dc.description.abstract | The Kakamega - Kapsabet study area located in western Kenya, occurs at the tectonic
contact zone between the Archaean Nyanza Cratonic Segment (NCS) an extensive
segment of the larger Tanzanian Craton (TC), and the Neoproterozoic Mozambique
Mobile Belt (MB). The area covering about 1330km2, and lying between latitudes 0005'N
and 0025'N, and longitudes 34°45'E and 35°05'E, is marked in the middle by the NNW-SSE
oriented Nandi fault escarpment. This escarpment also marks the middle of the
approximately 7 to 15 km wide Neoproterozoic Nandi Fault and Shear Zone (NFSZ)
which constitutes the tectonic boundary between the craton and the mobile belt.
The NCS of the Tanzanian craton in the study area is predominantly composed of
Archaean granite-greenstone lithelogies. The greenstones include both metavolcanics and
metasediments. The metavolcanics consist of andesites, rhyolites, agglomerates and tuffs
with a conspicuous absence of basalts in the study area. Through petrological studies the
metavolcanics are interpreted to be Archaean island arc volcanics. The metasediments
comprising basal polymictic conglomerates, greywackes, sandstones, shales and
mudstones are inter"preted to be Archaean back-arc and fore-arc turbiditic shallow and
deep marine sediments. Stratigraphic classification of the lithologies has led to the
establishment of new lithostratigraphic units and lithodemic classes in the study area. The
metavolcanics and metasediments have been assigned to the Nyanzian Supergroup and to
the unconformably overlying Kavirondian Supergroup respectively. The portion of the
Neoproterozoic Mozambique Mobile B-elt consists of biotite gneisses, augen gneisses,
banded gneisses, migmatites and granitoid gneisses, all of medium to high grade regional
metamorphism. Regional metamorphism in the Archaean terrane was largely within the
greenschist facies. Towards the end of the Archaean epoch thermal metamorphism was
superimposed on the earlier low grade regional metamorphism around the granitoid
plutons. The highest grade attained within the metamorphic aureoles was in the pyroxene
hornfels facies where sillimanite, cordierite and andalusite are the characteristic index . minerals.
The Kakamega-Kapsabet area has experienced a complex tectonic history spanning from
the Archaean to the Cenozoic. One depositional (DAD)and seven deformational (DAX,DAD,
DA1,DA2,DP1,Dp2,Dp3,DTl) events of different ages (Archaean, Neoproterozoic and
Tertiary) have been distinguished in the study area. DADis the Archaean Kavirondian
depositional event. The DAXis the pre-Kavirondian deformational event which affected
the older Nyanzian lithologies. DAl and DA2are folding events caused by Archaean
granitoid diaprism. DPl is the Neoproterozoic folding event associated with the
Mozambiquian orogenic event. Dp2and Dp3are later Neoproterozoic crustal extensional
events associated with post-collisional release of strain in the region. DTl is the Nandi
escarpment hinge faulting event during Tertiary/Quaternary period. It is interpreted as
the last tectonic event in the area and mainly affecting NFSZ. Rock blocks of the western
side of the Nandi escarpment experienced a downthrow while those to the east
underwent a tectonic uplift during DTl.
This study has established the presence of intra-granitoid and inter-granitoid multiepisodic
plutonism in the area. Detailed geochronological works, still will need to be
carried out to investigate and confirm the proper timing of these plutonic events.
Kavirondian turbidite-hosted gold mineralization in the study area is associated with
smoky quartz veins mainly in the NCS. Sulphide mineralisation has been identified
proximal to the granitoid plutons arid-is hosted by both the Nyanzian volcanics as well as
by the Kavirondian turbidites. Iron oxide bearing cherts form a marker bed along the
stratigraphic horizon capping the youngest members of the Nyanzian Group. These iron
rich cherts mark the waning of sub-marine volcanism in the area. The lithological and
tectonic evidence in this study has established that the Archaean Kavirondian marine
basin was shallowest to the east in the Nandi fault region and was progressively
deepening towards the west (or westwards). | en |