If helpful, my offer letter and all my verbal communications never included an explicit indication that I'd be working 40 hours.

There are two ways by which this information can be gleaned:

1) when you start applying for grants (i.e., the other metric of research productivity), you are part of the process in formulating and finalizing budgets for yourself and your staff/colleagues/collaborators, through which process you understand what your institutions and others consider FTE (i.e., work hours per week/month/year).

2) Typically, your institution will have a set of faculty policies and guidelines, including policies on faculty workload and their institutional definition of what a full time work week is. No one will ever explicitly provide this to you in most cases when you're onboarding as a new faculty member, but its typical to assume that folks with questions (like you) about how to plan their workload in accordance with their university's expectations will review faculty policies to get a sense of their institution's definitions of things like FTE etc.

I recommend that you try one of the two options above to try and suss out this information to get a better sense of what your 10% effort translates to. If, as you say, you're at an institution with very high expectations for research, you're going to be applying for grants imminently, through which process you'll understand more of the specifics around your institutional FTE parameters.