Characteristic $p$ analogues of the Mumford--Tate and André--Oort conjectures for products of ordinary GSpin Shimura varieties

Abstract: Let $p$ be an odd prime. We state characteristic $p$ analogues of the Mumford--Tate conjecture and the Andr\'e--Oort conjecture for ordinary strata of mod $p$ Shimura varieties. We prove the conjectures for arbitrary products of GSpin Shimura varieties (and their subvarieties). Important subvarieties of GSpin Shimura varieties include modular and Shimura curves, Hilbert modular surfaces, $\mathrm{U}(1,n)$ unitary Shimura varieties, and moduli spaces of principally polarized Abelian and K3 surfaces. The two conjectures are both related to a notion of linearity for mod $p$ Shimura varieties, about which Chai has formulated the Tate-linear conjecture. Though seemingly different, the three conjectures are intricately entangled. We will first solve the Tate-linear conjecture for single GSpin Shimura varieties, above which we build the proof of the Tate-linear conjecture and the characteristic $p$ analogue of the Mumford--Tate conjecture for products of GSpin Shimura varieties. We then use the Tate-linear and the characteristic $p$ analogue of the Mumford--Tate conjectures to prove the characteristic $p$ analogue of the Andr\'e--Oort conjecture. Our proof uses Chai's results on monodromy of $p$-divisible groups and rigidity theorems for formal tori, as well as Crew's parabolicity conjecture which is recently proven by D'Addezio.