"Here" is not a prepositional phrase in this construction; it is an adverbial of place that fulfills the additional function of deictic pro-form, more specifically called "reference signal". "Here" could be replaced by a prepositional phrase of the type "in this paragraph", "in this sentence", "in these deductions", etc.; the deictic function would then be taken up by the demonstrative.
"Here" does not constitute a phrase with "the level of maturity in argument". This can be seen from the mobility of this word, which is typical of adverbials; all three constructions below have the same meaning and "here" is the same word with the same function.
- Here the level of maturity in argument is astounding.
- The level of maturity in argument here is astounding.
- The level of maturity in argument is astounding here.
(CoGEL 19.46) Discourse reference
There are numerous signals marking the identity between what is being said
and what has been said before. […] We concentrate here on devices
that have a special value in referring less to concrete entities than to
constituents or aspects of discourse itself. The signals can be divided into two
groups, […]:
(a) sentence or clause reference signals
(b) noun-phrase reference signals
(CoGEL 19.47) Clausal reference
Common signals for sentence or clause reference include:
anaphoric and cataphoric: here, it, this
(CEG, R. Carter & M. McCarthy, 2006) Deixis (adjective = deictic) A term for words or expressions that depend for their interpretation on the immediate external situation in which they are uttered.
Deictic words are orientational features and are typically realised by
determiners (a, the, this, that, these, those), adverbs (here, there), personal pronouns (I, you, them).