Complex sentence: In the period following the 1973 oil crisis, immigration controls in European states were tightened, but the effect of this was not to reduce migration from North Africa but rather than encourage permanent settlement of previously temporary migrants, and associated family migration.
Split into simpler sentences: In the period following the 1973 oil crisis, immigration controls in European states were tightened. The effect of this was not to reduce migration from North Africa but rather to encourage permanent settlement of previously temporary migrants and associated family migration.