Development of (all forms of..) Argentine Spanish

Can anyone establish the main influences which forged the Spanish spoken in Uruguay and Argentina in terms of immigration, population trends etc.?

And I do not want to limit myself to Rioplatense dialect.

From what I understand there are three broad zones...

1) Rio Platense: From Buenos Aires Province to Rosario, and at least half of Uruguay. Characterized by 'sh' sound for 'll' and 'y-'. Spreading due to media influence (e.g. in Patagonia or areas which were previously sparsely populated)

2) Central zone: from Cordoba and other provinces. 'zh' sound for 'll' and 'y-'
3) Peripheral regions; more similar to other Latin American SPanish variants in that 'y' is used for 'll' and 'y-' (e.g. places like Jujuy, Misiones province..)

My Spanish teacher was Uruguayan, and spoke as you describe those from the Rio Platense zone, with the "sh" sound for "ll". She ascribed the dialect to influence of Italian immigration (she was herself of Italian descent). I know that's a very superficial answer, sorry, and I'll be watching for some more knowledgable responses.

I really liked her accent; it seemed to add a softness to an otherwise quite staccato language, as my ears hear it.

<<Characterized by 'sh' sound for 'll' >>
Spanish "llama"-Portuguese " chama" (ch sounds like English sh)-flame.

<<1) Rio Platense: From Buenos Aires Province to Rosario, and at least half of Uruguay. Characterized by 'sh' sound for 'll' and 'y-'. Spreading due to media influence (e.g. in Patagonia or areas which were previously sparsely populated)

2) Central zone: from Cordoba and other provinces. 'zh' sound for 'll' and 'y-'
3) Peripheral regions; more similar to other Latin American SPanish variants in that 'y' is used for 'll' and 'y-' (e.g. places like Jujuy, Misiones province..) >>

The "zh" sound used in parts of Argentina and Uruguay are not unique to those regions of Latin America. That pronunciation is also used in parts of Chile and is standardized in the speech of Caribbean Panamanian Spanish. So I doubt that particular pronunciation is Italian influenced.

I think that the influence of Italian on Argentinian Spanish may be limited to intonation and a few words here and there. It's Argentinian intonation that is really unique within the hispanosphere if I'm not mistaken.