Emeishan vs. Deccan: Field evidence for and against
regional pre-volcanic uplift, and implications for tectonic
models
Hetu C. Sheth
Department of Earth Sciences, Indian Institute of Technology
(IIT) Bombay, Powai,
Mumbai 400 076, hcsheth@iitb.ac.in)
The mantle plume model predicts one to few kilometres
of regional (~1000 km across), domal lithospheric
uplift 5-10 million years before flood basalt volcanism.
The uplift is due to heat conduction out of and dynamic
support by the hot, buoyant, rising plume head. Field
evidence for such uplift would comprise sedimentary
sequences that reflect progressive basin shallowing
before volcanism, or widespread conglomerates derived
from the basement rocks and underlying the first lavas
(indicative of differential tectonic uplift along
faults). Local uplifts and subsidences cannot be used
to prove or disprove plume-derived uplift. In the
Permo-Triassic Emeishan flood basalt province of China,
domal, kilometre-scale regional pre-volcanic uplift
has been identified based on the palaeogeographic
shallowing trends recorded in sediments, and conglomerate
horizons underlying the initial basalt lavas. This
is consistent with the plume model. Such uplift has
been claimed for the Deccan flood basalt province
of India as well, but field geological data from the
Deccan, and India, paint a very different picture
of crustal-lithospheric uplift than has been presented
in plume models. Over large areas of the Deccan province,
the base of the lava pile is in the subsurface. Basalt-basement
contacts are observed along the periphery of the province
and in central India. In central India (Satpura and
Vindhya ranges), long, flat-lying Deccan basalt flows
directly overlie extensive planation surfaces cut
on various older rocks with different internal structures.
Thin, patchy Late Cretaceous clays and limestones
(Lameta Formation) separate the basalts and basement
locally, but these sediments are known to have been
locally derived from the nearby basalts themselves.
Thus, (1) the eruption and flowage of the earliest
Deccan basalt lava flows onto extensive, flat planation
surfaces, (2) the usual absence of basement-derived
conglomerates under the first lavas throughout the
province, and (3) the usual absence of sedimentary
sequences that show shallowing-upward palaeogeography,
are all important lines of evidence against pre-volcanic
lithospheric uplift and thereby the plume head model
for the Deccan Traps. On the other hand, there is
much evidence for major (1-2 km) post-volcanic uplift
of the Indian peninsula, including the Satpura region,
and this is not domal. The easterly drainage of the
peninsula, speculated to be dome-flank drainage produced
by the plume head, is antecedent to plateau uplift.
Evidence for pre-volcanic, regional, domal uplift
from several flood basalt provinces of the world (such
as Siberia, Emeishan, Ontong Java, Deccan, Columbia
River) is variable, even mutually contradictory. Flood
basalts of the world are clearly different from each
other in more than one way, often significantly so.
Obviously, no single model, whether plume-based or
not, should be considered alone, at the exclusion
of alternative models.
Presentation type: Oral
Session: 3
|