Boulder, Colo., USA – Researcher John Sinton of the University of Hawai'i along with colleagues from the Monterrey Bay Aquarium and the French National Center for Scientific Research have announced the discovery of an ancient Hawaiian volcano. Now located in a region of shallow bathymetry extending about 100 km WNW from Ka'ena Point at the western tip of O'ahu, this volcano, which they have named Ka'ena, would have risen about 1,000 meters above sea level 3.5 million years ago.
Sinton and colleagues have found compelling evidence beneath the sea that this long-lived volcano was the first to contribute to the formation the island of O'ahu, and that the younger Wai'anae and Ko'olau volcanoes were built on its flanks. Geological observations of the seafloor and studies of volcanic rocks collected from Ka'ena provide evidence for its age and details of its lava chemistry and volcanic evolution. This GSA Bulletin paper was posted online on 2 May 2014.
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Ka'ena Volcano -- A precursor volcano of the island of O'ahu, Hawai'i
J.M. Sinton et al., University of Hawai'i, 1680 East-West Road, Honolulu, Hawaii 96822, USA. Posted online 2 May 2014; http://dx.doi.org/10.1130/B30936.1.
Age and provenance of a Paleoproterozoic to Devonian Canadian Cordilleran sequence of metasedimentary rocks, Thor-Odin dome, southeastern British Columbia
Y. Kuiper et al., Dept. of Geology and Geological Engineering, Colorado School of Mines, 1516 Illinois Street, Golden, Colorado 80401, USA, http://dx.doi.org/10.1130/B31031.1.
The Thor-Odin dome is an area that has been subject to high pressures and temperatures between ~160 and ~50 million years ago as a result of the formation of the Canadian Cordillera. The rocks are now metamorphic rocks, but they were originally sedimentary rocks deposited along the western margin of what is currently the North American continent. They were brought at deeper levels during Cordilleran mountain building and back at the surface through erosion. The sediment was derived from eroded material that contains a mineral zircon. Numerous of these zircon grains per sample were dated by U-Pb radiometric dating, to investigate the age of the original sediments and the source areas of the zircon grains and sediment. The grains have young overgrowths formed during the Cordilleran mountain building event, but older original cores are preserved that allowed us to conduct out study. The oldest units in the Thor-Odin dome are some of the oldest known sedimentary rocks in the Canadian Cordillera after the formation of the current Canadian Shield. The youngest rocks were deposited shortly before the onset of the Cordilleran mountain building. Parts of as many as ~1.4 billion years of sedimentary history are preserved.
The Pinal Schist of southern Arizona: A Paleoproterozoic forearc complex with evidence of spreading ridge-trench interaction at ca. 1.65 Ga and a Proterozoic arc obduction event
Arend Meijer, 9870 E. Fire Agate Place, Tucson, Arizona 85749, USA. Published online 18 Apr. 2014; http://dx.doi.org/10.1130/B31002.1.
This article by Arend Meijer proposes a new model for the origin of the basement rocks of southern Arizona. In most respects, the model is similar to models for modern-day subduction zones, such as the Cascade subduction zone. However, the model also involves the subduction of an oceanic spreading ridge beneath Arizona 1.65 billion years ago. In this sense, the model is more similar to models for the evolution of the California coast between 10 and 25 million years ago when the East Pacific Rise was subducted beneath California. The advantage of the Arizona terrane over the modern-day subduction terranes is that in Arizona, the internal workings of a subduction zone that experienced a spreading ridge subduction event are exposed at the present-day surface.
Hematite replacement of iron-bearing precursor sediments in the 3.46-b.y.-old Marble Bar Chert, Pilbara craton, Australia
B. Rasmussen et al., Dept. of Applied Geology, Curtin University, Kent Street, Bentley, WA 6102, Australia. Published online 15 May 2014; http://dx.doi.org/10.1130/B31049.1.
The ancient history of atmospheric oxygen is not well understood but is of the utmost importance because its accumulation profoundly transformed our planet. The presence of the iron-oxide mineral, hematite, in the Marble Bar Chert from a NASA-funded drill hole in the Pilbara Craton, Australia, has been cited as evidence for an oxygen-bearing ocean 3.46 billion years ago, however, others argue that isotopic data from hematite in the same drill-hole indicates that the ocean contained no oxygen. Both groups agree that the hematite formed in the ocean or just below the seafloor. Our study shows that hematite in jasper bands from the Marble Bar Chert formed long after deposition. The hematite replaced preexisting iron-bearing minerals during the infiltration of oxygen-bearing fluids, imparting the characteristic red color long after deposition. A secondary origin for hematite invalidates arguments for oxygen in the ocean ~3.46 billion ago and provides a new explanation for the formation of ancient jasper bands. We conclude that misinterpretations about the origin of hematite in ancient cherts could lead to false conclusions about the chemistry of the ocean and atmosphere on the early Earth.
Heat flow data in the Four Corners area suggests Neogene crustal warming resulting from partial-lithosphere replacement in the Colorado Plateau interior, southwest USA
M. Reiter, New Mexico Bureau of Geology and Mineral Resources, Emeritus, New Mexico Institute of Mining and Technology, Socorro, New Mexico 87801, USA. Published online 2 May 2014; http://dx.doi.org/10.1130/B30951.1.
Colorado Plateau uplift is of great geologic interest; and heat flow data can provide boundary conditions which need to be met by uplift models. The relative structural continuity of the Plateau interior with respect to the neighboring Southern Rockies and Basin and Range suggests a distinctive Neogene lithosphere evolution. The Four Corners region represents a most unique location in the Colorado Plateau, and perhaps the world, where both low-noise heat flow data, and mineralogical studies relating to pre-Neogene lithosphere temperatures are available. Using a model for heat flow-temperature-depth relations, the mineralogical data allow heat flow and lithosphere-asthenosphere boundary depth (LAB) estimates for about 25 million years ago. With present-day heat flow measurements the present LAB can be approximated. From these calculations the Neogene change in LAB depth (lithosphere thickness) is suggested to be ~ 100 km. The derived Neogene change in near surface heat flow is attributed to non-radioactive heat sources below the crust. The change in lithosphere thickness of ~ 100 km suggests a similar amount of lithosphere replacement may be responsible for the Neogene increase in near surface heat flow. Straightforward models imply partial upper-mantle lithosphere replacement can produce the calculated Neogene heat flow change.
Detrital zircon geochronology of the Grenville/Llano foreland and basal Sauk Sequence in west Texas, USA
Christopher J. Spencer et al., Dept. of Earth and Environmental Sciences, University of St. Andrews, North Street, St. Andrews KY16 9AL, UK. Published online 18 Apr. 2014; http://dx.doi.org/10.1130/B30884.1.
Christopher J. Spencer and colleagues present ages of zircons from approx. 1.1 billion and approx. 520 million-year-old sedimentary units in west Texas. These zircons provide evidence that the approx. 1.1-billion-year-old sedimentary rocks (the Lanoria and Hazel formations) were deposited in a foreland basin of the Grenvillian mountain system that stretches from west Texas to southern Norway. The younger, approx. 520-million-year-old unit (the Van Horn Formation) was deposited near the shoreline of Laurentia as it rifted away from the supercontinent Rodinia. This study revises the age of the Van Horn Formation from the assumed Neoproterozoic Era to the Middle Cambrian Era. This new age further implies the flooding of the western margin of present-day North America during the Cambrian took place 20 million years earlier than previously thought.
Constraining chronology and time-space evolution of Holocene volcanic activity on the Capelo Peninsula (Faial Island, Azores): The paleomagnetic contribution
Anita Di Chiara et al., Istituto Nazionale di Geofisicae Vulcanologia Roma, Via Vigna Murata, 605, 00143 Roma, Italy, and Università degli Studi di Bologna, Dipartimento di Fisica e Astronomia, Viale Berti Pichat, 6/2, 40127, Bologna, Italy. Published online 18 Apr. 2014; http://dx.doi.org/10.1130/B30933.1.
Faial is one of the nine islands of the Azores Archipelago (central-northern Atlantic Ocean); it is an active volcano where two historical eruptions occurred during A.D. 1672-1673 and in A.D. 1957-1958 (the famous Capelinhos eruption). Both the eruptions took place on the Capelo Peninsula (westernmost sector of the island). This study by Anita Di Chiara and colleagues aims at constraining the age of the older exposed volcanic products, so far loosely dated within last 10,000 years (Holocene). Di Chiara and colleagues used paleomagnetism to correlate scoria cones and lava flows yielded by the same eruption, sampling 31 sites (10 basaltic scoriae, 21 basaltic lava flows). In the investigated products, the team recognizes six prehistoric clusters of volcanic activity, correlating 11 lava sites with 4 scoria cones. Dating was assigned by comparing their data with values expected from European relocated reference curves of the paleosecular variation of the geomagnetic field. Di Chiara and colleagues suggest that the studied volcanic rocks of the Peninsula are younger than previously believed, entirely formed in the last 8,000 years, and the activity intensified over the last 3,000 years. This study confirms paleomagnetism as a powerful tool for unraveling the chronology and characteristics of recent activity at volcanoes where geochronological age constraints are lacking.
Magnetic polarity stratigraphy and palynostratigraphy of the Mississippian-Pennsylvanian boundary interval in eastern North America and the age of the beginning of the Kiaman
N.D. Opdyke et al., Dept. of Earth Sciences, University of Florida, Gainesville, Florida 32611, USA, http://dx.doi.org/10.1130/B30953.1.
From the abstract: Magnetic polarity data from the Maritimes Basin in eastern Canada reveal patterns of magnetic reversal through approx. 17 million years of late Mississippian and early Pennsylvanian time. We collected samples from three sections on Cape Breton Island, a fourth that extends a section studied previously, and a fifth from a deep cored well on Prince Edward Island. We compared the polarity data from these sections to previous results from lower parts of the Joggins section of eastern Canada. … Field mapping and magnetic polarity data show closely comparable Mississippian-Pennsylvanian relationships in the Maritimes Basin more than 1500 km northeast of the U.S. sections. … We believe that this polarity boundary is the base of the Kiaman. An early Langsettian position (ca. 318 Ma) for the base-Kiaman superchron is indicated by these new data. In eastern Nova Scotia and New Brunswick, the sedimentary rocks just above the unconformity are of reversed polarity. It is clear that the sedimentation in the two areas is affected by the unconformity, and no magnetic or palynological correlation is possible.
U-Pb geochronology of the type Nazas Formation and superjacent strata, northeastern Durango, Mexico: Implications of a Jurassic age for continental-arc magmatism in north-central Mexico
T.F. Lawton and R.S. Molina Garza, Centro de Geociencias, Universidad Nacional Autónoma de México (UNAM), Boulevard Juriquilla No. 3001, Querétaro 76230, México. Published online 2 May; http://dx.doi.org/10.1130/B30827.1.
U-Pb ages on volcanic ignimbrites and U-Pb detrital ages on associated sandstone in the type Nazas Formation and overlying Upper Jurassic strata in northern Durango, Mexico, demonstrate convincingly that a volcanic arc was present on the continental margin of north-central Mexico in Middle Jurassic time. The magmatism is equivalent in age to similar magmatism in the southwestern US and indicates that the volcanic arc likely continued from the US across northern Mexico. Wind-blown sand that is interbedded in the Middle Jurassic magmatic rocks in southern Arizona and northern Sonora is absent in Durango, despite the presence of abundant red sandstone and shales in Durango. This provides evidence that the rocks in Durango and Sonora were not originally deposited in adjacent settings and subsequently separated by a large-displacement strike-slip fault known as the Mojave-Sonora megashear.
Multiple intrusions and remelting-remobilization events in a magmatic arc: The St. Peter Suite, South Australia
N.J. Symington et al., R. Weinberg (corresponding), Monash University School of Geosciences Clayton, Victoria 3800 Australia. Published online 15 May 2014; http://dx.doi.org/10.1130/B30975.1.
The ring of fire surrounding the Pacific Ocean is made of volcanoes fed from magmas formed deep beneath the continental crust. These magmas intrude the crust and cool to form granitic bodies or they keep rising and are extruded from volcanoes giving rise to magmatic arcs. During this process magmas interact with their surroundings exchanging heat and exchanging mass giving rise to a large variety of rocks, mineral deposits and styles of volcanism. In the coast of South Australia, we have the roots of a 1.6 billion year old magmatic arc where we have documented this heat and mass transfer. This exchange leads to a dynamic setting where younger magmas intrude, disrupt and mix with older magmas, or cause the remelting of solidified older batches. Although widely expected on theoretical grounds and indirect evidence, cannibalization of the magmatic arc itself, through remelting has seldom been documented. We concluded that the continuous influx of new magmas reworking slightly older intrusions explains the large variety of end products that so characterizes the ring of fire and all other magmatic arcs formed across history of Earth.
Origin and evolution of the Bainaimiao arc belt: Implications for crustal growth in the southern Central Asian Orogenic Belt
S.-H. Zhang et al., Institute of Geomechanics, Chinese Academy of Geological Sciences, MLR Key Laboratory of Paleomagnetism and Tectonic Reconstruction, Beijing 100081, China. Published online 15 May 2014; http://dx.doi.org/10.1130/B31042.1.
Recent results show that evolution of the huge Central Asian Orogenic Belt (CAOB) can be explained in terms of southwest Pacific-style accretion of arcs and microcontinents. As a most important Early Paleozoic arc system south to the Solonker suture zone, origin and evolution of the Bainaimiao arc belt are still not well constrained. New geochronological and geochemical results on magmatic rocks indicate that the Bainaimiao arc was active from 0.52 to 0.42 billion years ago and can extend to east Siping in NE China. Zircon U-Pb geochronological results of metasedimentary rocks in the Bainaimiao arc belt indicate that they are Early Paleozoic in age, not Precambrian as previously regarded. Detrital zircon analysis of metasedimentary rocks and Sr-Nd-Hf geochemical results of magmatic rocks indicate that the Bainaimiao arc was built upon a Precambrian microcontinent with tectonic affinity to the Tarim or Yangtze cratons. The arc was separated by a wide ocean from the northern NCC during Cambrian-Ordovician period. Successive northward subduction resulted in contraction of the ocean and final accretion of the Bainaimiao arc to the northern NCC during Late Silurian-earliest Devonian by arc-continent collision. Arc-continent collision could be an important mechanism for continental crustal growth and formation of huge CAOB.
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