In a mixing bowl, combine olive oil, ground cumin, chili powder, garlic powder, salt, black pepper, and lime juice. Add chicken breasts and coat well. Marinate for at least 15 minutes.

Preheat the grill to medium-high heat. Remove chicken from marinade and grill for about 6-7 minutes on each side, or until fully cooked. Let rest for a few minutes before slicing.

Slice the grilled chicken into strips. On each tortilla, layer shredded lettuce, diced tomatoes, sliced avocado, and grilled chicken. Top with sour cream, cilantro, and jalapeño slices if desired.