Layer the chicken breasts on the bottom of the Crockpot, and season with salt and pepper to taste.
Pour the marinara sauce over the chicken.
Cook on high for 3 ½ hours.
Melt the butter in a pot over medium heat.
Stir in the garlic, and saute for 1 minute.
Pour in the heavy whipping cream.
Stir in the basil, and simmer until the sauce thickens.
Add in salt and pepper to taste as well as the parmesan cheese.
Pour the alfredo mixture into the Crockpot, and toss to combine well. Cover and cook on low for 30 minutes.
Sprinkle on the mozzarella cheese, and place the lid on the Crockpot until the cheese has melted. Serve immediately.