Perlin Noise detail level. How to zoom in on a landscape?

Assuming that you're generating the map files statically rather than on the fly I think your simplest solution is to generate the entire map at the highest resolution you require. In effect you're then zooming out and back in to that level of granularity. It's more computationally expensive and will yield larger map files. Depending upon your implementation for the viewer you can either use one single map file or build some de-resolutioned files for the bigger view.

You need to generate using noise at successively higher frequencies to avoid the smoothness. Try http://www.arendpeter.com/Perlin_Noise.html for a clearer explanation of how to manipulate noise frequency. Octaves naturally fit together with a level of detail (LOD) implementation, which as long as you support on the fly generation, means you only have to generate more detail as you move close to it. Be aware that there are draw backs to on the fly generation of terrain, such as complicating generation of features like rivers and roads, so you may end up with pre-generation of terrain to a fixed minimum level of detail (e.g. 64 km grid squares), and then on the fly generation of more detailed features.

I'd also recommend looking at Simplex noise which has lower computational overhead (see https://en.wikipedia.org/wiki/Simplex_noise for more details).