Plutonic foundation of a slow-spreading ridge segment : oceanic core complex at Kane Megamullion, 23°30′N, 45°20′W

Author Posting. © American Geophysical Union, 2008. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry Geophysics Geosystems 9 (2008): Q05014, doi:10.1029/2007GC001645.We mapped the Kane megamullion, an oceanic core complex on the west flank of the Mid-Atlantic Ridge exposing the plutonic foundation of a ∼50 km long, second-order ridge segment. The complex was exhumed by long-lived slip on a normal-sense detachment fault at the base of the rift valley wall from ∼3.3 to 2.1 Ma (Williams, 2007). Mantle peridotites, gabbros, and diabase dikes are exposed in the detachment footwall and in outward facing high-angle normal fault scarps and slide-scar headwalls that cut through the detachment. These rocks directly constrain crustal architecture and the pattern of melt flow from the mantle to and within the lower crust. In addition, the volcanic carapace that originally overlay the complex is preserved intact on the conjugate African plate, so the complete internal and external architecture of the paleoridge segment can be studied. Seafloor spreading during formation of the core complex was highly asymmetric, and crustal accretion occurred largely in the footwall of the detachment fault exposing the core complex. Because additions to the footwall, both magmatic and amagmatic, are nonconservative, oceanic detachment faults are plutonic growth faults. A local volcano and fissure eruptions partially cover the northwestern quarter of the complex. This volcanism is associated with outward facing normal faults and possible, intersecting transform-parallel faults that formed during exhumation of the megamullion, suggesting the volcanics erupted off-axis. We find a zone of late-stage vertical melt transport through the mantle to the crust in the southern part of the segment marked by a ∼10 km wide zone of dunites that likely fed a large gabbro and troctolite intrusion intercalated with dikes. This zone correlates with the midpoint of a lineated axial volcanic high of the same age on the conjugate African plate. In the central region of the segment, however, primitive gabbro is rare, massive depleted peridotite tectonites abundant, and dunites nearly absent, which indicate that little melt crossed the crust-mantle boundary there. Greenschist facies diabase and pillow basalt hanging wall debris are scattered over the detachment surface. The diabase indicates lateral melt transport in dikes that fed the volcanic carapace away from the magmatic centers. At the northern edge of the complex (southern wall of the Kane transform) is a second magmatic center marked by olivine gabbro and minor troctolite intruded into mantle peridotite tectonite. This center varied substantially in size with time, consistent with waxing and waning volcanism near the transform as is also inferred from volcanic abyssal-hill relief on the conjugate African plate. Our results indicate that melt flow from the mantle focuses to local magmatic centers and creates plutonic complexes within the ridge segment whose position varies in space and time rather than fixed at a single central point. Distal to and between these complexes there may not be continuous gabbroic crust, but only a thin carapace of pillow lavas overlying dike complexes laterally fed from the magmatic centers. This is consistent with plate-driven flow that engenders local, stochastically distributed transient instabilities at depth in the partially molten mantle that fed the magmatic centers. Fixed boundaries, such as large-offset fracture zones, or relatively short segment lengths, however, may help to focus episodes of repeated melt extraction in the same location. While no previous model for ocean crust is like that inferred here, our observations do not invalidate them but rather extend the known diversity of ridge architecture.NSF Grants OCE-0118445, OCE-0624408 and OCE-0621660 supported this research. B. Tucholke was also supported by the Henry Bryant Bigelow Chair in Oceanography at Woods Hole Oceanographic Institution

The Laramie project [a play]

"On October 7, 1998, a young gay man was discovered bound to a fence in the hills outside Laramie, Wyoming, savagely beaten and left to die in an act of brutality and hate that shocked the nation. Matthew Shepard's death became a national symbol of intolerance, but for the people of Laramie the event was deeply personal, and it is their voices we hear in this stunningly effective theater piece.""Moises Kaufman and fellow members of the Tectonic Theater Project made six trips to Laramie over the course of a year and a half in the aftermath of the beating and conducted more than 200 interviews with people of the town. From these interviews as well as their own experiences, Kaufman and the Tectonic Theater members have constructed a deeply moving theatrical experienceThe Laramie Project chronicles the life of the town of Laramie in the year after the murder, using eight actors to embody more than sixty different people in their own words - from rural ranchers to university professors. The result is a complex portrayal that dispels the simplistic media stereotypes and explores the depths to which humanity can sink and the heights of compassion of which we are capable."--BOOK JACKE

The crustal and upper mantle structure around the Gulf of California, inferred from surface wave data and receiver functions

This thesis reports on studies that have been carried out to better image the crustal and upper mantle shear velocity structure around the Gulf of California, Mexico. The Gulf of California forms a part of the plate boundary between the Pacific and North-American plates, where transform motion in the North converts into oceanic spreading in the South. Although currently oblique extension occurs within the gulf, in the past subduction of the oceanic Farallon plate took place along a trench west of what is currently the Baja California Peninsula. Around 12 Ma subduction and spreading ceased and the remaining parts of the Farallon plate were captured by the Pacific plate. After this the Gulf of California started to develop and around 5 Ma the Pacific-North America plate boundary shifted to its present location within the gulf. The study is subdivided into three parts. In the first part we use a spectral-element code to model seismic wave propagation and the adjoint technique to calculate finite-frequency sensitivity kernels for two-station surface wave measurements. We found a dominant sensitivity in the interstation region, close to what is assumed in the two-station method. However, we also show that anomalous structures even far from the assumed propagation path can significantly affect the measurements. To improve regional tomographic inversions using the two-station method, sensitivity kernels should therefore be taken into account. At the regional scale we are interested in this requires high computational costs, which are difficult to reach in practice. In the second part, instead of improving the lateral resolution using the method described above, we improve the vertical resolution by 1-D joint inversions of surface wave and receiver function data recorded by NARS-Baja stations. We use a Neighbourhood Algorithm to obtain shear velocity models beneath the stations. We are able to find sediment and crustal thicknesses below the stations in the area. In the upper mantle we find indications for the presence of subducted plate remnants below the middle and southern parts of the Baja California Peninsula and we identify the top as a sharp interface. The absence of plate remnants below the northern part explains why the middle and southern parts of the Baja California Peninsula move faster to the Northwest with respect to the North-American plate than the northern part. In the third part we investigate the effects of dipping crustal layers on the receiver functions. We find that azimuthal differences is the receiver function data can be explained by a westward dipping Moho below the Baja California Peninsula, and a dipping mid-crustal layer throughout the entire area. For detailed crustal imaging of the Baja California area using receiver functions more complex modelling than a 1-D inversion is recommended

Late Quaternary Activities in the Central Part of Itoshizu Tectonic Line : Excavation Study at Wakamiya and Osawa Faults, Nagano Prefecture, Central Japan

1983年に,糸静線活断層系中部の若宮断層(富士見町)と大沢断層(茅野市)の発掘調査を行った.Aトレンチ(若宮断層),Bトレンチ(大沢断層)ともに,北西方向にのびる丘の西縁に地形的に推定された断層を横切ってつくられた.Aトレンチでは約100,000年前以降の,Bトレンチでは約14,000年前以降の地層群をきる断層(群)が見いだされ,両断層が活断層であることが地質学的にも確認された.しかし,Aトレンチでは丘側の高角断層群と低地側の低角逆断層群とが存在するのに対し,Bトレンチでは一つの顕著な断層帯が存在する.Aトレンチの高角断層およびBトレンチの断層は,いずれも東側の隆起を伴う左横ずれを示す.断層東側の丘は,このような変位の繰り返しの結果として形成されたものと考えられる.Aトレンチでは断層の活動期を明確に決定することはできなかったが,Bトレンチでは,6層(トレンチ内最下部の地層)堆積以降少なくとも5回の同じ向きの変位が繰り返されたことが明らかになった.おもな断層活動の再来周期は約4000～5000年と考えられる.最新の活動は,約1200年前に起こった.この活動は,A.D.841年の,松本付近に被害を与えた歴史地震に対応すると思われる.A series of active faults developes in the central part of the Itoshizu Tectonic Line (ITL), which is one of the most important tectonic lines in Japan, running northwestward through central Japan (Figs. 1-3). In 1983, two trenches, A and B, were excavated across geomorphologically estimated active faults, the Wakamiya and Osawa Faults, belonging to the ITL active fault system. The purpose of this trenching survey is to identify the precise position of fault traces and the nature of faults, and to establish the timing of faulting, particulaly the age of the latest event and recurrence interval of faulting. Both trenches are located at the base of the western slope of tectonic bulges, but trench A is on the middle of the bulge (Figs. 4 & 6) and trench B is on the southern end of the bulge (Figs. 5 & 7). Both trenches have similar direction, NEE-SWW, perpendicular to the fault trend (long axes of the bulges), and have similar scale, ca. 30m in length and ca. 3 m in depth, except for the eastern part of trench A (ca. 7 m in depth) (Table 1)

Reports on the scientific investigations.

Includes bibliographical references and index.v. 1. Glaciology, physiography, stratigraphy, and tectonic geology of south Victoria Land / by T.W. Edgeworth David and Raymond E. Priestley, with short notes on palæontology by T. Griffith Taylor and E.J. Goddard -- v. 2. Contributions to the palæontology and petrology of south Victoria Land / by W.N. Benson ... [et al.]Mode of access: Internet