Technical architect is a specific role as expectations for person fulfilling it differ much depending on organization and even with the same organization depending on a project.
In short technical architect should take full responsibility for, well, software architecture. Not only does it mean designing it in a way which allows to fulfill all non-functional requirements, e.g. scaling, performance, but also making it reasonably easy for developers to build software. You don't get points for fancy top-notch architecture if development team struggles to build the application and maintain them.
When I think about responsibility I consider it throughout the whole project. Architect's job isn't done when development starts as architecture may be, and usually should be, evolving over time.
Also in many agile projects you may find architecture which is built incrementally - you set only general borders up front and then add and change it as you add new functionality. Again, if you plan to adjust to changing requirements when it comes to features you should plan the same for architecture so the work isn't done at the beginning of the project.
I would also say that it's technical architect who should care about technical debt in project. Whether it is incurred and when, if ever, it is payed back. Of course as long as anyone even cares about such thing.
Usually it isn't direct architect's responsibility but I'd add responsibility for project maintenance as we often see projects which are even finished on time but then maintenance costs skyrocket and sometimes the main reason is either bad architecture design or poor execution, i.e. not ensuring that development team is following guidelines.
Now the vague thing here is how we understand architecture. It can be anything from just some general high-level guidelines up to detailed rules what should and what should not be done, which tools, libraries, design patterns, coding standard, etc. are used and which are not. Anyway, however architecture is defined in the organization or the project all architecture-related issues should be addressed to technical architect.