Geology Reference
In-Depth Information
effects are interpreted to guide patterns of
deformation and metamorphism.
Consider, for example, four attributes of the
Himalayan-Tibetan orogen (Fig. 1.9A) and how
they might be linked. First, thickening of the
crust beneath the Tibetan Plateau appears to
have caused partial melting of at least some of
the lower crust (Nelson
et al.
, 1996), which then
becomes weak and tends to flow outward as a
channel confined above and below by stronger
rock. Second, the rocks of the Greater Himalaya
are bounded by two large faults that have
been active at the same time: below the
Greater Himalayan rocks, the Main Central
Thrust is an up-to-the-south thrust, whereas
above them, the South Tibetan Detachment is a
down-to-the-north normal fault. Third, every
summer the Indian monsoon dumps about 4 m
of rainfall on the southern flank of the Greater
Himalaya, but less than 10% of that amount pen-
etrates into Tibet. Across this decreasing rainfall
gradient, current erosion rates also diminish by
three- to four-fold (Gabet
et al.
, 2008). Fourth,
combining these observations, some geologists
suggest that the intense, monsoon-driven erosion
on the southern flank of the Himalaya weakened
the underlying crust and “drew” the partial-melt
channel toward the surface, thereby determining
where the big faults bounding the channel are
located. Numerical modeling of lower crustal
flow tends to reinforce the importance of erosion
(Beaumont
et al.
, 2001, 2004). In the absence of
erosion, lateral tunneling is predicted as channel
flow remains confined within the lower crust
(Fig. 1.9B), whereas when erosion is introduced,
the flow migrates upward toward the region of
high erosion (Fig. 1.9C). A striking aspect of these
numerical models is that, if the pattern of rainfall
were reversed so that rain was focused on the
opposite side of the orogen, the models predict
that the orientation of the large faults could also
flip direction! Whereas more questions than
answers remain about potential linkages between
climate, erosion, and tectonics, their proposed
interactions set an exciting stage for future explo-
ration of the role of climate in mountain
building.
Looking ahead
Resolutions to these controversies are beyond
the scope of this topic, but they provide a frame-
work for thinking about many of the topics
discussed in the subsequent chapters. These
controversies illustrate some of the breadth of
modern tectonic geomorphological studies. Any
serious consideration of potential solutions to
these controversies quickly reveals the interdis-
ciplinary nature of the research required to
address them. Although certainly not unique in
its demands for interdisciplinary work, tectonic
geomorphology attains much of its current
vibrancy from the cross-pollination that is
occurring between specialists of many disci-
plines who are coming together to address
major unresolved issues. It is our intent that the
following chapters provide some insight into
the tools, approaches, and interpretational
techniques that are currently used in tectonic
geomorphological studies. We hope to convey
the striking innovation and creativity of past
researchers, upon whose shoulders future
advances will be made.
