climate change.
GSA TODAY
Colorado Plateau uplift and erosion evaluated using GIS.
Joel L. Pederson et al., Department of Geology, Utah State University, Logan, Utah 84322-4505, USA.
The high plateaus and mountains and the deeply dissected canyons of the Colorado Plateau have inspired geologists since the beginning of the science in the late 19th century. Most early researchers attempting to explain the formation of this spectacular landscape called upon recent uplift erosion, whereas recent work has suggested that the uplift took place mostly during the Laramide orogeny about 40 to 70 million years ago. Not only the timing but also the causal mechanisms for the uplift and the erosion of the region are just beginning to be explored: What is the role of the region's unusual mantle in its uplift? What part has climate change played in increasing erosion through time? This paper addresses the landscape development of the Colorado Plateau by focusing on the basic questions: How much uplift and erosion has their been in the first place? And how much uplift is actually caused by the erosion itself (as mass is removed from the plateau and the landscape rebounds isostatically)?
This paper uses a geographic information system (GIS) to map, interpolate, and calculate the Cenozoic rock uplift and erosion of the Colorado Plateau. Results provide mean values of 2117 m for rock uplift and 406 m for thickness of rock eroded since the region was near sea level about 70 million years ago. The authors estimate 843 m of erosion since about 30 million years ago (a larger value because sediment was deposited not eroded from the Plateau between 70 and 30 million years ago), which itself can account for 639 m of "post-Laramide" rock uplift by isostatic rebound. With these new data and the erosional source of uplift added to other proposed mechanisms, an interesting conundrum becomes evident. There is actually less uplift on the plateau than all the proposed sources can supply. To resolve this problem, some longstanding ideas must be changed. Either Laramide uplift of the plateau was significantly less than the neighboring Rocky Mountains (often assumed to be similar), and/or that there has been little or no recent uplift beyond erosional isostasy.
To review the abstracts for these articles, go to www.gsajournals.org. To obtain a complimentary copy of any GEOLOGY article, contact Ann Cairns at acairns@geosociety.org. To review the complete table of contents for the August issue of GEOLOGY, go to http://www.gsajournals.org/gsaonline/?request=get-toc&issn=0091-7613&volume=030&issue=08
Journal
Geology