  We study the formation of marginally trapped surfaces in the head-on
collision of two shock waves both in anti-de Sitter and Minkowski space-time in
various dimensions as a function of the spread of the energy density in
transverse space. For D=4 and D=5 it is shown that there exists a critical
value of this spread above which no solution to the trapped surface equation is
found. In four dimensions the trapped surface at criticality has a finite size,
whereas in five the critical size vanishes. In both cases we find scaling
relations characterized by critical exponents. Finally, when D>5 there is
always a solution to the trapped surface equation for any transverse spread.
