Hyrcania - Taparistan, Tabaristan, Mazandaran...
I have always known that Mazandaran (Home to House of Darvish) is different than the rest of Iran, however after researching it's roots, we see that there is a reason for it. It is a mountainous region that has always been isolated from the rest of the country. Here is some historical backround on "Hyrcania - Land of the Wolves" as the Greeks called it. The Persians later called it Taparistan, which was later Arabized to Tabaristan and more recently it was split into three provinces of Gilan, Mazandaran and Gorgan and even more recently to Gilan, Mazandaran, and Golestan.
Post-Sassanid Era (Islamic)
This movie is called "Ray Traveller" which is the story of the Prophet's (pbuh) progeny migrating to Northern Iran during the Abbasid Era: (This movie was originally in Persian, however it's been dubbed in to Urdu, I could not find the Persian version yet on youtube)
The Prophet's family eventually mixed in with the local population, however later due to other historic reasons, a huge portion of these Persian, Arab Seyyed, and Persian-Arab Seyyed hybrid Zaydi Shias had to escape to Yemen, which their descendants still live up until this day! These should not be confused with the Jafari Shias that live in the rest of Iran, Saudi Arabia or the Ismaili Shias in Egypt (Fatimids), who have been there from the time of Prophet Muhammad (pbuh) and his family. Up until modern history, the Zaydi Shias were the rulers of Yemen. In Northern Iran...Later the Seljuq and Ottoman Turks were mostly responsible in bringing Hanafi Sunnism. Then later the Safavid Turks were responsible for bringing a modified Shiasm which later turned into Jafari Shiasm by the more orthodox Shia Ulema. Today in Northern Iran you see a mix of both Shia and Sunni Muslims along with Christians and other minorities. Most Northern Iranians were of Zoroastrian or Christian stock before they became Muslim.
Regardless, the history of this area that has been civilized for the past 7,000 years is very interesting leading up to it's 1300-1400 years of Islamic history. This area was also the same place that King Darius escaped to after his defeat by Alexander in Gaugamela. He came to the north to recruit the strongest of the Northern Persian warriors in order to face Alexander again. However, two of his generals betrayed him to Alexander before he was able to raise a new army. Ironically Alexander put both of them to death after learning that they had betrayed their own King so easily (they could also do it to him as well). This same tactic was used again by the King and Generals of Sassanid Persia during the Arab Muslim Invasion that initially defeated the southern Iranian provinces.