Receiver function investigation of crustal structure in the Malawi and Luangwa rift zones and adjacent areas

doi:10.1016/j.gr.2020.08.015

Gondwana Research

Volume 89, January 2021, Pages 168-176

https://doi.org/10.1016/j.gr.2020.08.015 Get rights and content

Highlights

•
Crustal thickness and Vp/Vs of the Malawi rift zone (MRZ) and nearby areas are mapped
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A crustal stretching factor of about 1.05–1.08 is revealed beneath the non-volcanic MRZ
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High Vp/Vs (≥ 1.81) beneath the southern MRZ may indicate crustal partial melting
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Infiltration of magma-derived CO₂ may cause the observed low Vp/Vs in northern MRZ
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Normal crustal thickness and Vp/Vs suggest a complete recovery of Luangwa rift

Abstract

Stacking over 2300 P-to-S receiver functions recorded by 33 SAFARI (Seismic Arrays for African Rift Initiation) broadband seismic stations that we installed in the vicinity of the Malawi and Luangwa rift zones (MRZ and LRZ, respectively) reveals significant variations of crustal thickness (32.8–46.3 km) and V_p/V_s (1.69–1.85). The resulting crustal stretching factor is about 1.05–1.08 for the MRZ, which is approximately 10–40% lower than that observed in the mature segments of the East African Rift System (EARS). The low stretching factor is consistent with the general absence of volcanism in the MRZ, and the relatively high V_p/V_s (≥ 1.81) beneath the southern MRZ, when combined with observations from previous studies, indicate the possible existence of crustal partial melting, elevated temperatures or fluid-filled deep crustal faults that are likely associated with lithospheric stretching. In sharp contrast with the southern MRZ, low V_p/V_s measurements in the range of 1.69–1.72 are observed along the western boundary of the northern MRZ, which could be attributable to the infiltration of magma-derived CO₂ into the crust. The LRZ shows negligible crustal thinning and a V_p/V_s that is comparable to the globally averaged value for continental crust, suggesting a complete post-rifting recovery of crustal properties in terms of crustal thickness and V_pV_s.

Graphical abstract

Introduction

A typical continental rift is a fault-bounded narrow valley where the entire lithosphere has been pulled apart under extension (Sengor and Burke, 1978; Gregory, 1894). As an archetypal example of continental rifts, the East Africa Rift System (EARS), which extends from the northern Red Sea to the southern terminus of the Malawi rift zone (MRZ), is an ideal natural laboratory for investigating rifting mechanisms and the characteristics of continental rifts in various stages. Relative to most of the other segments of the EARS, the Cenozoic MRZ of the EARS and the nearby Paleozoic-Mesozoic Luangwa Rift Zone (LRZ; Ebinger et al., 2017) have been less adequately investigated. Consequently, the magnitude and extent of crustal deformation, the existence of partial melting or mafic intrusion in the crust, and important characteristics such as the depth penetration and possible CO₂ infiltration (Roecker et al., 2017) of the seismically active boundary faults, remain enigmatic.

Laboratory investigations of crustal rock samples (Holbrook et al., 1992) suggest that under average crustal temperature and pressure conditions, felsic, intermediate, and mafic rocks have V_p/V_s values of smaller than 1.76, between 1.76 and 1.81, and greater than 1.81, respectively. The existence of crustal partial melting can lead to a higher V_p/V_s due to a greater reduction of V_s than V_p (Greenfield et al., 2016). Similarly, intensive intrusion of mantle materials into the crust can also increase the bulk crustal V_p/V_s (Christensen, 1996). An increasing number of mineral physical and observational studies have suggested that CO₂ released from the mantle through deep and steep lithospheric faults can significantly reduce crustal V_p/V_s (Julian et al., 1998; Lee et al., 2016; Parmigiani et al., 2016; Roecker et al., 2017). Specifically, it has been suggested that CO₂ can decrease V_p through its strong effect on the pore-fluid compressibility of porous crustal rocks, and consequently, reduce the crustal V_p/V_s (Ito et al., 1979; Mavko and Mukerji, 1995). Therefore, observations of V_p/V_s values that are below the normal felsic rock value of 1.76 may suggest the presence of magma-derived CO₂, as well as the existence of lithospheric faults acting as conduits for the CO₂ (Lee et al., 2016).

In this study, we measure spatial variations of crustal thickness and V_p/V_s using P-to-S receiver functions (RFs) recorded at 33 broadband seismic stations (Gao et al., 2013), to unveil crustal characteristics and possible effects of CO₂ infiltration and partial melting on crustal V_p/V_s beneath the MRZ and LRZ and adjacent areas.

Section snippets

Tectonic setting

The Cenozoic MRZ is the southernmost segment of the magma-poor western branch of the EARS. It separates the Nubian plate and the Rovuma microplate (Fig. 1) and originated approximately 25 Ma (Roberts et al., 2012). The Rungwe Volcanic Province located at the northern tip of the rift zone is the only volcanic province within the MRZ (Ebinger et al., 1993). Kinematic GPS studies (Saria et al., 2014; Stamps et al., 2018) indicated that the spreading rate between the Nubian plate and the Rovuma

Data and methods

The teleseismic (epicentral distance ranging from 30° to 100°) data used in the study were recorded by 33 stations (Fig. 1) that we installed in Malawi, Mozambique, and Zambia over a 2 year period (2012–2014) as a component of the SAFARI (Seismic Arrays for African Rift Initiation; Gao et al., 2013) project. One of these stations (Q05MS) is a combination of two nearby stations Q05MJ and Q05ML. To balance the quality and quantity of the selected data, a variable cut-off magnitude (M_c) was set by

Results

Robust P-to-S arrivals are obtained from the migrated RFs (Fig. 4, Fig. 5), enabling reliable determinations of crustal thickness and V_p/V_s beneath the vast majority of the stations.

Constraints on crustal magmatic intrusion and partial melting beneath the MRZ

In this study, a relatively flat Moho (39.4 ± 2.7 km) was found under most stations in the MRZ including the central and southern parts of the MRZ. The average crustal thickness observed beneath the Mozambique Belt and SIB are 41.9 ± 2.8 km and 43.6 ± 0.8 km, respectively, which are consistent with the ≥40 km results from a recent ANT study (Wang et al., 2019). Therefore, along the rift-orthogonal profile, the crustal thickness in the MRZ is 2–3 km thinner than the surrounding orogenic belts (

Conclusions

Crustal thickness and V_p/V_s beneath 33 SAFARI stations located along two profiles in the vicinity of the MRZ and LRZ were imaged by stacking 2307 high-quality RFs. The crustal thickness measurements are generally consistent with sparsely spaced previous measurements. The new observations show that relative to the adjacent orogenic belts, the crust beneath the MRZ is thinned by about 3 km. This low magnitude crustal stretching is consistent with the absence of volcanisms in the main portions of

CRediT authorship contribution statement

M.S. wrote the paper with help from all the co-authors. S.G., K.L., and Y.Y. wrote the computer programs and provided financial supports.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

We thank the IRIS DMC for archiving the data used in the study, and the Portable Array Seismic Studies of the Continental Lithosphere Instrument Center for providing equipment and logistical support. All the data used in the study were obtained from the IRIS DMC (last accessed: August 2018; doi:https://doi.org/10.7914/SN/XK_2012). Field assistance provided by Cory Reed, Shane Ingate, Patrick R. N. Chindandali, Belarmino Massingue, Hassan Mdala, and Daniel Mutamina, as well as by various

References (58)

C.J. Ammon
The isolation of receiver effects from teleseismic P-waveforms
Bull. Seismol. Soc. Am.
(1991)
N.L. Banks et al.
Karoo rift basins of the Luangwa Valley, Zambia
Geol. Soc. Lond., Spec. Publ.
(1995)
D. Borrego et al.
Crustal structure surrounding the northern Malawi rift and beneath the Rungwe Volcanic Province, East Africa
Geophys. J. Int.
(2018)
M.R. Burton et al.
Deep carbon emissions from volcanoes
Rev. Mineral. Geochem.
(2013)
N.I. Christensen
Poisson’s ratio and crustal seismology
J. Geophys. Res. Solid Earth
(1996)
T.J. Craig et al.
Earthquake distribution patterns in Africa: their relationship to variations in lithospheric and geological structure, and their rheological implications
Geophys. J. Int.
(2011)
M.C. Daly et al.
Rift basin evolution in Africa: the influence of reactivated steep basement shear zones
Geol. Soc. Lond., Spec. Publ.
(1989)
C.J. Ebinger et al.
Tectonic controls on rift basin morphology: evolution of the Northern Malawi (Nyasa) Rift
J. Geophys. Res. Solid Earth
(1993)
C.J. Ebinger et al.
Crustal structure of active deformation zones in Africa: implications for global crustal processes
Tectonics
(2017)
B. Efron et al.
Bootstrap methods for standard errors, confidence intervals, and other measures of statistical accuracy
Stat. Sci.
(1986)

S.F. Foley et al.

An essential role for continental rifts and lithosphere in the deep carbon cycle

Nat. Geosci.

(2017)

F.R. Fontaine et al.

Crustal and uppermost mantle structure variation beneath La Reunion hotspot track

Geophys. J. Int.

(2015)

H. Fritz et al.

Orogen styles in the East African Orogen: a review of the Neoproterozoic to Cambrian tectonic evolution

J. Afr. Earth Sci.

(2013)

S. Gao et al.

Seismic anisotropy and mantle flow beneath the Baikal Rift Zone

Nature

(1994)

S. Gao et al.

SKS splitting beneath continental rift zones

J. Geophys. Res. Solid Earth

(1997)

S.S. Gao et al.

Seismic arrays to study African rift initiation

EOS Trans. Am. Geophys. Union

(2013)

T. Greenfield et al.

The magmatic plumbing system of the Askja central volcano, Iceland, as imaged by seismic tomography

J. Geophys. Res. Solid Earth

(2016)

J.W. Gregory

Contributions to the physical geography of British East Africa

Geogr. J.

(1894)

M. Gurnis et al.

Constraining mantle density structure using geological evidence of surface uplift rates: the case of the African superplume

Geochem. Geophys. Geosyst.

(2000)

W.S. Holbrook et al.

The seismic velocity structure of the deep continental crust

Continental Lower Crust

(1992)

H. Ito et al.

Compressional and shear waves in saturated rock during water-steam transition

J. Geophys. Res. Solid Earth

(1979)

S.P. Johnson et al.

A review of the Mesoproterozoic to early Palaeozoic magmatic and tectonothermal history of south-Central Africa: implications for Rodinia and Gondwana

J. Geol. Soc.

(2005)

S.P. Johnson et al.

U-P_b sensitive high-resolution ion microprobe (SHRIMP) zircon geochronology of granitoid rocks in eastern Zambia: terrane subdivision of the Mesoproterozoic Southern Irumide Belt

Tectonics

(2006)

S.P. Johnson et al.

Geochemistry, geochronology and isotopic evolution of the Chewore-Rufunsa Terrane, Southern Irumide Belt: a Mesoproterozoic continental margin arc

J. Petrol.

(2007)

B.R. Julian et al.

Seismic image of a CO₂ reservoir beneath a seismically active volcano

Geophys. J. Int.

(1998)

M. Kachingwe et al.

Crustal structure of Precambrian terranes in the southern African subcontinent with implications for secular variation in crustal genesis

Geophys. J. Int.

(2015)

B.L.N. Kennett et al.

Traveltimes for global earthquake location and phase identification

Geophys. J. Int.

(1991)

H. Lee et al.

Massive and prolonged deep carbon emissions associated with continental rifting

Nat. Geosci.

(2016)

C. Lithgow-Bertelloni et al.

Dynamic topography, plate driving forces and the African superswell

Nature

(1998)

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Receiver function investigation of crustal structure in the Malawi and Luangwa rift zones and adjacent areas

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Tectonic setting

Data and methods

Results

Constraints on crustal magmatic intrusion and partial melting beneath the MRZ

Conclusions

CRediT authorship contribution statement

Declaration of competing interest

Acknowledgments

The isolation of receiver effects from teleseismic P-waveforms

Bull. Seismol. Soc. Am.

Karoo rift basins of the Luangwa Valley, Zambia

Geol. Soc. Lond., Spec. Publ.

Crustal structure surrounding the northern Malawi rift and beneath the Rungwe Volcanic Province, East Africa

Geophys. J. Int.

Deep carbon emissions from volcanoes

Rev. Mineral. Geochem.

Poisson’s ratio and crustal seismology

J. Geophys. Res. Solid Earth

Earthquake distribution patterns in Africa: their relationship to variations in lithospheric and geological structure, and their rheological implications

Geophys. J. Int.

Rift basin evolution in Africa: the influence of reactivated steep basement shear zones

Geol. Soc. Lond., Spec. Publ.

Tectonic controls on rift basin morphology: evolution of the Northern Malawi (Nyasa) Rift

J. Geophys. Res. Solid Earth

Crustal structure of active deformation zones in Africa: implications for global crustal processes

Tectonics

Bootstrap methods for standard errors, confidence intervals, and other measures of statistical accuracy

Stat. Sci.

An essential role for continental rifts and lithosphere in the deep carbon cycle

Nat. Geosci.

Crustal and uppermost mantle structure variation beneath La Reunion hotspot track

Geophys. J. Int.

Orogen styles in the East African Orogen: a review of the Neoproterozoic to Cambrian tectonic evolution

J. Afr. Earth Sci.

Seismic anisotropy and mantle flow beneath the Baikal Rift Zone

Nature

SKS splitting beneath continental rift zones

J. Geophys. Res. Solid Earth

Seismic arrays to study African rift initiation

EOS Trans. Am. Geophys. Union

The magmatic plumbing system of the Askja central volcano, Iceland, as imaged by seismic tomography

J. Geophys. Res. Solid Earth

Contributions to the physical geography of British East Africa

Geogr. J.

Constraining mantle density structure using geological evidence of surface uplift rates: the case of the African superplume

Geochem. Geophys. Geosyst.

The seismic velocity structure of the deep continental crust

Continental Lower Crust

Compressional and shear waves in saturated rock during water-steam transition

J. Geophys. Res. Solid Earth

A review of the Mesoproterozoic to early Palaeozoic magmatic and tectonothermal history of south-Central Africa: implications for Rodinia and Gondwana

J. Geol. Soc.

U-Pb sensitive high-resolution ion microprobe (SHRIMP) zircon geochronology of granitoid rocks in eastern Zambia: terrane subdivision of the Mesoproterozoic Southern Irumide Belt

Tectonics

Geochemistry, geochronology and isotopic evolution of the Chewore-Rufunsa Terrane, Southern Irumide Belt: a Mesoproterozoic continental margin arc

J. Petrol.

Seismic image of a CO2 reservoir beneath a seismically active volcano

Geophys. J. Int.

Crustal structure of Precambrian terranes in the southern African subcontinent with implications for secular variation in crustal genesis

Geophys. J. Int.

Traveltimes for global earthquake location and phase identification

Geophys. J. Int.

Massive and prolonged deep carbon emissions associated with continental rifting

Nat. Geosci.

Dynamic topography, plate driving forces and the African superswell

Nature

U-P_b sensitive high-resolution ion microprobe (SHRIMP) zircon geochronology of granitoid rocks in eastern Zambia: terrane subdivision of the Mesoproterozoic Southern Irumide Belt

Seismic image of a CO₂ reservoir beneath a seismically active volcano