Transcript Geology of Malta

Geology of Malta
Peter Bird
UCLA
2016
Outline:
• Stratigraphy (layers of rocks)
• Environment of deposition (where they landed)
• Probable fault lines
• Puzzles and Speculations:
What is underneath what we see?
Sea level change, or local uplift?
Origin of faults?
Changing stress and fault slip directions
“Cart-ruts”: man-made, natural, or both?
Source of the
“Globigerina Limestone”
Miocene Blue Clay(?)
Nearby Pantellaria is an active volcano.
Geology of Malta, by Victoria Griffiths
shallow
a bit deeper
???
shallow
shallow
Limestone Isles in a Crystal Sea, by Martyn Pedley
thin oceanic crust
of Mediterranean
thick continental crust of Africa
oceanic crust
continental crust
Outline:
• Stratigraphy (layers of rocks)
• Environment of deposition (where they landed)
• Probable fault lines
• Puzzles and Speculations:
What is underneath what we see?
Malta is part of the continental shelf of Africa.
Sea level change, or local uplift?
Origin of faults?
Changing stress and fault slip directions
“Cart-ruts”: man-made, natural, or both?
Arguments that Sicily may have been part of a subduction zone:
(1) Its rocks have been shortened by thrust-faulting.
(2) Deep rocks in the pile match Africa; shallow ones don’t.
(3) Some thrusts connect to the Calabrian subduction zone.
(4) The volcanic Aeolian Islands indicate subduction SN.
Caveat: Young (1 Ma) Mount Etna
may be a red herring.
= ~500 m
N.B. Modeling studies show that an outer rise will ALWAYS form,
regardless of whether the subducting plate is elastic,
or viscous, or viscoplastic, …
Normal faulting leaves a few isolated blocks
(horsts)
standing higher than the rest
of the country.
Outline:
• Stratigraphy (layers of rocks)
• Environment of deposition (where they landed)
• Probable fault lines
• Puzzles and Speculations:
What is underneath what we see?
Malta is part of the continental shelf of Africa.
Sea level change, or local uplift?
Local uplift, in the outer rise of the Sicilian subduction zone.
Origin of faults?
Normal faults were generated by plate-bending stresses.
Changing stress and fault slip directions
“Cart-ruts”: man-made, natural, or both?
Carafa, Barba, & Bird [2015, J. Geophys. Res.]
Carafa, Barba, & Bird [2015, J. Geophys. Res.]
Outline:
• Stratigraphy (layers of rocks)
• Environment of deposition (where they landed)
• Probable fault lines
• Puzzles and Speculations:
What is underneath what we see?
Malta is part of the continental shelf of Africa.
Sea level change, or local uplift?
Local uplift, in the outer rise of the Sicilian subduction zone.
Origin of faults?
Normal faults were generated by plate-bending stresses.
Changing stress and fault slip directions
Sicilian subduction has jammed to S (but started to N).
“Cart-ruts”: man-made, natural, or both?
“Cart-Ruts”
[per David H. Trump, Malta: Prehistory and Temples, Midsea Books, Malta]
Across Malta and Gozo, wherever bare rock is exposed, paired grooves are found.
The gauge is fairly uniform, at ~1.41 m between ruts in a pair.
Depth is quite variable, up to 60 cm {24”}.
Only a few seem to radiate from Bronze Age village-sites; none from temple sites.
So far, no technology has been available to date them. {Cosmogenic nuclides?}
Limestone bedrock is continually dissolving away, because rain is pure water.
Any small hole or groove collects runoff, increasing local infiltration & dissolution.
Thus, any small hole or groove continually gets deeper, without any need for
further mechanical wear or friction.
Rates:
• In the Mediterranean climate, limestone is dissolved at about 0.01 to
0.04 mm/year [Cucchi et al., 1987, 1996; Gines et al., 2009]. The
higher rates apply to fine-grained limestones (like the Globigerina
Limestone).
• Since the start of the temple-building period on Malta (~4,500 BC),
the average surface of fine-grained limestone has probably been
lowered by about 260 mm (about 10 inches).
• If cart-ruts (initially in soft soil) concentrated the infiltration of rain,
the infiltration and dissolution in the ruts might easily have been two
to three times higher.
• This would allow natural dissolution to lower the cart-ruts by about
260-520 mm (10~20 inches) relative to the surrounding surface.
Probably (1) these cart-ruts WERE started by Bronze Age sleds and/or carts, but initially
as mere ruts in the overlying soil;
(2) The ruts continued to deepen (under the soil) without any further mechanical wear;
(3) Loss of topsoil then exposed the grooves. Some are now too deep for easy use.
To download this file:
http://PeterBird.name/
oldFTP/temporary/
Geology_of_Malta.pptx
Scientific American,
227(6), 26-26.
Bird, P., Y. Y. Kagan, D. D. Jackson, F. P. Schoenberg, & M. J. Werner [2009] Linear and nonlinear relations between relative plate velocity
and seismicity Bull. Seismol. Soc. Am., 99(6), 3097-3113, doi:10.1785/0120090082.
Howe, T. M., and P. Bird [2010], Exploratory models of long-term crustal flow and resulting seismicity across
the Alpine- Aegean orogen, Tectonics, 29, TC4023, doi:10.1029/2009TC002565.