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Title: Crustal structure of the ocean-continent transform margin off Ghana
Author: Edwards, Rosemary Anne
Awarding Body: University of Edinburgh
Current Institution: University of Edinburgh
Date of Award: 1994
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In January 1991 a suite of seven seismic refraction lines was acquired across the transform continental margin off Ghana. Six lines were shot parallel to the margin, from the continental shelf down to the deep Guinea Basin, while the seventh line was normal to, and bisected, these. Maslov travel-time and amplitude forward modelling of the refraction line normal to the margin shows a rapid transition from continental to oceanic crust over a 10 km wide high velocity zone at the foot of the continental slope. The high crustal P-wave velocities (5.8 - 7.3 km s-1) in this zone raise the possibility that this region may be heavily intruded by basic igneous rocks. Interpretation of the seismic refraction lines in the oceanic domain has shown a highly heterogeneous crust both parallel and normal to the margin. Oceanic layer 2 has a two layer structure and shows considerable variation in velocity, from 3.3 - 6.6 km s-1. The velocities are generally lower, and layer 2 thicker, to the northeast compared to the southwest ends of the lines. Layer 3 shows more homogeneity and has velocities of 6.6 - 7.3 kms s-1. The oceanic crust is anomalously thin to at least 70 km from the ocean-continent transition, with a thickness of 3.5 - 5.5 km, and slightly thicker crust is systematically observed to the northeast. This thin crust is caused by an abnormally thin layer 3 (2.0 - 3.5 km), while layer 2 attains a more normal thickness of 1.5 - 3.0 km. Two-dimensional gravity modelling along the line normal to the margin has confirmed the seismic refraction results. The continental crustal structure from the gravity modelling includes an abrupt increase in the depth to the Moho from 23 km below the continental slope and shelf edge to 37 km further inshore (northwest). This vertical boundary coincides with a southeast to northwest increase in the velocity of the upper crust from 5.00 km s-1 to 5.80 km s-1. The variation in crustal thickness and structure is thought to represent the oceanward extension of the Akwapim fault zone which is mapped onshore Ghana with a southwest trend. This is a Pan-African suture zone which separates the West African craton to the west from Pre-Cambrian Dahomeyan units to the east.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available