The Cameroon Volcanic Line

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The Cameroon Volcanic Line

• What is it? Where did it come from?

Jay Pulliam Institute for Geophysics The Univers
ity of Texas at Austin
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What is a mantle plume?

• Morgan (1971) originally proposed that plumes,
  which he described as hot upwellings of
  relatively primordial material, rise from the
  deep mantle and feed surface hot spots. Such
  plumes rise because of thermal buoyancy, and must
  originate at a thermal boundary layer. The only
  such layer known to exist in the deep mantle is
  the core-mantle boundary (D"), and thus
  Morgan-type plumes are generally assumed to
  rise from this layer.
• Irrefutable evidence for such plumes has not
  been confirmed, and contrary, or unexpected,
  observations are often reported. On the other
  hand, low-wave-speed seismic anomalies with
  different shapes, e.g., shallow, or very wide
  bodies have been found. Such observations have
  led to diversification of the range of features
  that scientists call plumes.
• A clear, widely accepted definition of a plume
  does not currently exist. As a result, scientific
  interaction is often hampered because individuals
  use the word plume to mean different physical
  models without each other realising. Furthermore,
  it is essentially impossible to disprove the
  plume hypothesis at any given location because
  the term plume has become so vague.

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What is a mantle plume?

• Griffiths and Campbell (1990)Basalts erupted at
  oceanic and continental hotspots originate from
  zones of melting having potential temperatures
  greater than normal melting and therefore are
  attributed to plumes of hot material upwelling
  from deep in the mantle
• Turcotte and Schubert (2002)Mantle plumes are
  quasi-cylindrical concentrated upwellings of hot
  mantle rock and they represent a basic form of
  mantle convection (p.15)
• Davies (1999)Mantle plumes are buoyant mantle
  upwellings that are inferred to exist under some
  volcanic centres.
  
• p. 513, Bates R.L. and J.A. Jackson (eds), 1987,
  Glossary of Geology, 3rd ed.  published by the
  American Geological Institute, Alexandria, VA,
  788 p  plume  (a) a localized body of
  volcanic rock rising into the crust from the
  mantle and thought to be the causal mechanism of
  a hot spot.  hot spot (p. 314)  a volcanic
  center, 100 to 200 km across and persistent for
  at least a few tens of millions of years, that is
  thought to be the surface expression of a
  persistent rising plume of hot mantle material. 
  Hot spots are not linked with arcs, and may or
  may not be associated with oceanic ridges.

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What is a mantle plume?

• p. 555 (glossary), W.K. Hamblin, 1989, The
  Earth's Dynamic Systems (A Textbook in Physical
  Geology), 5th ed.  Macmillan Publishing Co., NY,
  NY, 576 pp. 
• mantle plume  a buoyant mass of hot mantle
  material that rises to the base of the
  lithosphere.  Mantle plumes commonly produce
  volcanic activity and structural deformation in
  the central part of lithospheric plates.
• John Hernlund
• An upwelling hot solid body arising from a
  thermal boundary layer deep in the mantle, a
  natural and essential feature of bottom-heated
  convection.
• From Tozer (1973)A narrow buoyant upwelling in
  fluids of high Peclet number and near unity
  Prandtl number.
  
• Mark Jancin, March 2004
• Plume a term that amounts to a semantic
  garbage disposal that does nothing to clarify the
  thoughts of either the author or the reader.

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Distribution of Volcanism in Africa

• Distribution of all complexes for which
  radiometric dates are available (Woolley, 2001),
  divided into three categories.
  
• (1) Ages less than 135 m. yr (blue circles)
  cover the period following the final break up of
  Gondwanaland.
  
• (2) Between 500-135 m. yr (green triangles)
  Africa was the nucleus of the super-continent
  Gondwana-Pangea.
  
• (3) Ages greater than 500 m. yr (red squares)
  predate the end of the last plate-wide
  tectono-thermal event (Pan African) and broadly
  indicate cratonic regions.
• (Figure from Bailey and Woolley, 2005)

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Satellite-derived free air gravity map of the
Central, Equatorial and South Atlantic Oceans
(after Sandwell and Smith, 1997). Warm colors
indicate gravity highs and cool colors gravity
lows. The locations of the main aseismic ridges
(Walvis Ridge and Rio Grande Rise) and the
Cameroon Volcanic Line are indicated. The St.
Helena Seamount Chain forms the continuation of
the Cameroon Volcanic Line towards the axis of
the Mid-Atlantic Ridge and the young volcanic
island of St. Helena (considered to be the
current locus of the hotspot). The active
volcanic island of Tristan da Cunha is considered
to mark the current locus of the hotspot
associated with the Walvis Ridge. Figure from
Fairhead and Wilson (2005).
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Sketch map of the Cameroon Volcanic Line
Reported ages refer to the stratoid volcanism
of the continental sector (Gouhier et
al., 1974 Fitton Dunlop, 1985 Njilah, 1991
this study), and to the onset of the basaltic
volcanism on the oceanic islands (Lee et al.,
1994a). Inset, top left sketch map of Western
Cameroon Highlands Mt Bambouto caldera is shown,
as well as location of basaltic samples used for
40Ar/39Ar dating. Inset, bottom right the cra
tons are West African Craton (WAC), Congo Craton
(CC) and Kalahari Craton (KC).
Plomerova et al., 1993 Poudjom Djomani et al.,
1995, Ngaoundere Plateau (22 samples) volcanic
complexes
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Figure from Thorne Lay (2005)
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Scenarios for the thermo-chemical boundary layer
(TCBL) in the D" region based on seismic
observations. (a) The hybrid TCBL scenario for
the deep mantle, in which subducting slabs
penetrate to the CMB, providing thermal and
chemical anomalies that will eventually rise back
up in the mantle, while hot dense chemical
anomalies of either primordial or slab-related
origin are swept into large piles under
upwellings. Either a phase change or a radial
gradient in structural fabric exists as well,
giving rise to reflections from the top of D".
(b) The global TCBL scenario, in which the lowerm
ost mantle is a dense chemically distinct layer,
possibly of primoridal nature, which remains
unmixed, but thermally coupled to overlying flow.
Variable heat flow out of the chemical layer
occurs in response to the configuration of mantle
flow, leading to lateral variation in thermal
structure across the boundary layer. Topography
is induced on the chemical layer by the
mid-mantle flow as well. Either of these
scenarios may be overlain by thermal boundary
layer instabilities yielding plumes as in Figure
1, but at present the direct observational basis
for them is weak. (Modified from Lay et al.,
2004).
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Basins and swells of the African plate. The
Cameroon Line of 10 volcano-capped swells
straddles the continental margin close to long
10E. The line forms a distinctive part of the
African plates active swell population
(1) the swells are small, 100 km across (2) they
are arranged in a straight line and (3) half the
swells lie onshore and half offshore.
The ca. 75 swells of the African plate are
indicated as ellipses. Irregularly shaped basins
are interspersed among the swells. Volcano-capped
swells are shown with black marks indicating the
locations of volcanic rocks. The three swells at
the landward end of the Cameroon Line are, from
west to east, the Jos, Biu, and Ngaoundere
swells. Offshore swells are shown only where ther
e is evidence of dynamic support such as active
volcanism. In addition to the Cameroon Line, ca.
32 offshore and ca. 32 onshore swells are shown.
(Figure from Burke, 2001.)
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Summary of relative plate motions (large red
arrows) of Africa and South America based on data
compiled by GFZ Potsdam for the period 1993-2003.
GPS data (thin red arrows), estimates of motion
predicted by the NUVEL 1A model (black arrows),
and astronomical data (blue arrows). Black
dashed lines indicate major intra-continental
rift systems. (Figure from Fairhead and Wilson,
2005).
18
Plate reconstruction at 100 Ma illustrating the
onshore and offshore gravity data (left panel
data from GETECH) and the major tectonic features
within Africa and South America (right panel).
Major rift zones and intra-continental shear
zones are marked in red. Red and blue arrows
within the ocean basins indicate the major
extension directions. Grey arrows within the
continents indicate the dominant fault movement
directions. Green shading indicates areas of
oceanic crust. The black line marks the position
of the Mid-Atlantic Ridge at 100 Ma. (from
Fairhead and Wilson, 2005).
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Satellite-derived free air gravity image showing
the segmentation of the oceanic crust (segment
boundaries shown as thick white lines) between
the Central and South Atlantic Oceans. Gravity
data from Sandwell and Smith (1997). Figure from
Fairhead and Wilson, 2005).
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Detail of the free air gravity map of the African
continental margin in the southern Equatorial
Atlantic illustrating the disruption of the
flowline geometry. The left panel shows the
uninterpreted image the right panel shows the
interpreted image. The white dashed line marks
the trend of the flowlines the red polygon
probably represents an area of volcanic
over-production the black dashed line is a
structure yet to be interpreted. This area
provides a typical example of why better
resolution data would greatly improve the imaging
of these subtle structures.
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