During the period within a few million years of 8 Ma, a wide variety of changes took place in the region surrounding and including the Tibetan Plateau. These changes span a range of phenomena including the onset of folding and faulting of oceanic lithosphere and environmental changes north and south of Tibet. Together they motivate exploration of mechanisms by which the growth and development of Tibet might have affected regional environments, but because of various inconsistencies among the observations, it is perhaps no wonder that only a few of us pursue this topic.
Dating of several environmental
changes suggests that in the period between ~9 and ~6 Ma, much of eastern Asia
either became drier or precipitation became more seasonally concentrated. These
changes, however, were not synchronous. Increases in abundances of
microorganisms sensitive to winds over the Arabian and South China Seas, in
rates of aeolian deposition over China and the Pacific Ocean, and in values of
18O
in pedogenic carbonates near 8 Ma occurred before changes in
13C
and apparently before other evidence for increased aridity and/or seasonal
precipitation. Any understanding of these changes must account for the lack of
simultaneity, to say nothing of evidence inconsistent with such environmental
changes (e.g., Dettman et al. 2001).
Both the nature of tectonic events and their dating make their relationship to the growth of the Tibetan Plateau non-unique. First, dating is too imprecise to show that most occurred at, or just before, the environmental changes. Moreover, it appears that some tectonic developments definitely began before those changes. Thus, on the one hand, one can concoct explanations for differences in timing in various places, so that one can interpret the observations of tectonic development in terms of growth of Tibet, and then as the stimulus for resulting climate, or environmental, change. On the other hand, some, but not all, suggestions of abrupt onsets of tectonic development near or just before 8 Ma might be explained by some kind of climate change that accelerated erosion, river incision, and sedimentation. Suffice it to say that the rough simultaneity of these events will motivate us to pursue the link between these various phenomena, but that others should be justifiably skeptical about the speculative thinking that still must be employed to make such a link.
The most convincing test that a
significant part of the Tibetan Plateau rose ~1000 m (or more) at ~ 8 Ma would
be the demonstration of lower elevations before that time and confirmation that
the plateau was as high or higher (by ~500 m) at ~ 8 Ma. Present knowledge
suggests that carrying out such a test will be difficult. First, fossil leaves
that could be used to estimate paleo-elevations appear to be sparse on the
Tibetan Plateau, and finding enough to yield convincing paleo-elevations
appears, at present, to be impossible. Moreover, although oxygen isotopes can be
measured from pedogenic carbonates and from clay minerals on Tibet, how they
should be interpreted remains uncertain. As
Garzione et al. (2004) showed, diagenesis can destroy records of past local climate. More discouraging is the
absence of a comprehensible dependence of
18O
on elevation, measured both from precipitation over northeastern Tibet (Araguas-Araguas
et al. 1998) and from stream water that samples annual precipitation
(Garzione et al. 2004). Thus, extracting direct measures of paleo-elevations
appears difficult, at least on the northern side of the plateau.
Despite the numerous phenomena that seem to have occurred near 8 Ma, dating of many remains sufficiently imprecise to demonstrate simultaneity or its absence. In a search for the ideal field area, the northeast margin of the plateau might allow precise timing of both tectonic and local climatic events using the same material.
Tests of both a rise of Tibet at or just before 8 Ma and resulting climate changes may require theoretical developments that yield predictions to be tested. For instance, if we understood how Tibet perturbs regional climates and we could confidently predict how different distributions of elevations affected climate, we could use the paleoclimatic record outside of the plateau, such as records of loess deposition, pollen and other paleobotanical fossil organs, or stable isotopes, to test possible past elevation distributions. My impression is that we can calculate possible paleoclimates with atmospheric general circulation models, but we lack the understanding of how the many interrelated phenomena affect calculations; hence we cannot yet use such calculations to draw unique inferences.
A rise of the high interior of Tibet at ~8 Ma requires increased horizontal normal deviatoric stress for its support and therefore should facilitate compressive deformation of the flanks of the plateau. Thus, a documentation of rapid growth of the margins of the plateau beginning near 8 Ma would allow the inference of such growth to pass a test. Conversely, a demonstration that the plateau has grown outward steadily for tens of millions of years would cast doubt on an abrupt rise of its interior. Perhaps most convincing would be the demonstration that normal faulting and crustal extension became important at ~8 Ma. This demonstration requires the study of many normal faults.
If mantle lithosphere has been removed recently in geologic time (since ~10 Ma), cold material should lie beneath Tibet. Seismological confirmation of such material would allow the idea of such removal to pass a test, but a demonstration, for instance, of intact sub-lithospheric mantle beneath the majority of Tibet would cast doubt on this idea.
The variety of studies that can be used to test ideas of Tibet’s growth and its impact on Tibet call for a multidisciplinary approach, and it seems safe to assume that no single study will settle the debate conclusively. Moreover, although the best approach seems to be to pose tests of ideas, the non-unique interpretations of most observations will enable such tests to falsify suggested interrelationships between Tibetan growth and regional climate change, but not confirm them. Most likely, someone will stumble onto some new measurement or new observation that we have not anticipated, and it will provide the most definitive test.
