Download Mountain Building in the Red Centre

Week 2
Mountain Building in the Red Centre
We now have enough background to tell the story of the
Red Centre.
Mountain-building is called orogeny by the geologists.
The Alice Springs Orogeny lasted from 450 million to
300 million years ago, producing, among others, the
MacDonnell Ranges from 350 to 300 million years ago
ago, during the Gondwanda and Pangaea supercontinent
periods.
The story begins with the formation of the Amadeus
Basin in the coming together of the continent of Australia
during the assembly of Columbia around 1.8 billion years
ago. The Strangways mountain-building dates from this
period. The eastern part of Australia (Tasmanides) did not
exist.
Geodynamic Synthesis of the Gawler Craton and Curnamona Province Geoscience Australia Record 2010/27 N.
Kositcin (Ed.)
During this period, the Gawler Craton in South Australia,
the West Australian Craton and the North Australian
Craton came together at a convergent plate boundary.
The Amadeus Basin was formed as a foreland basin
during the Stangway mountain-building episode.
https://digital.library.adelaide.edu.au/dspace/handle/2440/16496
The continental shelves of the Gawler and North
Australian Cratons came together, resulting in a sea,
possibly open to the east, in what became the Amadeus
Basin.
Geology of The Amadeus Basin, Central Australia. Bulletin 100,
Department of National Development, Bureau of Mineral Resources, Geology and Geophysics, Commonwealth of
Australia, 1970
Over time, the Amadeus Basin has accumulated
sediments, up to 14 kilometres in some places. By
comparison, Mount Everest is less than 9 kilometres high,
and Alice Springs is 545 metres above sea level.
Fourteen kilometres seems like a lot, but it was over a
very long time. 10 kilometres is 10 million millimetres,
which in a billion years amounts to 1 millimetre every 100
years. A house, left untended, would accumulate more
than 1 millimetre of dust in a single year.
The geological record shows long gaps in accumulation,
and periods when the sediments have eroded.
In addition, we need to keep in mind that the continents
are floating on the oceanic crust, and that accumulation of
sediment causes the basin to subside.
For the first several hundred million years, the sediments
would have come largely from the Strangways mountains,
just to the north.
The Petermann Ranges were formed 550 to 500 million
years ago, during the assembly of Gonwanda. The
mountains were built by large and relatively rapid
movements of thrust faults, principally in the Woodroffe
Thrust. These faults force older strata of rock above
younger strata, by pressure from several directions. There
seems to be no generally accepted direct cause of the
episode.
http://austhrutime.com/musgrave_block.htm
http://www.ga.gov.au/corporate_data/81425/Jou1995_v16_n1-2_p147.pdf
The ranges were then eroded, with vast fans of sand and
gravel deposited, kilometres thick, in the Amadeus Basin.
There were glacial episodes, and a long period in which
the Amadeus basin was a sea, so there are many layers
of marine sediments over the sand and gravel fans,
contributing to the pressure that turned these, and earlier
sediments, into rock.
The rocks that were there when the basin formed are
called the basement. These rocks today are mostly
gneiss and granite. Gneiss is rock, sedimentary or
igneous, that has been metamorphosised under high
temperature and pressure. The pressure comes from the
rocks lying below much sediment, while the temperature
comes from the mantle, in the form of granite plumes and
other forms of non-volcanic molten rock. These influences
cause the rock to partially melt and re-crystallize, and
leaves bands.
Gneiss
The lowest level of sediment is the Heavitree quartzite,
100 to 300 metres thick. Quartzite is metamorphosised
sandstone. The backbone of the MacDonnell Ranges is
Heavitree quartzite, as are the gorges at Ellery Creek Big
Hole, Ormiston Gorge and Trephina Gorge, among others.
http://lrm.nt.gov.au/water/ground/people/alice
Corroboree Rock, East MacDonnells, is dolomite rock.
The processes that created dolomite rock seem no longer
to be operating. There are several competing theories as
to how the vast deposits of dolomite around the world
were formed, but they certainly involve marine sediments
of microscopic shelled organisms.
Most of the more recent layers are sandstone or siltstone,
although there are a few limestone layers dating from
more than 500 million years ago.
We now have the machinery to tell the story of the
Alice Springs landscape.
The MacDonnell Ranges mountain-building process
occurred while Australia was part of Gondwana, and while
Gondwana was in process of colliding with Laurasia to
form the supercontinent Pangaea.
The direction of movement of Gondwana would have been
from southeast to northwest in the Pangaea
reconstruction, which is from north to south in the present
orientation of Australia.
The movement would have been powered by a push from
a divergent plate boundary to the relative north. This push
would have placed enormous stress on the Gondwana
continent. This stress re-activated a pre-existing fault to
the north-west of Alice Springs called the Redbank Shear
Zone.
BMR Journal of Australian Geology & Geophysics, 13, 1993, 359-368
This shear zone is very deep, going down to the bottom of
the crust, 60 kilometres below the surface.
Australian Journal of Earth Sciences No. 44(2), 215-226, 1997
The Earth’s crust shortened by about 50 kilometres during
this event, lifting and folding the sediments under the
Amadeus basin to form the MacDonnell Ranges. This
folding has left the sediment layers nearly vertical.
Geological Society of America Special Papers, 509, 2015
Since the formation of the ranges about 300 million years
ago, Central Australia has been geologically fairly stable,
so that the erosion processes have done their work,
removing the softer upper sediments, leaving the hardest
metamorphosised Heavitree quartzite, formed from the
billion-year-old Strangways Range in the billion-year-old
Amadeus Basin, that we can see now. Leaving also the
exposed even older basement gneiss found at Alice
Springs.
The Finke River, which drained this mountain range,
remains, occasionally returning to its former flow, most
recently at Easter 1988, when the bridge on the Stuart
Highway south of Alice Springs was under water.
The whole Amadeus Basin was affected by the mountainbuilding period in which the MacDonnell ranges were
formed. The sediments formed after the Petermann
Ranges were involved.
Uluru and Kata Tjuta are eroded remnants of the sand and
gravel (respectively) fans which were uplifted and tilted in
that process.
But the southern Amadeus Basin mountains did not
expose the deep layers that characterise the MacDonnell
Ranges.
After the formation of the MacDonnells, Gonwanda broke
up.
The first break was the separation of Africa and South
America, which began about 184 million years ago, during
the Jurassic, the age of dinosaurs.
India and Madagascar began to break away and move
northward about 120 million years ago, during the early
Cretaceous, also during the dinosaur ages.
The final break was the separation of Antarctica,
beginning about 80 million years ago, during the late
Cretaceous. The separation became complete about 55
million years ago, during the Eocene, when the early
ancestors of modern mammals began to appear.
The Red Centre became much dryer from this time. The
Finke River system has not been permanently flowing for
about 55 million years.
Global climate change may have played a part on the
drying out of the Centre.
Australia has moved about 4000 kilometres to the
northwest since the separation from Antarctica, about the
distance from Kalgoorlie to Port Douglas.
Evolution of the Climate
323.2–330.9 ma Pangea forming, MacDonnells Building.
Rainforests covered the tropical regions of Pangea which
was bounded to the north and south by deserts. An ice
cap began to expand northward from the South Pole.
265-255 ma. Equatorial rainforest disappeared as deserts
spread across central Pangea. Though the southern ice
sheets were gone, an ice cap covered the North Pole.
Rainforests covered South China as it crossed the
Equator.
250ma MacDonnells at their peak. The interior of
Pangea was hot and dry during the Triassic. Warm
Temperate climates extended to the Poles. This may have
been one of the hottest times in Earth history. Rapid
Global Warming at the very end of the Permian may have
created a super - "Hot House" world that caused the great
Permo-Triassic extinction. 99% of all life on Earth perished
during the Permo-Triassic extinction.
140ma The Early Cretaceous was a mild "Ice House"
world. There was snow and ice during the winter
seasons, and Cool Temperate forests covered the polar
regions.
70ma. During the Late Cretaceous the global climate was
warmer than today's climate. No ice existed at the
Poles. Dinosaurs migrated between the Warm Temperate
and Cool Temperate Zones as the seasons changed
61.6 – 59.2 ma. The climate during the Paleocene was
much warmer than today. Palm trees grew in Greenland
and Patagonia. The Mangrove swamps of southern
Australia were located at 65 degrees south latitude.
34-38 mya. Global climate during the Late Eocene was
warmer than today. Ice had just begun to form at the
South Pole. India was covered by tropical rainforests, and
Warm Temperate forests covered much of Australia.
23-34 mya. During the Oligocene, ice covered the South
Pole but not the North Pole. Warm Temperate forests
covered northern Eurasia and North America.
7-23 mya. The climate during the Miocene was similar to
today's climate, but warmer. Well-defined climatic belts
stretched from Pole to Equator, however, there were palm
trees and alligators in England and Northern
Europe. Australia was less arid than it is now.
http://www.scotese.com/climate.htm
Readings
A. T. Wells, D. J. Forman, L. C. Ranford, and P. J. Cook, Geology of The Amadeus Basin,
Central Australia. Bulletin 100, Department of National Development, Bureau of Mineral
Resources, Geology and Geophysics, Commonwealth of Australia, 1970
Petermann Ranges and Surrounds - Site of Conservation Significance Northern Territory
Department Of Natural Resources, Environment, The Arts and Sport.
http://www.Lrm.Nt.Gov.Au/__Data/Assets/Pdf_File/0003/13881/60_Petermann.Pdf
John J.W. Rogers and M. Santosh, “Configuration of Columbia, a Mesoproterozoic
Supercontinent” Gondwana Research, V. 5, No. I , pp. 5-22. 2002 International Association
for Gondwana Research, Japan.
N. Kositcin (Ed.) “Geodynamic Synthesis of the Gawler Craton and Curnamona Province”
Geoscience Australia Record 2010/27Rec
A. Davidson “A review of the Grenville orogen in its North American type area” AGSO Journal
of Australian Geology & Geophysics, 16 (112), 3-24, 199527
J. F. Lindsay “Heavitree Quartzite, a Neoproterozoic (ca 800-760 Ma), high-energy, tidally
influenced, ramp association, Amadeus Basin, central Australia” Australian Journal of Earth
Sciences (1999) 46, 127-139.
M. H. Monroe Australia: The Land Where Time Began: A biography of the Australian
continent, 2015, http://www.austhrutime.com
C. Wright, et al. “Seismic structure and continuity of the Redbank Thrust Zone, central
Australia” BMR Journal of Australian Geology and Geophysics, 13, 359-368, 1993.
Fred Beekman and Randell A. Stephenson “Mechanical Stability of the Redbank Thrust Zone,
Central Australia: Dynamical and Rheological Implications” Australian Journal of Earth
Sciences No. 44(2), 215-226, 1997