Five abstracts were accepted for presentation in the 2015 Geological Society of America South Central Annual Meeting to be held in Stillwater, Oklahoma, USA. Students and faculty from the Tectonics and Geophysics Group will present their research in the Session: “T1. East African Rift, Southern Oklahoma Aulacogen, Rio Grande Rift, and other Continental Rifts: A Tribute to the Career of G. Randy Keller”
Here are the abstracts:
BORDER FAULT SEGMENTATION ALONG THE MALAWI RIFT
Border fault segmentation characterizes the early stage of continental rift zones. Previous studies suggest that border faults in the Malawi Rift vary in displacement along a segment, alternate half-graben polarities, and are connected through accommodation zones. This study uses Shuttle Radar Topography Mission (SRTM) Digital Elevation Model (DEM) data and published seismic sections to investigate the along strike variation in border faults along the Malawi Rift. Available SRTM-DEM data allowed more detailed examination of vertical distances, and relationships between Precambrian basement structures and border fault segments than was previously possible. Detailed E-W topographic profiles were extracted from the SRTM-DEM data at a spacing of 1.4 km along the entire length of the Malawi Rift to determine the fault scarp elevations. Seismic cross-sections were used to provide the subsurface structure and displacement of the border faults. Eight border fault segments were identified based on the location of full-grabens, which occur between two half-grabens of opposite polarities. The length of fault segments varied from ~230 to ~45 km. The general trend observed reveals a decrease in fault scarp elevation from ~2 km in the north to ~0.2 km in the south. Three broad regions were identified based on different fault geometries along the rift. The Northern Area shows the overlap model that contains the transition of half-graben to full-graben and to half-graben of opposing polarity. In some places, border faults follow the general structural grain of the basement. In the Central Area, zones of segmentation coincide with Permo-Triassic grabens that strike at high angle to the trend of the rift. There are no well-defined escarpments in this area. The Southern Area exposes basement structure and low elevations, indicative of no subsidence and rift termination. The well-developed overlap model to the north, the less well-defined escarpments in the center, and rift termination to the south suggest that the rift becomes younger to the south in the direction of propagation. Results from this study suggest that the overlap model of border fault segmentation is only well defined in the northern part of the Malawi Rift, and not in other parts where pre-existing structures at high angle occur, and rifting is young and undeveloped.
UPLIFT HISTORY ALONG THE MALAWI RIFT THROUGH MORPHO-TECTONIC ANALYSIS
Previous work on the Malawi Rift has shown at least two distinct phases of rifting, switching from an extensional tectonic regime to a strike-slip regime around 400-500 ka. It has been proposed that the rift is opening in a zipper-like motion from north to south, though the impact of the extension on rift flank uplift remains unknown. We used morpho-tectonic analysis of streams in the rift flanks to compare the uplift history from north to south along the Malawi Rift. The main geomorphic indicators we used were the normalized steepness index generated by the Stream Profiler ArcMap extension and knickpoints that were picked by visually examining sharp changes in steepness along drainage basins that feed into Lake Malawi (Nyasa). These indicators cannot constrain uplift rates but can instead show changes in tectonic regimes along the rift. The middle and northern portions of the rift exhibit high normalized steepness values across all studied border faults, indicating fast uplift rates and a distinct onset of rifting to the north. The southern portion of the rift shows varying levels of response to border faults, with some drainage basins showing little change upstream of the border fault and others showing a distinct increase in normalized steepness. The contrast between morpho-tectonic indicators in the northern and southern sections of the rift suggests a more mature and dynamic rifting process to the north. Previous workers in the Malawi Rift have also noted the influence of pre-existing structures on rift development, which may be expressed through these geographic variations in geomorphic response to border faults. Similarly-oriented pre-existing structures may have provided a weakened pathway for incipient rifting in the north, though no such evidence to confirm a similar pattern exists in the south. Future mapping of these structures and their corresponding dips will show if the decrease in uplift rate to the south can be explained by pre-existing structure orientation.
INFLUENCE OF PRE-EXISTING STRUCTURES ON STRAIN LOCALIZATION AND EVOLUTION AT THE SOUTHERN TERMINATION OF THE MALAWI RIFT
The Malawi rift is the youngest segment of the East Africa Rift System (EARS) and has been suggested to be the southern termination of the western branch of the EARS. Rifting in Malawi mostly occurs in the Precambrian basement rock, which is well exposed at the surface. The Malawi rift is an ideal location to determine the controls of pre-existing structures on incipient rifting. Our objectives include mapping the rift surface structures in southern Malawi using remote sensing in the form of Shuttle Radar Topography Mission (SRTM) digital elevation model, which records elevation over the rift; Landsat, an Earth imaging satellite that records the electromagnetic radiation resulting from reflected and emitted energy from Earth; and Radio Detecting And Ranging Satellite (RADARSAT), which measures surface roughness. We will map the basement structures over the southern portion of Malawi using a magnetic anomaly map, produced from aeromagnetic data. Magnetic anomalies are often reflected through the distribution of magnetic minerals, primarily magnetite. The Precambrian basement is rich with magnetite and this high concentration, juxtaposed against rocks with lower concentrations of magnetite can produce a sharp boundary, showing as a linear trend. Preliminary remote sensing analysis show a northeast-southwest trend, truncated against the border faults, largely from the Mesozoic Karoo rifting, and Precambrian basement structures showing a WNW-ESE trend as well as a similar northeast-southwest trend to that of the Karoo rifting events. There are also some north-south trends that side step along pre-existing Precambrian faults, showing that current structural evolution is influenced by pre-existing structures. By comparing basement structures to superficial surface structural expression, we can better understand how pre-existing basement structures influence the initiation, evolution, and termination of amagmatic rifting, which will contribute to hazard mitigation, rift modeling, and the construction of more accurate geologic maps of southern Malawi.
MAGMATIC UNDER-PLATING BELOW THE KAROO AGED LUANGWA RIFT VALLEY?
We used aeromagnetic data, moderate resolution satellite and high resolution ground gravity data to examine the thermal and crustal structure beneath the Luangwa Rift Valley (LRV) in Zambia in order to determine the geodynamic controls of its formation. We computed Curie Point Depths (CPD) using two-dimensional (2D) power spectrum analysis and three-dimensional (3D) inversion of the aeromagnetic data, and the results were used to calculate heat flow under the LRV. We also determined the crustal thickness under the LRV by calculating the depths to the Moho using 2D power spectrum analysis of the satellite gravity data. We found that (1) There is no elevated CPD beneath LRV, and as such no high heat flow. (2) 3D inversion of aeromagnetic data suggests the presence of magnetic bodies below the LRV. (3) Moho depth results suggests a thicker crust and deeper Moho (~50 km) beneath the rift, compared to 35-45 km depths away from the rift. The gravity-Moho depths differ from Moho depths determined from passive seismic studies especially beneath the rift but are comparable at locations outside the rift. (4) The high resolution ground gravity data suggests the presence of a broad long-wavelength positive anomaly, possibly related to deep dense material at the Moho. This anomaly is modified by shorter-wavelength positive anomalies at the rift shoulders and floor (possibly related to denser bodies in agreement with the 3D inversion results of aeromagnetic data), and negative short-wavelength anomalies (related to rift sediment infill). We subsequently developed a 2D forward model from the high resolution gravity data to reconcile the thermal and crustal characteristics of the LRV. Our model suggests the deeper Moho below the rift to be the result of magmatic under-plating. The difference between the gravity and passive seismic Moho depths estimates may be because the passive seismic data imaged the Moho as the top of the under-plated mafic body whereas the gravity data considered the base of the under-plated mafic body as the Moho. Magmatic under-plating may have introduced a rheological weakness that may have helped to localize strain during rift initiation in Karoo times. It is possible that other palaeo-rifts may have similar thermal and crustal characteristics as the LRV.
GRAVITY AND MORPHO-TECTONIC ANALYSES OF THE BROADLY RIFTED ZONE IN SOUTHERN ETHIOPIA
The Broadly Rifted Zone (BRZ) is a diffused rift in southern Ethiopia between the Main Ethiopian Rift and the Eastern Branch of the East African Rift System (EARS). Rifts width within the EARS ranges from 50 to 65 km with the exception of the 300 km-wide BRZ, which is a wide zone of narrow graben and horst structures that form 10-50 km wide basins. Individual grabens within the BRZ are bounded by steeply-dipping normal faults with no low angle detachments exposed on the surface. The anomalous width of the BRZ is unusual to be developed through extension associated within a narrow rift system. We propose that mantle upwelling caused uplifting and subsequent gravitational collapse resulting in the broadly distributed extensional deformation. To test this hypothesis, we used two-dimensional (2D) power-density spectral analysis of ~ 9 km spatial resolution satellite gravity data and morpho-tectonic analysis of streams extracted from the 90 m spatial resolution Shuttle Radar Topography Mission (SRTM) Digital Elevation Model (DEM) to examine the relationship between the BRZ, Moho depths, and topography. To estimate the Moho depth, 2D power density spectral curves were computed from 1° X 1° windows of the gravity data with 50 % N-S and E-W overlaps. The morpho-tectonic analysis was used for the calculation of normalized steepness indices of streams in the BRZ. The gravity data shows shallow Moho depths under the BRZ with the shallowest depth of ~ 27.5 km in its northern limit. The areas surrounding the BRZ have Moho depths that reach up to 43 km. These results are consistent with previous seismic Moho depth estimations. The morpho-tectonc analysis showed that stream longitudinal profiles with the BRZ have high normalized steepness values following the ~ 1 km and ~ 2 km contour lines, which suggest two uplifting episodes. This is consistent with the presence of exhumed Precambrian basement rocks in the northern part of the BRZ at an elevation as high as 3.4 km. These results suggest that the development of the BRZ is related to uplifting due to the thermal effects of mantle upwelling which was followed by gravitational collapse to form the wide zone of extensional deformation. There are only few examples in the world of wide rifts besides the BRZ, including the Basin and Range of the western US, which also show heterogeneous crustal thinning.
