"Then" is in this context an adverb meant to provide a connection; this is why in A Comprehensive Grammar of the English Language (Quirk et al., 1985) the type of adverbial that "then" embodies in this sort of sentence is called a conjunct. There is no rhetorical device involved, and the discussion remains in the domain of syntax.
(OLD) then adverb /ðen/
used to show the logical result of a particular statement or situation
- If you miss that train then you'll have to get a taxi.
- ‘My wife's got a job in Glasgow.’ ‘I take it you'll be moving, then.’
- ‘You haven't done anything to upset me.’ ‘So what's wrong, then?’
- Why don't you hire a car? Then you'll be able to visit more of the area.
(CoGEL) § 8.135
[…] conjuncts indicate how the speaker 'views the connection between two linguistic units', [but] such an indication does
not conversely entail the use of a conjunct. The semantic role of expressing a
relation between two units can frequently be fulfilled by an adjunct. Compare
the following (where a pronoun, as frequently, functions as a linking device):
- It was snowing, and in spite of this Mona went cycling.
The adverbial here which conjoins and indicates a concessive relation is an
adjunct, as we see from its propensity to be focused in a cleft sentence:
- . . . and it was in spite of this that Mona went cycling.
By contrast, in:
- It was snowing, and nonetheless Mona went cycling.
the same conjoining function with the same concessive relation is fulfilled
grammatically by a conjunct :
*. . . and it was nonetheless that Mona went cycling. (user LPH: grammatically unacceptable, impossibility of writing cleft sentences shows the non-adjunct nature of "nonetheless")
Conjuncts thus both indicate the relation and are demonstrably outside the
syntactically integrated clause structure which admits adjuncts.
Moreover, there is one significant respect in which we must relax the
semantic characteristic concerning the conjoining of linguistic units. It is
possible for conjuncts to be used as discourse-initial items. Thus speech may
actually begin, in the following way, given a particular context of situation:
- So you're LÈAVing, then! (user LPH: "LÈAV" represents a stressed syllable with falling intonation)
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