And other Geosphere articles posted online June 10, 2015
The age of the Grand Canyon (USA) has been studied for years, with recent technological advances facilitating new attempts to determine when erosion of this iconic canyon began. The result is sometimes conflicting ages based on different types of data; most data support the notion that the canyon began to erode to its current form about six million years ago. Then even newer, "high-tech," data became available and questions were again raised about whether the western end of the canyon could be older.
This is Figure 2 from Darling and Whipple: Photographs taken from Twin Point overlook by Rich Rudow. (A) View to the southeast, showing the Shivwitz Plateau escarpment above the Sanup Plateau. (B) View to the south, showing the Sanup Plateau in the foreground and the Hualapai Plateau in the background. Surprise and Spencer canyons are prominent recesses in the plateau.
Photographs taken from Twin Point overlook by Rich Rudow; Geosphere, 10 June 2015.
Two numbers are used as general time markers for these alternate hypotheses. The first suggests that the canyon may have started incising 17 million years ago. The second suggests that the canyon may have looked largely as it does today 70 million years ago. The time contrast between these hypotheses is striking, and any accurate concept of the canyon would have to be consistent with all observations.
Other researchers have studied the Grand Wash Fault, which truncates the western Grand Canyon. The fault runs north to south, nearly perpendicular to the Canyon. The fault slides in such a way that the west side of the fracture moves down relative to the east side, leaving a cliff face called the Grand Wash Cliffs. This slip, called "normal slip," has led to the opening of a valley called the Grand Wash trough along the east end of Lake Meade. Erosion of hillslopes and canyons in the Grand Wash Cliffs is driven by the fault movement exposing the rock at the surface. These hillslopes and canyons are similar to the Colorado River's tributaries in Grand Canyon, except hills and side streams are all steeper in Grand Canyon.
This comparison is useful because the Grand Wash fault has been studied extensively, and other scientists have shown that the fault completed most of its sliding between 18 and 12 million years ago. The rocks and climate in both regions are similar, so the difference in landform shape is most likely due to when the landforms started eroding.
In this new article for Geosphere, Andrew Darling and Kelin Whipple focus on the western Grand Canyon, west of the Hurricane fault. Their data show that the Grand Canyon must be younger than the fault slip that occurred 18 to 12 million years ago. Comparing their data to other datasets suggests that the notion that the canyon starting eroding around six million years ago is still the best scientific idea for the age of the Grand Canyon.
Geomorphic constraints on the age of the western Grand Canyon
Andrew Darling and Kelin Whipple, Arizona State University, Tempe, Arizona, USA. Published online on 10 June 2015; http://dx.
Other new GEOSPHERE articles posted online on 10 June 2015 are summarized below.
A global perspective on the topographic response to fault growth Magdalena A. Ellis and Jason B. Barnes, University of North Carolina, Chapel Hill, North Carolina, USA. Published online on 10 June 2015; http://dx.
How do tectonics and topography coevolve in mountain ranges formed by fault movement? The complex feedbacks involved in the evolution of mountain topography make it difficult to understand the dominant variables. In this paper we analyze the topography of >40 fault-driven mountain ranges from across the globe and compare the results with tectonic data, climate data, and lithologic information. Our results show that in all ranges, relief becomes limited at some distance from the fault tip, reaching a uniform value in the center of the mountain range. The scales over which this transition from increasing to uniform relief occurs is (1) related to long-term rock uplift rate and (2) further modulated by climate and lithology. This contribution is the first comprehensive and global analysis of patterns of along-strike relief development for active dip-slip faults. Implications of our work include estimation of the temporal scale of fault-driven mountain range growth and the ability to make 1st-order predictions of tectonic rates in regions lacking detailed estimates. We include with the paper a Google Earth accessible database with the imagery and variables for every analyzed mountain range.
Laurentian and Amazonian sediment sources to Neoproterozoic-lower Paleozoic Maryland Piedmont rocks
Aaron J. Martin et al., Department of Geology, University of Maryland, College Park, Maryland 20742, USA. Published online on 10 June 2015; http://dx.
The prevailing paradigm to explain the Ordovician to Mississippian pulses of Appalachian mountain building is collision of small continental fragments (ribbon continents) with the eastern edge of Laurentia (Laurentia is the name for North America prior to the Jurassic). This model is based on the presence of exotic terranes in the northern and southern Appalachians that accreted to the eastern edge of Laurentia at this time. The Piedmont region between central Virginia and New York City also experienced early Paleozoic orogeny, but no accreted terranes have been recognized between central Virginia and New York City, raising the question, "What caused early Paleozoic mountain building in the central Appalachians?" We used uranium-lead dates of zircon grains that were washed into Maryland Piedmont sandstones at the time of deposition to answer this question. We found that in one of our eighteen samples, the detrital zircon age spectrum is consistent with sediment derivation from Amazonia (a component of Gondwana) but not Laurentia. This sample may indicate the presence of a previously unrecognized exotic terrane in the central Appalachian Piedmont, though we caution that more work is necessary to confirm its presence and delineate its extent.
Controls on the expression of igneous intrusions in seismic reflection data
Craig Magee, Basins Research Group (BRG), Imperial College, London, UK. Published online on 10 June 2015; http://dx.
Seismic reflection data provides X-ray-like images of Earth's subsurface, presenting a novel method for analyzing ancient, solidified magma conduits and storage sites (intrusions) in three-dimensions. To make sense of these images, it is important to compare interpreted intrusion morphologies with intrusions observed exposed at the Earth's surface, which are commonly partially eroded. For the first time, this work presents a technique whereby synthetic seismic data is generated for intrusion geometries reconstructed from field examples. This allows us to test the controls on the seismic expression of igneous intrusions, helping inform future academic and industrial initiatives to map intrusions in seismic reflection data.
Dynamic deep-water circulation in the northwestern Pacific during the Eocene: Evidence from Ocean Drilling Program Site 884 benthic foraminiferal stable isotopes (d18O and d13C)
C. Borrelli and M.E. Katz, Rensselaer Polytechnic Institute, Troy, New York, USA. Published online on 10 June 2015; http://dx.
The Pacific Ocean has been a key component of climate evolution since the early Cenozoic (65-0 million years ago [Ma]). Today, the Pacific Ocean is Earth's largest basin; considering that it was even larger in the early Cenozoic than it is today, the investigation of ocean circulation in the Pacific during the past 65 million years (Myr) is a very important step towards understanding the impact of the Pacific Ocean on Earth's climate today, as well as in the geological past. The goal of this study is the investigation of ocean circulation in the northwestern Pacific from the early middle Eocene to the early Oligocene (49-33 Ma). Studying the isotopic composition of shells of microfossils called foraminifera, a picture has emerged of a very dynamic ocean circulation in the northwestern Pacific during this time. In particular, the data collected during this study indicate that the northwestern Pacific was bathed by three different deep-water masses over 16 Myr: (1) a water mass probably originating from the Southern Ocean, or a water mass having an oxygen isotopic signature similar to that of the water masses bathing other locations in the Pacific, Atlantic, and Southern Oceans, making the northwestern Pacific part of a "global" ocean circulation route; (2) a "regional" deepwater mass, possibly originating from downwelling of nutrient-rich surface waters at the higher latitudes of the North Pacific or warm saline deep waters originating at low latitudes; and (3) a water mass originating again from the Southern Ocean. In particular, we speculate that this last change was linked to global tectonic movements that opened key ocean gateways, which isolated Antarctica from the South America (Drake Passage) and from Australia (Tasman Rise), allowing the development of the Antarctic Circumpolar Current (ACC), a very important component in today's ocean circulation and climate system. It has been demonstrated that the development of the ACC impacted the ocean circulation in the northwestern Atlantic; the data collected during this study indicate that the ACC influenced ocean circulation in the northwestern Pacific, as well.
All GEOSPHERE articles are available at http://geosphere.
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