Hi! I’m Bob Stern
and I’m very pleased to contribute to the GeoPRISMS synthesis workshop:
Extensional Processes Across Tectonic Settings and Time Scales.
I’ve been invited to talk about ancient rifts, which is a very big topic.
The topic of ancient systems is often overlooked,
partly because GeoPRISMS and its predecessor MARGINS emphasized active systems.
But ancient rifts and rifts on other planets provide important insights into the rifting process
and to the body’s tectonic history and should be kept in mind.
There are a number of ways to broach this topic, but the perspective I’d like to offer is this one.
My talk is entitled “Folks are arguing about when Plate Tectonics began. Should rifts care?”
The answer to this question is “Yes!”
because rifts behave differently depending on the planet’s thermal structure and tectonic regime.
Most importantly, rifts can evolve into seafloor spreading if and only if there are associated subduction zones,
and this requires plate tectonics.
Rifts on silicate planets without plate tectonics are fundamentally different from rifts on planets with plate tectonics.
This begs the question: what other tectonic regime besides plate tectonics is there?
To answer that question, we need to consider the 3 other large, active rocky planets and moons of our solar system.
All three have some kind of single lid tectonics;
Only Earth has plate tectonics.
Jupiter’s innermost moon Io is very active because of strong tidal forces and experiences heat pipe tectonics
where lavas erupt from scattered volcanoes.
Young lava flows bury older flows which eventually are depressed, ultimately reaching the molten zone where they are remelted and re-erupted.
Tectonics on the Hadean Earth may have been like this.
Another single lid tectonic style is that of Venus, which is characterized by many lithospheric drips and mantle plumes.
This is a vigorous style of single lid tectonics.
Something like this might have been Earth’s tectonic style after it cooled enough to begin growing mantle lithosphere.
Mars is the third example of active single lid tectonics in our solar system.
It seems to be characterized by a few long-lived mantle plumes and is an example of sluggish single lid tectonics,
perhaps like that of Mesoproterozoic Earth before plate tectonics started.
We will discuss this possibility further.
One insight we can take away from the planetary perspective is that there are other tectonic styles than plate tectonics
that Earth likely experienced at different times over its 4.56 billion year history.
How would rifts behave during these times?
Again it is useful to examine the evidence from Venus and Mars.
The vigorously active single lid planet Venus has many rifts, 59 in all.
These are associated with volcanoes and plume-related upwellings called coronas.
The sluggishly active single lid planet Mars has fewer rifts, most of which radiate from the most volcanically active region, the Tharsis Rise.
The biggest Martian rift by far is Valles Marineris,
which is more than 4000 km long, 200 km wide, and up to 700 km deep
but does not seem to be associated with any volcanoes.
There are some interesting differences between modern Earth rifts and those of Mars.
For example, Martian rifts tend to be narrower than those of the same length on Earth.
A take-away lesson from comparing rifts on active rocky planets is that hotter bodies have more and wider rifts
because the lithosphere is thinner and weaker and these rifts have more igneous activity than rifts on cooler active bodies.
This insight is useful in thinking about rifts through Earth history.
It seems that rifts are associated with all tectonically active rocky planets, whether these have single lid or plate tectonic styles,
and that these rifts differ in ways that we are just beginning to understand.
Given that Earth must have evolved from heat pipe to vigorous single lid to plate tectonics as its mantle cooled, Earth rifts must have evolved as well.
The next questions are when did plate tectonics began on Earth and what was Earth’s tectonic style before that?
These are big questions, and what do we do when we have a big, unanswered question? We ask Google!
Google says plate tectonics began 800 million years ago but in fact this is a much more controversial topic than Google appreciates .
Most geoscientists think plate tectonics began much earlier, in Archean times.
I along with increasing numbers of geoscientists think they are wrong.
This controversy is great fun and can’t be examined properly here. We do need to briefly consider the evidence that
Mesoproterozoic Earth was characterized by a sluggish single lid episode that transformed in Neoproterozoic time
into the modern plate tectonic regime and then using this to explore how Mesoproterozoic rifts differed from modern ones.
The Mesoproterozic is the most underappreciated era in Earth history
encompasses 600 million years and is part of what some geoscientists call the Boring Billion,
a time when little of interest happened.
I think the Mesoproterozoic was not boring, just different, and will explain why.
Before we give some love to the Mesoproterozoic let’s look at the evidence for plate tectonics that is preserved in the rock record and how these vary through time.
There are 3 groups of plate tectonic indicators: those for seafloor spreading and subduction initiation, those for subduction, and those for continental collision.
Seafloor spreading, subduction initiation, subduction, and continental collision are all hallmarks of plate tectonics and do
not happen during single lid tectonic regimes. Let’s look at these three groups separately.
Ophiolites are slices of oceanic lithosphere preserved on land and these are excellent proxies
for seafloor spreading and subduction initiation. These are abundant in Neoproterozoic and younger
times and missing from older times, except for a few at 1.9 to 2.1 Ga. There are 3 metamorphic
rocks and minerals that only form in the unique subduction zone environment and thus are subduction
proxies: blueschists, lawsonite-bearing eclogites and the pyroxene variety of jade called jadeitite.
These show a similar distribution in time to ophiolites. Finally, there are 2 indicators
of continental collision, ultra-high pressure metamorphic rocks or UHP metamorphic rocks and the
gemstone ruby. UHP metamorphic rocks contain minerals – diamond or stishovite – that
can only form at depths of 100 km or more in the Earth, demonstrating deep subduction
of continental crust and return of this material to the surface. Rubies form in silica-poor
environments where there is a lot of chromium, an environment only found in continental collision
zones. These continental collision proxies are limited to Late Neoproterozoic and younger
times, although isolated ruby deposits are found earlier in Earth history. The six plate
tectonic indicators – with the exception of one ruby deposit – are missing from the
600 million years of Mesoproterozoic time. The 600 million year gap in plate tectonic
proxies during Mesoproterozoic is powerful evidence that plate tectonics did not happen
then. In a paper published in the December 2020
issue of GSA Today, I argued that the Mesoproterozoic was a protracted episode of single lid tectonics.
Half of the argument comes from the absence of plate tectonic proxies, as just discussed.
The other half of the argument comes from unusual igneous rocks, thermal gradients inferred
from metamorphic terranes, and the lack of evidence for formation of passive margins.
Unusual igneous rocks include (unusually) abundant A-type granites and massif-type anorthosites.
Both of these rock types reflect unusually dry magmas, as expected for a tectonic regime
that lacks subduction zones. Another expectation for single lid tectonic regimes is that the
mantle will be more insulated and cool less efficiently than for plate tectonic regimes.
In fact, the mantle might heat up and this is reflected in calculated thermobarometric
ratios determined for metamorphic rocks. Finally, there are almost no confidently-identified
passive margins identified for this time, as expected for a single-lid tectonic regime.
Continental rifts can widen into new oceans only if there is a plate tectonics; formation
of passive continental margins is impossible on a planet with single lid tectonics.
The implications for our understanding of rifts if the Mesoproterozoic single lid hypothesis
is correct are profound. First of all, the mantle beneath these rifts should be hotter
than beneath modern rifts, and therefore should be associated with more igneous rocks. Secondly,
Mesoproterozoic single lid rifts should evolve differently than modern plate tectonic rifts,
never opening into a true ocean with subsiding passive margins. Is there any evidence for
these predictions? Let’s look at the best example we have of a Mesoproterozoic rift,
the Mid-continent Rift, which stretches at least 3000 km from exposures around Lake Superior
south along two buried arms, the eastern arm heading towards Tennessee and Alabama and
the western arm heading towards Oklahoma and Texas. The Mid-continent rift was actively
extending ~1100 Ma, about the same time as strong compression affected the Grenville
Orogen to the south and east. The MCR contains an unusually large amount of igneous rocks
in a very focused region compared to Phanerozoic large igneous provinces, as expected for a
rift on a single lid planet. On a planet with plate tectonics, such an energetic system
with long lithospheric weaknesses would be expected to continue opening into an ocean
but that was not possible on single lid Mesoproterozoic Earth.
The unusual nature of the Mid-continent rift is apparent when comparing gravity profiles
over this and two younger rifts, the Southern Oklahoma Aulacogen of SW Oklahoma and the
Reelfoot Rift of NW Mississippi, eastern Arkansas, and western Tennessee. The strong gravity
high over the western arm of the Mid-continent rift reflects the large volume of mafic igneous
rocks it contains, which is similar to the situation for the Southern Oklahoma Aulacogen.
It would be interesting to compare gravity profiles for modern rifts as well and begin
exploring what these similarities and differences are telling us about Earth’s tectonic evolution
and cooling history. In conclusion, there is a lot of useful work
that could be done with thoughtful study of ancient continental rifts and comparing these
with modern rifts. Let’s get started! Oh, by the way, if you are looking for a simple
video explaining how continental rifts sometimes evolve into new oceans and passive continental
margins, check out our 7 minute video “Continental Rifting, New Oceans, and Passive Continental
Margins for Beginners”. You can find this easily on our Youtube channel “UTD Geoscience
Studios”. Please subscribe to our channel while you are there.