Aubreya Adams | Colgate University (original) (raw)

Papers by Aubreya Adams

An expanded model of the 3-D shear wave velocity structure of the uppermost mantle beneath easter... more An expanded model of the 3-D shear wave velocity structure of the uppermost mantle beneath eastern Africa has been developed using earthquakes recorded by the AfricaArray East African Seismic Experiment in conjunction with data from permanent stations and previously deployed temporary stations. The combined data set comprises 331 earthquakes recorded on a total of 95 seismic stations spanning Kenya, Uganda, Tanzania, Zambia and Malawi. In this study, data from 149 earthquakes were used to determine fundamental-mode Rayleigh wave phase velocities at periods ranging from 20 to 182 s using the two-plane wave method, and then combined with the similarly processed published measurements and inverted for a 3-D shear wave velocity model of the uppermost mantle. New features in the model include (1) a low-velocity region in western Zambia, (2) a high-velocity region in eastern Zambia, (3) a low-velocity region in eastern Tanzania and (4) low-velocity regions beneath the Lake Malawi rift. When considered in conjunction with mapped seismicity, these results support a secondary western rift branch striking southwestwards from Lake Tanganyika, likely exploiting the relatively weak lithosphere of the southern Kibaran Belt between the Bangweulu Block and the Congo Craton. We estimate a lithospheric thickness of ∼150–200 km for the substantial fast shear wave anomaly imaged in eastern Zambia, which may be a southward subsurface extension of the Bangweulu Block. The low-velocity region in eastern Tanzania suggests that the eastern rift branch trends southeastwards offshore eastern Tanzania coincident with the purported location of the northern margin of the proposed Ruvuma microplate. Pronounced velocity lows along the Lake Malawi rift are found beneath the northern and southern ends of the lake, but not beneath the central portion of the lake.

Six earthquakes within the Zagros Mountains with magnitudes between 4.9 and 5.7 have been studied... more Six earthquakes within the Zagros Mountains with magnitudes between 4.9 and 5.7 have been studied to determine their source parameters. These events were selected for study because they were reported in open catalogs to have lower crustal or upper mantle source depths and because they occurred within an area of the Zag- ros Mountains where crustal velocity structure has been constrained by previous stud- ies. Moment tensor inversion of regional broadband waveforms has been combined with forward modeling of depth phases on short-period teleseismic waveforms to con- strain source depths and moment tensors. Our results show that all six events nucleated within the upper crust (<11 km depth) and have thrust mechanisms. This finding supports other studies that call into question the existence of lower crustal or mantle events beneath the Zagros Mountains.

This thesis addresses crustal and upper mantle structure in three regions in and around the Afric... more This thesis addresses crustal and upper mantle structure in three regions in and around the African Superswell: the Zagros Mountains, southern Africa, and the East African Plateau, each representing a different tectonic regime. In the Zagros Mountains of southwestern Iran, source mechanisms for six moderate-sized earthquakes are investigated using a combination of moment tensor inversion and depth phase analysis. The six earthquakes that are studied were reported in global earthquake catalogs as having lower crustal or upper mantle source depths, but upon further study, it was found that all six nucleated within the upper crust. This finding contributes to a growing number of studies indicating that seismicity in the Zagros Mountains is limited to the upper crust. In both southern and eastern Africa, upper mantle structure was investigated to evaluate the thermal state of the upper mantle and implications for the source of uplift in each area. Rayleigh wave phase velocities were measured for these regions using a two plane wave approximation method and were then inverted for a quasi-three dimensional shear wave velocity model. In southern Africa, it was found that the lithospheric lid structure and the sublithospheric velocity reduction for the Kaapvaal Craton is comparable to the upper mantle structure beneath other Archean Cratons. Thus, little seismic evidence was found of an upper mantle thermal anomaly sufficient to support high elevations in that area. In East Africa, evidence was found for a broad thermal anomaly across the study region that extends from the base of the lithosphere into the transition zone. This anomaly is larger than previous studies have indicated and its presence and size imply a possible connection to the African Superplume.

Few undergraduate students had the opportunity to go to the field and image the subsurface struct... more Few undergraduate students had the opportunity to go to the field and image the subsurface structure of the eastern Bushveld Complex in South Africa. We applied the seismic refraction method which is a method based on a seismic wave refracted by 90 degrees (critical angle) and will generate a head wave that will be picked up by geophones as detectors. Sledgehammer was used to generate seismic waves (artificial pulses). Software called SeisModule Controller was used to record and process the seismic data. The idea is to model the subsurface structure and the modeling gave us the depth and velocity of different layers. We also utilized aeromagnetic and borehole information to make comparison with the results obtained. Each student in the field had a chance to utilize four different geophysical methods (electrical resistivity, magnetic, gravity and seismic refraction) but focused more on one of those methods in this geologically remarkable place that is rich in minerals.

Geophysical Journal International, 2012

The shear wave velocity structure of the upper mantle beneath the East African plateau has been i... more The shear wave velocity structure of the upper mantle beneath the East African plateau has been investigated using teleseismic surface waves recorded on new broadband seismic stations deployed in Uganda and Tanzania, as well as on previously deployed stations in Tanzania and Kenya. Rayleigh wave phase velocities at periods between 20 and 182 s, measured with a two-plane wave method, have been used to create phase velocity maps, and dispersion curves extracted from the maps have been inverted to obtain a quasi-3-D shear wave velocity model of the upper mantle. We find that phase velocities beneath the Tanzania Craton and areas directly north and west of the craton are faster, at all periods, than those beneath the Western and Eastern branches of the East African Rift System. At periods <50 s, the western branch is slower than the Eastern Branch, but at periods greater than 50 s, this relationship is reversed. Anisotropy is found at all periods, with a generally north-south fast polarization direction. The shear wave velocity model shows a seismically fast lithosphere (lid) beneath the Tanzania Craton to depths between 150 and 200 km. The fast velocities in this depth range extend to the north beneath the Uganda Basement Complex and to the east beneath the northern Tanzania divergence zone, indicating that these regions together form a rigid block around which rifting has occurred within weaker mobile belt lithosphere. The Eastern and Western branches are slower than the craton at lithospheric mantle depths, and both branches show variable structure in the upper 200 km of the mantle, with the lowest velocities found beneath areas of Cenozoic volcanism. At depths greater than ∼225 km, a low velocity anomaly is present beneath the entire East African plateau that may extend into the mantle transition zone. Velocities in the low velocity region are reduced by ≥10 per cent relative to lid velocities, and if attributed only to temperature variations, would represent an unrealistic thermal perturbation of >400 K. Consequently, it is likely that the velocity reduction reflects a combination of thermal and compositional changes, and also possibly the presence of partial melt. The width and thickness of the low velocity anomaly is greater than typically expected for a plume head and is more easily attributed to an upward continuation of the lower mantle African superplume structure into the upper mantle.

Geophysical Journal International, 2011

Broad-band seismic data from the southern African seismic experiment and the AfricaArray network ... more Broad-band seismic data from the southern African seismic experiment and the AfricaArray network are used to investigate the seismic velocity structure of the upper mantle beneath southern Africa, and in particular beneath the Kaapvaal Craton. A two-plane approximation method that includes a finite frequency sensitivity kernel is employed to measure Rayleigh wave phase velocities, which are inverted to obtain a quasi-3-D shear wave velocity model of the upper mantle. We find phase velocities for the Kaapvaal Craton and surrounding mobile belts that are comparable to those reported by previous studies, and we find little evidence for variation from east to west across the Namaqua-Natal Belt, a region not well imaged in previous studies. A high-velocity upper-mantle lid is found beneath the Kaapvaal Craton and most of southern Africa. For the Kaapvaal Craton, the thickness of the lid (∼150-200 km) is consistent with the lid thicknesses reported in many previous studies. The cratonic lid is underlain by a ∼100-km thick low-velocity zone with a 3.9 per cent maximum velocity reduction. By comparing the velocity model to those published for other Archean cratons, we find few differences, and therefore conclude that there is little evidence in the shear wave velocity structure of the mantle to indicate that the southern African plateau is supported by an upper-mantle thermal anomaly.

The Cameroon Volcanic Line (CVL) is an 1800km long volcanic chain, extending SW-NE from the Gulf ... more The Cameroon Volcanic Line (CVL) is an 1800km long volcanic chain, extending SW-NE from the Gulf of Guinea into Central Africa, that lacks the typical age progression exhibited by hotspot-related volcanic tracks. This study investigates the upper mantle seismic structure beneath the CVL and surrounding regions to constrain the origin of volcanic lines that are poorly described by the classic plume model. Rayleigh wave phase velocities are measured at periods from 20 to 182 seconds following the twoplane wave methodology, using data from the Cameroon Seismic Experiment, which consists of 32 broadband stations deployed between 2005 and 2007. These phase velocities are then inverted to build a model of shear wave velocity structure in the upper mantle beneath the CVL. Results show that phase velocities beneath the CVL are reduced at all periods, with average velocities beneath the CVL deviating more than -2% from the regional average, and +4% beneath the Congo Craton. This distinction is observed for all periods, but is less pronounced for the longest periods measured.

An expanded model of the 3-D shear wave velocity structure of the uppermost mantle beneath easter... more An expanded model of the 3-D shear wave velocity structure of the uppermost mantle beneath eastern Africa has been developed using earthquakes recorded by the AfricaArray East African Seismic Experiment in conjunction with data from permanent stations and previously deployed temporary stations. The combined data set comprises 331 earthquakes recorded on a total of 95 seismic stations spanning Kenya, Uganda, Tanzania, Zambia and Malawi. In this study, data from 149 earthquakes were used to determine fundamental-mode Rayleigh wave phase velocities at periods ranging from 20 to 182 s using the two-plane wave method, and then combined with the similarly processed published measurements and inverted for a 3-D shear wave velocity model of the uppermost mantle. New features in the model include (1) a low-velocity region in western Zambia, (2) a high-velocity region in eastern Zambia, (3) a low-velocity region in eastern Tanzania and (4) low-velocity regions beneath the Lake Malawi rift. When considered in conjunction with mapped seismicity, these results support a secondary western rift branch striking southwestwards from Lake Tanganyika, likely exploiting the relatively weak lithosphere of the southern Kibaran Belt between the Bangweulu Block and the Congo Craton. We estimate a lithospheric thickness of ∼150–200 km for the substantial fast shear wave anomaly imaged in eastern Zambia, which may be a southward subsurface extension of the Bangweulu Block. The low-velocity region in eastern Tanzania suggests that the eastern rift branch trends southeastwards offshore eastern Tanzania coincident with the purported location of the northern margin of the proposed Ruvuma microplate. Pronounced velocity lows along the Lake Malawi rift are found beneath the northern and southern ends of the lake, but not beneath the central portion of the lake.

Six earthquakes within the Zagros Mountains with magnitudes between 4.9 and 5.7 have been studied... more Six earthquakes within the Zagros Mountains with magnitudes between 4.9 and 5.7 have been studied to determine their source parameters. These events were selected for study because they were reported in open catalogs to have lower crustal or upper mantle source depths and because they occurred within an area of the Zag- ros Mountains where crustal velocity structure has been constrained by previous stud- ies. Moment tensor inversion of regional broadband waveforms has been combined with forward modeling of depth phases on short-period teleseismic waveforms to con- strain source depths and moment tensors. Our results show that all six events nucleated within the upper crust (<11 km depth) and have thrust mechanisms. This finding supports other studies that call into question the existence of lower crustal or mantle events beneath the Zagros Mountains.

This thesis addresses crustal and upper mantle structure in three regions in and around the Afric... more This thesis addresses crustal and upper mantle structure in three regions in and around the African Superswell: the Zagros Mountains, southern Africa, and the East African Plateau, each representing a different tectonic regime. In the Zagros Mountains of southwestern Iran, source mechanisms for six moderate-sized earthquakes are investigated using a combination of moment tensor inversion and depth phase analysis. The six earthquakes that are studied were reported in global earthquake catalogs as having lower crustal or upper mantle source depths, but upon further study, it was found that all six nucleated within the upper crust. This finding contributes to a growing number of studies indicating that seismicity in the Zagros Mountains is limited to the upper crust. In both southern and eastern Africa, upper mantle structure was investigated to evaluate the thermal state of the upper mantle and implications for the source of uplift in each area. Rayleigh wave phase velocities were measured for these regions using a two plane wave approximation method and were then inverted for a quasi-three dimensional shear wave velocity model. In southern Africa, it was found that the lithospheric lid structure and the sublithospheric velocity reduction for the Kaapvaal Craton is comparable to the upper mantle structure beneath other Archean Cratons. Thus, little seismic evidence was found of an upper mantle thermal anomaly sufficient to support high elevations in that area. In East Africa, evidence was found for a broad thermal anomaly across the study region that extends from the base of the lithosphere into the transition zone. This anomaly is larger than previous studies have indicated and its presence and size imply a possible connection to the African Superplume.

Few undergraduate students had the opportunity to go to the field and image the subsurface struct... more Few undergraduate students had the opportunity to go to the field and image the subsurface structure of the eastern Bushveld Complex in South Africa. We applied the seismic refraction method which is a method based on a seismic wave refracted by 90 degrees (critical angle) and will generate a head wave that will be picked up by geophones as detectors. Sledgehammer was used to generate seismic waves (artificial pulses). Software called SeisModule Controller was used to record and process the seismic data. The idea is to model the subsurface structure and the modeling gave us the depth and velocity of different layers. We also utilized aeromagnetic and borehole information to make comparison with the results obtained. Each student in the field had a chance to utilize four different geophysical methods (electrical resistivity, magnetic, gravity and seismic refraction) but focused more on one of those methods in this geologically remarkable place that is rich in minerals.

Geophysical Journal International, 2012

The shear wave velocity structure of the upper mantle beneath the East African plateau has been i... more The shear wave velocity structure of the upper mantle beneath the East African plateau has been investigated using teleseismic surface waves recorded on new broadband seismic stations deployed in Uganda and Tanzania, as well as on previously deployed stations in Tanzania and Kenya. Rayleigh wave phase velocities at periods between 20 and 182 s, measured with a two-plane wave method, have been used to create phase velocity maps, and dispersion curves extracted from the maps have been inverted to obtain a quasi-3-D shear wave velocity model of the upper mantle. We find that phase velocities beneath the Tanzania Craton and areas directly north and west of the craton are faster, at all periods, than those beneath the Western and Eastern branches of the East African Rift System. At periods <50 s, the western branch is slower than the Eastern Branch, but at periods greater than 50 s, this relationship is reversed. Anisotropy is found at all periods, with a generally north-south fast polarization direction. The shear wave velocity model shows a seismically fast lithosphere (lid) beneath the Tanzania Craton to depths between 150 and 200 km. The fast velocities in this depth range extend to the north beneath the Uganda Basement Complex and to the east beneath the northern Tanzania divergence zone, indicating that these regions together form a rigid block around which rifting has occurred within weaker mobile belt lithosphere. The Eastern and Western branches are slower than the craton at lithospheric mantle depths, and both branches show variable structure in the upper 200 km of the mantle, with the lowest velocities found beneath areas of Cenozoic volcanism. At depths greater than ∼225 km, a low velocity anomaly is present beneath the entire East African plateau that may extend into the mantle transition zone. Velocities in the low velocity region are reduced by ≥10 per cent relative to lid velocities, and if attributed only to temperature variations, would represent an unrealistic thermal perturbation of >400 K. Consequently, it is likely that the velocity reduction reflects a combination of thermal and compositional changes, and also possibly the presence of partial melt. The width and thickness of the low velocity anomaly is greater than typically expected for a plume head and is more easily attributed to an upward continuation of the lower mantle African superplume structure into the upper mantle.

Geophysical Journal International, 2011

Broad-band seismic data from the southern African seismic experiment and the AfricaArray network ... more Broad-band seismic data from the southern African seismic experiment and the AfricaArray network are used to investigate the seismic velocity structure of the upper mantle beneath southern Africa, and in particular beneath the Kaapvaal Craton. A two-plane approximation method that includes a finite frequency sensitivity kernel is employed to measure Rayleigh wave phase velocities, which are inverted to obtain a quasi-3-D shear wave velocity model of the upper mantle. We find phase velocities for the Kaapvaal Craton and surrounding mobile belts that are comparable to those reported by previous studies, and we find little evidence for variation from east to west across the Namaqua-Natal Belt, a region not well imaged in previous studies. A high-velocity upper-mantle lid is found beneath the Kaapvaal Craton and most of southern Africa. For the Kaapvaal Craton, the thickness of the lid (∼150-200 km) is consistent with the lid thicknesses reported in many previous studies. The cratonic lid is underlain by a ∼100-km thick low-velocity zone with a 3.9 per cent maximum velocity reduction. By comparing the velocity model to those published for other Archean cratons, we find few differences, and therefore conclude that there is little evidence in the shear wave velocity structure of the mantle to indicate that the southern African plateau is supported by an upper-mantle thermal anomaly.

The Cameroon Volcanic Line (CVL) is an 1800km long volcanic chain, extending SW-NE from the Gulf ... more The Cameroon Volcanic Line (CVL) is an 1800km long volcanic chain, extending SW-NE from the Gulf of Guinea into Central Africa, that lacks the typical age progression exhibited by hotspot-related volcanic tracks. This study investigates the upper mantle seismic structure beneath the CVL and surrounding regions to constrain the origin of volcanic lines that are poorly described by the classic plume model. Rayleigh wave phase velocities are measured at periods from 20 to 182 seconds following the twoplane wave methodology, using data from the Cameroon Seismic Experiment, which consists of 32 broadband stations deployed between 2005 and 2007. These phase velocities are then inverted to build a model of shear wave velocity structure in the upper mantle beneath the CVL. Results show that phase velocities beneath the CVL are reduced at all periods, with average velocities beneath the CVL deviating more than -2% from the regional average, and +4% beneath the Congo Craton. This distinction is observed for all periods, but is less pronounced for the longest periods measured.