Geochemistry of volcanic glass from Oligocene detrital sediments at DSDP Site 296, Kyushu Palau Ridge: Interpreting the magmatic evolution of the early northern Izu–Bonin–Mariana Island Arc

Understanding the petrologic and geochemical evolution of island arcs is important for interpreting the timing and impacts of subduction and processes leading to the formation of a continental crust. The Izu–Bonin–Mariana (IBM) Arc, western Pacific, is an outstanding location to study arc evolution. The IBM first arc (45–25 Ma) followed a period of forearc basalt and boninite formation associated with subduction initiation (52–45 Ma). In this study, we present new major and trace element data for the IBM first arc from detrital glass shards and clasts from DSDP Site 296, located on the northernmost Kyushu Palau Ridge (KPR). We synthesize these data with published literature for contemporaneous airfall ash and tephra from the Izu–Bonin forearc, dredge and piston core samples from the KPR, and plutonic rocks from the rifted eastern KPR escarpment, locations which lie within or correlate with KPR Segment 1 of Ishizuka, Taylor, Yuasa, and Ohara (2011). Our objective is to test ways in which petrologic and chemical data for diverse igneous materials can be used to construct a complete picture of this section of the Oligocene first arc and to draw conclusions about its evolution. Important findings reveal that widely varying primary magmas formed and differentiated at various depths at this location during this period. Changes in key trace element ratios such as La/Sm, Nb/Yb, and Ba/Th show that mantle sources varied in fertility and in the inputs of subducted sediment and fluids over time and space. Plutonic rocks appear to be related to early K‐poor dacitic liquids represented by glasses sampled both in the forearc and volcanic fronts. An interesting observation is that the variation in magma compositions in this relatively small segment encompasses that inferred for the IBM Arc as a whole, suggesting that sampling is a key factor in inferring temporal, across‐arc, and along‐strike geochemical trends.     

海洋核杂岩

为了解释洋壳中大量铲形正断层及垂直洋中脊的大量线理(如大西洋中脊的巨型窗棱构造)等现象,通过与大陆上变质核杂岩对比,近来提出了一种新的海底构造类型———海洋核杂岩。在洋底深地震剖面上核杂岩结构形态可以分为3部分,其中第1部分为层1和层2,以脆性变形为特征;第2部分为脆-韧性过渡层,拆离带发育其中,由白色结壳式碳酸盐岩和强烈蛇纹石化的橄榄岩或玄武岩、超镁铁质糜棱岩、糜棱状辉长岩等组成。拆离面之上为未变质的薄层海洋沉积层,其下为热洋幔的退变质岩石组成;第3部分为核部,以塑性变形为特征,常被超基性岩体(尤其是辉长岩侵入体)底辟侵入。与大陆变质核杂岩相比,海洋核杂岩具有明显的独特性。海洋核杂岩的拆离断层同样有数十千米的位移量,因而,可能导致出现海底磁条带的局部错位现象,使得洋壳磁条带的平面结构复杂化。     

Rupture and delamination of arc crust rupture and delamination of island arc crust due to the arc–arc collision in the South Fossa Magna, central Japan

Three collisional cycles, the Tanzawa, Izu and Shichito, are known to have occurred in the South Fossa Magna, central Japan, since the late Miocene, based on geologic evidence. The cycles consist of six stages. At present the South Fossa Magna is in the later part of stage 5 of the Izu Cycle and stage 2 of the Shichito Cycle. Because the collisional processes are ongoing we can observe, measure and correlate them with the geologic records of the former cycles. The collisional processes are progressing intermittently because of the rupture and deformation of the collided and colliding island arc crusts. Rupture in the subducting crust can be explained by the geometry of the subducting plate along a boundary that is not straight. The delamination of the upper crust is detected from the geologic and crustal structure in the collided Tanzawa Block; it is an important factor in the deformation of the crust.     

大兴安岭古利库金矿区落马湖群变质岩系及其含矿性

大兴安岭古利库金矿区出露的新元古代-早寒武世落马湖群变质岩系,系一套片麻岩、片岩、变粒岩以及大理岩、板岩组合,属低绿片岩相、高绿片岩相和低角闪岩相.恢复原岩为中基性、中酸性火山岩及其碎屑岩和粉砂岩-砂岩、灰岩等.主要成矿元素Bi、Au、Ag、Hg对比大陆地壳相对富集,在遭受韧性剪切作用后,Au、Ag、Bi明显富集,As、Sb亦有富集.由此推断,落马湖群变质岩系应属矿源岩,韧性剪切作用使Au、Ag等成矿元素预富集.     

Detailed Study of Three Contiguous Segments of the Mid-Atlantic Ridge,South of the Azores (37° N to 38°30′ N), Using Acoustic Imaging Coupled with Submersible Observations

Using a new tool of seafloor characterisation (sonar images from FARA-SIGMA cruise; Needham et al., 1992), coupled with submersible observations (DIVA1 cruise) we compare, at different scales of observation, three contiguous segments of the Mid-Atlantic Ridge, South of the Azores Triple Junction, between 37° N and 38°30 N.The two northernmost segments (38°20 N and Menez-Gwen) show unusual morphological features for the MAR; the rift valley is absent and the present-day magmatism is focused on shallow axial volcanoes. On the third segment (Lucky Strike), the morphology is the one usually found on the MAR. On the Menez-Gwen and 38°20 N segments, volcanic constructional activity can obliterate, during periods of high magmatic supply, the morphology inherited from tectonic activity. The dive results constrain the recent evolution of each segment and show that a temporal variability in volcanic dynamics exists. On the three segments, outcrops of eruptive lavas alternate with large areas of explosive volcanic ejecta. This cycle in volcanic activity is influenced by changes in water depth, both spatially (i.e. between segments) and temporally (i.e. for the same segment through time).Each segment has known a specific history in its accretionary processes with a succession of tectonic and volcanic predominance and changes in its volcanic phases between volcanic ejecta and effusive dynamics.The hydrothermal activity is focused at the central part of each segment and is controlled by the presence of fresh lava and major tectonic features.     

Seismic phases from the Moho and its implication on the ultraslow spreading ridge

The Moho interface provides critical evidence for crustal thickness and the mode of oceanic crust accretion. The seismic Moho interface has not been identified yet at the magma-rich segments （46°-52°E） of the ultra- slow spreading Southwestern Indian Ridge （SWIR）. This paper firstly deduces the characteristics and do- mains of seismic phases based on a theoretical oceanic crust model. Then, topographic correction is carried out for the OBS record sections along Profile Y3Y4 using the latest OBS data acquired from the detailed 3D seismic survey at the SWIR in 2010. Seismic phases are identified and analyzed, especially for the reflected and refracted seismic phases from the Moho. A 2D crustal model is finally established using the ray tracing and travel-time simulation method. The presence of reflected seismic phases at Segment 28 shows that the crustal rocks have been separated from the mantle by cooling and the Moho interface has already formed at zero age. The 2D seismic velocity structure across the axis of Segment 28 indicates that detachment faults play a key role during the processes of asymmetric oceanic crust accretion.     

Lithologic control of bedrock meander dimensions in the Appalachian Valley and Ridge province

Marked differences in bedrock meander dimensions in the Appalachian Valley and Ridge province, at times in adjacent reaches of a single stream, are related to differences in relative erodability of the bedrock. Meanders cut in thick-bedded to massive lithologies, typically carbonates, are distinctly smaller overall than meanders cut into shaly lithologies. Other factors that could affect bedrock meander dimensions were considered and none appeared to offer any help in explaining the dimensional differences observed above. In plateau regions underlain by essentially horizontal strata, the meander form characteristic of one lithology may become superimposed upon another as incision progresses, producing anomalous relationships between bedrock meander dimensions and lithology. Empirical relations developed from meander geometry measurements and estimated bedrock erodability for 78 bedrock meander reaches, containing a total of 1089 individual meander loops, show that meanders cut in shaly lithologies (ML = 105 Q^0.50_f, where ML = meander length and Q_f = most probable annual flood) are about twice the length of meanders cut in non-shaly lithologies (ML = 39.3^0.56_f). The valley floor width of the meanders cut in shaly bedrock (VF = 28 Q^0.43_f) is two to three times wider than the valley floor width of the meanders cut in the more resistant non-shaly bedrock (VF = 8 Q^0.43_f). The mean and median meander length values for individual reaches typically differ by less than 10 per cent.     

岭型组合主成分估计及误差影响

将岭型组合主成分估计用于测量平差,定义了平差参数的岭型组合主成分估计,推导了它的验后精度─均方误差及其误差影响函数,讨论了它的优良性质,并且证明了在一定条件下,它具有比最小二乘估计和岭估计更好的抗干扰性和抗差性。最后,用实例验证了其理论的正确性。     

Morphology of a ‘superfast’ Mid-Ocean Ridge crest and flanks: The East Pacific Rise, 7°–9° S

Detailed bathymetric data from a Hydrosweep multibeam sonar survey of a 250 km-long portion of the superfast-spreading southern East Pacific Rise crest and flanks show that the along-axis variation in morphology and axial depth differs significantly from that observed at the fast-spreading northern East Pacific Rise. While the deep mantle upwelling pattern is similar under the northern and southern East Pacific Rise, our observations require that the connectivity of the shallow, subcrestal plumbing system be more efficient beneath the super-fast spreading southern East Pacific Rise than beneath the slower spreading northern East Pacific Rise.