What is obsessive defilement?

This is exactly a common behaviour during practice. There are certain ways you could identify this

• These thoughts originated from a deep level of yours (chilhood/dark)
• Your defilment has become less during that day/week/time (If you didn't do unique or something memorable on that day as Vinyana hasn't stored much)
• Your defilement had actually become less (But it should stay blank when you are practicing )

The best practice is obsessive defilment (Always!). You can try watching television and practice the meditation and check those thoughts appear as before. If it does, you must continue to make the defilment less.

If you are following Four Satipattana, and if you tend to see less thoughts; you are in the 3rd level arūpajhāna and trying to settle in the arūpajhāna. It is best if you could get Kamatahan (advices) from a teacher/someone superior to you. This helps you to clarify your level and the path to your ultimate goal.

Edit

If you add something in there about sankharas being the scheme of the unconscious mind which influence unwholesome thoughts, and clear-up the contention with arupajhana,

When following Four Sattipattana while meditating, in simple terms:

• Kāyānupassanā : Focussing on the body and its' actions
• Vedānupassanā : Focussing on the contemplation on feelings occuring by the Kāyānupassanā
• Chiththānupassanā : Focussing on mind and thoughts
• Dhammānupassanā : Focuss on Dhamma/teachings

You are no longer talking (Vachi Sankhara), your eyes are not open, you are not focussing on other conversations happening, You are in one place and in the meditation position (kāya,saṅkhāra), You are not thinking of anything else rather than the meditation practice(citta saṅkhāra).

No saṅkhāra is happening when you are meditating as all the doors are closed and the subconscious mind is unable to save photocopies what's happening around you (cannot see, hear, feel). The subsconscious mind (or the wizard) is the photocopier of your mind. Therefore the subconcious mind can only distract you from your memories, hallucinations, illusions.

When someone is following the Four satipattana,

• You are following to clear the Khlesaa or Defilment
• You are aware of the subconcious thoughts flowing to your as you are not doing any sankhara
• You are not attached when practicing (Kayanupassana), and you are looking at Rupa Kalāpa (Grouping of Matter) unattached which you are in 1st dhyana

Quite secluded from sense pleasures, secluded from unwholesome states of mind, he enters and dwells in the first jhana, which is accompanied by applied thought and sustained thought with rapture and happiness born of seclusion. (M.i,1818; Vbh.245)

• When you practice the four satipattana, feelings, pains occur for your past khlesa(Mental Impurities)/akusal(unwholesome deeds). Eg: You will feel pinching, tearing you apart, burning and many more according to your Karma. You tend to focus on Grouping of Matter and Maitri the groupings which is Vedānupassanā

• Since you are not attached, and the mind is not attached to Kaya(body), you are overviewing the Grouping of matter unattached. Therefore you are in the second Dhyana by Sukha

• Since you are not attached to the Kaya, you are feeling the emptiness (But not always with the Vedānupassanā).

• Until you clear the defilments with Maitri Grouping of matter, you will not be in the 4th Dhyana

• When you are not attached you are feeling consciousness, a sensation (1 of the Chiththakshana -Mental factors[1/27]) which is the 1st level of arupa dhyana but you are experiencing the 3rd level of arupdhyana ("The Base of No-thingness") when your defilment is getting almost cleared. But it doesn't settle there unless you are sotapanna.

This is very hard to explain when translating to english. You could experience it to yourself by you will need to read abhidhamma pitaka and apply those into your exercise with the help of a Kamatahan(Advicing) Teacher.