From the relevant section of Sacrosanctum Concilum, with my emphasis added.

37. Even in the liturgy, the Church has no wish to impose a rigid uniformity in matters which do not implicate the faith or the good of the whole community; rather does she respect and foster the genius and talents of the various races and peoples. Anything in these peoples' way of life which is not indissolubly bound up with superstition and error she studies with sympathy and, if possible, preserves intact. Sometimes in fact she admits such things into the liturgy itself, so long as they harmonize with its true and authentic spirit.

38. Provisions shall also be made, when revising the liturgical books, for legitimate variations and adaptations to different groups, regions, and peoples, especially in mission lands, provided that the substantial unity of the Roman rite is preserved; and this should be borne in mind when drawing up the rites and devising rubrics.

39. Within the limits set by the typical editions of the liturgical books, it shall be for the competent territorial ecclesiastical authority mentioned in Art. 22, 2, to specify adaptations, especially in the case of the administration of the sacraments, the sacramentals, processions, liturgical language, sacred music, and the arts, but according to the fundamental norms laid down in this Constitution.

It should be noted:

-Enculturation is intended primarily concerning mission lands. While it is debatable whether modern America counts as a "mission land," it is worth observing that the Habitual Music of the Roman Rite is not employed (typically) with a missionary focus, but rather because it is liked and enjoyed by people already attending Catholic mass.

-Anything done in the process of enculturation must remain in harmony with the Roman Rite, the integrity of which must be preserved. Whether and how contemporary popular music harmonizes with and preserves the integrity of the Roman Rite is a question worth discussing, but most of the advocates of this type of music don't seem to give the issue much thought at all.

-It is not mentioned in the above excerpt, but in other places (I seem to recall) there is a general principle that what should be brought into the Roman Rite for the purposes of enculturation are those things which themselves are considered worthy of sacred use in the culture being evangelized. So, for example, it may make sense to adapt the chants of the Mass to Buddhist melodies, or adopt a particular style of sacred artwork used in Mosques. The idea is to bridge the gap between an ingrained notion of "sacred" and the true sacredness of the liturgy. This is very different from importing secular musical styles in an attempt to specifically make the liturgy feel and sound more like secular culture. (And while I can imagine someone making the argument that they find the Beatles or Elton John to be particularly sacred forms of American popular music, I have never heard anything approaching that kind of justification for the use of popular music styles in Catholic liturgy.)