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1.
A 2‐D crustal velocity model has been derived from a 1997 364 km north‐south wide‐angle seismic profile that passed from Ordovician volcanic and volcaniclastic rocks (Molong Volcanic Belt of the Macquarie Arc) in the north, across the Lachlan Transverse Zone into Ordovician turbidites and Early Devonian intrusive granitoids in the south. The Lachlan Transverse Zone is a proposed west‐northwest to east‐southeast structural feature in the Eastern Lachlan Orogen and is considered to be a possible early lithospheric feature controlling structural evolution in eastern Australia; its true nature, however, is still contentious. The velocity model highlights significant north to south lateral variations in subsurface crustal architecture in the upper and middle crust. In particular, a higher P‐wave velocity (6.24–6.32 km/s) layer identified as metamorphosed arc rocks (sensu lato) in the upper crust under the arc at 5–15 km depth is juxtaposed against Ordovician craton‐derived turbidites by an inferred south‐dipping fault that marks the southern boundary of the Lachlan Transverse Zone. Near‐surface P‐wave velocities in the Lachlan Transverse Zone are markedly less than those along other parts of the profile and some of these may be attributed to mid‐Miocene volcanic centres. In the middle and lower crust there are poorly defined velocity features that we infer to be related to the Lachlan Transverse Zone. The Moho depth increases from 37 km in the north to 47 km in the south, above an underlying upper mantle with a P‐wave velocity of 8.19 km/s. Comparison with velocity layers in the Proterozoic Broken Hill Block supports the inferred presence of Cambrian oceanic mafic volcanics (or an accreted mafic volcanic terrane) as substrate to this part of the Eastern Lachlan Orogen. Overall, the seismic data indicate significant differences in crustal architecture between the northern and southern parts of the profile. The crustal‐scale P‐wave velocity differences are attributed to the different early crustal evolution processes north and south of the Lachlan Transverse Zone.  相似文献   

2.
A deep seismic‐reflection transect in western Victoria was designed to provide insights into the structural relationship between the Lachlan and the Delamerian Orogens. Three seismic lines were acquired to provide images of the subsurface from west of the Grampians Range to east of the Stawell‐Ararat Fault Zone. The boundary between the Delamerian and Lachlan Orogens is now generally considered to be the Moyston Fault. In the vicinity of the seismic survey, this fault is intruded by a near‐surface granite, but at depth the fault dips to the east, confirming recent field mapping. East of the Moyston Fault, the uppermost crust is very weakly reflective, consisting of short, non‐continuous, west‐dipping reflections. These weak reflections represent rocks of the Lachlan Orogen and are typical of the reflective character seen on other seismic images from elsewhere in the Lachlan Orogen. Within the Lachlan Orogen, the Pleasant Creek Fault is also east dipping and approximately parallel to the Moyston Fault in the plane of the seismic section. Rocks of the Delamerian Orogen in the vicinity of the seismic line occur below surficial cover to the west of the Moyston Fault. Generally, the upper crust is only weakly reflective, but subhorizontal reflections at shallow depths (up to 3 km) represent the Grampians Group. The Escondida Fault appears to stop below the Grampians Group, and has an apparent gentle dip to the east. Farther east, the Golton and Mehuse Faults are also east dipping. The middle to lower crust below the Delamerian Orogen is strongly reflective, with several major antiformal structures in the middle crust. The Moho is a slightly undulating horizon at the base of the highly reflective middle to lower crust at 11–12 s TWT (approximately 35 km depth). Tectonically, the western margin of the Lachlan Orogen has been thrust over the Delamerian Orogen for a distance of at least 25 km, and possibly over 40 km.  相似文献   

3.
The magnetotelluric (MT) method was used to image the crust and upper mantle beneath the Delamerian and Lachlan orogens in western Victoria, Australia. During the Cambrian time period, this region changed from being the extended passive margin of Proterozoic Australia into an Andean-style convergent margin that progressively began to accrete younger oceanic terranes. Several broadband MT transects, which were collected in stages along coincident deep (full crust imaging) seismic reflection lines, have now been combined to create a continuous 500 km east–west transect over the Delamerian–Lachlan transition region in the Stawell Zone. We present the electrical resistivity structure of the lithosphere using both 3D and 2D inversion methods. Additionally, 1D inversions of long-period AusLAMP (Australian Lithospheric Architecture Magnetotelluric Project) MT data on a 55 km regionally spaced grid were used to provide starting constraints for the 3D inversion of the 2D profile. The Delamerian to Lachlan Orogen transition region coincides with the Mortlake Discontinuity, which marks an isotopic discontinuity in Cenozoic basalts, with higher strontium isotope enrichment ratios in the Lachlan Orogen relative to the Delamerian Orogen. Phase tensor ellipses of the MT data reveal a distinct change in electrical resistivity structure near the location of the Mortlake Discontinuity, and results of 3D and 2D inversions along the MT profile image a more conductive lower crust and upper mantle beneath the Lachlan Orogen than the Delamerian Orogen. Increased conductivity is commonly ascribed to mantle enrichment and thus supports the notion that the isotope enrichment of the Cenozoic basalts at least partially reflects an enriched mantle source rather than crustal contamination. Fault slivers of the lower crust from the more conductive Lachlan region expose Cambrian boninites and island arc andesites indicative of subduction, a process that can enrich the mantle isotopically, and also electrically, by introducing carbon (graphite) and water (hydrogen).  相似文献   

4.
利用长江中下游成矿带多学科深部探测剖面于2009年11月至2011年3月间采集的天然地震数据,通过天然地震接收函数成像等分析研究,得到了研究区地壳和上地幔结构的清晰图像。接收函数成像结果显示研究区内Moho面深度存在着明显的起伏变化,在长江中下游成矿带(指剖面穿过的长江中下游成矿带宁芜矿集区,下同)下方存在着"幔隆构造"。在剖面东南端(即扬子克拉通北缘),Moho面相对稳定,深度约为30km;在茅山和江南断裂附近,Moho面存在上下起伏现象;在剖面中部或宁芜矿集区下方,Moho面存在明显隆起,深度只有28km;在郯庐断裂带下方,Moho面明显加深,深度达到36km;进一步向北到华北地台南缘,Moho面深度逐渐恢复到了32km左右的平均深度水平。其次,我们在接收函数成像结果中发现,长江中下游成矿带与其周边下地壳结构存在着明显的差异,成矿带的下地壳具有显著的地震波方位各向异性。扬子克拉通北缘的下地壳呈高速的近水平状结构,地震波各向异性特征不明显;与此相比,长江中下游成矿带的下地壳虽然也呈近水平状结构特征,但是,对于沿成矿带走向方向传播的地震波,其下地壳具有高速特征,而对于垂直于成矿带走向方向上传播的地震波,其下地壳却又表现为低速特征,这意味着成矿带的下地壳存在着平行于成矿带走向(即近北东—南西)方向的地震波各向异性,我们解释其是下地壳熔融并沿成矿带走向水平流动导致矿物晶体定向排列的结果。最后,在郯庐断裂以西的华北地台南缘观测到一条从上地壳延伸到中下地壳的南南东向倾斜的转换震相,我们推测它可能是合肥盆地内地壳伸展构造的反映。此外,我们发现接收函数成像结果中观测到的"幔隆构造"与远震P波层析成像结果在成矿带下方150km深度上显示的上地幔低速异常(江国明等,另文发表)存在着良好的对应关系,我们解释它们是软流圈物质上涌的遗迹。综合天然地震接收函数成像、远震P波层析成像和前人关于岩浆岩等方面的研究成果,我们认为长江中下游成矿带现今的下地壳可能是中生代发生成矿作用的多级岩浆房系统的一部分,成矿带的形成可能是类似MASH过程的产物。首先,软流圈物质上涌导致了长江中下游成矿带及其周边拉张环境的形成,在其上部地壳中形成了一系列伸展构造;然后,软流圈物质通过底侵进入长江中下游成矿带的原下地壳并与原下地壳物质发生同化作用,形成类埃达克质岩浆;接着,类埃达克质岩浆沿着伸展、拆离构造上升到地壳浅部形成不同层次的岩浆房和侵入岩体,并与围岩作用形成矿床。  相似文献   

5.
Seismic reflection and refraction data were collected west of New Zealand's South Island parallel to the Pacific–Australian Plate boundary. The obliquely convergent plate boundary is marked at the surface by the Alpine Fault, which juxtaposes continental crust of each plate. The data are used to study the crustal and uppermost mantle structure and provide a link between other seismic transects which cross the plate boundary. Arrival times of wide-angle reflected and refracted events from 13 recording stations are used to construct a 380-km long crustal velocity model. The model shows that, beneath a 2–4-km thick sedimentary veneer, the crust consists of two layers. The upper layer velocities increase from 5.4–5.9 km/s at the top of the layer to 6.3 km/s at the base of the layer. The base of the layer is mainly about 20 km deep but deepens to 25 km at its southern end. The lower layer velocities range from 6.3 to 7.1 km/s, and are commonly around 6.5 km/s at the top of the layer and 6.7 km/s at the base. Beneath the lower layer, the model has velocities of 8.2–8.5 km/s, typical of mantle material. The Mohorovicic discontinuity (Moho) therefore lies at the base of the second layer. It is at a depth of around 30 km but shallows over the south–central third of the profile to about 26 km, possibly associated with a southwest dipping detachment fault. The high, variable sub-Moho velocities of 8.2 km/s to 8.5 km/s are inferred to result from strong upper mantle anisotropy. Multichannel seismic reflection data cover about 220 km of the southern part of the modelled section. Beneath the well-layered Oligocene to recent sedimentary section, the crustal section is broadly divided into two zones, which correspond to the two layers of the velocity model. The upper layer (down to about 7–9 s two-way travel time) has few reflections. The lower layer (down to about 11 s two-way time) contains many strong, subparallel reflections. The base of this reflective zone is the Moho. Bi-vergent dipping reflective zones within this lower crustal layer are interpreted as interwedging structures common in areas of crustal shortening. These structures and the strong northeast dipping reflections beneath the Moho towards the north end of the (MCS) line are interpreted to be caused by Paleozoic north-dipping subduction and terrane collision at the margin of Gondwana. Deeper mantle reflections with variable dip are observed on the wide-angle gathers. Travel-time modelling of these events by ray-tracing through the established velocity model indicates depths of 50–110 km for these events. They show little coherence in dip and may be caused side-swipe from the adjacent crustal root under the Southern Alps or from the upper mantle density anomalies inferred from teleseismic data under the crustal root.  相似文献   

6.
江南造山带位于华南大陆扬子块体和华夏块体之间，其深部地壳结构与变形特征记录了扬子块体与华夏块体拼合与相互作用的痕迹，且在其内部与邻区发育了丰富的多金属矿床，并形成了巨型Cu-Au-Pb-Zn-Ag多金属成矿带，是深化认识华南大陆地壳演化、岩浆作用与成矿系统的关键地域。针对华南大陆地区的地壳结构与成矿过程，国家科技重点研发计划“华南陆内成矿系统的深部过程与物质响应”项目在该区实施了一条密集宽频带地震流动探测剖面，旨在探测其深部结构与物性变化特征和深部成矿背景。本文利用其中江西广昌-湖南浏阳段长320km的宽频带地震流动台站数据开展了远震P波接收函数研究，获得了剖面辖区深部地壳结构和Vp/Vs变化特征。研究结果表明：(1)剖面Moho界面深度在29～35km之间变化，呈近穹窿状分布，平均Moho界面深度为31km左右，低于全球大陆地壳平均值，且与地形高程在整体上呈镜像相关，均衡程度较好；(2)剖面沿线地壳Vp/Vs在1.64～1.83之间呈波浪状起伏变化，平均值为1.72左右，且华夏块体略高于江南造山带...  相似文献   

7.
Teleseismic body waves from broadband seismic stations are used to investigate the crustal and uppermost mantle structure of Stromboli volcano through inversion of the receiver functions (RFs). First, we computed RFs in the frequency domain using a multiple-taper spectral correlation technique. Then, the non-linear neighbourhood algorithm was applied to estimate the seismic shear wave velocity of the crust and uppermost mantle and to define the main seismic velocity discontinuities. The stability of the inversion solution was tested using a range of initial random seeds and model parameterizations. A shallow Moho, present at depth of 14.8 km, is evidence of a thinned crust beneath Stromboli volcano. However, the most intriguing and innovative result is a low S velocity layer in the uppermost mantle, below 32 km. The low S velocity layer suggests a possible partial melt region associated with the volcanism, as also recently supported by tomographic studies and petrological estimations.  相似文献   

8.
David E. James  Fenglin Niu  Juliana Rokosky   《Lithos》2003,71(2-4):413-429
High-quality seismic data obtained from a dense broadband array near Kimberley, South Africa, exhibit crustal reverberations of remarkable clarity that provide well-resolved constraints on the structure of the lowermost crust and Moho. Receiver function analysis of Moho conversions and crustal multiples beneath the Kimberley array shows that the crust is 35 km thick with an average Poisson's ratio of 0.25. The density contrast across the Moho is 15%, indicating a crustal density about 2.86 gm/cc just above the Moho, appropriate for felsic to intermediate rock compositions. Analysis of waveform broadening of the crustal reverberation phases suggests that the Moho transition can be no more than 0.5 km thick and the total variation in crustal thickness over the 2400 km2 footprint of the array no more than 1 km. Waveform and travel time analysis of a large earthquake triggered by deep gold mining operations (the Welkom mine event) some 200 km away from the array yield an average crustal thickness of 35 km along the propagation path between the Kimberley array and the event. P- and S-wave velocities for the lowermost crust are modeled to be 6.75 and 3.90 km/s, respectively, with uppermost mantle velocities of 8.2 and 4.79 km/s, respectively. Seismograms from the Welkom event exhibit theoretically predicted but rarely observed crustal reverberation phases that involve reflection or conversion at the Moho. Correlation between observed and synthetic waveforms and phase amplitudes of the Moho reverberations suggests that the crust along the propagation path between source and receiver is highly uniform in both thickness and average seismic velocity and that the Moho transition zone is everywhere less than about 2 km thick. While the extremely flat Moho, sharp transition zone and low crustal densities beneath the region of study may date from the time of crustal formation, a more geologically plausible interpretation involves extensive crustal melting and ductile flow during the major craton-wide Ventersdorp tectonomagmatic event near the end of Archean time.  相似文献   

9.
莫霍面地震反射图像揭露出扬子陆块深俯冲过程   总被引：21，自引：0，他引：21  
近垂直深地震反射剖面对莫霍面变化的观测 ,强有力地说明大陆莫霍面的复杂特征记录了岩石圈的构造历史。横过大别山造山带前陆的深地震反射剖面长约 1 4 0km ,记录时间达 3 0s ,探测深度超过莫霍面深达岩石圈地幔。深地震反射剖面揭示出扬子陆块与大别山造山带结合部位的岩石圈精细结构、清晰的莫霍面及其变化特征。作为相关解释的第一步 ,我们将探测到的莫霍面变化特征与其他特殊反映不同地质年代和岩石圈构造历史的深地震反射剖面进行对比 ,以追索扬子陆块与大别山造山带的岩石圈构造过程。总体北倾的莫霍面和同样北倾的下地壳结构记录了中生代扬子陆块的向北俯冲。北倾的莫霍面错断、叠置现象描述出扬子陆块的俯冲过程。大别山前向北和向南倾斜的交叉反射图像 ,反映了扬子陆块与大别山造山带岩石圈尺度的碰撞关系  相似文献   

10.
A ～400 km long deep crustal reflection seismic survey was acquired in central Victoria, Australia, in 2006. It has provided information on crustal architecture across the western Lachlan Orogen and has greatly added to the understanding of the tectonic evolution. The east-dipping Moyston Fault is confirmed as the suture between the Delamerian and western Lachlan Orogens, and is shown to extend down to the Moho. The Avoca Fault, the boundary between the Stawell and Bendigo Zones, is a west-dipping listric reverse fault that intersects the Moyston Fault at a depth of about 22 km, forming a V-shaped geometry. Both the Stawell and Bendigo Zones can be divided broadly into a lower crustal region of interlayered and imbricated metavolcanic and metasedimentary rocks and an upper crustal region of tightly folded metasedimentary rocks. The Stawell Zone was probably part of a Cambrian accretionary system along the eastern Gondwanaland margin, and mafic rocks may have been partly consumed by Cambrian subduction. Much of the Early Cambrian oceanic crust beneath the Bendigo Zone was not subducted, and is preserved as a crustal-scale imbricate thrust stack. The seismic data have shown that a thin-skinned structural model appears to be valid for much of the Melbourne Zone, whereas the Stawell and Bendigo Zones have a thick-skinned structural style. Internal faults in the Stawell and Bendigo Zones are mostly west-dipping listric faults, which extend from the surface to near the base of the crust. The Heathcote Fault Zone, the boundary between the Bendigo and Melbourne Zones, extends to at least 20 km, and possibly to the Moho. A striking feature in the seismic data is the markedly different seismic character of the mid to lower crust of the Melbourne Zone. The deep seismic reflection data for the Melbourne Zone have revealed a multilayered crustal structure that supports the Selwyn Block model.  相似文献   

11.
C. Bois 《地学学报》1992,4(1):99-108
Deep seismic reflection images from a set of profiles shot in Western Europe have been reviewed and compared, and tentative conclusions have been proposed concerning the evolution of the layered lower crust and the Moho. The disappearance of Variscan mountain roots is related to the set-up of a new Moho at a typical 30-km depth and the creation of seismic layering in the lower crust. Deep seismic profiles suggest that these processes resulted, at least in part, from magmatic intrusion, partial crustal melting and metamorphism of deep crustal rocks into eclogite. On the other hand, the layered lower crust is greatly attenuated beneath Cretaceous basins and Tertiary rifts in relation to prominent Moho upwellings. The unusual amplitude of the Moho reflection and the presence of anomalously high seismic velocities in the lowermost crust beneath the Tertiary rifts suggest that the Moho and part of the layering are comparatively young features related to interactions between crust and mantle. Beneath Triassic-Jurassic basins, the layered lower crust was not affected by the subsidence of the basement, with the whole crustal thinning being entirely concentrated in the upper crust. This indicates that the layered lower crust and the Moho were formed or restored during or after the main rifting phase. Seismic data reveal constraints on the processes that affect the crust-mantle transition and seem to restore the Moho to its typical depth after any mechanical deformation of the lithosphere.  相似文献   

12.
Distant earthquake data recorded by seven sub-arrays of the ongoing WOMBAT rolling seismic array deployment in southeast Australia are combined for the first time to constrain 3-D variations in upper mantle P-wavespeed via teleseismic tomography. The seven arrays comprise a total of 276 short period recorders spaced at intervals of approximately 50 km, thus allowing unprecedented resolution of the upper mantle over a large region. In the mantle lithosphere immediately below the crust (~ 50 km depth), dominant variations in velocity tend to strike east–west, and share little resemblance to Palaeozoic boundaries in the shallow crust inferred from surface geology and potential field data. A broad region of elevated wavespeed beneath northern Victoria may represent the signature of underplated igneous rocks associated with detachment faulting during the break-up of Australia and Antarctica. A distinct low velocity anomaly in southern Victoria appears to correlate well with the Quaternary Newer Volcanic Provinces. Towards the base of the mantle lithosphere, the dominant structural trend becomes north–south, and five distinct velocity zones become apparent. Of particular note is a transition from higher wavespeed in the west to lower wavespeed in the east beneath the Stawell Zone, implying that the Proterozoic lithosphere of the Delamerian Orogen protrudes eastward beneath the Western subprovince of the Lachlan Orogen. This transition zone extends northwards from southern Victoria into central New South Wales (the northward limit of the arrays), and is one of the dominant features of the model. Further east, there is a transition from lower to higher wavespeeds in the vicinity of the boundary between the Western and Central subprovinces of the Lachlan Orogen, which has several plausible explanations, including the existence of a Proterozoic continental fragment beneath the Wagga–Omeo Zone. The presence of elevated wavespeeds beneath the Melbourne Zone in Victoria, although not well constrained due to limited data coverage, provides some support to the Selwyn Block model, which proposes a northward extension beneath Bass Strait of the Proterozoic core of Tasmania.  相似文献   

13.
The Otway Basin in southeastern Australia formed on a triangular‐shaped area of extended continental lithosphere during two extensional episodes in Cretaceous to Miocene times. The extent of the offshore continental margin is highlighted by Seasat/Geosat satellite altimeter data. The crustal architecture and structural features across this southeast Australian margin have been interpreted from offshore‐onshore wide‐angle seismic profiling data along the Otway Continental Margin Transect extending from the onshore Lake Condah High, through the town of Portland, to the deep Southern Ocean. Along the Otway Continental Margin Transect, the onshore half‐graben geometry of Early Cretaceous deposition gives way offshore to a 5 km‐thick slope basin (P‐wave velocity 2.2–4.6 km/s) to at least 60 km from the shoreline. At 120 km from the nearest shore in a water depth of 4220 m, sonobuoy data indicate a 4–5 km sedimentary sequence overlying a 7 km thick basement above the Moho at 15 km depth. Major fault zones affect the thickness of basin sequences in the onshore area (Tartwaup Fault Zone and its southeast continuation) and at the seaward edge of the Mussel Platform (Mussel Fault). Upper crustal basement is interpreted to be attenuated and thinned Palaeozoic rocks of the Delamerian and Lachlan Orogens (intruded with Jurassic volcanics) that thin from 16 km onshore to about 3.5 km at 120 km from the nearest shore. Basement rocks comprise a 3 km section with velocity 5.5–5.7 km/s overlying a deeper basement unit with velocity 6.15–6.35 km/s. The Moho shallows from a depth of 30 km onshore to 15 km depth at 120 km from the nearest shore, and then to about 12 km in the deep ocean at the limits of the transect (water depth 5200 m). The continent‐ocean boundary is interpreted to be at a prominent topographic inflection point 170 km from shore at the bottom of the continental slope in 4800 m of water. P‐wave velocities in the lower crust are 6.4–6.8 km/s, overlying a thin transition zone to an upper mantle velocity of 8.05 km/s beneath the Moho. Outstandingly clear Moho reflections seen in deep‐marine profiling data at about 10.3 s two‐way time under the slope basin and continent‐ocean boundary place further strong controls on crustal thickness. There is no evidence of massive high velocity (>7 km/s) intrusives/underplate material in the lower crust nor any synrift or early post‐rift subaerial volcanics, indicating that the Otway continental margin can be considered a non‐volcanic margin, similar in many respects to some parts of the Atlantic Ocean margins e.g. the Nova Scotia ‐ Newfoundland margin off Canada and the Galicia Bank off the Iberian Peninsula. Using this analogue, the prominent gravity feature trending northwest‐southeast at the continent‐ocean boundary may indicate the presence of highly serpentinised mantle material beneath a thin crust, but this has yet to be tested by detailed work.  相似文献   

14.
Strain reversal of structural/stratigraphic profiles at different scales in the western Lachlan Orogen provides a perspective on original crustal thickness estimates, the former depositional basin width of the proto-western Lachlan Orogen, the original sedimentary-fan thickness, and the possible length extent of lower crust lost by subduction. Retrodeformation using strain-reversal techniques allows basin reconstruction giving an original width of the western Lachlan Orogen basin receptor of between 800 km (minimum) and ～1150 km (maximum), depending on the amount of stratal duplication allowed in the turbidites. Crude area balancing of the regional cross-section, adding in sectional volume lost by erosion and assuming strain compatibility between the upper and lower crust, suggests that the predeformation crustal thickness ranges between 15 km and ～21 km, with a lower crustal thickness (oceanic lithosphere) of ～9 km and a turbidite fan thickness of ～6 km (minimum) and ～12 km (maximum allowable), respectively. Disparity between the calculated fan thickness and that derived from measured stratigraphic sections adjusted for strain (～6 km) indicates that some form of crustal stacking must be important in structural thickening of the turbidite crustal component. By varying shortening due to fault stacking, mass balance dictates the mismatch of the upper crustal (uc) and lower crustal (lc) retrodeformed lengths, and therefore provides an estimate of lower crustal loss by subduction. End members range from: (i) a 12 km-thick fan without fault duplication, a basin width of ～800 km where uc = lc giving no lower crustal loss by subduction; to (ii) a ～6 km fan, requiring duplication by faulting, a basin of ～1150 km where uc > lc, and ～360 km of lower crust length (～30%) lost by subduction. This suggests that the total thickness of underplated igneous material in the western Lachlan Orogen is low, probably < ～2 km.  相似文献   

15.
青藏高原东部壳幔速度结构和地幔变形场的研究   总被引：16，自引：0，他引：16  
在青藏高原东部地球动力学问题中,笔者在文中主要考虑与地壳上地幔速度结构和地幔变形场有关的问题,它涉及当前流行的下地壳流动模型和壳-幔的耦合-解耦模型。在2000年完成的穿过川西高原和四川盆地的深地震测深剖面,揭示了川西高原的地壳结构具有地壳增厚(主要是下地壳增厚)、地壳平均速度低等特点,显示地壳的缩短与增厚的碰撞变形特征。根据川西高原上设置各爆炸点的记录截面图共同呈现PmP(莫霍界面反射波)弱能量的特点,推断在川西高原的下地壳介质具有强衰减(Qp=100～300)的性质,支持存在下地壳流动的模型。青藏高原东部和川滇西部地区的上地幔各向异性(SKS波快波偏振方向和快慢波延迟时间)的初步结果表明,这两个地区的壳-幔变形特征是不同的,尽管它们在地理位置上属于同一个板块碰撞带。在青藏高原内部的壳幔变形属于垂直连贯变形,它以缩短为主,而高原外部的地壳(或岩石圈)则相对于其下方地幔运动。在高原内部和外部之间存在一个重要的地幔变形过渡带。然而,高原内部的垂直连贯变形与高原内部存在大范围下地壳流动的模型不一致。笔者在该地区开展了近两年的宽频带流动地震观测,试图从地震记录中确定过渡带的位置和探讨它的流变性质。文中扼要回顾已经取得的结果,并介绍正在进行的研究。  相似文献   

16.
Abstract

The origin of elevated geothermal gradients in the subsurface Thomson Orogen and the nature of the crustal basement beneath it, whether oceanic or continental, remain enigmatic. Previous studies have demonstrated that a higher crustal radiogenic input is required to explain these anomalous thermal gradients. In this study, we have investigated the nature and age of this crustal input by undertaking geochemical, geochronological and Hf and O isotope analyses of buried granitic rocks as well as evaluating the heat-producing potential of metasedimentary rocks. The mineralogy, composition and Neoproterozoic/Cambrian to Devonian age of the low to moderate heat-producing I- and S-type granitic rocks strongly contrast with the Carboniferous A-type high-heat-producing granites of the Big Lake Suite, which have been suggested to be an important contributor to the elevated geothermal gradients, near the southwest corner of the Thomson Orogen. These differences suggest the Big Lake Suite rocks do not extend into the Queensland part of the temperature anomaly. Heat production of the metasedimentary rocks is also low to moderate. Based on Hf isotope compositions of zircons characterised by mantle-like oxygen signature (?Hf(t) = –12 to +2), we propose the temperature anomaly results from the occurrence of Mesoproterozoic and/or Paleoproterozoic high-heat-producing rocks beneath the Thomson Orogen. Precambrian crust, therefore, lies well east of the Tasman line. The results do not support a Neoproterozoic to Cambrian oceanic crust, as previously suggested, but instead point to a continental substrate for the Thomson Orogen. Hf isotopes indicate an overall trend towards more isotopically juvenile compositions with a progressive reduction in the contribution of older crustal sources to granitic magmas towards the present time. Different Hf isotopic signatures for the Lachlan (?Hf(t) = –13 to +15), Thomson (?Hf(t) = –14 to +5) and Delamerian (?Hf(t) = –7 to +4) orogens highlight lateral variations in the age structures of crustal basement beneath these orogens.  相似文献   

17.
ABSTRACT

The land-sea transition zone in the northern South China Sea (SCS) records important information from the continental rifting to the seafloor spreading. The crustal structure is the key to explore the deep tectonic environment and the evolution of the SCS. In 2015, the onshore-offshore 3D deep seismic experiment was carried out on the Pearl River Estuary (PRE). Explosions and air guns were used as sources on land and at sea respectively in this experiment.Onshore seismic stations and Ocean Bottom Seismographs (OBSs) synchronously recorded the seismic signals. We focus on an onshore-offshore seismic profile (L2, SE-trending) along the eastern side of the PRE. By modelling the seismic travel times, we constructed a P-wave velocity model along the profile. The model shows that the sediment on land is thin and has seismic velocities of 4.5–5.5 km/s. In contrast, thickness of the offshore sediment gradually increases to more than 4.0 km, and the velocities vary between 2.0 km/s and 4.5 km/s. The onshore and offshore crustal velocities are 5.8–6.8 km/s and 5.5–6.8 km/s, respectively. At depth between 15 km and 20 km, a low-velocity layer (LVL; only 5.9 km/s) is detected, pinching out under the Littoral Fault Zone (LFZ). The LVL has probably accommodated the crustal extension beneath the land area, resulting in low extent of the crustal thinning. A slightly uplifted Moho exists beneath the Dongguan fault depression zone, representing a place where hot mantle materials ascend. Localized thickening of the sediments and rapid thinning of the crust characterize the LFZ, and it can be regarded as a tectonic boundary between the South China (SC) with normal continental crust and the northern SCS margin with extended continental crust.  相似文献   

18.
The Borborema Province of northeastern Brazil is a major Proterozoic crustal province that, until now, has never been explored using deep crustal seismic methods. Here are reported the first results obtained from a high-quality seismic refraction/wide-angle reflection profile that has defined the internal seismic velocity structure and thickness of the crust in this region. Almost 400 recording stations were deployed in the Deep Seismic Refraction (DSR) experiment through an NW–SE ca. 900 km linear array and 19 shots were exploded at every 50 km along the line. Data from the 10 southeastern most shots of the seismic profile were processed in this work. The main features and geological structures crossed by the studied portion of the profile belong to the so-called Central Sub-province of the Borborema tectonic province. The crustal model obtained is compatible with a typical structure of extended crust. The model was essentially divided into three layers: upper crust, lower crust, and a half-space represented by the shallower portion of the mantle. The Moho is an irregular interface with depth ranging between 31.7 and 34.5 km, and beneath the Central Sub-province it varies from 31.5 to 33 km depth, where its limits are related to major crustal discontinuities. The distribution of velocities within the crust is heterogeneous, varying vertically from 5.7 to 6.3 km/s in the upper crust and from 6.45 to 6.9 km/s in the lower crust. From the average crustal velocity distribution it is evident that the Central Sub-province has seismic characteristics different from neighboring domains. The crust is relatively thin and crustal thickness variations in the profile are subtle due to stretching that occurred in the Cretaceous, during the fragmentation of Pangaea, opening of the South Atlantic Ocean and separation of South America from Africa.  相似文献   

19.
The North China Craton (NCC), which is composed of the eastern NCC and the western NCC sutured by the Palaeoproterozoic Trans‐North China Orogen, is one of the oldest continental nuclei in the world and the largest cratonic block in China. The eastern NCC is widely known for its significant lithospheric thinning and destruction during the Late Mesozoic. Models on the destruction of the eastern NCC can be principally grouped into two: (1) thermal/mechanical and/or chemical erosion, and (2) lower crustal and (or) lithospheric delamination. The erosion model suggests that the NCC lithospheric thinning resulted from chemical and/or mechanical interactions of lithospheric mantle with melts or hydrous fluids derived from the asthenosphere, whereas the delamination model proposes lithospheric destruction through foundering of eclogitic lower crust together with lithospheric mantle into the underlying convecting mantle. However, those models lack seismic evidence to explain the destruction process. Here, we analyse the crustal structure and upper mantle discontinuity by employing the H–k stacking technique of receiver function as well as the depth domain receiver function. Our results indicate deep mantle upwelling and lower crustal delamination beneath the eastern NCC, and suggest that either or both of these processes contributed to the unique lithospheric thinning and destruction of the eastern NCC. © 2013 The Authors. Geological Journal published by John Wiley & Sons, Ltd.  相似文献   

20.
The Moho topography is strongly undulating in southern Scandinavia and northeastern Europe. A map of the depth to Moho shows similarities between the areas of the Teisseyre–Tornquist Zone (TTZ) in Poland and the Fennoscandian Border Zone (FBZ), which is partly coinciding with the Sorgenfrei–Tornquist Zone (STZ) in Denmark. The Moho is steeply dipping at these zones from a crustal thickness of approximately 32 km in the young Palaeozoic Platform and basin areas to approximately 45 km in the old Precambrian Platform and Baltic Shield. The Moho reflectivity (PMP waveform) in the POLONAISE'97 refraction/wide-angle seismic data from Poland and Lithuania is variable, ranging from ‘sharp’ to strongly reverberating signals of up to 2 s duration. There is little or no lower crustal wide-angle reflectivity in the thick Precambrian Platform, whereas lower crustal reflectivity in the thin Palaeozoic Platform is strongly reverberating, suggesting that the reflective lower crust and upper mantle is a young phenomena. From stochastic reflectivity modelling, we conclude that alternating high- and low-velocity layers with average thicknesses of 50–300 m and P-wave velocity variations of ±3–4% of the background velocity can explain the lower crustal reflectivity. Sedimentary layering affects the reflectivity of deeper layers significantly and must be considered in reflectivity studies, although the reverberations from the deeper crust cannot be explained by the sedimentary layering only. The reflective lower crust and upper mantle may correspond to a zone that has been intruded by mafic melts from the mantle during crustal extension and volcanism.  相似文献   

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