Seek & Destroy is basically just a mild change of Sucking My Love (and while I don't see others mentioning it, I think Phantom Lord very strongly resembles Priest's Rapid Fire as well), the early Exodus song Warlords steals its main riff right out of Holocaust's Death or Glory, Crionics takes directly from Transylvania, I mean it's pretty clear that the early thrash bands worshiped NWOBHM above all. Obviously they took things to a higher level of intensity, stripped away any remnants of hard rock, gradually sped things up, etc, but it's still a direct evolution. I think the "thrash = hardcore + metal" meme is heavily exaggerated, whereas Nirvana was full of punk/hardcore influence from the beginning.