Goodness, it's covered in tabs!
K5, 53 45 bytes

{{,/(b+a&amp;~b:x*&amp;\a:9=y){$[x;x#" ";y]}'y}[x]'y}


In action:

  {{,/(b+a&amp;~b:x*&amp;\a:9=y){$[x;x#" ";y]}'y}[x]'y}[4;(,"a";"\t\tb\tc";,"d")]
(,"a"
 "        b c"
 ,"d")


I just want the record to show that this question is morally reprehensible.