Studying extreme protoclusters is necessary to test if the IMF origin can be independent of cloud local characteristics. The W43-MM1 ridge, being extreme in terms of cloud concentration and star formation activity, is a case-study to confront models up to their limits. In Cycle 2, ALMA performed a deep, large mosaic of the 5 pc2 ridge. The 1mm image reveals an exquisite hub of spiraling filaments and a rich cluster of about 300 cores with 2000 AU sizes. A temperature model was built from the knowledge of the main hot core characteristics and the heating of protostars identified with ALMA. The resulting core mass function (CMF) definitively is ’top-heavy’, for both the low- and high-mass regimes. For the first time, one can question the origin of stellar masses for solar-type to O-type (1-100 Msun) stars in a single cloud. I will present various interpretations for this ‘top-heavyCMF in the framework of mini-starburst events.
