There has been active volcanism over a large part of western Kenya for the last 25 my. This has been accompanied by the development of the Kenyan Dome and Rift. The volcanics can be considered in terms of several space/time groupings based on their age and geographic position relative to the site of the present day rift valley. There is considerable variety in the petrography and chemistry of the volcanics. By examination of the more basic volcanics (> 4 wt. % MgO) it can be demonstrated that these variations are systematic through both time and space. As the alkalinity of the volcanics increases there is a sympathetic increase in the abundance of incompatible trace elements. Within the rift zone there is a decrease in alkalinity through time, and during the Miocene and Pliocene a north to south increase in alkalinity. Volcanics to the East and West of the rift zone are more alkaline than contemporaneous volcanics within the rift zone. The chemistry of the Kenyan Basic Volcanics is comparable to that of volcanics from ocean islands. North of the Kenyan Province, but associated with the same rift system, are the Ethiopian Volcanics, the chemistry of which is more akin to continental flood basalts. It is demonstrated that the chemical variety displayed by the Kenyan Volcanics can be produced by variable degrees of partial melting of and homogenous source mantle, and that this source must be garnet Iherzolite containing carbonate phases. The source must be enriched in Nb relative to other incompatible trace elements, but is not necessarily enriched in the light REE relative to the heavy REE. There may be variations in the Ba, P and Sr contents of the source. The trace element abundances of the source are similar to the source tapped by ocean islands. However, the ratios La/Nb and K/Nb show that unlike ocean islands the parental magmas to the Kenyan Volcanics are probably contaminated during transit through the sub-continental lithosphere. Geophysical and geotectonic evidence suggest the presence of a hotspot operating beneath Kenya. The variations in the chemistry, however, require a more sophisticated melting regime than that which would be produced a simple hotspot. It is possible that the volcanism is the result of partial melting in more than one melting regime and that these melting regimes have or have had a strong linear element to them, possibly related to upwelling of the mantle caused by passive stretching of the lithosphere.

Phreatomagmatic volcanoes form when ascending magma explosively interacts with surface or groundwater at shallow depths. Three types of phreatomagmatic activity are recognised- phreaticp phreatomagmatic (s. s. ) and surtseyan - based on the degree of involvement of magma with water and the depth of the interaction. Phreatic maars and phreatomagmatic tuff-rings are underlain by pipe-like diatremes but these structures are poorly developed or absent in surtseyan tuff-rings. Comparisons of phreatomagmatic volcanoes with their eroded diatreme equivalents, which contain subsided subaerially-deposited material, allow a model for activity of this type to be constructed. The Saefell tuff-ring SW Iceland is a surtseyantype structure whose crater remained open to the sea during most of its activity, allowing easy access of water to the magma. Base-surges sourced partly from directed blasts, formed large dunes with internal structures indicating deposition by density currents whose flowpower decreased with time and with distance from the vent. Syndepositional slumping and minor en masse collapse of crater deposits formed a pile of massive tuffs above which subsequent surge and airfall activity deposited a nestedg inner crater rim. The Medano tuff-ring Tenerife, is a phreatomagmatictype structure whose crater contains reworked tuffs deposited during subsidence into the underlying diatreme. Initial activity ejected much country rock material as magma contacted groundwater at depth but with time eruptions became more strombolian, as water was used up or failed to gain access to the vent. Surges were less common than in the Saefell eruption because the Medano water: magma ratio and explosion depth less often fulfilled the optimum conditions for surge production. The East Lothian diatremes in Scotland are subdivided into two groups on the basis of their infilling. The Red group diatremes contain high proportions of sediment and represent the subsided products of phreatic maars which erupted into a pile of water-rich poorly-consolidated alluvial plain sediments. The later Green group diatremes contain mainly juvenile basalt fragments and formed as phreatomagmatic or sometimes surtseyan tuff-rings, due to magma contacting water at shallow depths or in marginal lakes respectively. The Parade diatreme, Dunbar, contains over 300m of largely base-surge tuffs thought to represent the subsided inner flank deposits of a large maar. The Heads of Ayr and the East Fife diatremes expose different levels in subsided phreatomagmatic tuff-rings due to collapse-ahd erosion. Deep levels such as that exposed at Lundin Links, contain unbedded tuffs and abundant intrusive material. Shallower levels, such as at Elie Ness contain high proportions of bedded tuffs which are often centroclinally orientated. Base-surge, airfall, slumped and reworked tuffs in the Scottish diatremes are directly comparable to deposits in the modern tuff-rings studied proving their origin. A model for the formation of surtseyan tuff-rings is presented, with phreatomagmatic explosions resulting from steam expansion jets which disrupt an already vesiculating magma as it engulfes subsiding water-laden ash. A base surge model is also presented, involving deposition of tuffs with characteristic bedforms and structures by the head, body and tail of each surge analogous to turbidity currents. Cooling of hot, dry steam to cool moist steam towards the rear of surge pulses leads to lag breccias and progressive dune deposits being succeeded by regressive dunes and plastering structures with time. Juvenile sideromelane fragments erupted by phreatomagmatic volcanoes rapidly alter to palagonite as heated pore-waters circulate through the newly-deposited tuffs. Palagonitization results in cation mobility within unstable glass and precipitation of authigenic minerals in voids. Non-equilibrium growth of such minerals results in variable compositions and crystal forms. Subsequent alteration occurs slowly as a weathering process whose rate is greatly reduced as authigenic precipitation closes pore spaces within the tuffs. On diagenesis, unstable alteration products are commonly replaced by chlorite, calcite and clay. Reddening of some tuffs occurs by in situ breakdown of ironbearing minerals and release of Fe to solution although groundwater exchange with red country rock sediments may also occur. Unless present in diatremes phreatomagmatic products have a low preservation potential due to :- extreme alteration, rapid syn- and post-volcanic reworking, low ejecta volumes and breaching and burial beneath later lavas. In contrast the sedimentary structures petrography, morphology and grain size characteristics of diatreme tuffs are shown to be often sufficiently well preserved to permit the identification of their original surface volcanoes and their eruptive histories.

Wrinkle ridges are among the most common tectonic structures on the terrestrial planets and provide important records of the history of planetary strain and geodynamics. The observed broad arches and superposed narrow wrinkles are thought to be the surface manifestation of blind thrust faults, which terminate in near-surface volcanic sequences and cause folding and layer-parallel shear. However, the subsurface tectonic architecture associated with the ridges remains a matter of debate. Here we present direct observations of a wrinkle ridge thrust fault where it has been exposed by erosion in the southern wall of Melas Chasma on Mars. The thrust fault has been made resistant to erosion, likely due to volcanic intrusion, such that later erosional widening of the trough exposed the fault plane as a 70km long ridge extending into the chasma. A plane fit to this ridge crest reveals a thrust fault with a dip of 13 degrees (+8 degrees, -7 degrees) between 1 and 3.5km depth below the plateau surface, with no evidence for listric character in this depth range. This dip is significantly lower than the commonly assumed value of 30 degrees, which, if representative of other wrinkle ridges, indicates that global contraction on Mars may have been previously underestimated.

No description available.

No description available.

No description available.

No description available.

Abstract not available.

No description available.

No description available.