Boulder, Colo., USA - Granitic rocks make up much of Earth's continental crust and many of the planet's most iconic landscapes. However, granite's formation is poorly understood because it happens tens of kilometers below the surface. In this unique study, authors Roger Putnam and colleagues combine decimeter-scale field mapping, rock climbing, and new dating and geochemical analyses to evaluate the timing and intrusive dynamics of the granitic rocks that make up El Capitan in Yosemite National Park, California, USA.
The comparatively accessible southeast face of El Capitan provides a clean, ~1-km-tall exposure of the interior of a granitic system. Putnam and colleagues found this vertical landscape to be a perfect place to test hypotheses regarding the formation of granitic rocks. In their paper published in Geosphere on 1 July 2015, the authors use climbing route designations as landmarks in describing the geology, along with both official and unofficial (e.g., North America; The Alcove) local place names.
They write that many models of granite formation rely on processes such as crystal/liquid segregation that should present a signature visible in the vertical dimension of a granitic system. They found that El Capitan is made up of seven different granitic units that episodically intruded over about three million years. Their chemical and textural analyses of samples collected along vertical transects of the two dominant rocks there, the El Capitan and Taft Granites, reveal no systematic patterns in rock composition. In fact, they conclude, "These data reveal [3 million years of] assembly of the plutonic system and show no evidence for gravity-driven separation of crystals and liquid over the 1 km vertical extent of the cliff," which, they write, is "hard to reconcile with models of granite formation that envision magma chambers as large, mostly liquid, fractionating bodies."
Plutonism in three dimensions: Field and geochemical relations on the southeast face of El Capitan, Yosemite National Park, California
Roger Putnam et al., University of North Carolina, Chapel Hill, North Carolina, USA. Published online on 1 July 2015; http://dx.
Other GEOSPHERE articles (see below) cover such topics as
- 1. Tsunami generation in the Alaska-Aleutian arc (open-access paper);
2. The southern California Inner Continental Borderland; and
3. The Franciscan Complex of Sonoma and Marin counties, California, USA.
Tsunami recurrence in the eastern Alaska-Aleutian arc: A Holocene stratigraphic record from Chirikof Island, Alaska
Alan R. Nelson et al., Geologic Hazards Science Center, USGS, Golden, Colorado, USA. Published online on 1 July 2015; http://dx.
As demonstrated by recent subduction-zone earthquakes of unexpected magnitude accompanied by tsunamis of unanticipated height in Sumatra and Japan, the frequency of Earth's largest earthquakes and tsunamis is difficult to estimate because such events occur hundreds to thousands of years apart. One of the shortest and most incomplete histories of megathrust earthquakes and tsunamis comes from one of the longest and most seismically active subduction zones: the Alaska-Aleutian arc, extending 1600 km from southeast Alaska westward to Kamchatka. Based on its history of magnitude-8-to-9 earthquakes in the mid-20th century, prior statistical analyses showed that the Alaska-Aleutian arc poses the greatest tsunami hazard to much of the coast of California. On the southwest coast of Chirikof Island, the closest land to the megathrust in the eastern Alaska-Aleutian arc, as many as 23 or as few as 13 sandy beds in boggy lowland peat were deposited by tsunamis. Stratigraphy, sedimentology, diatoms, and radiocarbon and optically stimulated luminescence analyses from cores at two sites -- combined with unpublished island historical data -- show the sandy deposits date from the past 13,000 years into the 20th century. Because this is the first long record of prehistoric Aleutian tsunamis, geoscientists cannot estimate source earthquake locations or magnitude for most of the tsunami-deposited beds. Nor can they determine what proportion of beds were deposited by tsunamis caused by faulting in Earth's shallow upper crust or submarine landslides near Chirikof Island that produced minimal tsunami inundation far from the island. In this open-access article for Geopshere, Alan R. Nelson and colleagues argue that the proportion of non-megathrust-related tsunamis is probably small and that this part of the arc sent high tsunamis southward every 180-270 years for at least the past 3,500 years.
Late Miocene-Quaternary fault evolution and interaction in the southern California Inner Continental Borderland
Christopher C. Sorlien et al., Earth Research Institute, University of California, Santa Barbara, California, USA. Published online on 24 June 2015; http://dx.
The right-lateral motion between the Pacific and North American plates is distributed across a broad area in southern California. This broad plate boundary includes the San Andreas fault proper, and extends westward across southern California, and the 200 km-wide offshore area of stretched continental crust known as the California Continental Borderland. Careful analysis of dense grids of seismic reflection profiles from sub-surface acoustic imaging reveals the 3D geometry of the offshore fault system between San Clemente and San Diego. The eastern limit of the study includes the steep Newport-Inglewood-Rose Canyon strike-slip fault, responsible for the deadly 1933 Long Beach earthquake. This study by Christopher C. Sorlien and colleagues indicates that the Newport-Inglewood fault is physically linked with faults that lie as far as 20 km offshore, both in map view and cross section. These faults that link in map view have a geometry that causes an extensional component of deformation during much or all of the last five-million years from the Mexican Border 50 km northward. It also highlights the history of deformation on large flatter, more gently-dipping faults that underlie and interact with the above fault system. These include the Oceanside detachment and the deeper Thirtymile detachment faults. Previous studies have suggested that these are large, active thrust faults, with serious implications for associated seismic and tsunami hazards to coastal southern California. Instead, this study shows that the upper part of the Oceanside detachment has been inactive since about four million years ago, and, much of the Thirtymile Bank detachment is not a thrust fault. Overall, the predominant motion on moderately to steeply-dipping NNW-SSE-oriented faults has been right-lateral strike-slip for at least the last two million years. Evidence for thrust faulting is not seen beneath the ocean within 30 km of San Diego.
Sandstone-matrix mélanges, architectural subdivision, and geologic history of accretionary complexes: A sedimentological and structural perspective from the Franciscan Complex of Sonoma and Marin counties, California, USA
Loren A. Raymond and David A. Bero, Coast Range Geologic Mapping Institute, Santa Rosa, California 95405, USA. Published online on 1 July 2015; http://dx.
The plates making up the outer Earth collide to yield mountains. Between 20 million years ago and 125 million years ago, collision of an ocean plate with western North America set in motion the events that produced the California Coast Ranges. Rocks were broken and mixed along huge faults to form tectonic mélanges, a sort of plastic pudding of hard rocks in a softer rock matrix. Deeply buried and metamorphosed rocks were uplifted from the fault zones, fragmented by erosion, and carried to the ocean by landslides to form sedimentary mélanges, similar to those formed by faulting. These mélanges formed parts of submarine deposits of sand, gravel, and mud that made up seafloor submarine fan deposits. Detailed mapping of the Franciscan Complex with submarine fan deposits containing sedimentary mélanges have been found in western Marin and Sonoma counties and these and associated rocks reveal some details of the mountain forming processes that created the Coast Ranges of the San Francisco Bay Region. Since the Franciscan Complex is referred to as a prime example of this type of mountain building, the discoveries and the method of mapping reported here are important to understanding all similar mountain belts in the world.
Stratigraphy and structural development of the southwest Isla Tiburón marine basin: Implications for latest Miocene tectonic opening and flooding of the northern Gulf of California
Scott E.K. Bennett et al., University of California, Davis, California, USA; and U.S. Geological Survey, Geologic Hazards Science Center, Denver, Colorado, USA. Published online on 1 July 2015; http://dx.
The geologic history of the youthful Gulf of California rift illustrates in detail the processes of continental breakup and formation of a new ocean basin. Northwestward wrenching of Baja California away from mainland Mexico obliquely opened the Gulf of California seaway about six to eight million years ago. However, there is controversial evidence for an older marine incursion event about 13 million years ago that predates much of the known faulting related to the Gulf of California rift. Understanding of this early marine incursion is essential for evaluating the mechanisms and degree of crustal thinning that led to continental breakup. For over 40 years, geologists contended that marine rocks exposed on Isla Tiburón, the largest of the Midriff Islands of the central Gulf of California, were related to this early (proto-Gulf of California) seaway. Scott Bennett and colleagues use new, detailed geologic mapping and dating of rocks, integrated with existing microfossil age data, to show that marine deposition occurred on Isla Tiburón only ~6.2 million years ago, consistent with widespread evidence for opening of the northern Gulf of California at this time. These revised timing constraints support a regional-scale marine flooding from Isla Tiburón to southern California and into the lower Colorado River corridor during latest Miocene time, the final phase of a punctuated south-to-north flooding of the sea into the narrow, fault-controlled core of the greater-than 1000-km-long Gulf of California rift.
Controls on submarine canyon activity during sea-level highstands: The Biobio canyon system offshore Chile
Anne Bernhardt et al., Universitat Potsdam, Potsdam, Germany. Published online on 15 July 2015; http://dx.
Submarine canyons are the most important transport conduits for terrestrial sediments, including associated pollutants, nutrients, and organic carbon, from the continents to the ocean floor. When sea level is high, as it was during the past 10,000 years, terrestrial sediment transported by rivers is often stored on the continental shelf close to the coast. Thus, the sediment does not reach the submarine canyon heads and cannot be transported to the deep ocean. Offshore of the city of Concepción in south-central Chile, the submarine Biobío Canyon and its tributary canyon were analyzed using sediment cores and rarely available, high-resolution bathymetry data, in which objects as small as a few meters can be imaged on the seafloor in a few hundred meters of water depth. The aim of this study is to evaluate whether the Chilean canyon system has been transporting sediment to the ocean floor during the past millenia of rising and high sea level. In comparison with global examples, the Biobío submarine canyon served as a moderately active conduit during the past 5,000 years, but its tributary system has been inactive. The factors that control canyon activity and sediment export to the deep sea during high sea level have been identified. These include (1) currents on the seafloor that transport sediment into the canyon; (2) tectonic deformation of the seafloor that controls the location and the orientation of a submarine canyon with respect to terrestrial sediment sources and promotes failure of sediment into the canyon; (3) the gradient of the seafloor offshore coast; and (4) the location of river systems on the continent. These controlling factors can be used for predictions of submarine canyon activity worldwide, where no high-resolution data are available.
Paleodischarge of the Mojave River, southwestern United States, investigated with single-pebble measurements of 10Be
Andrew J. Cyr et al., USGS, Menlo Park, California, USA. Published online on 15 July 2015; http://dx.
From the abstract: The paleohydrology of ephemeral stream systems is an important constraint on paleoclimatic conditions in arid environments but remains difficult to measure quantitatively. For example, sedimentary records of the size and extent of pluvial lakes in the Mojave Desert (southwestern USA) have been used as a proxy for Quaternary climate variability. Although the delivery mechanisms of this additional water are still being debated, it is generally agreed that the discharge of the Mojave River, which supplied water for several Pleistocene pluvial lakes along its course, must have been significantly greater during lake highstands ...
Revealing the hidden Milankovitch record from Pennsylvanian cyclothem successions and implications regarding late Paleozoic chronology and terrestrial-carbon (coal) storage
Frank J.G. van den Belt et al., University of Utrecht, P.O. Box 80021, 3508 TA Utrecht, Netherlands. Published online ahead of print on 1 July 2015; http://dx.
From the abstract: The widely held view that Pennsylvanian cyclothems formed in response to Milankovitch-controlled, glacio-eustatic, sea-level oscillations lacks unambiguous quantitative support and is challenged by models that are based on climate-controlled precipitation-driven changes in depositional style. This study shows that cyclothem successions do in fact contain a clear record of Milankovitch-controlled oscillating sea level, but that it is prerequisite that besides cyclothem thickness, cyclothem composition is taken into account.
All GEOSPHERE articles are available at http://geosphere.