In a large pot, heat the oil over medium. Add the garlic and cook until just fragrant, about 30 seconds. Add the tomatoes, stock and thyme; simmer for flavors to meld, about 5 minutes. Add the potatoes and simmer until tender, about 12 minutes; season with salt and pepper.
Stir in the cod, vegetables and orange juice. Simmer, stirring occasionally, until the fish is warmed through, about 5 minutes. Divide the chowder among bowls and sprinkle with the orange zest and basil.