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Imaging Precambrian Lithospheric Structure in Zambia Using Electromagnetic Methods

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Imaging Precambrian Lithospheric Structure in Zambia Using Electromagnetic Methods Gondwana Research 54 (2018) 38–49 Contents lists available at ScienceDirect Gondwana Research journal homepage: www.elsevier.com/locate/gr Imaging Precambrian lithospheric structure in Zambia using electromagnetic methods Emily Sarafian a,b,⁎,1, Rob L. Evans b, Mohamed G. Abdelsalam c,EstellaAtekwanac, Jimmy Elsenbeck b, Alan G. Jones d,2, Ezekiah Chikambwe e a Massachusetts Institute of Technology-Woods Hole Oceanographic Institution Joint Program in Oceanography/Applied Ocean Science and Engineering, Woods Hole, MA 02543, USA b Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA c Boone Pickens School of Geology, Oklahoma State University, Stillwater, OK 74078, USA d Dublin Institute for Advanced Studies, Dublin, Ireland e Geological Survey Department of Zambia, P.O. Box 50135, Lusaka, Zambia article info abstract Article history: The Precambrian geology of eastern Zambia and Malawi is highly complex due to multiple episodes of rifting and Received 19 March 2017 collision, particularly during the formation of Greater Gondwana as a product of the Neoproterozoic Pan-African Received in revised form 8 August 2017 Orogeny. The lithospheric structure and extent of known Precambrian tectonic entities of the region are poorly Accepted 14 September 2017 known as there have been to date few detailed geophysical studies to probe them. Herein, we present results Available online 05 October 2017 from electromagnetic lithospheric imaging across Zambia into southern Malawi using the magnetotelluric meth- Handling Editor: N. Rawlinson od complemented by high-resolution aeromagnetic data of the upper crust in order to explore the extent and ge- ometry of Precambrian structures in the region. We focus particularly on determining the extent of Keywords: subcontinental lithospheric mantle (SCLM) beneath the Archean-Paleoproterozoic cratonic Bangweulu Block Geology and the Mesoproterozoic-Neoproterozoic Irumide and Southern Irumide Orogenic Belts. We also focus on imag- Lithosphere ing the boundaries between these tectonic entities, particularly the boundary between the Irumide and Southern Aeromagnetics Irumide Belts. The thickest and most resistive lithosphere is found beneath the Bangweulu Block, as anticipated Magnetotellurics for stable cratonic lithosphere. Whereas the lithospheric thickness estimates beneath the Irumide Belt match Zambia those determined for other orogenic belts, the Southern Irumide Belt lithosphere is substantially thicker similar to that of the Bangweulu Block to the north. We interpret the thicker lithosphere beneath the Southern Irumide Belt as due to preservation of a cratonic nucleus (the pre-Mesoproterozoic Niassa Craton). A conductive mantle discontinuity is observed between the Irumide and Southern Irumide Belts directly beneath the Mwembeshi Shear Zone. We interpret this discontinuity as modified SCLM relating to a major suture zone. The lithospheric geometries determined from our study reveal tectonic features inferred from surficial studies and provide impor- tant details for the tectonothermal history of the region. © 2017 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved. 1. Introduction of the Neoproterozoic was accompanied by widespread remobilization (metacratonization) of either entire cratonic blocks or the cratonic mar- The assembly of the Greater Gondwana supercontinent at ~600 Ma, gins (Abdelsalam et al., 2002; Liégeois et al., 2013). following the breakup of Rodinia at ~830 Ma, marks an important tec- There are substantial uncertainties in detailing the tectonic elements tonic event in Earth history (Stern, 2008). It is estimated that of the super-continent, especially the spatial distribution of cratonic Neoproterozoic volcanism formed ~20% of the continental crust that is fragments, the orogenic belts surrounding them, and the portions of currently preserved in the form of orogenic belts circling older cratonic the cratonic fragments that have been metacratonized. This is largely fragments that collided to create Greater Gondwana (Maruyama and because many of the present-day fragments of Greater Gondwana are Liou, 1998). Collision between different cratonic fragments at the end covered with Phanerozoic sedimentary rocks formed in large intra-con- tinental sag basins (Artemieva and Mooney, 2001). Sub-continental lithospheric mantle (SCLM) thickness varies between tectonic ele- ⁎ Corresponding author at: 266 Woods Hole Road, WHOI MS#22, Woods Hole, MA ments, with typical estimates of a few tens of kilometers beneath active 02543, USA. rifts to N250 km under certain cratons, though refertilization of old cra- E-mail address: [email protected] (E. Sarafian). 1 Now at Corning, Incorporated, Corning, NY, USA. tonic SCLM or cooling of young SCLM can cause delamination and thin- 2 Now at Complete MT Solutions Inc., Ottawa, Canada. ning of the lithosphere (Begg et al., 2009; Khoza et al., 2013; Miensopust https://doi.org/10.1016/j.gr.2017.09.007 1342-937X/© 2017 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved. E. Sarafian et al. / Gondwana Research 54 (2018) 38–49 39 et al., 2011). These lithospheric heterogeneities facilitated strain locali- craton to the south (Fig. 1B) because few detailed studies of lithospheric zation during Phanerozoic rifting (McConnell, 1972; Nyblade and structures exist in this area. Brazier, 2002), which further complicates the surface expression of In this paper, we present results from two-dimensional (2D) isotro- the different Precambrian tectonic structures. pic inversion of a NW-SE magnetotelluric (MT) regional profile collect- In Africa, which in many reconstructions is put as the heart of Great- ed across Zambia and southern Malawi (Figs. 1B and 2A). Stations in er Gondwana (Irving and Irving, 1982; Smith and Hallam, 1970), surface Zambia are labeled “W” and “E” in Fig. 2B whereas stations in Malawi geological mapping, geochemical and geochronological studies together are labeled “M” in the same figure. This regional profile crosses major with low resolution seismic tomography (Abdelsalam et al., 2011; Begg Precambrian entities including the Bangweulu Block, the Irumide and et al., 2009; Deen et al., 2006; Lebedev et al., 2009; Pasyanos, 2010; Southern Irumide Orogenic Belts (separated by the Mwembeshi Shear Pasyanos and Nyblade, 2007; Ritsema and van Heijst, 2000; Shapiro Zone), and the foreland sedimentary rocks portion of the Lufilian Oro- and Ritzwoller, 2002) have resulted in establishing the extent of major genic Belt (referred to here as the Lufilian Foreland Basin) (Fig. 1B). cratons and orogenic belts of Greater Gondwana (Fig. 1A). However, Our MT profile crosses the NE-trending Paleozoic - Mesozoic (Permian identifying the geologic framework is particularly difficult in the region to Triassic) Karoo-age Luangwa Rift, the southwestern part of which fol- between the Bangweulu Block in northern Zambia and the Zimbabwe lows the Mwembeshi Shear Zone whereas its northeastern part extends within the Irumide Belt. The profile extends to the Malawi Rift at the eastern margin of the Southern Irumide Belt (Figs. 1Band2A). MT is a natural-source electromagnetic imaging method, and maps the lateral and vertical subsurface resistivity structure, and is sensitive to variations in temperature, composition, and interconnected conduc- tors, such as fluid, melt, or metals. As a result, MT is a powerful tool to investigate lithospheric-scale structures (Evans et al., 2011; Jones, 1999; Khoza et al., 2013; Miensopust et al., 2011; Muller et al., 2009). In addition to the mantle-probing MT data, we also use high-resolution aeromagnetic data over the Mwembeshi Shear Zone and surrounding Irumide and Southern Irumide Belts, provided by the Geological Survey Department of Zambia, to unveil the upper crustal structure (Fig. 3). The combination of electrical models derived from our MT data and high- resolution aeromagnetic data allows us to reveal the extent of the SCLM and improve our understanding of the Precambrian structures in the region – those obvious in surface geology, and those oblivious to surface geology as they are buried at depth. In particular, our work fa- cilitates delineation between stable cratonic blocks with thick SCLM from those with thinned SCLM, possibly due to partial delamination and regional metacratonization, and provides detailed images of the lithospheric boundaries (suture zones) between different Precambrian entities. Our work also touches upon the possible influence of the Pre- cambrian structures on the evolution of the Karoo rifts represented by the Luangwa Rift, although it is not our primary focus. Before discussing the data, results, and geological implications of our study, we first define the key terms of craton and metacratons, and then briefly discuss the geological background and previous geophysical studies of the region of study. 2. Cratons and metacratons Cratons primarily occupy the interior of continental plates. They are Archean to Paleoproterozoic in age, and have been tectonically stable for long periods of time, possibly since the time of their formation, albeit with frequent modification in many cases due to influx of metasomatizing fluids. Such long-lasting stability is attributed to the presence of a thick, cold and mostly anhydrous SCLM beneath the cra- tonic continental crust (Black and Liégeois, 1993; Peslier et al., 2010). The thick, depleted nature of the SCLM makes it sufficiently buoyant to be isolated
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