I use the Tara mantra as my focus during Samatha meditation prior to beginning Vipassana but lately I am beginning to wonder...am I falling asleep? I can't tell, really. I don't get drowsy but my mantra becomes so quiet that it begins to merge with the thought-stream that it is resting on, and then I lose awareness of both until my consciousness returns to the fact that I am still chanting, but without any awareness of what, if anything, I was actually thinking about-the thoughts being too quiet to catch or to get caught in. But just as there is no going, there is no returning either (no waking up) with this experience, just very subtle ebb and flow. When I began a regular meditation practice about five months ago discursive thought would become loud enough to break my concentration. It still does, of course, more often than not, and I thoroughly enjoy the challenge of "getting caught up in the flow" only to return to the mantra. But lately it is like meditation has lost its urgency. It has become so much a part of my life. It has become like that ebb and flow and I just want to rest in the stillness of that. It is just so beautiful...but then I start to think that maybe it is an escape from being present and that my mind is playing with me. I would really like to know what is going on, and if there is somewhere I should be going with this, some way that I should be practice with it skillfully.
Ah, the bliss plateau. Good that you caught this now. Some practicioners never get past it, the quietitude is too enticing for them to push past. In Zen, we call this "dead sitting" and it can completely ruin the practice of even experienced monks.
What you're experiencing now is step two on your way to absorption. Step one was the monkey mind that you put to rest. Here, correct me if I'm wrong, but I bet you can sit longer than you could in stage one. Your body is probably moving less, and, like you mentioned, your thoughts aren't as intrusive.
In order to go beyond, you need an anchor point. Unless if you have one to apply your mind to, you are going to drift off into that see of oceanic bliss and never find your way to the other shore.
You're working with a mantra. That's similar to how we work with a koan in Rinzai. One thing I've found effective is to hold the vowel sounds. Keep them going so the mantra takes up the entirely of a single breath. Wait for that exhalation to stop. Don't force the breath out. The technique I have been using more often these days is to simply find a spot and anchor your attention there. Don't force the concentration point. Sit on it. Just like you don't have to force your butt down to stay seated, just plop your attention down on that point and let it sit there. Dont let your mind shift off that spot.
If you're doing this right, a feeling of energy is going to start to emerge. It may feel like tension or discomfort in the body. It can also start as a feeling of pressure in the head. Theres going to be a temptation to move your body or slide off your seat of concentration. Resist that. Stay with that energy. Eventually it will crack - but please don't try to crack it.
When that happens, you'll have this meditation thing pretty well sorted!