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91.
Submersible observations and photogeology document dramatic variations in the distribution of young volcanic rocks, faulting,
fissuring, and hydrothermal activity along an 80 km-long segment of the Mid-Atlantic Ridge south of the Kane Transform (MARK
Area). These variations define two spreading cells separated by a cell boundary zone or a small-offset transform zone. The
northern spreading cell is characterized by a median ‘neovolcanic’ ridge which runs down the axis of the median valley floor
for 40 km. This edifice is as much as 4 km wide and 600 m high and is composed of very lightly sedimented basalts inferred
to be < 5000 years old. It is the largest single volcanic constructional feature discovered to date on the Mid-Atlantic Ridge.
The active Snake Pit hydrothermal vent field is on the crest of this ridge and implies the presence of a magma chamber in
the northern spreading cell. In contrast, the southern cell is characterized by small, individual volcanos similar in size
to the central volcanos in the FAMOUS area. Two of the volcanos that were sampled appear to be composed of dominantly glassy
basaltic rocks with very light sediment cover; whereas, other volcanos in this region appear to be older features. The boundary
zone between the two spreading cells is intensely faulted and lacks young volcanic rocks. This area may also contain a small-offset
( < 8 km) transform zone. Magmatism in the northern cell has been episodic and tens of thousands of years have lapsed since
the last major magmatic event there. In the southern cell, a more continuous style of volcanic accretion appears to be operative.
The style of spreading in the southern cell may be much more typical for the Mid-Atlantic Ridge than that of the northern
cell because the latter is adjacent to the 150 km-offset Kane Transform that may act as a thermal sink along the MAR. Such
large transforms are not common on the MAR, therefore, lithosphere produced in a spreading cell influenced by a large transform
may also be somewhat atypical. 相似文献
92.
93.
Laura S. L. Kong Robert S. Detrick Paul J. Fox Larry A. Mayer W. B. F. Ryan 《Marine Geophysical Researches》1988,10(1-2):59-90
High-resolution Sea Beam bathymetry and Sea MARC I side scan sonar data have been obtained in the MARK area, a 100-km-long
portion of the Mid-Atlantic Ridge rift valley south of the Kane Fracture Zone. These data reveal a surprisingly complex rift
valley structure that is composed of two distinct spreading cells which overlap to create a small, zero-offset transform or
discordant zone. The northern spreading cell consists of a magmatically robust, active ridge segment 40–50 km in length that
extends from the eastern Kane ridge-transform intersection south to about 23°12′ N. The rift valley in this area is dominated
by a large constructional volcanic ridge that creates 200–500 m of relief and is associated with high-temperature hydrothermal
activity. The southern spreading cell is characterized by a NNE-trending band of small (50–200 m high), conical volcanos that
are built upon relatively old, fissured and sediment-covered lavas, and which in some cases are themselves fissured and faulted.
This cell appears to be in a predominantly extensional phase with only small, isolated eruptions. These two spreading cells
overlap in an anomalous zone between 23°05′ N and 23°17′ N that lacks a well-developed rift valley or neovolcanic zone, and
may represent a slow-spreading ridge analogue to the overlapping spreading centers found at the East Pacific Rise. Despite
the complexity of the MARK area, volcanic and tectonic activity appears to be confined to the 10–17 km wide rift valley floor.
Block faulting along near-vertical, small-offset normal faults, accompanied by minor amounts of back-tilting (generally less
than 5°), begins within a few km of the ridge axis and is largely completed by the time the crust is transported up into the
rift valley walls. Features that appear to be constructional volcanic ridges formed in the median valley are preserved largely
intact in the rift mountains. Mass-wasting and gullying of scarp faces, and sedimentation which buries low-relief seafloor
features, are the major geological processes occurring outside of the rift valley. The morphological and structural heterogeneity
within the MARK rift valley and in the flanking rift mountains documented in this study are largely the product of two spreading
cells that evolve independently to the interplay between extensional tectonism and episodic variations in magma production
rates. 相似文献
94.
地震层析资料表明,南海地区,自红河口向南经南海、苏录海到苏拉威西海,岩石圈速度低,底部横波速度仅4.4km/s,岩石圈厚度在60~80km,为薄岩石圈地区;而软流层的速度也较低,在4.2~4.4km/s,但厚度较大,大于200km。从红河—莺歌海断裂带经南海到苏录海,存在一条北西向宽约200km的上地幔北西向低速带,速度在4.05~4.25km/s(面波速度)。它反映了新生代南海地区上地幔的动力学过程。南海岩石圈厚度变化存在明显差异,南海陆缘,岩石圈厚度在70~80km,而在南海洋盆之下,岩石圈厚度超过100km,岩石圈底部存在高速岩石层,并且洋盆下的岩石圈之厚度比大陆边缘厚,在海盆岩石圈下部的60~80km深度上存在一高速层,纵波速度为8.2~8.3km/s。特别是中央海盆及西北海盆与西南海盆,其下部岩石圈中均存在一高速岩石层,这是非常具有构造意义的。由此笔者提出大陆岩石圈裂离、上地幔因减压而部分熔融所产生的基性岩浆形成南海新生代洋壳的猜想。 相似文献
95.
Tectonic controls on a large landslide complex: Williams Fork Mountains near Dillon, Colorado 总被引:2,自引:0,他引:2
An extensive ( 25 km2) landslide complex covers a large area on the west side of the Williams Fork Mountains in central Colorado. The complex is deeply weathered and incised, and in most places geomorphic evidence of sliding (breakaways, hummocky topography, transverse ridges, and lobate distal zones) are no longer visible, indicating that the main mass of the slide has long been inactive. However, localized Holocene reactivation of the landslide deposits is common above the timberline (at about 3300 m) and locally at lower elevations. Clasts within the complex, as long as several tens of meters, are entirely of crystalline basement (Proterozoic gneiss and granitic rocks) from the hanging wall of the Laramide (Late Cretaceous to Early Tertiary), west-directed Williams Range thrust, which forms the western structural boundary of the Colorado Front Range. Late Cretaceous shale and sandstone compose most footwall rocks. The crystalline hanging-wall rocks are pervasively fractured or shattered, and alteration to clay minerals is locally well developed. Sackung structures (trenches or small-scale grabens and upslope-facing scarps) are common near the rounded crest of the range, suggesting gravitational spreading of the fractured rocks and oversteepening of the mountain flanks. Late Tertiary and Quaternary incision of the Blue River Valley, just west of the Williams Fork Mountains, contributed to the oversteepening. Major landslide movement is suspected during periods of deglaciation when abundant meltwater increased pore-water pressure in bedrock fractures.A fault-flexure model for the development of the widespread fracturing and weakening of the Proterozoic basement proposes that the surface of the Williams Range thrust contains a concave-downward flexure, the axis of which coincides approximately with the contact in the footwall between Proterozoic basement and mostly Cretaceous rocks. Movement of brittle, hanging-wall rocks through the flexure during Laramide deformation pervasively fractured the hanging-wall rocks. 相似文献
96.
从区域地质背景、岩石学、岩石化学、微量元素、氧同位素及稀土元素组成等方面论证东川铜矿含铜“礁—硅岩组合”中的含铜硅质条带—硅质白云岩为海底喷流热水沉积成因;提出了该类层状铜矿的深源含铜富硅混合热液海底喷流—生物富集的沉积成岩成矿模式。并提出东川落雪式层状“马尾丝”铜矿含矿岩系为“含铜礁—硅岩组合”的新观点。 相似文献
97.
构造重力扩展机制的地质力学模拟研究 总被引:3,自引:0,他引:3
本文采用相似材料的地质力学机制模拟试验再现并验证了西安地裂缝的构造重力扩展机制,得出了构造重力扩展机制是西安地裂缝形成的根本成因,同时还得出了工程建筑物只要避开地裂缝一定距离就可以安全布置,以及西安地裂缝最大可能延深为老第三系地层的中上部等重要结论,初步揭示了现今西安市异常地面沉降的成因,具有一定的理论价值和社会效益。 相似文献
98.
正断层生长和侧向连接是最基本的三维地质构造问题,深入剖析其三维时 空演化过程对理解沉积盆地内正断层系统的形成与演化至关重要。
南海扩张时间和影响范围一直是国内外地学界研究的热点问题之一。南海北部珠江口盆地陆丰凹陷发育大量正断层,并覆盖有高精度二维地震反射剖面,成为研究上述问题的理想地区之一。本研究以陆丰凹陷为研究对象,开展三维构造解析和建模研究,为正断层系统三维时 空演化过程提供一个实例。研究结果表明:陆丰凹陷主控断层F1的生长和连接过程主要分三个阶段:晚白垩世—早古新世,两条断层分别形成并以孤立断层模式分别向两侧生长;早古新世—早渐新世,断层发生侧向连接作用合并为F1断层,连接处发育侧断坡;晚渐新世—中中新世,F1断层继续向两侧生长。基于F1断层定量分析(生长指数和断距),发现F1断层活动强度表现为先增加然后突然降低,其转折时间对应于T60界面,~23 Ma,较好地对应了西北次海盆扩张终止时间,说明南海北部扩张已经影响到陆丰凹陷。结合前人对珠江口盆地构造变形和演化研究,初步认为南海北部扩张波及的范围已跨过陆丰凹陷向大陆方向继续延伸,其北部边界可能对应于华南大陆南缘的滨海断裂带。本研究可为正断层系统三维生长过程研究提供案例,同时为南海扩张过程研究提供基础数据支撑。 相似文献
99.
通过大比例尺活动断裂地质填图,查明了宝鸡高家村──高店断裂的几何展市及内部结构特征。在多处发现断层露头剖面,断层错断了晚更新世和全新世沉积物,反映出断层具有很新的活动性。 相似文献
100.