Mount Targon has been shown with their worship of the Sun, as shown with Leona. They believe that the Sun is an entity, whose power is great.
Shurima's lore tells us the power of the Sun Disc and their worship of it.
So are the followers of Mount Targon the remnants of the Shurima Culture, those who might've survived the fall of Shurima? Not the current followers, but the people who started it all.
Lost R3 votesI don't think there is a relation between the two cultures. There were a few oceans between the Aztecs, Japan, China and Rome and they all have sun gods and goddesses. Even the Judeo-Christian religion has a basis in sun worship. It's a universal constant that ancient religions worship the sun. The Sun provides light and heat for life to grow. In my eyes, it's just a coincidence that both Targon and 
SHURIMA happen to worship the sun, but as a writer, I know that in fiction there are rarely any coincidences...
ModAcademy Kayn1 votesDespite not having a current-canon map, the Map we currently know has Mount Targon located near on the mountatin range that borders Shurima.
Lost R1 votesCould be a shared cultural background. That's another thing that happened in neighboring countries; they tended to have very similar cultures and customs.
DwarvenGiant1 votesI wonder what does Leona think about it, if she knows what Shurima's sun disc is (or what she would've thought, if she doesn't know about Shurima's sun disc), and the opposite (what will Shuriman people think about her).
turkeyBACON1 votesI have a bit of head canon that Shurima and Targon were once sister trade cities with a similar/intermingled worship of the sun. Targon was a place of pilgrimage being the tallest and thus closest place to the sun. Shurmia in turn was the true trade hub being a desert oasis at the center of southern Valoran. This is mixed with my other head canon that Kalamanda is a secret underground city that is at the base of Targon that provides passage between and to other places to those in the know cough Taric cough.
UomoAfide1 votesInteresting, but i have another theory. Imagine if Shuriman and Rakkor (or better, Solari) culture might have originated from a common ancestor civilization. We don't know when this civilization (or tribe) has split into different groups (which doesn't necessarily means only two) but we can imagine them retaining some common elements through the ages in their beliefs and culutral heritages. Let's look closely, Leona is known as the Radiant Dawn and can be considered an enbodiment of the Sun itself, kind of fantasy-style avatara. In Shurima we have the Ascendants; The Ritual of Ascension suppose to harness the power of the Sun through a device of long lost technology or magic (in the present day of Runeterra of course) leading to incredible transformation and aquisition of tremendous powers, usually granted to those deemed worthy of carrying the burden of such power. What we can see is the difference between the use of solar powers between the two civilizations. While Shurima uses the power of the Runeterran star to choose the people worthy of becoming ascendant and basically "forcing" the Sun's hand in giving his powers. Looking at the Solari we see as the Sun chooses personally a single person that becomes the living embodiment of the solar greatness. How this works is right now only pure speculation by fan, but let me give you my other theory. If the sun is a definite deity it seems rather strange that it could give his powers to several people throughout history even if they weren't exactly chosen ones, then why could Shurimans take the Sun power for themselves (although only when really in time of dire need) like they were forcing the Sun's choice? Is it possible for Leona to be, not exactly the chosen of an almighty god, but rather some kind of powerful magic "magnet", capable of harnessing the star's power with her presence only? Who knows, but apart from this, it is interesting to notice that although the sun was venerated by both people there were some relevant differences on how this veneration was acted. So, does that mean Shurima = Sunlight's catchers and Solari = Chosen ones?
Solideus1 votesI think there's potential for awesome interactions between those two factions, the precedent being the real-life relations between Rome (Targon is greco-roman inspired) and Egypt (Shurimas inspiration) by way of Caesar and Cleopatra. It's exciting to theorize how something similar could translate into Valroan's setting!