This is a complicated subject and I may start a new thread to warn other expats of the potential problems they may face.
First of all your monotribustia is for basically what they call social security, its not the income tax nor is the wealth tax.
The income tax is completely separate and has been around since the thirties in Argentina. Basically non-residents have to pay on any income they earn in Argentina. Residents are expected to pay the tax on their world-wide income.
The tax rates for Argentina are significantly higher than the U.S. The top rate is 35% and kicks in at $40k and above income per year. The top rate in the U.S. is also 35% but applies only to income above $357k. On the same income your taxes in Argentina would be much higher than in the U.S.
Since you are not aware of this tax I assume you are not paying it. I would advise you to seek professional advice. Maybe people don't pay what they should in Argentina but if you are paying nothing now you are asking for big trouble.
I read the penalty for non-payment is between 50% and 100% on the taxed owed. If they decide to prosecute someone for income tax evasion in Argentina the penalty is 3 1/2 to 10 years in prison.
This is no laughing matter.
The attached link provides the best summary on taxes in Argentina I've seen.
http://www.pwc.com/us/eng/hr/argentina.pdf