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首页> 外文期刊>South African Journal of Geology: Being the Transaction of the Geological Society of South African: Syndie die Verhandelinge van die Geologiese Vereniging van Suid-Afrika >DEEP CRUSTAL PROFILE ACROSS THE SOUTHERN KAROO BASIN AND BEATTIE MAGNETIC ANOMALY, SOUTH AFRICA: AN INTEGRATED INTERPRETATION WITH TECTONIC IMPLICATIONS
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DEEP CRUSTAL PROFILE ACROSS THE SOUTHERN KAROO BASIN AND BEATTIE MAGNETIC ANOMALY, SOUTH AFRICA: AN INTEGRATED INTERPRETATION WITH TECTONIC IMPLICATIONS

机译：南部卡鲁盆地和贝蒂磁异常的深部地壳剖面：构造意义的综合解释

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Two geophysical onshore-offshore lines on the southern margin of Africa form the Agulhas-Karoo Geophysical Transect (AKGT) and cross prominent geological features such as the Karoo Basin, Cape Fold Belt (CFB) and the Beattie Magnetic Anomaly (BMA).Geophysical data acquired along this AKGTransect between 2004 and 2007 within the Inkaba yeAfrica (lyA) framework, provide the platform for constructing a deep crustal section (IyA-200501) for the centre 100 km of the western AKGT-transect in order to resolve these features at depth. We present a detailed deep crustal model constructed from the joint interpretation of:i. archive data comprising surface geology, aeromagnetic data, nearby deep boreholes, teleseismic receiver functions and regional seismic reflection profiles, and ii. line coincident newly acquired high-resolution geophysical data consisting of near vertical seismic reflection data, shallow P- and S-wave velocity data, wide-angle refraction data, high resolution magnetotelluric data and impedance spectroscopy measurements on borehole samples. Our model differentiates four components in the up to 45 km thick crust:1. a ~2 to 5 km thick folded Karoo Supergroup, disrupted by low-angle thrust faults rooted in a zone of local décollements in the lower Ecca Group and resting paraconformably on 2. a continuous undeformed sub-horizontal ~1.5 to 10 km thick wedge of the Cape Supergroup (CSG). This CSG wedge stretches from the Escarpment in the north to the tectonic front of the CFB in the south, and rests on an unconformity that dips about three degrees to the south. The angular unconformity is interpreted as an erosional peneplain that separates the CSG wedge from component 3. the ~13 to 21 km mid-crust basement below. The mid-crust contains a distinct north-dipping seismic fabric, here interpreted as~1.4 to 1.0 Ga Mesoproterozoic Namaqua-Natal Metamorphic Belt (NNMB) crust. A south-dipping mid-crustal detachment, interpreted as a ductile thrust zone, separates the mid-crust from component 4. a highly reflective ~10 to 24 km thick lower crust. The latter is interpreted as an older Palaeoproterozoic section of the NNMB (or even Archean cratonic basement), and bounded by a ~2 to 5 km thick, highly reflective bottom layer below that lies subparallel to a clear Moho. This bottom layer is interpreted as a mafic underplate, metasomatic reaction zone, or lower-crust to mantle transition zone. Collectively the seismic reflection and wide-angle refraction data support an interpretation that the NNMB mid-crustal layer contains the BMA source, possibly connected to two zones of strong reflectivity: a ~10 to 12 km wide northern zone and a ~5 to 7 km wide southern zone, both about 5 km thick and 7 to 8 km below surface. We interpret the BMA source to be at least in part, a Namaqua-like massive to disseminated, deformed/metamorphosed stratiform sulphide-magnetite ore body with metasomatic overprint.The seismic reflection and -refraction data support an interpretation that a Pan-African suture zone at the BMA is absent and that instead, the NNMB continues below the CFB tectonic front, probably up to the continental margin and the Agulhas Fracture Zone. The seismic reflection data also supports a thin-skinned tectonic thrust model for the evolution of the CFB without significant fore-deep stratigraphic thickening of the Karoo Basin strata. A compatible tectonic model implies a Palaeozoic collision orogen setting, coupled to a south verging subduction zone much farther south of the CFB. Similarly, the geophysical data support a south dipping subduction zone during the amalgamation of the NNMB in the Mesoproterozoic. Current reconstructions of the Rodinia supercontinent link the NNMB and the Grenville Province of North America across the Grenville-Kibaran orogen. Our seismic section tests this reconstruction through a direct comparison with seismic profiles on the opposite flank of the orogen. Although the once adjacent continental blocks are no
机译：非洲南部边缘的两条地球物理陆上-近海线形成了Agulhas-Karoo地球物理样带（AKGT），并跨越了诸如Karoo盆地，Cape Fold带（CFB）和Beattie磁异常（BMA）等突出的地质特征。于2004年至2007年期间在Inkaba yeAfrica（lyA）框架内沿该AKGTransect收购，为在西部AKGT样带中心100 km处构造深地壳剖面（IyA-200501）提供了平台，以便深入解析这些特征。我们提出了根据i的联合解释构造的详细的深层地壳模型。存档数据，包括地表地质，航空磁数据，附近的深孔，远震接收器功能和区域地震反射剖面，以及ii。线重合的新近获得的高分辨率地球物理数据，包括近垂直地震反射数据，浅P和S波速度数据，广角折射数据，高分辨率大地电磁数据以及对井眼样品的阻抗谱测量。我们的模型区分了长达45 km厚的地壳中的四个成分：1。一个约2至5 km厚的折叠的Karoo超级群，被低角度的逆冲断层打断，该断层根植于下部Ecca组的局部斜度带中，并顺次地搁置在2上。连续的未变形的亚水平约1.5至10 km厚的楔形Cape Supergroup（CSG）。该CSG楔形物从北部的悬崖延伸到南部的CFB构造前缘，并搁置在不整合面上，向南倾斜约3度。角度不整合被解释为侵蚀性的平原，将CSG楔块与分量3分开。下面的地壳中部基底约13至21 km。中地壳包含一种独特的北倾地震构造，在这里被解释为〜1.4〜1.0 Ga中元古代纳马夸-纳塔尔变质带（NNMB）地壳。向南倾斜的中地壳脱离，被解释为延性冲断带，将中地壳与组分4分开。高反射性的约10至24 km厚的下地壳。后者被解释为是NNMB（甚至是太古宙克拉通基底）的旧元古界断层，并由一个约2至5 km厚，高度反射的底层围绕，该底层与纯净的莫霍面平行。该底层被解释为镁铁质底板，交代反应区或下地壳到地幔的过渡带。总的来说，地震反射和广角折射数据支持以下解释：NNMB中地壳层包含BMA源，可能与两个强反射率区域相连：北部约10至12 km，北部约5至7 km较宽的南部地带，厚约5公里，地表以下7至8公里。我们认为BMA来源至少部分是Namaqua状块状，散布，变形/变质的层状硫化物-磁铁矿矿体，并具有交代叠印。地震反射和折射数据支持了泛非洲缝合带的解释。在BMA上没有，而是NNMB继续在CFB构造锋面以下，可能一直到大陆边缘和Agulhas断裂带。地震反射数据还支持了CFB演化的薄皮构造逆冲模型，而Karoo盆地地层没有明显的前深层地层增厚。兼容的构造模型意味着古生代碰撞造山带背景，再加上CFB南部更南的俯冲带。同样，在中元古生代的NNMB合并过程中，地球物理数据支持了一个南倾俯冲带。 Rodinia超大陆的当前重建过程将NNMB和北美Grenville省横越了Grenville-Kibaran造山带。我们的地震剖面通过与造山带对侧的地震剖面直接比较来测试这种重建。虽然曾经相邻的大陆块没有

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