Imaging and modelling the subsurface structure of the Rungwe Volcanic Province in SW Tanzania with aeromagnetic data: An improved structural map to support geothermal exploration

IF 2.2 4区 地球科学 Q2 GEOSCIENCES, MULTIDISCIPLINARY
Makoye M. Didas , Egidio Armadillo , Gylfi Páll Hersir , William Cumming , Daniele Rizzello , Halldór Geirsson
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引用次数: 0

Abstract

The Rungwe Volcanic Province (RVP) in the East African Rift System, SW Tanzania, provides a unique opportunity to investigate geothermal resources in the context of particularly complex continental rifting processes. To support geothermal resource targeting in the RVP, we present a revised neotectonic structural map based on an interpretation of aeromagnetic data constrained by 2D-forward modelling of magnetic anomalies integrated with the distribution of previously reported faults, seismic epicentre locations, 3D magnetotelluric resistivity models and surface geothermal manifestations. Magnetic anomalies in the RVP, including the nationally prominent Mbeya anomaly, are related to the high magnetic susceptibility or remanent magnetism of Precambrian rocks and Cretaceous carbonatite intrusions buried in the rift under a varying thickness of non-magnetic sediments and volcanic rocks. Magnetic lineaments are related to structures controlling the geometry of the Precambrian rocks and concealed dikes and the thickness of the sediments and volcanics. The recent Ngozi and Rungwe trachyte volcanics have relatively low magnetic susceptibility comparable to the low susceptibility of the sediments in the rift basins. The revised neotectonic structural map shows prominent NW, NE and NS-trending magnetic lineaments that correlate with previously reported faults and alignments of seismic epicentres in the study area and with the regional trend of the rift segments. The NE-trending magnetic lineaments are consistent with interpretations of the current stress field in the RVP. The main volcanoes in the RVP, Ngozi, Rungwe and Kiejo (also known as Kyejo and Kieyo), are aligned with the NW-trending linear magnetic feature joining the Lupa and Livingstone rift border faults. This lineament is intersected and frequently displaced by the NE and NS-trending lineaments, suggesting that the NE to NS-striking structures are younger. The Rungwe and Ngozi volcanoes as well as numerous ‘‘monogenetic’’ eruption centres and the Mwakaleli caldera, which originated ca. 2 Ma ago (Ebinger et al., 1989) following a large explosive eruption emplacing widespread ignimbrite deposits, are located within a zone of low-intermediate magnitude magnetic features forming a basin-like structure surrounded by magnetic high anomalies of the Precambrian basement structures. We interpret the intersections between the NW-trending intra-rift faults and the NS and NE-trending faults as favourable locations for wells to target high permeability within the geothermal resource conceptual models previously constructed using 3D MT resistivity imaging integrated with supporting geoscientific data. The intersections provide a focus area for follow-up ground mapping of subtle features that may be associated with very recent fault movement.
利用航磁数据对坦桑尼亚西南部 Rungwe 火山省的地下结构进行成像和建模:支持地热勘探的改进结构图
坦桑尼亚西南部东非裂谷系统中的隆圭火山省(RVP)为研究特别复杂的大陆裂谷过程背景下的地热资源提供了一个独特的机会。为了支持 RVP 的地热资源目标定位,我们在对航空磁数据进行解释的基础上,结合之前报告的断层分布、地震震中位置、三维磁测电阻率模型和地表地热表现,提出了经修订的新构造结构图,并对磁异常进行了二维前向建模。RVP 中的磁异常,包括全国著名的姆贝亚异常,与前寒武纪岩石和白垩纪碳酸盐岩侵入体的高磁感应强度或剩磁有关,这些岩石和侵入体埋藏在裂谷中厚度不等的非磁性沉积物和火山岩之下。磁力线条与控制前寒武纪岩石和隐伏岩钉的几何形状以及沉积物和火山岩厚度的结构有关。近期的恩戈齐和隆圭岩屑火山岩的磁感应强度相对较低,与裂谷盆地沉积物的低磁感应强度相当。修订后的新构造结构图显示了突出的西北、东北和北北走向磁力线,这些磁力线与之前报告的研究区域内的断层和地震震中线以及裂谷段的区域走向相关。东北走向的磁力线与对裂谷地段当前应力场的解释一致。该地区的主要火山--恩戈齐火山、隆圭火山和基约火山(又称基耶约火山和基约火山)--与连接卢帕断裂和利文斯通断裂边界的西北走向线性磁力特征相一致。这条线与 NE 和 NS 走向的线相交,并经常发生位移,这表明 NE 至 NS 走向的构造更年轻。Rungwe 火山和 Ngozi 火山以及众多的 "单源 "喷发中心和 Mwakaleli 破火山口(起源于约 2 Ma 前(Ebinger 等人,1989 年)的一次大爆发,形成了广泛的火成岩沉积)都位于中低磁级磁性特征区内,形成了一个盆地状结构,周围环绕着前寒武纪基底结构的磁性高异常区。我们认为,西北走向的断裂内断层与 NS 和 NE 走向的断层之间的交汇处,是先前利用三维 MT 电阻率成像技术和支持性地球科学数据构建的地热资源概念模型中针对高渗透性打井的有利位置。这些交叉点为后续地面测绘可能与近期断层运动有关的细微特征提供了重点区域。
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来源期刊
Journal of African Earth Sciences
Journal of African Earth Sciences 地学-地球科学综合
CiteScore
4.70
自引率
4.30%
发文量
240
审稿时长
12 months
期刊介绍: The Journal of African Earth Sciences sees itself as the prime geological journal for all aspects of the Earth Sciences about the African plate. Papers dealing with peripheral areas are welcome if they demonstrate a tight link with Africa. The Journal publishes high quality, peer-reviewed scientific papers. It is devoted primarily to research papers but short communications relating to new developments of broad interest, reviews and book reviews will also be considered. Papers must have international appeal and should present work of more regional than local significance and dealing with well identified and justified scientific questions. Specialised technical papers, analytical or exploration reports must be avoided. Papers on applied geology should preferably be linked to such core disciplines and must be addressed to a more general geoscientific audience.
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