Second Language Development (SLD) research has investigated aspects of complexity, accuracy, and fluency (CAF) to understand how English language learners (ELLs) develop linguistic faculties related to language production, especially in academic writing (Biber & Gray, 2010; Biber, Gray & Ponpoon, 2011; Kyle & Crossley, 2018; Lu & Ai, 2015). Contributing to the current body of SLD research, this study addresses complexity in expert academic argumentative essays by investigating language at the phrasal level through phrase-frames (PFs). Phrase-frames are fixed or semi-fixed syntactic structures with a variable slot that might be considered another measure of syntactic complexity. This study aimed to identify if native and non-native expert writers in the Michigan Corpus of Upper-Level Student Papers (MICUSP) displayed significant differences in their use of these frames concerning their predictability, variability, and function. Results of the study indicated expert academic writers in this corpus use highly variable, highly unpredictable structures which confirm the complexity, elaboration, and explicitness unique to the academic register. Understanding of these register features through the identified PFs was used to develop sample pedagogical materials to promote more complex writing in academic genres.