Geology Reference
In-Depth Information
Mantle Drip & Dynamic Topography
B
A
s
i t y
y
Tule
Lake
B
Fig. 10.45
Dynamic topography associated
with a mantle drip in the southern Sierra
Nevada, California.
A. Topographic overview of the Central Valley
and adjacent Sierra. Tule Lake lies in a closed
depression that is generated by subsidence in
response to a lithospheric drip. Sediment onlap
dominates above the drip, whereas offlap
predominates elsewhere. B. Drowned
topography along the Sierra foothills where
steep “islands” of bedrock rise abruptly above
the aggrading basin floor. C. Schematic
cross-section of subsidence in the Central
Valley and uplift of the Sierra that is driven by
the mantle drip. Modified after Saleeby and
Foster (2004).
C
Central Valley
Sierra Nevada
Owens
Valley
area of anomalous
topography
0
mafic batholith/terranes
f
felsic batholith
S U
50
L I
E
100
convecting asthenosphere
interpreted
mantle
lithospheric
drip
150
-200
0
50
Distance (km)
100
250
Bay. This closed depression clearly indicates that,
today, the pace of subsidence above the drip is
outstripping the rate of sediment delivery from
the Sierra. Both north and south of the anomaly,
more typical offlap of Holocene sediment prevails,
as is expected given the general late Quaternary
decrease in sediment flux from the Sierra.
At the scale of tectonic plates, subduction of
dense lithosphere tends to flex plates downward,
whereas upwelling and outward push at spread-
ing ridges tends to flex them upwards. The
geoid, in contrast, should be high-standing
above the denser lithospheric bodies associated
with subduction zones. A spectacular example
of flexure at the plate scale is found in the
coastal record of Australia (Sandiford and
Quigley, 2009) (Fig. 10.46). The Indo-Australian
Plate is bounded on the north and east largely
by subduction zones and on its south and west
by mid-ocean ridges. The down-to-the-northeast
pull and up-to-the-southwest push of dynamic
lithospheric stresses exerted by these boundaries
are interpreted to cause the plate to tilt down
to the north-northeast. The topographic conse-
quence of this tilt is that the southern part of
Australia is emerging, whereas the northern part
is sinking. The geomorphic manifestation of
this long-term tilt is that the 15-Ma Miocene
