||29 June 2007
GSA Release No. 07-32
Boulder, CO, USA - Topics include: first images of an active oceanic detachment fault; new theory of Transantarctic Mountains formation; why western Siberian rivers flow into the Arctic Ocean via estuaries rather than coastal deltas; and cause of the large earthquake and tsunami that destroyed coastal cities of sixth-century Phoenicia (modern-day Lebanon). The GSA TODAY science article presents new evidence of an advanced civilization in Alexandria, Egypt, at least seven centuries prior to the arrival of Alexander the Great.
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- Aegean Sea as driver of hydrographic and ecological changes in the eastern Mediterranean
Gianluca Marino et al., National Research Council, Institute for Coastal Marine Environment, Porto di Napoli, Calata Porta di Massa, Naples, 80133; Italy. Pages 675-678.
- The process of deep-water formation involves the sinking of aerated surface waters down to the bottom of the ocean. This process replenishes the deep ocean with oxygen, which is otherwise consumed by decomposition of organic material raining down from the surface waters where primary productivity occurs. However, releases of freshwater to the ocean reduce the sea surface density, which in turn suppresses new deep-water formation. A failure of deep-water formation eventually leads to oxygen depletion at the seafloor, eliminating benthic life and promoting the accumulation of organic carbon in the sediments. In the eastern Mediterranean sedimentary archive, the (quasi) periodic occurrence of dark, organic-rich layers (so-called sapropels) bears witnesses to past failures in the deep-water formation process. Marino et al. investigated a sapropel layer deposited between 119,000and 124,000 years ago in the Aegean Sea, an important region for eastern Mediterranean deep-water formation. They found that injections of large volumes of freshwater into the eastern Mediterranean led (within about 40 years) to a collapse of Aegean deep-water formation, causing a termination of benthic life and an onset of high rates of organic carbon accumulation at the seafloor. Within about 650 years, the oxygen starvation had expanded all the way up through the water column toward the surface layer where light penetrates (the so-called photic zone), which they recognized by the occurrence of organic compounds synthesized by green sulphur bacteria that require both sulphide and light. Marino et al.'s study provides new insight into the great sensitivity of the Aegean Sea deep-water overturn system to substantial perturbations in the basin's freshwater cycle, and highlight the associated profound ecological implications.
- Dissolution of biogenic ooze over basement edifices in the equatorial Pacific with implications for hydrothermal ventilation of the oceanic crust
Barbara A. Bekins et al., U.S. Geological Survey, Water Resources Division, 345 Middlefield Road, Mailstop 496, Menlo Park, CA 94025, USA. Pages 679-682.
- Hydrothermal circulation in the oceanic basement has widespread effects on seafloor heat flow, ocean chemistry, and geologic carbon cycling. The occurrence of vigorous water exchange between the ocean and igneous crust depends on whether the high-permeability basaltic basement is hydrologically connected to the seafloor or blocked by low-permeability sediments. Typically, basement edifices that outcrop at the seafloor form the discharge sites for circulating fluids. The recent observation of curious closed depressions in carbonate sediments overlying basement edifices suggests a process that prolongs seafloor exposure of hydrothermally active basement in thickly sedimented areas. Formation of these depressions may involve dissolution by discharging basement fluids that have cooled to bottom-water temperatures and have become undersaturated with carbonate. Bekins et al. provide a model of carbonate solubility to estimate the discharging fluid flux required to keep pace with typical equatorial Pacific carbonate mass accumulation rates. The resulting values are comparable to other published basement fluxes. Recent data from other researchers showing widespread occurrence of these dissolution features in the eastern equatorial Pacific help explain why there are so many indications of vigorous basement ventilation in this region, despite the thick sediment cover that would normally block basement outcrops, including anomalously low regional heat flux, and aerobic and nitrate-reducing microbial activity at the base of the sediments.
- Plateau collapse model for the Transantarctic Mountains–West Antarctic Rift System: Insights from numerical experiments
Robert W. Bialas et al., Lamont Doherty Earth Observatory of Columbia University, Earth and Environmental Science, PO Box 1000, 61 Route 9W, Palisades, New York 10964, USA. Pages 687-690.
- The Transantarctic Mountains are the highest and longest rift-related mountain belt on Earth and divide the Antarctic continent into its eastern and western halves. Despite their prominence, no model has been able to adequately explain their formation or their juxtaposition with the adjacent West Antarctic Rift System, a broad region of thin, extended continental crust exhibiting wide rift characteristics. Bialas et al. explore the possibility that the Transantarctics represent a remnant edge of a high-elevation plateau that has since rifted and subsided to create the West Antarctic Rift System. This concept revolutionizes the thinking about the Transantarctic Mountains. Previous models discuss ways to make the mountains go up; Bialas et al. propose a scenario where the Transantantarctic Mountains were already high, and the adjacent rift system goes down. They use numerical models and geological and geophysical data to back this new theory of Transantarctic Mountain development.
- Late Oligocene initiation of the Antarctic Circumpolar Current: Evidence from the South Pacific
Mitchell Lyle et al., Texas A&M University, Department of Oceanography, 3146 TAMU, College Station, TX 77843-3146, USA. Pages 691-694.
- The Earth has gone through many major cooling steps during the past 65 million years of the Cenozoic Era. Of these, one of the most prominent occurred at the Eocene-Oligocene boundary (33.7 million years ago) when large permanent ice sheets first appeared on the Antarctic continent. One major hypothesis for Antarctic cooling proposes that it was caused by the formation of the Antarctic Circumpolar Current (ACC) after Southern Ocean gateways had opened. The hypothesis states that the ACC started after the northward movement of South America and Australia opened a deep water path around Antarctica for this major ocean current. According to this hypothesis, the formation of the ACC thermally isolated Antarctica and caused its glaciation. Lyle et al. show that the ACC did not form until about 8 million years after the Antarctic ice cap formed. Thus, the ACC formation was not the driver of Antarctic cooling. Instead, a decrease in atmospheric greenhouse gases, an alternate hypothesis, may have cooled Antarctica to cause its glaciation.
- Plutonic xenoliths reveal the timing of magma evolution at Hualalai and Mauna Kea, Hawaii
J.A. Vazquez et al., California State University-Northridge, Department of Geological Sciences, 18111 Nordhoff St., Los Angeles, CA 91330-8266, USA. Pages 695-698.
- Vazquez et al.'s high-resolution ion microprobe dating of zircons from basalt-hosted plutonic xenoliths provides new insight into the timing and paths of magma evolution in Hawaiian volcanoes. The ages of diorite xenoliths and the compositions of zircon-hosted melt inclusions from Hualalalai volcano reveal episodes of extreme differentiation that are only partly reflected in the volcanic stratigraphy, suggesting deep storage and fractionation of shield and post-shield basalts. In contrast, diorite xenoliths from Mauna Kea volcano formed contemporaneously with eruptions of differentiated post-shield lavas. The results indicate that the timing of magma differentiation and the assembly of plutonic complexes at Hawaiian volcanoes is variable and may be decoupled from volcanic activity.
- Multiple early Eocene hyperthermals: Their sedimentary expression on the New Zealand continental margin and in the deep sea
Micah J. Nicolo et al., Rice University, Earth Science, MS-126, 6100 Main St., Houston, TX 77005, USA. Pages 699-702.
- Over a seven-million-year interval of the early Paleogene (ca. 58-51 million years ago), Earth underwent a gradual warming trend. At the Paleocene/Eocene boundary (ca. 55.5 million years ago), in the midst of slowly increasing global temperatures, a phenomenally rapid carbon cycle perturbation similar to what is being anthropogenically driven today, and an associated transient extreme warming (or "hyperthermal") event , known as the Paleocene-Eocene Thermal Maximum (PETM), severely impacted many global systems. Nicolo et al. studied sediments deposited on the New Zealand continental margin and at various deep-sea locations and suggest that the PETM was not a unique event, but rather the most pronounced of a series of similar events that punctuated the early Paleogene warming trend. These results imply that any appropriate explanation for what caused the PETM must also be able to explain multiple similar events during the early Paleogene.
- Queen Maud block: A newly recognized Paleoproterozoic (2.4-2.5 Ga) terrane in northwest Laurentia
Michael E.J. Schultz et al., University of Alberta, Department of Earth and Atmospheric Sciences, 1-26 Earth Sciences Building, Edmonton, Alberta T6G 2E3, Canada. Pages 707-710.
- Schultz et al. describe a large, remote, and understudied bedrock terrane in Arctic Canada. The sedimentary basins found within the terrane, and the age and timing of high-temperature metamorphism of the rocks found there, challenge previous models and offer new insight into how the different continental fragments of North America assembled billions of years ago. Specifically, Shultz et al. contend that the development of this area likely occurred 500 million years earlier than previously thought and involved the incipient break-up and reassembly of an ancient continent.
- Kinematics and geometry of active detachment faulting beneath the Trans-Atlantic Geotraverse (TAG) hydrothermal field on the Mid-Atlantic Ridge
Brian J. deMartin et al., Brown University, Geological Sciences, 324 Brook Street, Box 1846, Providence, RI 02912, USA. Pages 711-714.
- The Trans-Atlantic Geotraverse (TAG) hydrothermal field on the Mid-Atlantic Ridge is one of the largest, longest-lived regions of focused, high-temperature fluid flow on the seafloor. Encompassing an area of roughly six square miles, the hydrothermal field includes relict mounds containing vast stores of precious metals and sulfide ores, and at least one active mound that vents fluids in excess of 360 °C (680 °F) and hosts a flourishing deep-sea ecosystem. The TAG hydrothermal field lies above an active oceanic detachment fault, a long-lived zone of weakness in Earth's crust where extension has been focused for the past 175,000-350,000 years. Although numerous inactive detachment faults have been identified on the seafloor, the subsurface morphology of an active detachment fault and the relationship between detachment faulting and fluid flow has remained speculative. deMartin et al. report results from an eight-month ocean bottom seismometer experiment that provide important new perspectives on the nature of fluid circulation at the TAG hydrothermal field and show a detailed image of the subsurface structure of an active oceanic detachment fault for the first time. Their results also indicate that entrained seawater extracts the energy necessary for hydrothermal circulation near the crust/mantle interface at least 7 kilometers (4.3 miles) below the seafloor. This finding dramatically effects the geologic understanding of hydrothermal flow, because it has been widely supposed that hydrothermal convection within the seafloor is driven by much shallower, crustal heat sources. These results suggest high-temperature hydrothermal circulation may be a natural consequence of the detachment faulting process, and may indicate that significant numbers of hydrothermal fields await discovery along the slowly diverging plate boundaries in the Atlantic, Indian, and Arctic oceans.
- First field evidence of southward ductile flow of Asian crust beneath southern Tibet
Jess King et al., Open University, Department of Earth Sciences, Walton Hall, Milton Keynes, Buckinghamshire MK7 6AA, UK. Pages 717-720.
- Major mountain ranges like the Himalaya have thick crustal roots that heat up rapidly due to radioactive decay. The India-Asia collision, which started about 55 million years ago, would have caused rocks in the deep crust beneath the Himalaya to begin to melt about 20 million years later. Theoretical models suggest that these small amounts of magma dramatically weakened a deep layer of rock so that it could flow as a narrow channel, oozing out toward the surface where the summer monsoon vigorously eroded the mountain front. King et al. present the first geological evidence for a southward flow of partially molten deep crust from Asia, north of the line of collision, to beneath Indian crust during the Miocene. Vertical sheets of magma, called dikes, emplaced into Indian crust south of the collisional suture 12-9 million years ago have a geochemical and isotopic fingerprint indicating a source in the Asian plate to the north. The partially molten Asian source material for the dikes must therefore have been involved in a southward flow at depth, so that the dikes could be intruded vertically into Indian crust south of the line of collision. The peculiar geochemistry of the dike magmas also implies that the Indian crustal slab sliding under Asia around 10 million years ago was diving down into the mantle at a very steep angle, rather than almost horizontally.
- Coastal dunes in Westland, New Zealand, provide a record of paleoseismic activity on the Alpine fault
Andrew Wells and James Goff, Geological and Nuclear Sciences, Geohazard Solutions, 2 Mclennan Road, Hawea Flat, Otago 7979, New Zealand. Pages 731-734.
- Wells and Goff studied coastal dune sequences in southwestern New Zealand that have built up over the past several thousand years. For at least the past 800 years, new dune ridges have formed in the few decades immediately following large earthquakes on the Alpine fault. This is a response to huge volumes of sediment being brought down to the coast, released from the mountainous catchments through landsliding during the earthquakes. The dunes may provide a valuable tool for identifying and dating prehistoric ruptures on the Alpine fault over the past few thousand years. This direct response of coastal plains to earthquakes may be unique to this part of New Zealand, but it is likely that tectonic activity has left some sort of record in other coastal regions too.
- Extensive thin sequences spanning Cretaceous foredeep suggest high-frequency eustatic control: Late Cenomanian, Western Canada foreland basin
A. Guy Plint and Michael A. Kreitner, University of Western Ontario, Earth Sciences, London, Ontario N6A 5B7, Canada. Pages 735-738.
- Much controversy surrounds the mechanism(s) responsible for meter-scale relative sea-level oscillations in the Cretaceous, which have a frequency of much less than one million years. Various lines of evidence are converging to suggest that a small ice cap existed on the Antarctic craton during the Cretaceous 'greenhouse' period. Plint and Kreitner show that meter-scale deepening and shallowing successions can be traced for hundreds of kilometers across the Western Canada foreland basin, and include discrete high- and low-stand deposits. The extent, frequency, and geometry of these sequences strongly suggest an eustatic control, supporting the Antarctic ice cap hypothesis.
- Pacing the post-Last Glacial Maximum demise of the Animas Valley glacier and the San Juan Mountain ice cap, Colorado
Zackry S. Guido et al., University of Colorado, Geology, INSTAAR, 1560 30th Street, 450 UCB, Boulder, CO 80309, USA. Pages 739-742.
- The lengths of alpine glaciers quickly adjust to changes in temperature and precipitation, making them sensitive indicators of climate. Recording changes in length of current glaciers is a matter of visual observation. However, for glaciers that existed in the past, documenting the changes in length often requires rare geological and glaciological circumstances. The Animas River drainage of Colorado's San Juan Mountains allows documentation of the demise of a large alpine valley glacier from its Last Glacial Maximum extent. Approximately 20,000 years before present (B.P.), the Animas Valley glacier lobe of the San Juan Ice cap extended 91 kilometers from the ice divide, and pervasively polished crystalline bedrock. To document the retreat history of this glacier, Guido et al. measured the concentration of cosmogenically produced 10Be in polished bedrock to deduce the duration of exposure to cosmic rays since the glacier receded past each of eight locations. This yielded a record of retreat that began at approximately 19,500 years B.P and ended at roughly 12,500 years B.P., when the San Juan Mountains became largely devoid of ice. This history implies that the demise of the Animas Valley glacier was protracted, coincided with a gradual rise in solar radiation, and was perhaps fastest during a time of regional drying recorded in shoreline elevations of lakes in western North America.
- Tectonic uplift, threshold hillslopes, and denudation rates in a developing mountain range
Steven A. Binnie et al., University of Edinburgh, Institute of Geography, Drummond Street, Edinburgh, Midlothian EH8 9XP, UK. Pages 743-746.
- Mountains form by competing processes of tectonic uplift and erosion, whereby rivers cut valleys and transport the eroded sediments. One theory suggests that when mountainous slopes reach a critical steepness angle, their evolution will occur predominantly through landsliding. This critical, or 'threshold,' angle will exist where the stream erosion in the bottom of valleys keeps pace with the rate of tectonic uplift, and has important implications for geologic understanding of how mountains are formed and how they respond to changing climatic and tectonic conditions. However, this theory has been little studied, and the operation of 'threshold hillslopes' has not been not adequately shown to exist in nature. Binnie et al. test whether such a process can be observed in the San Bernardino Mountains, California, by comparing erosion rates and hillslope gradients. Their data show that increases in gradients are matched by increases of erosion rates, until slope angles reach 30 degrees. Above this angle, increasing erosion rates do not correspond to increasing hillslope gradients, suggesting they cannot steepen further, and thus marking the emergence of slopes at their threshold angle. Binnie et al. use these results to illustrate how mountainous topography evolves over time in response to tectonic uplift.
- Glacial isostatic adjustment as a control on coastal processes: An example from the Siberian Arctic
Pippa L. Whitehouse et al., University of Missouri, Columbia, Department of Geological Sciences, 101 Geological Sciences Building, Columbia, MO 65211-1380, USA. Pages 747-750.
- The vast majority of the world's major rivers terminate at extensive coastal deltas. The rivers of western Siberia, however, do not. The Ob' and Yenisei rivers, most notably, flow into the Arctic Ocean via estuaries several hundreds of kilometers long. Whitehouse et al. investigated the reason for this departure from the rule. During the last glaciation, the land in central Scandinavia was depressed beneath the weight of the Fennoscandian ice sheet, and consequently the surrounding land, such as that in western Siberia, was elevated above its present height. Since the ice sheets melted, the Earth has been slowly returning to its original shape, and continues to do so in the present day. Whitehouse et al. demonstrate via geophysical modeling that the present-day subsidence of the land in western Siberia is still sufficiently rapid to cause apparent sea-level rise at rates that prohibit the development of coastal features such as river deltas.
- Vapor segregation and loss in basaltic melts
Marie Edmonds and Terrence M. Gerlach, University of East Anglia, School of Environmental Sciences, Norwich, NR4 7TJ, UK. Pages 751-754.
- Volcanic eruptive activity is strongly dependent on the style of magma degassing. Remote spectroscopic measurements of volcanic gas composition at Kilauea volcano, in Hawaii, by Edmonds and Gerlach have revealed aspects of shallow volcanic degassing processes. Bubbles in magma change in composition as they ascend a volcanic conduit, owing to decompression, changes in magma and vapor composition, and the degree of vapor segregation and loss from the host melt (open- or closed-system degassing). Open-path Fourier transform infrared spectroscopy measurements of the volcanic gases, using incandescent lava vents as infrared sources, have revealed changes in volcanic gas composition over time that correspond to distinct degassing regimes. Gas piston events are associated with the emission of CO2-rich gases (up to 70 mol%) and are caused by the accumulation of vapor at depths of 400-900 meters beneath Pu'u 'Ō'ō. Lava spattering is associated with the bursting of H2O-rich bubbles, which are formed by open-system degassing in the shallow (<150 meters) conduit. Large bubbles ascend through the melt, harvesting smaller bubbles and accelerating, bursting at the surface, causing spatter. Static gas accumulation and dynamic bubble coalescence are both manifestations of vapor segregation in low viscosity basaltic melts.
- Active thrusting offshore Mount Lebanon: Source of the tsunamigenic A.D. 551 Beirut-Tripoli earthquake
Ata Elias et al., IPGP, Tectonique, Boite 89, 4 Place Jussieu, Paris 75252, Cedex 05, France. Pages 755-758.
- On 9 July A.D. 551, a large earthquake, followed by a tsunami, destroyed most of the coastal cities of Phoenicia (modern-day Lebanon). This was one of the most devastating historical earthquakes in the eastern Mediterranean. New marine geophysical data help unveil its source: the rupture of a previously unknown, albeit major, submarine fault, offshore of Mount Lebanon. This active fault is also responsible for the build-up of the Mount Lebanon range that towers around 3100 meters above sea level. Sets of uplifted shorelines along the Lebanese coast, attributed to repeated earthquakes on this fault, hint to a 1500-1750 year recurrence period for similar A.D. 551-sized earthquakes. The next destructive, tsunamigenic earthquake may occur soon.
- Geophysical insights into the Transition fault debate: Propagating strike slip in response to stalling Yakutat block subduction in the Gulf of Alaska
Sean P.S. Gulick et al., University of Texas, Jackson School of Geosciences, Institute for Geophysics, J.J. Pickle Research Campus, 10100 Burnet Road, Mail Code R2200, Austin, TX 78758, USA. Pages 763-766.
- New mapping of faults in the Gulf of Alaska by Gulick et al. reveal that an important response to tectonic collision of the Yakutat block into North America over the last 5 million years is the recent creation of a new plate boundary. The unusually thick Yakutat block is resisting underthrusting beneath the continent, and thus the Queen Charlotte-Fairweather fault system that stretches from British Columbia to the Denali fault in central Alaska has recently (within the last few hundred thousand years) started extending offshore to propagate strike-slip faulting at the base of the continental slope. The base of the slope in this area has been called the Transition zone or Transition fault, and has been interpreted as everything from not existing, to being a thrust fault, to being a strike-slip fault. Gulick et al.'s new bathymetry and seismic data show it to be a young strike-slip fault lying at the boundary between normal Pacific Ocean crust and the Yakutat block. This research suggests that plate boundaries may have been created similarly in the geologic past when thickened regions of crust collided with continents.
- Alexandria, Egypt, before Alexander the Great: A multidisciplinary approach yields rich discoveries
Jean-Daniel Stanley et al., Geoarchaeology Program, Rm. E-206, Paleobiology, Smithsonian Institution National Museum of Natural History (NMNH), Washington, D.C. 20013-7012, USA.
- Our modern western civilization traces its roots to the Mediterranean region, and determining exactly when and where civilizations took hold remains an ongoing quest. For example, the armies of Alexander the Great swept across the region, leading to the establishment of the city of Alexandria on the shores of the Mediterranean in BC 332. But what came before Alexander? Was there a settlement that preceded Alexandria, and if so, what can we learn about the people who lived and died there? These are some of the questions addressed by Jean-Daniel Stanley of the Smithsonian Institution, Washington, D.C., and his co-workers in a paper in the August GSA TODAY. By applying a multidisciplinary approach, involving archeology, sedimentology and geochemistry, to the study of sediment cores collected from Alexandria's Eastern Harbor, Stanley and his colleagues have demonstrated that a settlement occupied the region for at least seven centuries prior to the arrival of Alexander. Ceramic shards, high lead levels, and the use of building stones imported from other regions all attest to a once flourishing urban center as far back as BC 1000. These discoveries indicate that much is still to be learned about the early development of western civilization, and an effective means of achieving this is by integrating geologic and archaeological methodologies.
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