It seems to me somehow odd to have Usurpation without having a corresponding Usurpate. I know about back-formation, but in that case both words are present. Am I missing something? Are there more examples of the same situation? Is there a specific term for this?
There are many verbs that don't end in -ate that have corresponding nouns in -ation: converse, conversation; retard, retardation; expect, expectation; inform, information; observe, observation; consult, consultation; exult, exultation; derive, derivation; excite, excitation; provoke, provocation; administer, administration.
The source of the English ending -ation is the Latin ending -ātiō, -ātiōnis. This was used to form nouns related to verbs of the first conjugation, which have a fourth principle part ending in -āt- (containing the first-conjugation theme vowel A).
English verbs taken from Latin are often based on the fourth principle part (giving us -ate verbs from Latin first-conjugation verbs), but not always. It depends on things like how old the verb is and whether it came through French. Latin-derived verbs that do not end in -ate usually have a stressed final syllable that either contains a long vowel or ends in a consonant cluster (but there are other less common forms for these verbs, like administer in the list above).
There are even a few -ation nouns that were formed in English by analogy to the ones taken from Latin. The Oxford English Dictionary entry for -ation mentions the examples starvation and flirtation.
This is called morphological blocking in linguistics. Basically, if there are two or more possible ways to form a word with a particular meaning (for example, by adding derivational affixes), then only one of these words will actually be used. And as a corollary, if two such words do exist, then there are likely to be different shades of meaning between then ("no absolute synonyms").
You will find plenty of examples in English due to its having incorporated French Latinate words and morphology on top of the native words and morphology.
If you google the right jargon you'll find examples discussed in scholarly literature.