T41D-1245 0800h
Inferred Variations in Crustal Accretion Processes Along the Southwest Indian Ridge Near the Marion Hotspot
The Southwest Indian Ridge (SWIR) presents a unique opportunity to study hotspot/ridge interactions in an ultra-slow spreading, highly segmented environment. We investigate the effects of the Marion hotspot on SWIR crustal accretion processes. More specifically, the goal of this study is to constrain the history of Marion/SWIR interactions by looking at temporal variations in SWIR magmatism, inferred from global gravity and bathymetry databases, for the last approximately 35 Myr. Marion lies in the central portion of the SWIR on 28 Ma crust, about 250 km south of the ridge axis and approximately 360 km to the east of the extremely long (750 km) offset Andrew Bain FZ. The region of study encompasses eight ridge segments and approximately 425 km of ridge axis, from the Andrew Bain FZ to the Discovery II FZ. Ridge offsets in the study area include both long-lived, well-developed transform faults as well as oblique discontinuities. To constrain temporal variability in crustal accretion, we consider shipboard bathymetry, satellite free-air gravity, and seafloor age data. We also calculate residual bathymetry, by removing a predicted lithospheric subsidence curve from GEBCO seafloor depth. Further, we utilize published calculations of mantle Bouguer and residual mantle Bouguer anomalies from Georgen et al. (2001). We examine both two-dimensional patterns using map projections and one-dimensional variations using quasi-axis-perpendicular profiles along GEBCO ship tracks. The data suggest that large inferred variations in rates of crustal accretion have occurred over the past approximately 35 Myr. Moreover, these variations are not always temporally correlated between the segments in the study region, nor are they symmetric about the ridge axis. To better constrain the style of crustal accretion processes along this highly-segmented, ultra-slow spreading ridge, we also compare the Marion-hotspot-influenced region of the SWIR to non-hotspot-affected sections of the ridge and to other hotspot-ridge systems.
T41D-1246 0800h
Nanometre sized inclusions in microdiamonds: a new source of information about diamond genesis and fluid composition.
Inclusions in diamond provide the unique opportunity investigating the mantle rocks where diamond has formed and the fluids from which they have crystallized. Most of all natural diamonds are microcrystalline diamonds (< 0.5 mm). They contain many submicrometer sized or even nanometre sized inclusions. Spectroscopy was the only method to get information about these inclusions. TEM investigations of inclusions in microdiamond were nearly impossible because of preparation problems. With the availability of the focused ion beam (FIB) technique situation has changed. FIB allows preparing electron transparent foils even from microdiamonds. With TEM it is possible to determine the structure, chemical composition and paragenesis of micro- and nano-inclusions. The excellent electron transparency of diamond allows keeping the FIB prepared foils rather thick (up to 300 nm). Thick foils make it likely to find and investigate such inclusions, which are still closed. With analytical electron microscopy (EDX) or electron energy-loss spectroscopy (EELS) and diffraction analysis the content of the inclusions can be studied. After that, the inclusion is opened drilling a hole with the focused electron beam releasing the fluid and gas content. A second analysis of the remaining material provides an estimate of the fluid and gas composition in the inclusion. The most efficient investigation of inclusions in microdiamonds is the combination of Raman- and IR-spectroscopy and TEM studies of one and the same sample. This is demonstrated on microdiamonds from Udachnaya (Siberia) and from Ukrainia.
T41D-1247 0800h
Tectonomagmatic relationship between the Sierra Madre Occidental ignimbrite flare-up and the southern Basin and Range province
The Sierra Madre Occidental (SMO) is a Mid-Tertiary, large-volume, ignimbrite province at least 1,200 km long and 200-500 km wide, extending continuously from the U.S.-Mexico border (31\deg N) to its intersection with the Mexican Volcanic Belt (21\deg N). Considering the average thickness of 1,000 m for the ignimbrite plateau, based on several measured sections along the province, and the average wide of the province of 300 km, a conservative estimate of the physical volume of the SMO ignimbrites is about 360,000 km$^{3}$. The southern part of the Basin and Range province is in Mexico. This extensional province overlaps in space and time with the SMO ignimbrite flare-up and formed NW- to NE-trending normal faults that bound many large grabens, which are particularly long and deep in the southern SMO. Basin and Range faulting occurred between at least 32 Ma and 12 Ma with both limits probably extending until the Eocene and the Quaternary. Ignimbrite activity can be as old as 51 Ma and as young as 17-16 Ma, but most of the ignimbrite volume was erupted in the 38-23 Ma period. Thus, the ignimbrite flare-up can be defined as a period of intense explosive volcanic activity that produced enormous volumes of silicic ignimbrite sheets, which took place mainly between 38 and 23 Ma in Mexico. The ignimbrite flare-up coincided in time with peaks in Basin and Range faulting, and the ignimbrite activity apparently migrated from the east-northeast to the west-southwest, i.e., from central Chihuahua (38-27 Ma) to Durango-Tayoltita-Nazas (32-29 Ma) to Zacatecas-Tepic (24-23 Ma), finishing by 16 Ma at Jalisco-Nayarit, as deduced from the compilation of geologic works done in the SMO. It is unknown yet whether there was a west-southward migration of Basin and Range faulting and if the ignimbrite flare-up occurred episodically as peaks (38-27 Ma, 32-29 Ma, and 24-23 Ma) or was continuous. Nevertheless, by the time that the ignimbrite flare-up started, the Basin and Range extension was already active in Mexico. Therefore, it can be concluded that the emplacement of the SMO ignimbrites and the Basin and Range extensional regime coincided in time and space, and this coincidence occurred in different times and different places throughout the SMO volcanic province.
T41D-1248 0800h
Emplacement and Growth of Serpentinite Seamounts on the Mariana Forearc
Seamounts comprised primarily of serpentinite muds are found on the outer forearc of the Izu-Bonin-Mariana subduction system. They represent some of the first material outputs of the recycling process that takes place in subduction zones. Therefore, understanding their evolution is necessary to correctly quantify the flux of material through the subduction system. Serpentinite seamounts have been described as mud diapirs, mud volcanoes, uplifted blocks of mantle material, and a composite of the latter two. Multi-channel seismic (MCS) data collected in 2002 from the outer Mariana forearc imaged, for the first time, the large-scale internal structure of these seamounts. These data, combined with new bathymetry, have provided insight into how the seamounts grow and deform with time and have allowed us to evaluate proposed models for their formation. The serpentinite seamounts rest on faulted and sedimented Mariana forearc basement. Flank flows of serpentinite muds downlap existing forearc substrate, leaving the underlying stratigraphy largely undisturbed. Reflections located 3.5-5 km beneath forearc basement may represent Moho, suggesting that the seamounts are built on anomalously thin forearc crust. A strong reflection at the summit of Big Blue, the largest serpentinite seamount in the Mariana Forearc, represents a collapse structure that has been partially in-filled by younger muds, supporting the idea that serpentinite seamount growth is episodic. Basal thrusts that incorporate forearc sediments at the toe of Turquoise Seamount provide evidence for seamount settling and lateral growth. We are conducting numerical simulations of seamount growth and evolution using the discrete element method (DEM), previously used to examine gravity spreading phenomena in magmatic volcanoes. Simulations employing distinctly low basal and internal friction coefficients provide a good match to the overall morphology of the serpentinite seamounts, and offer insight into their internal structure and dynamics, including the formation of inward-dipping reflections and basal thrust faulting. Although the models do not capture central conduit processes that build the seamounts, they do address flank processes that may explain why some seamounts flow passively over pre-existing sediments while others uplift them.
T41D-1249 0800h
The High P-T Densities of Melt and Residual Mantle in the Archean and Implications for Models of Komatiite Genesis
An open question concerning the Archean Earth is the origin of ultramafic komatiites with MgO $>$ 25 wt.% and melting extents $>$ 35%. One endmember model that has been proposed is anhydrous decompression melting of an ultra-hot mantle plume. A key physical constraint for this model is whether the deep melts are less dense than the surrounding mantle, so that they can rise buoyantly out of the mantle. In this study, we use the results of high pressure melting experiments to predict the compositions and densities of both melt and residual mantle in a self-consistent decompression melting column, to determine the cross-over depth at which melt becomes buoyant. We calculate a model 1-D adiabat and the composition of incremental melts formed at each depth (Sparks and Cheadle, GCA, 2002). The parameterized melt composition is based on the melting experiments of Herzberg and Zhang (JGR, 1996). A typical model adiabat that generates a good match to an Archean komatiite has a potential temperature of 1875\deg C beneath a 90 km thick lithosphere. The uniqueness of this model is its ability to yield melt compositions which are variable and cumulative over the height of the melting column. This model assumes fractional melting where the partial melt is instantaneously separated from the rock matrix as each increment of melt is generated. The measured densities of the Komatiitic composition of Courtial et al., (GCA, 1997) were adjusted for the temperatures on our typical model adiabat, using reported values and uncertainties in molar volume, thermal expansion coefficient, isothermal compression coefficient and its pressure derivative. However, the initial near-solidus melt compositions that we predict at a depth are not the same as the composition of erupted komatiite. Our preliminary estimate, based on partial molar volumes for the individual major element oxides indicate that these deep melts are 0.5-1.0% less dense than what a typical erupted komatiite would be at 12 GPa. The modal composition of the rock can be determined from the bulk chemistry assuming simple elemental partitioning among the four most abundant mantle phases, and appropriate phase changes, particularly the conversion to majorite. We will also calculate a range of possible densities for the residual mantle, assuming a range of thermodynamic parameters and extent of phase changes.
T41D-1250 0800h
Crustal growth of oceanic island arc inferred from seismic structure of Mariana arc-backarc system
The Izu-Ogasawara-Marina arc (IBM arc) is one of the typical oceanic island arcs and it has developed repeating magmatic arc volcanisms and backarc spreading since Eocene. Because tectonics of the IBM arc is relatively simple and does not include collisions between the arc and a continent, it is one of best targets to research crustal growth. In 2003, wide-angle seismic survey using 106 ocean bottom seismographs had been carried out as a part of Margin program in collaboration between US and Japan in Mariana region. The seismic line runs from a serpentine diaper near the trench to Parece Vela basin through the Mariana arc, the Marina trough and the West Mariana ridge. We present the characteristics of the seismic structure of the Mariana arc-backarc system and discuss the crustal growth process by comparison with a structure of the northern Izu-Ogasawara arc. Main structural characteristics of the Mariana arc-backarc system are (1) variation of the crustal thickness (Mariana arc: 20 km, West Mariana ridge: 17 km, Mariana trough and Parece Vela basin: 6 km), (2) distribution of an andesitic middle crust with about P-wave velocity of 6 km/s, (3) variation of P-wave velocity in the middle crust (4) velocity anomalies of the lower crust in transition area between the arc and the backarc, (5) thickening of the lower crust under the Mariana trough axis and (6) slow mantle velocities under the Mariana arc, Mariana trough axis and the West Mariana ridge. Above characteristics from (1) to (4) are common to the seismic structure of the northern Izu-Ogasawara arc. In particular, the vertical P-wave velocity gradients of the middle crust under the forearc in both regions tend to become large rather than those under the arc. Main differences of seismic structures between both regions are the velocity gradients and an existence of a thin transition layer between the middle and lower crust. These differences and similarities of the velocity gradient might originate the age and indicate a difference of a crustal differentiation relating each tectonic stage.
T41D-1251 0800h
Cretaceous Igneous Activity of the US Gulf Coast: Evidence for a Post-Albian Crustal Event
Post-Albian igneous rocks are recognized along the US Gulf Coast from South Texas to Mississippi. This alkalic suite of intrusive and extrusive rocks are characterized by a variety of structural features which include surface volcanoes, phreatic explosion craters, sills, dikes, laccoliths, and diatremes and many have associated tuffs and volcaniclastic sediments. Generally these rocks occur along crustal block boundaries that resulted from rifting in this area during the Triassic. Collectively, the compositions of these igneous rocks would suggest a deep crustal origin (depth $>$40 km). Reactivation and crustal cracking along these old block boundaries is the most likely explanation for the composition and location of the rocks. In the absence of associated large scale tectonic or thermal events, we suggest that upper Jurassic to lower Cretaceous sedimentary loading of the continental margin was followed by a "crustal event", much like the failure of a mechanical beam, which resulted in the igneous activity.
T41D-1252 0800h
Cooling and Exhumation of the Sierra Nevada Batholith in the Mount Whitney Area (California) Based on (U-Th)/He Thermochronometry.
Plutonic rocks of the Mount Whitney area were intruded at about 83-87 Ma during the latest stages of emplacement of the Sierra Nevada Batholith. The region is located in the eastern part of the batholith, and is bounded on the east by the currently active Sierra Nevada Frontal Fault, a normal fault. Here we report new apatite and zircon (U-Th)/He ages on a dense 3.5 km vertical transect 20 km S of Whitney. Published K/Ar and U/Pb ages indicate that this area experienced very rapid cooling, $>$ $100\deg$C/km, from 83 Ma to 79-80 Ma. Our new ages suggest that this period was followed by a progressive decrease of the cooling rate from 79 to 55 Ma. Between 79 and 75 Ma the cooling rate was $\sim$ $15\deg$C/km, and $<$ $10\DEG$C/km from 75 to 55 Ma. Considering the relatively shallow emplacement depth of the Mount Whitney area plutons (2-3 kbar), and the duration of the progressive cooling event ($\sim$ 30 Ma) we consider that post-intrusion thermal re-equilibration is achieved in a few million years and that exhumation of the batholith must have also occurred between 83 and 55 Ma. This period was followed by a long episode of very slow apparent exhumation, $<$ 0.03 mm/yr, between 55 and 11 Ma, associated with a very slow apparent cooling rate ($<$ $1\deg$C/Ma). This event, recorded by the apatite (U-Th)/He ages, could either be a period of very slow exhumation, or related to the residence of the dated samples within the zone of partial retention for Helium. While the vertical age profile cannot distinguish between these possibilities, efforts in progress to measure the concentration profiles of these apatites using the $^{4}$He/$^{3}$He method should be diagnostic. The occurrence of a period of slow apparent exhumation until as late as 11 Ma implies that the Sierra Nevada Frontal Fault was only active after 11 Ma. This result is in accord with estimates based on tilted lithologies in the Central and Northern part of the Sierra Nevada. New dating in progress will allow better constraints on this initiation age. Considering the other dated normal faults in the vicinity of the Sierra Nevada Frontal Fault, a westward propagation of fault initiation is suggested.
T41D-1253 0800h
New SHRIMP-Zircon Data Provide Evidence for a Complex Polyphase Magmato-Metamorphic History of the Western Bohemian Massif
Zircons from three orthogneiss complexes and a Sill-granitoid from the Saxothuringian-Moldanubian transition zone (Western Bohemian Massif) have been studied using SHRIMP-RG. The results suggest a complex polyphase magmatic and metamorphic history for the rocks in this area. The latest metamorphic overprint occurred during Variscan LP/HT regional metamorphism between 330-320 Ma. Conventional U-Pb zircon dating of several orthogneiss and pegmatite complexes of the Saxothuringian, the Moldanubian and the nappe complexes (ZEV, ZTD, MM) produced ages between 460 and 530 Ma which were interpreted as emplacement and crystallization ages for these rocks. Upper concordia intercept ages between 2000 Ma and 2700 Ma were interpreted to be ages of source materials. Conventional U/Pb analyses of zircon from the polyphase overprinted Saxothuringian-Moldanubian transition zone often result in extremely discordant ages influenced by lead loss, high common lead contents, and uranium gain (Wiegand 1997). Hence, conventional U-Pb age determinations should be used with caution. SHRIMP analyses provide more insight into the age pattern in zircons from the Saxothuringian-Moldanubian transition zone. The results show a widespread distribution of ages between 200 Ma and 3200 Ma with main peaks at approximately 470 Ma and 570 Ma. Both ages reflect magmatic stages and are found together in the investigated zircon separates. For the Wunsiedler Gneis and the Epigneis of Wernersreuth (Saxothuringian) the youngest age peak is 490 Ma and reflects the last magmatic stage and emplacement of these rocks. Two major older age peaks reflect an earlier magmatic stage at approximately 570 Ma and an inherited magmatic event at about 610 Ma. An additional small amount of older inherited zircons suggest previous magmatic events at approximately 1000 Ma and 1950 Ma. The latest metamorphic overprint occurred during Variscan regional metamorphism at about 320 Ma ago. The age pattern of the Tachov Orthogneiss and the Sill-granitoid (Moldanubian) is far more complex. Magmatic events are comparable to the Saxothuringian orthogneisses at approximately 460 Ma and 570 Ma. In addition, a widespread older age pattern of approximately 650 Ma, 800 - 1200 Ma, 2700 Ma, and 3200 Ma reflects inherited ages of earlier crustal events. The Sill-granitoid intruded Moldanubian rocks during the Variscan orogeny which is evident from a strongly zonated zircon population formed at about 320 Ma ago and monazites of the same age. The last magmatic stage of the Tachov Orthogneiss appears to be at about 460 Ma with later metamorphic overprint during Variscan LP/HT metamorphism which was followed by late stage hydrothermal activity. Lead loss during metamorphism is believed to be responsible for apparent ages between 330 and 200 Ma. B. Wiegand (1997) Geotektonische Forschungen 88, Schweizerbart, Stuttgart, Germany.
T41D-1254 0800h
Transregional lineament of Central Asia, its magmatism, metallogeny and seismicity
The analysis of the place and role of such large fault as Central Kyzylkum, North Nurata and South Ferghana, Atbashin, which were regarded earlier as separate independent structures led us to the idea that they are parts of a single global structure. We suggest that it should be called "Transregional lineament of Central Asia". Transregional lineament of Central Asia is multisutured long-term, and in the nodal points of some parts it is complicated by deep fault zones of Anti Tien-Shan trend.There are large gold ore deposits (Muruntau, Kokpatas, Kumtor) in the intersection of some of these faults. Within the lineament there are 4 mafite - ultramafite associations of different age, that are presented as isolated or combined blocks, zones and regions. The most ancient is ophiolite one (I association). Best of all it is developed in Sultanuvais and Northern Tamdytau, Uzbekistan. The second, rift association of this belt is picrite-gabbro-diabase-alkali-olivine-basalt is widespread within the belt (northern Bukantau, northern Nuratau, northern slope of the Altay ridge).The third association is peridotite-gabbroic. It is represented by the Tebinbulak intrusive of Sultanuvais. Coverings, small stocks, dikes and explosion tubes formed by potassic mafite-ultramafites ore related to much later inter-plate (P-T) occurrences of mafite-ultramafite magmatism (IY association). On Kyrgyzstan's territory the studied lineament is observed as a system of regional deep faults -Atbashi-Inylchek and Southern Ferghana, with which the ophiolite ultramafite-mafite formation is associated. The rocks have the traces of tectonic movements, which can be the ground to regard them as protrusions. Tectonically, the vast territory of Mongolia is divided into two large blocks: northern and southern. This part of the lineament called Transmongolian. This part is week studied-a special investigation was only carried out in its western part - Bulgan fault. Thus, in the presence of linear-stretched sublatitudinal metallogenic belts within the area of the studied transregional lineament in the Central Asian region, the most prospective ore deposits'assemblages are located in the intersection of these belts as deep faults of meridional, north-eastern and north-western trend. The following deposits are situated in such intersection zones: the largest in Eurasia Muruntau gold deposit, a large deposit Kumtor, Kyrgyzstan; and the gigantic class deposit - Ashi in China.Anomalously increased heat flow density (HFD) values both Muruntau deposit (according to our data- up to 85 mW/m2), and for Kumtor deposit (according to E.A.Ljubimova data - up to 130 mW/m2 ) indicate the deep character of dislocations. Interpretation of seismic observations revealed a transregional zone with the width of up to 80 km in the upper part of Kyzylkums crust. The strip is located in the relief of Moho surface, and is discordant to the general trend of premesozoic structures. Abrupt variations of the Moho boundary with vertical movement of up to 10 km are observed in the eastern end of the Issyk-Kul valley and at the boundary of Northern and Middle Tien Shan, along the Nikolaev line. In conclusion we'd like to note that such structures are well-known in American, Australian and other continents.They are recognized as deep structures and served as channels for the heat energy, magmas and fluids to come out of the core and mantle of the Earth. In this connection, activity of these structures are accompanied by the presence of various magamatic, endogenous as well as gigantic ore deposits.