June media highlights: Geology and GSA Today

May 21, 2001

BOULDER, Co. - Following are highlights from the June issue of GEOLOGY and a summary of the science article from the June issue of GSA TODAY, published by the Geological Society of America. Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to GEOLOGY or GSA TODAY in stories published. Contact Ann Cairns at GSA for copies of articles and for additional information or assistance.


Rapid Eocene extension in the Robinson district, White Pine County, Nevada: Constraints from 40Ar/39Ar dating. Phillip B. Gans, University of California, Santa Barbara, et al. Pages 475-478.
Rhyolitic volcanic rocks representing at least five eruptive episodes in the Robinson district of east-central Nevada were deposited prior to, synchronous with, and following a period of extreme crustal extension (~400%). Dating of sanidine grains from these rhyolitic units by the 40Ar/39Ar method indicates that normal faulting occurred in <900 k.y. during the eocene, comparable to or shorter than other highly extended areas in western north america. robinson district lies within a large extensional domain east-central nevada that is composite feature produced by localized, episodic, rapid extension began early eocene and continued at least middle miocene time.

Oxygen isotope and trace element zoning in hydrothermal garnets: Windows into large-scale fluid-flow behavior. Douglas E. Crowe, University of Georgia, et al. Pages 479-482.
This paper describes the oxygen isotope zonation measured in hydrothermal garnets from the Dalnegorsk B-skarn deposit. The distribution of oxygen isotopes reveals striking and apparently rapid changes in the fluid source from almost pure magmatic fluid to dominantly meteoric fluids. Trace element analysis shows consistent positive correlations between oxygen isotope composition and both boron and iron, suggesting that these elements were sourced from the intrusion that produced the hydrothermal system. This microanalytical approach allows for the temporal tracking of fluid histories and could be used in many other geologic settings.

New constraints on the origin of the Australian Great Barrier Reef: Results from an international project of deep coring. International Consortium for Great Barrier Reef Drilling. Pages 483-486.
The Australian Great Barrier Reef is the largest living organism on Earth, a distinctive landmark even for astronauts viewing the planet from space. Until recently, scientists had very little idea of the age and origin of this impressive natural feature. An international team of geologists report that the Great Barrier Reef as we know it today is < 1 Ma--a baby by geological standards. In contrast, the coral atolls of Enewetak and Bikini are at least 45 Ma, and the reefs and banks of the Bahamas date from ca. 150-200 Ma--the age of the dinosaurs. The team's work is based on two new, deep boreholes drilled with care into the central Great Barrier Reef system. Results suggest that reef initiation in this region may have been related to the onset of Earth's large-scale interglacial-glacial cycles in global climate and sea level that occurred ca. 600-700 ka.

Forearc-basin sedimentary response to rapid Late Cretaceous batholith emplacement in the Peninsular Ranges of southern and Baja California. David L. Kimbrough, San Diego State University, et al. Pages 491-494.
The emplacement of continental-margin batholiths is an important mechanism of crustal growth. However, the dynamic linkage between the timing and magnitude of magma addition to the middle and lower crust, and upper-crustal responses such as uplift, erosion, and sedimentation, is not well understood. The Cretaceous Peninsular Ranges batholith of southern and Baja California and adjacent forearc basin strata in the Vizcaino Peninsula region of Baja California present excellent examples of the dynamic linkage between large-volume batholith emplacement, arc denudation, and forearc basin sedimentary response. This paper documents the synchronicity of these events and provides insight into the mid- to upper-crustal response to large-volume magma emplacement along continental margins.

Elastic rebound following the Kocaeli earthquake, Turkey, recorded using synthetic aperture radar interferometry. Larry Mayer, University of Arizona, and Zhong Lu, Raytheon STX. Pages 495-498.
Earthquakes result from the catastrophic release of strain, which has accumulated over many millennia, in a matter of seconds. The August 17, 1999, Kocaeli earthquake (Turkey), which occurred on the North Anatolian fault, caused tremendous damage and loss of life, but also provided scientists with an image of the strain that had accompanied the earthquake. Captured by European radar satellites, the deformation was quantified and analyzed by the authors. The deformation associated with the Kocaeli earthquake reflects the strain conditions just prior to the earthquake; therefore, it may provide a robust method to measure strain in real time in order to quantify the behavior of the seismogenic crust and to provide data that may guide earthquake predictions in the future.

Response of the East African climate to orbital forcing during the last interglacial (130-117 ka) and the early last glacial (117-60 ka). Martin H. Trauth, Universitat Potsdam, et al. Pages 499-502.
East African climate is mainly controlled by the position and seasonal migration of the Intertropical Convergence Zone. The zone of maximum rainfall follows the latitudinal position of the overhead sun with a time lag of ~4-6 weeks. Accordingly, over most of East Africa there are two rainy seasons--the "long rains" during April and May and the "short rains" during October and November. Because the historical record used to calibrate climate prediction models is short for East Africa, the authors studied the up to 175 ka deposits of Lake Naivasha (Kenya). The authors' reconstructions show a dramatic alternation between deep, fresh water and shallow, highly alkaline lake conditions in Lake Naivasha during this time. Because the lake is a closed-basin lake, these changes were most likely triggered by changes in climate. The precise radiometric dating of these fluctuations suggests that periods of wetter climate in East Africa mainly follow maximum equatorial solar radiation in spring, causing more intense April-May rains. This happens every 23 k.y. and is known as one of the Milankovitch cycles resulting from changes in Earth's orbital geometry.

Generation of oceanic-island basalt-type volcanism in the western Trans-Mexican volcanic belt by slab rollback, asthenosphere infiltration, and variable flux melting. Luca Ferrari, Universidad Nacional Autónoma de México, et al. Pages 507-510. The occurrence of lavas with chemical composition unrelated to subduction in the western Mexican Volcanic Belt has been explained with the presence of a mantle plume beneath this continental volcanic arc since 10 Ma or by flowing of material from the back-arc region and its subsequent decompression meltings. According to new geologic and geochemical data and plate tectonics reconstructions we propose instead that asthenosphere material may have flowed laterally into the mantle wedge from the Gulf of California area during a period of very low convergence between the Rivera and North America plate (between 7.2 and 4.7 Ma) when the former plate start to sink into the mantle.

Crustal growth by magmatic underplating: Isotopic evidence from the northern Sherman batholith. C.D. Frost, University of Wyoming, et al. Pages 515-518.
Today most additions to the continents take place along plate margins above subduction zones, where mantle-derived magmas are emplaced into the crust. One additional, possible mechanism of crustal growth is underplating and intrusion of mantle-derived magmas in continental interiors during extension. By its very nature, this mechanism is less easily identified than additions that take place along continental margins. In this study the authors suggest that distinctive iron-rich, potassic granites found in continental interiors may represent a mid- to upper-crustal record of magmatic underplating at depth. Nd, Sr, and Pb isotopic characteristics of one such granite batholith, the northern Sherman batholith of southeastern Wyoming, require that the granite magmas originated by melting of the upper mantle or relatively young mafic magma emplaced at the base of a continent.

Isotopic evidence for cooler and drier conditions in the tropical Andes during the last glacial stage. Germán Mora, Johns Hopkins University, and Lisa M. Pratt, Indiana University, Bloomington. Pages 519-522.
Relatively little is known about the climate of tropical regions during the last glaciation. One method of determining environmental conditions of the past is measuring the relative abundance of stable isotopes of hydrogen and oxygen contained in soil minerals. Soils near the city of Bogota (tropical Andes of Colombia) contain kaolinite, a common clay mineral found in tropical soils. Compared to the kaolinite formed in modern Colombian soils, the kaolinite separated from soils formed between 15 and 30 ka near the city of Bogota shows an enrichment of the heavier isotopes (deuterium and oxygen-18). This isotopic enrichment suggests a mean annual temperature of ~7.5 °C, which is 6 °C less than the mean annual temperature of 13.5 °C for the modern city of Bogota. Moreover, the kaolinite bears an isotopic signature similar to that of water evaporating from the Caribbean Sea, suggesting not only a change in the source of moisture, but also drier conditions in northern South America during the last glaciation. These results indicate that the climate of the Colombian Andes was significantly cooler and drier during the last glaciation when compared to modern weather patterns.

Geochronology and calibration of global Milankovitch cyclicity at the Cenomanian-Turonian boundary. Andreas Prokoph, University of Ottawa, et al. Pages 523-526.
The authors discuss the global preservation of orbital-forced cyclicity and marine paleo-environmental changes during the Mesozoic based on high-resolution data. Improved Ar-Ar-dating of volcanic ash layers resulted in a new Cenomanian-Turonian Boundary age of ca. 96.4 Ma. The authors present evidence for climatic cyclicity of ~20-100 k.y. periodicity in various sedimentary basins across the paleo-North Atlantic, occurring synchronously over ~320 k.y. with the global oceanic anoxic event. The authors' new Ar-Ar dating suggests that marine mass extinction and global oceanic anoxia occurring around the Cenomanian-Turonian Boundary could not have been caused by oceanic plateau basalt volcanism, as was previously assumed, because of a significant age difference.

Continental breakup in magmatic provinces: An Ethiopian example. C.J. Ebinger, Royal Holloway College, and M. Casey, University of Leeds, Pages 527-530.
The relative importance of faulting and magmatic processes during the breakup of continents can be examined in the Ethiopian rift, Africa, where the style of faulting and magmatism has undergone a dramatic change in the past 1.6 m.y. Most of the strain across the ~100-km-wide rift is now accommodated across ~20-km-wide and ~60-km-long magmatic segments within the central rift valley. The dimensions, faulting styles, and aligned magmatic centers within the magmatic segments are very similar to those of slow-spreading mid-ocean ridges. This comparison suggests that magmatic segments, rather than the original rift-bounding faults, mark the ocean-continent boundary in rifts with a ready magma supply. The authors' model predicts strips of mafic crust transitional to oceanic crust, but without coherent seafloor-spreading magnetic anomalies.

Geochemical evolution of arc magmatism during arc-continent collision, South Mayo, Ireland. Amy E. Draut and Peter D. Clift, Woods Hole Oceanographic Institution. Pages 543-546.
This paper documents the chemical changes in the composition of volcanic material that erupted during the collision of the Lough Nafooey island arc with the continent of Laurentia ca. 470 Ma. This collision, which resulted in a mountain-building event known as the Grampian Orogeny in the British Isles and the Taconic Orogeny in North America, was studied using rock samples collected in the South Mayo area of western Ireland. The results of this study show that the volcanic rocks erupted before and during this collision event gradually evolved to contain more material derived from the continent with which the island arc collided. Some of the volcanic rocks analyzed were found to contain higher concentrations of certain trace elements than are present in the rock material that composes continental crust. The authors' results indicate that this type of arc-continent collision may be an important step in generating continental crust.

Knickzone propagation in the Black Hills and northern High Plains: A different perspective on the late Cenozoic exhumation of the Laramide Rocky Mountains. Brent J. Zaprowski, Lehigh University, et al. Pages 547-550.
Traditionally, geologists have used uplift to explain the increase in landscape incision that has occurred during the late Cenozoic in the western United States. Within the past decade, a group of geologists has suggested that the increased incision is the result of changes between glacial and interglacial climates during the late Cenozoic. The authors' research suggests that knickpoint propagation is a third mechanism that can contribute to landscape incision. Research conducted in the Black Hills of South Dakota demonstrates that migrating knickpoints are responsible for the formation of terraces, and hence, the incision of the landscape. If this relationship is true for the northern Rocky Mountains in general, knickpoint migration must be considered as an alternative hypothesis to explain late Cenozoic landscape incision of the Laramide Ranges.

Isotope fractionation by sulfate-reducing natural populations and the isotopic composition of sulfide in marine sediments. Kirsten S. Habicht, NASA Ames Research Center, and Donald E. Canfield, Danish Center for Earth System Science. Pages 555-558.
The sulfur isotope composition of reduced sulfur compounds in marine sediments is an important tool for understanding how the biogeochemical sulfur cycle works today and how it may have evolved over geological time. Sedimentary sulfides are generally depleted in 34S compared to seawater sulfate. The microbial sulfate reduction is the best know reaction leading to a depletion in 34S of sedimentary sulfides. We have compared the sulfur isotope fractionation during sulfate reduction by natural populations of sulfate-reducing bacteria in a wide range of marine sediments to the isotopic composition of solid sulfide preserved in the same sediment. We demonstrate that even in their natural environment sulfate reducers cannot provide sufficient fractionation to explain the isotopic composition of sedimentary sulfides. With a simple model, we show that reactions in the oxidative part of the sulfur cycle are active in enhancing the fractionations to various degrees in the different sediments.

Exhumation of the west-central Alborz Mountains, Iran, Caspian subsidence, and collision-related tectonics. Gary J. Axen, University of California, Los Angeles, et al. Pages 559-562.
This paper provides new insights into the timing and tectonic style of mountain building in northern Iran. The pluton ages and cooling histories presented are compared to available south Caspian sedimentation rate data and to regional tectonics.


A moderate translation alternative to the Baja British Columbia hypothesis. Robert Butler, University of Arizona, et al.
This paper continues a debate on the Baja British Columbia (Baja B.C.) controversy, a debate that has also been explored in past GSA TODAY articles (Cowan, 1994; Hollister and Andronicos, 1997). The Baja B.C. hypothesis states that western portions of British Columbia were previously attached to the southwestern margin of North America at about the latitude of present-day Baja California. Between 90 million and 50 million years ago, these terranes moved to their present positions by sliding north, up the coast, along strike-slip shear zones. Most workers agree that substantial motion of these terranes is required, but there has been intense debate on the distance of northerly transport. Some paleomagnetic data have been interpreted to suggest up to 4000 km of motion. This paper provides an alternative interpretation of the paleomagnetic observations. Based on both geologic and paleomagnetic data, Butler et al. propose that western British Columbia was translated on the order of 1000 km, a "moderate translation" compared to Baja B.C., but still an impressive distance of transport. This interpretation is compatible with paleomagnetic data if tilting of crustal blocks (often unrecognized) and shallowing of inclination during compaction of sedimentary rocks are invoked to help to explain observed shallow paleomagnetic inclinations in some rocks. The authors argue that this model is more consistent with the entire geologic and paleomagnetic record and that the resulting Cretaceous paleogeography of western North America features a continuous Andes-like magmatic arc.
To view abstracts and the complete table of contents of GEOLOGY, as well as that of the GEOLOGICAL SOCIETY OF AMERICA BULLETIN, see http://www.gsajournals.org. To obtain full text of these articles and articles from back issues, contact Ann Cairns, acairns@geosociety.org.

Geological Society of America

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