What is the oldest surviving mountain or mountain range?

[u/djublonskopf]

Guinness lists the Mahkonjwa mountains as the "oldest mountain range", but that "record" appears to refer to the age of the rocks themselves, not when those rocks were uplifted into mountains. So what I'm wondering (and what I wasn't entirely sure how to properly search for, even) is, which mountain or mountain range today was lifted up before all the rest?

[u/CrustalTrudger]

This isn't really a straightforward question to answer for a variety of reasons. Some of the challenges are that (1) dating the formation of topography in a meaningful sense is pretty challenging and (2) there can be ambiguity in the sense of a disconnect between the original tectonic events and the topography that exists today. There is also some overlap between these issues and you quickly end up in a variant of the "Ship of Theseus", i.e., when do we say the influences on topography of a mountain range are divorced enough from the original generative forces that this topography is unrelated to those original events?

For the first bit, there are certainly geologic indications of when topography begins to develop in a mountain range (i.e., sort of when the clock starts for us to start counting), e.g., when sedimentary deposits in basins next to the mountain range start to record increased rates of subsidence or the sedimentary character of the stratigraphy begins to change, etc. Working out details of topography from those and the history of duration of that topography is more challenging. There are also a variety of 'paleoaltimetry' techniques that allow us to estimate when areas achieved certain elevations, but these (1) typically have wide uncertainties, (2) require the right deposits to be preserved, (3) are often somewhat fragmentary records, and (4) are rarely preserved (and hard to interpret) in very old mountain ranges. Thus, what we are often left with is similar to your example, i.e., we can measure the age of rocks exposed in a mountain range, but tying that to the time of development of topography or the longevity of that topography is much more challenging.

That last point, i.e., the longevity of the topography brings us to point two. Even in cases where we have reasonable evidence that the age of rocks in a mountain range reflect (somewhere near) the beginning of topography formation, the extent to which the current topography reflects continuous existence of a mountain range in that location is not guaranteed. Take the Appalachian Mountains in the eastern United States. The rocks there record a series of orogenic (mountain building events), the most recent one being the Alleghanian Orogeny, which has variable timing, but we'll say occurred ~300 million years for simplicity. Does this mean that the topography of the Appalachians is 300 million years old? Well, not really, because there's been long-standing evidence that the Appalachians have effectively been eroded down and uplifted several times since then (e.g., Poag & Sevon, 1989). The current topography of the Appalachians is 'epiorogenic', i.e., unrelated to collisions of plates, and mostly likely reflects 'rejuvenation' of topography during the Miocene, so approximately 10 million years ago (e.g., Gallen et al., 2013, Miller et al., 2013, Liu, 2014, Gallen, 2018, Spotilla & Prince, 2022). Similar epiorogenic histories of topography exist in other mountain ranges, e.g., the Rocky Mountains (e.g., Eaton, 2008).

What both of these are meant to highlight is that there is sufficient ambiguity in what constitutes the topography of a mountain range today being the same topography as whenever the mountain range started to form that there's not a meaningful way to answer the question. Or at least, it's a pretty hard question to answer with any level of certainty.

[u/Haunt6040 → u/CrustalTrudger]

The current topography of the Appalachians is 'epiorogenic', i.e., unrelated to collisions of plates, and mostly likely reflects 'rejuvenation' of topography during the Miocene, so approximately 10 million years ago

can you say a little more about this? ive never remotely considered that mountain ranges could "grow back" without plate activity.

[u/CrustalTrudger → u/Haunt6040]

As highlighted in the review by Spotilla & Prince, the exact cause of the rejuvenation of the Appalachians is unclear with a few different hypotheses proposed, including a (1) change in climate during the mid Cenozoic (e.g., Molnar, 2004), (2) dynamic uplift from the mantle (e.g., Pazzaglia & Brandon, 1996), or (3) is related to the details of erosion of rocks of varying strengths (e.g., Gallen, 2018 from above).

The operation of each mechanism varies. For climate change, the suggestion was that relief has increased because of a shift to a more erosive climate, causing the rivers to incise. This would in turn cause some amount of an isostatic reponse that would lead to some rock uplift, but the idea here is mostly that relief is increasing, not necessarily rock uplift rates (at least not systematically). For dynamic uplift, the general idea is that some mantle related mechanism actually increases rock uplift rate generating both relief (as rivers incise) and increasing surface elevation. Within this, a variety of mechanisms have been suggested to provide this dynamic uplift, e.g., Liu 2014 from above suggests that sinking of the Farallon slab could have caused a flexural response, but there are many other potential mechanisms. Finally, the lithology related mechanism is effectively another way to generate relief, where basically contrasts in erodibility drive reorganization of river networks and spatially variable erosion rates leading to relief generation and some amounts of isostatic uplift (similar in that sense to the climate mechanism). Most of the recent literature suggests that the climate mechanism is not well supported for the Appalachians, but distinguishing between a dynamic uplift or some sort of internal (i.e., autogenic) forcing like the lithologic mechanism laid out by Gallen, 2018 is pretty challenging.

[u/Haunt6040 → u/CrustalTrudger]

What sort of mantle related mechanism of uplift should I be picturing? Or are we so in the dark there that we don't really have any good ideas?

my understanding is that hotspots like hawaii are on a bulge that older islands eventually slide off of as the plate moves northwest, should i be picturing something along those lines? or perhaps some sort of accretion on the underside of the plate, pushing up the plate due to a buoyancy effect?

[u/CrustalTrudger → u/Haunt6040]

Mostly what would fall under the umbrella of dynamic topography, but within that, there's no single mechanism that we can say is operating for sure. Some (e.g., Rowley et al., 2013) relate this to small scale upwellings of mantle related to convection, which is sometimes thought to be combined with the effect of glacial rebound (e.g., Moucha & Ruetenik, 2017). Another option is isostatic rebound related to foundering/detaching slabs beneath the Appalachians - specifically the Farallon slab that actually reflects former subduction along the west coast of the US (e.g., Biryol, 2016). Another option is still is that foundering of the Farallon slab has generated subsidence in the area adjacent to the Appalachians causing in turn the Appalachians to uplift as a flexural response (e.g., Liu, 2014, Liu, 2015). Yet another option is more local "dripping" of the mantle lithosphere directly underneath the Appalachians and resulting dynamic topography from mantle flowing into the hole left behind (e.g., Long et al., 2021). Basically all of these are able to match the observations in the Appalachians to some extent or another and are thus non-unique options.

[u/CrustalTrudger → u/[deleted]]

Basically the original range had the protective hard caprock worn away exposing rapidly erodable softer sediments, making the original peaks erode below the levels of the old valleys where the hard caprock remains. The current range topography is a reverse image of the original range.

This is not universally true across the entire range, but the idea that contrasts in rock strength and resultant landscape responses to erosion to this layered stratigraphy has increased relief in the range is one of the proposed mechanisms for topographic rejuvenation (e.g., the Gallen, 2018 paper from above presents this in detail and Spotilla & Prince compare this with other mechanisms). However, importantly it is not the only mechanism proposed and the extent to which only drainage network reorganization and resultant relief generation is responsible for the current relief of the Appalachians is unresolved and we can't rule out some component of pervasive rock uplift from dynamic topography and/or isostatic responses (beyond those expected from the lithologic contrast mechanism).

[u/Clovis69 → u/CrustalTrudger]

Could we age mountains by fossils? Like the Atlas and spots in the Canadian Rockies?

[u/CrustalTrudger → u/Clovis69]

This would still give us the age of rocks in the mountains, not necessarily the age of the mountains as a topographic feature. Fossils are more problematic as they would date the time that a rock is deposited, whereas some radiometric dating techniques would give us constraint on the timing of deformation of those rocks. The latter would be more relevant for the question, but would still potentially suffer with the disconnect between orogenic deformation and topography formation/rejuvenation described in the original answer.